JP4143576B2 - MRI equipment - Google Patents

Description

  The present invention relates to an MRI (magnetic resonance imaging) apparatus, and in particular, a pulse sequence incorporating a plurality of high frequency reversing pulses (RF refocusing pulses) such as a CPMG pulse sequence and a phase reversal CP pulse sequence, , RARE method (high-speed SE method), GRASE method (Hybrid EPI method)).

  Magnetic resonance imaging (MRI), which utilizes the magnetic resonance phenomenon of nuclear spins, can be obtained noninvasively in an image of a subject. Therefore, it is increasingly being used in the field of medical technology. With the advancement and sophistication of the above, the qualitative requirements for MR images and the degree of high-speed imaging are also increasing.

  Conventionally, various pulse sequences for magnetic resonance imaging have been implemented or proposed in an attempt to satisfy these conditions. As one of them, a pulse sequence called RARE method (for example, Non-Patent Document 1: “Magnetic Resonance in Medicine Vol. 3, 823-833, 1986”, Non-Patent Document 2: “Journal of Magnetic Resonance Imaging Vol. 78,” 397-407, 1988 ”) and a pulse sequence called GRASE method (for example, see Patent Document 1:“ US Pat. No. 5,270,654 ”).

  FIG. 35 shows a pulse sequence of the RARE method, and FIG. 36 shows a pulse sequence of the GRASE method. In these drawings, a pulse RFex indicates a high frequency excitation pulse (RF excitation pulse), and pulses RFre1 to RFre5 indicate high frequency inversion pulses (RF refocusing pulses). Subsequent to the high frequency excitation pulse RFex, high frequency inversion pulses RFre1 to RFre5 are sequentially applied. The pulse Gss is a slice direction gradient magnetic field, the pulse Gpe is a phase encode direction gradient magnetic field, the pulse Gro is a read direction gradient magnetic field, and waveforms E (1) to E (5) (or E (1,1) to E (5,3) represents an NMR echo signal (waveform after phase detection) generated by the CPMG pulse sequence, and the phase φ when the high frequency pulses RFex and RFre1 to RFre5 are applied is the high frequency excitation pulse RFex. On the other hand, φ = 0 ° and φ = 90 ° are set for the high frequency inversion pulses RFre1 to RFre5, and the flip angle θ for depressing the magnetization spin by the high frequency pulse is θ = for the high frequency excitation pulse RFex. α (generally 90 °), θ = β1 to β5 (generally 180 °) with respect to the high-frequency inversion pulses RFre1 to RFre5, both of the pulse sequences in FIGS. A plurality of echo signals are generated, and each echo signal is subjected to phase encoding (phase encoding direction gradient magnetic field pulse Gpe) having a different magnitude (pulse area) to obtain echo data necessary for reconstruction of one frame image. As a result, the imaging time can be reduced from a fraction to a hundredth compared with the conventional spin echo method.

  In the case of a pulse sequence using a plurality of high frequency inversion pulses such as this CPMG pulse sequence and phase inversion CP pulse sequence, the respective echo signals collected between the respective high frequency inversion pulses have passed through a plurality of paths on the phase diagram. It has been clarified that it is the sum of echo components (for example, Non-Patent Document 2: “Journal of Magnetic Resonance Imaging Vol. 78, 397-407, 1988”, Non-Patent Document 3: “Magnetic Resonance in Medicine Vol. .30, 183-191, 1993 ”).

  When these pulse sequences are used, the main causes are eddy currents generated in the conductor part of the MRI apparatus due to the switching of the gradient magnetic field pulses, manufacturing errors of the gradient magnetic field coil, and other imperfect calibration of the MRI apparatus. In each echo component, a phase shift having a different spatial distribution occurs. Specifically, for example, Patent Document 2: JP-A-6-121777, Patent Document 3: JP-A-6-54827, and Patent Document 4: US Pat. No. 5,378,985. Are listed. For this reason, ghost artifacts occur in the phase encoding direction of the reconstructed image, the signal value of the image decreases locally, the S / N ratio of the entire image decreases, and the image quality deteriorates significantly. I might let you. This inconvenience is particularly noticeable in the case of imaging with high spatial resolution where the intensity of the gradient magnetic field pulse must be changed greatly during a short switching time or imaging with a very large number of echoes.

  One method (first prior art) for solving the problem of image quality degradation is disclosed in the above-mentioned Patent Document 4: US Pat. No. 5,378,985. Prior to actual imaging (hereinafter referred to as “main scan”) for acquiring an MR image of the subject, a scan called pre-scan is performed. In this pre-scan, scanning is performed with the intensity of the gradient magnetic field pulse in the phase encoding direction being zero, and one-dimensional Fourier transform is performed on each echo signal obtained thereby. The zeroth-order and first-order components of the phase distribution thus obtained are calculated as correction data, and the gradient magnetic field pulse waveform and the phase in which the high-frequency pulse is applied (hereinafter referred to as “application phase”) are corrected. . As shown in FIG. 38, this applied phase is the phase φ of the magnetization M when a high frequency pulse is applied to the reference coordinate axis on the X ′, Y ′ plane of the rotating coordinate system X ′, Y ′, Z ′, for example, X ′. Represented as:

  By the way, in the imaging using a plurality of high frequency inversion pulses such as the CPMG pulse series and the phase inversion CP pulse series, another physical property is also known. That is, the transverse magnetization receives a phase error between the high-frequency excitation pulse and the first high-frequency inversion pulse, and the transverse magnetization is uniformly uniform between the adjacent high-frequency inversion pulses thereafter. When received, each echo signal (the sum of echo components that have passed through a plurality of paths) is divided into two groups of echo groups. This property is shown, for example, in Non-Patent Document 4: “Magnetic Resonance in Medicine Vol. 30, 251-255, 1993”.

  When a pulse sequence such as the RARE method or the GRASE method is used, the gradient magnetic field pulse having substantially the same shape is repeatedly applied as described above, so that not only the gradient magnetic field pulse itself but also the magnitude of the gradient magnetic field pulse is used. It is considered that the eddy magnetic field having a proportional property also causes a phase error based on the pulse repetition pattern and is divided into two echo component groups. Therefore, it can be considered that the phase shift pattern of each echo signal and the image quality degradation of the reconstructed image are caused approximately by mutual interference between the echo components of the two groups.

The phase error that the transverse magnetization of the nuclear spin receives between the high frequency excitation pulse and the first high frequency inversion pulse is Δφ1, and the phase error that the transverse magnetization uniformly receives between the adjacent high frequency inversion pulses is Δφ2. Among the two groups of echo components observed between the nth and (n + 1) th high frequency inversion pulses, the main echo component Emain includes an echo component of a path that is phase-reversed by all high frequency inversion pulses. (n) If the other echo components are sub-echo components Esub (n), the phases of both echo components Emain (n) and Esub (n) are

It is represented by

  Since the echo signals generated at each point in the imaging region have such a correspondence, the phase dispersion of the echo signal, which is the sum (observed) of the echo signals, and the magnitude of phase dispersion indicating the degree of phase dispersion is also included. There is a similar correspondence. FIG. 38 is a phase diagram schematically illustrating changes in the magnitude of the phase dispersion of each echo signal due to the gradient magnetic field in the read direction between the high frequency inversion pulses. The vertical axis of the figure is the magnitude (relative value) of the phase dispersion of the echo signal due to the gradient magnetic field in the lead direction, so that the path showing the change in the figure of the echo component with the same phase dispersion can be easily understood. Intentionally, the time integral value of the gradient magnetic field in the read direction between the high frequency excitation pulse and the first high frequency inversion pulse is increased by ΔA from the normal value A (normally 1/2 of pulse B in the figure). ing. As shown in the above-mentioned document 2), a transverse magnetization component (echo component) having a certain phase dispersion is preserved by a high-frequency inversion pulse as a transverse magnetization component whose dispersion relation is reversed, a transverse magnetization component that is not reversed, and a longitudinal magnetization. It is known to differentiate into three of the components. Since the application interval of the subsequent high-frequency inversion pulse and the gradient magnetic field waveform to be applied are regular, the echo components of several paths on the phase diagram overlap with the same phase dispersion. It becomes a regular dispersion pattern. In the figure, the thick solid line is a group of echo components in phase with the main echo component Emain, and the thin solid line is a group of echo components in phase with the sub-echo component Esub. Also, during the echo collection period, most of the echo components whose phase dispersion is greatly separated in the positive or negative direction are as echo signals due to the phase interference between the spins in the group to which the echo component belongs. It is in a so-called spoiled state that cannot be observed. Therefore, the portion actually collected as an echo signal is mainly the portion indicated by the arrow in the figure (a component having a moment when the phase dispersion becomes zero during data collection).

On the other hand, looking at the phase of the echo signal, the relationship between the phase of the echo signal and the applied phase of the high frequency pulse satisfies the conditions of the applied phase of the CPMG pulse sequence or the phase-inverted CP pulse sequence, and other hardware incompleteness If there is no spin phase error due to the nature, the phases of the main echo component Emain (n) and the sub-echo component Esub (n) coincide. The actually collected echo signal is the sum of both echo components,
[Equation 3]
E (n) = Emain (n) + Esub (n)
It becomes. Even if unadjusted, ΔA in FIG. 38 is sufficiently small, so the peaks of the two echo components often appear to overlap.

  Further, it has been confirmed that the amplitudes of both echo components Emain (n) and Esub (n) depend on the number n of high-frequency inversion pulses, the slice characteristics of the high-frequency pulses, the flip angle, etc. (for example, Non-Patent Document 5). : "Society of Magnetic Resonance in Medicine, Pro. Of Annual Meeting in 1992 No. 4508", Non-Patent Document 6: "Society of Magnetic Resonance in Medicine, Pro. Of Annual Meeting in 1991 No. 1025").

  Therefore, the phase error cannot be extracted from the collected echo signal unless the amplitude ratio R (n) of both echo components Emain (n) and Esub (n) is known.

Under such circumstances, the magnetic resonance imaging method and apparatus described in Japanese Patent Laid-Open No. 6-54827 described above is another prior art (second prior art) for solving the above-mentioned problems. Is disclosed. When the correction processing according to this disclosure is indicated by the same symbol as above,
[Equation 4]
｜ Esub (1) | = 0
And if the flip angle of the first high frequency inversion pulse is approximately 180 °,
[Equation 5]
| Esub (2) | = nearly zero. For this reason,

It becomes. Using this, the gradient magnetic field waveform of the main scan and the application phase of the high frequency pulse are corrected only from the information of the first echo signal and the second echo signal obtained from the pre-scan similar to the above, and it is relatively simple. A correction is to be made.

  The second prior art will be described in detail with reference to FIGS. FIG. 39 shows a pre-scan pulse sequence, FIG. 40 shows a main-scan pulse sequence, and FIG. 41 shows a correction processing flowchart.

As shown in FIG. 41, prescanning is first performed according to the pulse sequence of FIG. 39 (step 101). In this prescan, the gradient magnetic field in the phase encoding direction is always zero (Gpe = 0). If there is a phase shift due to some cause, each echo signal except the first echo signal E (1) is divided into two echo components Emain (n) and Esub (n) as shown in FIG. Actually, the difference between the two echo components Emain (n) and Esub (n) is small, the echo peak is not divided into two, and the echo peak position shift p (n) and the phase shift φ (n) It changes so as to vibrate alternately for each echo. According to the second prior art, the positional deviations p (1) and p (2) of the echo peaks of the first and second echo signals E (1) and E (2) and the phase deviations φ (1) and φ ( 2) is measured (step 102). When the distance from the magnetic field center to the selected slice plane is xs, the correction amount Δφ of the application phase of the high-frequency pulse (or “ΔGss · xs” with respect to the correction amount ΔGss of the slice direction gradient magnetic field pulse Gss) is the phase shift φ (1 ), Φ (2), and ΔGro is proportional to the difference between the positional deviations p (1), p (2). Based on this relationship, the correction amount Δφ of the applied phase of the high frequency pulse and the slice direction gradient magnetic field pulse Gss during the main scan are corrected from the obtained p (1), p (2), φ (1), φ (2). One of the amounts ΔGss and the correction amount ΔGro of the read direction gradient magnetic field pulse Gro are calculated (step 103). Next, the pulse sequence is corrected with these correction amounts Δφ (or ΔGss) and ΔGro (see FIG. 40), and the main scan is executed by this pulse sequence (step 104).
US Pat. No. 5,270,654 JP-A-6-121777 JP-A-6-54827 US Pat. No. 5,378,985 “Magnetic Resonance in Medicine Vol.3, 823-833, 1986” “Journal of Magnetic Resonance Imaging Vol.78, 397-407, 1988” “Magnetic Resonance in Medicine Vol.30, 183-191, 1993” “Magnetic Resonance in Medicine Vol.30, 251-255, 1993” “Society of Magnetic Resonance in Medicine, Pro. Of Annual Meeting in 1992 No. 4508” “Society of Magnetic Resonance in Medicine, Pro. Of Annual Meeting in 1991 No.1025”

  The above-described first and second prior arts are not intended to improve problems such as artifacts in the phase encoding direction caused by the phase shifts of the respective echo components having different spatial distributions. However, as shown below, there were unsolved problems.

  (1) First, when the flip angle of the high frequency inversion pulse is deviated from 180 °, there is a problem in that the correction accuracy of the application phase of the high frequency pulse and the gradient magnetic field waveform with respect to the main scan is lowered.

  For example, if | Emain (n) | = | Esub (n) |, no matter how large the phase difference between the two echo components Emain (n) and Esub (n) is, the two echo components Emain (n) and Esub The phase of the nth echo signal collected as the vector sum of (n) does not change. For this reason, even if the first and second conventional techniques are implemented, the correction is not performed accurately, and the image quality of the MR image is not improved at all, or even if improved, it is insufficient. Often ends.

  The situation where the flip angle deviates or deviates from 180 ° is nothing special. For example, as described in the aforementioned Non-Patent Document 5: “Society of Magnetic Resonance in Medicine, Pro. Of Annual Meeting in 1992 No. 4508”, even in the case of the current FSE (FastSE) method, It is reported that the amplitudes of the main echo component Emain (n) and the sub-echo component Esub (n) are substantially equal in the eighth echo. In Non-Patent Document 3: “Magnetic Resonance in Medicine Vol. 30, 183-191, 1993”, each slice characteristic and flip angle of the high-frequency inversion pulse are individually changed in order to stabilize the amplitude of each echo signal. I am letting. Furthermore, an attempt to reduce attenuation of each echo signal due to T2 relaxation using this technique has been reported (see “Society of Magnetic Resonance, Proc. Of 2nd Annual Meeting in 1994 No. 27”). Furthermore, in the case of multi-slice imaging, there is a report that the flip angle of the high-frequency inversion pulse can be set to a value of 180 ° or less to reduce the SAR (RF exposure) and the change in tissue contrast due to the MTC effect ( (See “Society of Magnetic Resonance in Medicine, Proc. Of Annual Meeting in 1993 No. 1244”).

  As described above, the flip angle of the high-frequency inversion pulse often deviates from 180 °, and the correction according to the first and second conventional techniques described above is insufficient in terms of accuracy and stability under such circumstances. .

  This problem is also pointed out in Patent Document 3: Japanese Patent Laid-Open No. 6-54827 described above (see the 11th page, lines 18-27 of the same publication). To solve this problem, the technique of this patent publication shows a method of avoiding such as dephasing one echo component by making full use of the gradient magnetic field in the slice direction, but the waveform of the gradient magnetic field pulse is deformed. There is a problem that it is often not suitable for automation of sequence adjustment, such as being affected by imperfection of system calibration.

  (2) The second problem is that, among a plurality of causes of phase shift, correction by the conventional method is due to some factors and not all. In both the first and second prior arts described above, the gradient magnetic field pulse Gpe in the pre-scan phase encoding direction is always zero. For this reason, the echo signal obtained by the pre-scan does not include a phase shift component caused by the gradient magnetic field pulse in the phase encoding direction in the main scan. That is, a): a deviation of the gradient magnetic field pulse itself in the phase encoding direction, b); a phase deviation having a zero-order or first-order spatial distribution due to the eddy magnetic field caused by the gradient magnetic field pulse in the phase encoding direction, etc. It cannot be corrected in principle. In recent years when the spatial resolution of scans is becoming higher and faster, depending on the type of pulse sequence, the phase shift component caused by the gradient magnetic field in the phase encoding direction cannot be completely ignored. But I want to break it down.

  (3) The third problem is that the accuracy of the correction value changes depending on the state of the imaging target. In the above-described first and second conventional techniques, correction information is obtained based on an echo signal from an imaging target. Therefore, an accurate phase error may not be measured depending on the shape and state of the imaging target. There is a problem in terms of stability.

  For example, in the case of a medical diagnostic device, the main signal source in the T2-weighted image of an axial image of the cervical spine is cerebrospinal fluid (CSF) that flows in a direction substantially perpendicular to the cross section. Due to the gradient magnetic field pulse, the phase change of the nuclear spin related to the flow velocity occurs. When this phase change is corrected by the methods shown in the conventional first and second prior arts, the phase change appears almost as it is in the pre-scan, so the correction amount required from the pre-scan is not accurate and reliable. The correction accuracy is low and the stability is inferior. For example, the target phase change may be erroneously corrected. For this reason, even when the nuclear spin is stationary, it may result in problems such as ghost artifacts appearing, signal value decreases and S / N ratio decreases, and a high echo signal is generated. When the movement of the camera is intense, the image quality may deteriorate. This is a problem that cannot be avoided to some extent in the case of a correction method characterized in that correction information (correction amount) of a pulse sequence at the time of a main scan is obtained in advance from an imaging target whose shape and state are unknown. It is a problem.

  The present invention has been made in view of the above problems and problems. (1); In magnetic resonance imaging which presupposes processing for performing a pre-scan and obtaining a correction amount necessary for the pulse sequence during the main scan, it exhibits higher resistance to fluctuations in the flip angle of the high-frequency inversion pulse than before, Provided an MRI pulse sequence automatic correction method and an MRI apparatus capable of performing a pre-scan of stable operation, correcting a pulse sequence at the time of a main scan with higher accuracy, and suppressing image quality deterioration of a reconstructed MR image This is the first object of the present invention. In addition, (2); in the magnetic resonance imaging that presupposes the processing to obtain the correction amount necessary for the pulse sequence during the main scan, the phase shift component caused by the gradient magnetic field pulse in the phase encoding direction is also accurate. It is a second object of the present invention to improve the image quality of the reconstructed MR image so that it can be corrected well. Further, (3) in the magnetic resonance imaging which presupposes the processing for obtaining the correction amount necessary for the pulse sequence during the main scan by performing the prescan, the main scan is performed even when the main signal source is moving at high speed. It is a third object of the present invention to prevent a reduction in correction accuracy of a pulse sequence at the time and to suppress deterioration in image quality of a reconstructed MR image.

  Summarizing the above, at least one of the first to third objects described above is achieved, and it is necessary for imaging using a pulse sequence using a plurality of high-frequency inversion pulses such as a CPMG pulse sequence and a phase inversion CP pulse sequence. By collecting and correcting correction information more systematically than before, improving the accuracy and stability of such correction and providing high-quality MR images in terms of spatial resolution, ghost artifacts, image contrast, etc. is there.

In order to achieve the above-described problem , according to the MRI apparatus of the present invention , a high-frequency excitation pulse and a plurality of high-frequency inversion pulses are provided before the main scan for obtaining the MR image of the subject. The first pre-scan of the pulse sequence in which the application phase of the high-frequency inversion pulse is set to the reference phase value, and the even-numbered high-frequency wave having the high-frequency excitation pulse and the plurality of high-frequency inversion pulses. Scanning means for separately performing a second prescan of a pulse sequence in which the application phase of the inversion pulse is set to a value having a phase difference of 180 ° with respect to the reference phase value, and obtained by the first prescan A first echo data group comprising a plurality of echo signals and a second echo data group comprising a plurality of echo signals obtained by the second pre-scan; Based a correcting means for correcting the pre-pulse sequences of the present scan, said first, phase-encoding gradient field pulse sequence of the second pre-scanning is always zero, the correction means, said first A sub-echo composed of a main echo assembly composed of a main echo component and an echo component other than the main echo component obtained by adding or subtracting the corresponding echo data of one echo data group and the second echo data group. means for separating and extracting the assembly, is characterized in that it has a said main, it means for automatically correcting the pulse sequence of the main scan by the correction data determined on the basis of the sub echo aggregate .
In order to achieve the above-described problem, the MRI apparatus according to the present invention includes a high-frequency excitation pulse and a plurality of high-frequency inversion pulses before the main scan for obtaining the MR image of the subject. A first pre-scan of a pulse sequence in which an application phase of a plurality of high-frequency inversion pulses is set as a reference phase value, and an even-numbered one of the plurality of high-frequency inversion pulses having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses Scanning means for separately performing a second pre-scan of a pulse sequence in which the application phase of the high-frequency inversion pulse is set to a value having a phase difference of 180 ° with respect to the reference phase value, and the first pre-scan A first echo data group comprising a plurality of obtained echo signals and a second echo data comprising a plurality of echo signals obtained by the second pre-scan. Correction means for correcting the pulse sequence of the main scan in advance based on the group, the gradient magnetic field for phase encoding of the pulse sequences of the first and second pre-scans is always zero, and the correction means From the main echo aggregate consisting of main echo components by adding or subtracting the corresponding echo data in the corresponding order of the first echo data group and the second echo data group and the echo components other than the main echo component Means for separating / extracting the sub-echo assembly, and at least the phase shift and the position shift of the echo peak of the odd-numbered and even-numbered echo data in each set of the main echo set and the sub-echo set Means for calculating one of them, and means for automatically correcting the pulse sequence of the main scan based on correction data obtained from the calculated value. It is a sign.
Furthermore, in order to achieve the above-described problem, the MRI apparatus according to the present invention has a high-frequency excitation pulse and a plurality of high-frequency inversion pulses before the main scan for obtaining the MR image of the subject. A first pre-scan of a pulse sequence in which an application phase of a plurality of high-frequency inversion pulses is set as a reference phase value, and an even-numbered one of the plurality of high-frequency inversion pulses having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses Scanning means for separately performing a second pre-scan of a pulse sequence in which the application phase of the high-frequency inversion pulse is set to a value having a phase difference of 180 ° with respect to the reference phase value, and the first pre-scan A first echo data group comprising a plurality of echo signals obtained and a second echo data comprising a plurality of echo signals obtained by the second pre-scan. Correction means for correcting the pulse sequence of the main scan in advance based on a group of data, and the phase encoding gradient magnetic field of the first and second pre-scan pulse sequences is always zero, and the correction means Is a main echo aggregate composed of main echo components by adding or subtracting the corresponding echo data of the first echo data group and the second echo data group and echo components other than the main echo component Means for separating / extracting a sub-echo assembly comprising: means for comparing 0th-order or first-order phase distribution of at least a part of each of the main echo assembly and the sub-echo assembly; And a means for automatically correcting the pulse sequence of the main scan based on the correction data obtained in accordance with the comparison result.

For example, the first and second pre-scan pulse sequences and the main scan pulse sequence are CPMG pulse sequences or phase-inverted CP pulse sequences. Further, for example, the first and second pre-scan pulse sequences and the main scan pulse sequence are pulse sequences according to the GRASE method or the high-speed SE method .

In order to achieve the above-described problem, the MRI apparatus according to the present invention includes a high-frequency excitation pulse and a plurality of high-frequency inversion pulses before the main scan for obtaining the MR image of the subject. A first pre-scan of a pulse sequence in which an application phase of a plurality of high-frequency inversion pulses is set as a reference phase value, and an even-numbered one of the plurality of high-frequency inversion pulses having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses Scanning means for separately performing a second pre-scan of a pulse sequence in which the application phase of the high-frequency inversion pulse is set to a value having a phase difference of 180 ° with respect to the reference phase value, and the first pre-scan A first echo data group comprising a plurality of obtained echo signals and a second echo data comprising a plurality of echo signals obtained by the second pre-scan. Correction means for correcting the pulse sequence of the main scan in advance based on the group, and the gradient magnetic field for phase encoding of the pulse sequences of the first and second pre-scans on the k-space in the main scan The waveform of the gradient magnetic field for phase encoding is the same as that of a shot at the time of partial or entire echo data collection, and the correction means is on the time axis of the first echo data group and the second echo data group. A main echo assembly consisting of the main echo component and an auxiliary echo assembly consisting of an echo component other than the main echo component with respect to the echo data corresponding to the effective echo time by adding or subtracting the echo data corresponding to the effective echo time of The phase of the echo peak of the echo data corresponding to the effective echo time between the main echo assembly and the secondary echo assembly is separated and extracted. And a means for calculating at least one of the deviation and the positional deviation, and means for automatically correcting the pulse sequence of the main scan with correction data obtained from the calculated value.

Furthermore, in order to achieve the above-described problem, an MRI apparatus according to the present invention includes a high-frequency excitation pulse and a plurality of high-frequency inversion pulses and a plurality of the high-frequency inversion pulses before the main scan for obtaining an MR image of the subject. The first pre-scan of the pulse sequence in which the application phase of the high-frequency inversion pulse is set to the reference phase value, and the even-numbered high-frequency wave having the high-frequency excitation pulse and the plurality of high-frequency inversion pulses. Scanning means for separately performing a second prescan of a pulse sequence in which the application phase of the inversion pulse is set to a value having a phase difference of 180 ° with respect to the reference phase value, and obtained by the first prescan A first echo data group composed of a plurality of echo signals and a second echo data group composed of a plurality of echo signals obtained by the second pre-scan. Correction means for correcting the pulse sequence of the main scan in advance based on the phase sequence, the gradient magnetic field for phase encoding of the first and second pre-scan pulse sequences is a part of the main scan in the k space. Alternatively, the waveform of the gradient magnetic field for phase encoding is the same as that of the shot at the time of collecting echo data of the entire surface, and the correction means adds or subtracts the echo data of the first echo data group and the second echo data group. Means for separating / extracting a main echo assembly composed of a two-dimensional main echo component in k-space and a two-dimensional sub-echo assembly in k-space composed of echo components other than the main echo component; The phase shift of the echo peak of the desired main echo component from the main echo assembly and the secondary echo assembly, the two-dimensional positional deviation vector, and the desired secondary echo component And means for calculating at least one of an echo peak phase shift and a two-dimensional position shift vector, and means for automatically correcting the pulse sequence of the main scan with correction data obtained from the calculated value. It is what.

  The correction means further includes means for obtaining information related to the phase error inherent to the combination of the imaging channel and the physical channel of the gradient coil based on the first and second echo data groups, and the main scan. Sometimes, it may be configured to include means for obtaining correction data of the pulse sequence of the main scan from the information related to the phase error and the imaging conditions related to the gradient magnetic field.

  As described above, according to the MRI apparatus of the present invention, the first pre-scan of the pulse sequence in which the application phase of the high frequency inversion pulse is set to the reference phase value, and the even number among the plurality of high frequency inversion pulses. A plurality of echoes obtained by performing the second pre-scan of the pulse sequence in which the application phase of the high-frequency inversion pulse is set to a value having a phase difference of 180 ° with respect to the reference phase value is obtained separately. In order to correct the pulse sequence of the main scan in advance based on the first echo data group of signals and the second echo data group of a plurality of echo signals obtained by the second pre-scan, Unlike conventional methods, sequence correction is performed based on well-separated main and sub-echo components. Can exhibit high resistance to changes in the flip angle of the wave inversion pulse becomes a stable pre-scan, by force has a high correction accuracy can provide high-quality MR image.

  In particular, the phase encoding gradient magnetic field of the first and second pre-scan pulse sequences has the same phase encoding gradient as the shot at the time of echo data collection arranged on the data line including the center position of the k space in the main scan. By using a magnetic field waveform, it is possible to correct a spatially uniform phase shift component (zeroth order component) among the phase shift components caused by the gradient magnetic field in the phase encoding direction. The phase encoding gradient magnetic field of the first and second pre-scan pulse sequences is the same as the waveform of the phase encoding gradient magnetic field that is the same as the shot at the time of echo data collection on part or the entire k-space in the main scan. By doing so, it is possible to correct a component having a first-order spatial distribution in the phase encoding direction among the phase shift components caused by the gradient magnetic field in the phase encoding direction. As described above, the phase shift component caused by the gradient magnetic field in the phase encoding direction, which could not be measured conventionally, can be suitably corrected, the correction accuracy can be further improved, and a high-quality MR image can be provided.

  In particular, an ideal phantom having a uniform MR signal intensity distribution without movement can be prepared in advance by setting the target for performing the first and second pre-scans to be a phantom different from the subject. Even when the main MR signal source in the subject moves violently, accurate correction data can be obtained and the correction accuracy can be improved. Therefore, the restraint time of the subject is reduced, and MR diagnosis can be facilitated.

  As described above, according to the present invention, in the imaging using a pulse sequence using a plurality of high-frequency inversion pulses such as a CPMG pulse sequence and a phase inversion CP pulse sequence, collection and correction of necessary correction information is more systematic than in the past. In this way, it is possible to improve the accuracy and stability of such correction and provide a high-quality MR image in terms of spatial resolution, ghost artifacts, image contrast, and the like.

  Hereinafter, various embodiments of the present invention will be described with reference to the drawings. First, the correspondence between the embodiment described in detail below and the first to third objects according to the present invention will be described.

  The first embodiment is the first object, the second embodiment and its modified form is the second object, the third embodiment is the second object, the fourth embodiment and its modified form. Tries to achieve each of the third objectives. The fifth embodiment takes an approach different from the first to fourth embodiments, and provides high-quality MR images in terms of spatial resolution, ghost artifacts, image contrast, and the like.

(First embodiment)
A first embodiment according to the present invention will be described with reference to FIGS.

  A schematic configuration of a magnetic resonance imaging (MRI) apparatus according to this embodiment is shown in FIG. This MRI apparatus takes charge of system control and image reconstruction, a magnet part for generating a static magnetic field, a gradient magnetic field part for adding position information to the static magnetic field, a transmitting / receiving part for selective excitation and MR signal reception. And a control / arithmetic unit.

The magnet section includes, for example, a superconducting magnet 1 and a static magnetic field power source 2 that supplies current to the magnet 1, and the Z-axis direction (longitudinal direction) of a cylindrical diagnostic space into which a subject P as an imaging target is inserted. generating a static magnetic field H ₀ in the direction).

The gradient magnetic field unit includes three sets of gradient magnetic field coils 3x to 3z in the X, Y, and Z axis directions incorporated in the magnet 1, a gradient magnetic field power supply 4 for supplying current to the gradient magnetic field coils 3x to 3z, and the power supply 4 is provided. When this gradient magnetic field sequencer 5a includes a computer and receives a signal for instructing a collection sequence such as FSE method and GRASE method for performing pre-scan and main scan from a controller 6 (equipped with a computer) of the entire apparatus, In response to this, the process shown in FIG. 2 is executed. Thus, the gradient sequencer 5a is, X in accordance with the commanded sequence, Y, and controls the application and its intensity of each gradient magnetic field in the Z axis direction, those of the gradient magnetic field is enabled superposed on the static magnetic field H ₀ Yes. In this embodiment, a slice gradient G _S the gradient magnetic field in the Z-axis direction of the three mutually orthogonal axes, its X-axis direction and a readout gradient field G _R, further Y-axis direction that the phase The gradient magnetic field for encoding _GE is used.

  The transmission / reception unit includes a high-frequency coil 7 disposed in the vicinity of the subject P in the imaging space in the magnet 1, a transmitter 8T and a receiver 8R connected to the high-frequency coil 7, and the transmitter 8T and reception. And an RF sequencer 5b (equipped with a computer) for controlling the operation timing of the machine 8R. The RF sequencer 5b forms the sequencer 5 together with the gradient magnetic field sequencer 5a. The RF sequencer 5b can instruct the application of an RF pulse in a state synchronized with the gradient magnetic field sequencer 5a. The transmitter 8T and the receiver 8R supply an RF current pulse having a Larmor frequency for exciting nuclear magnetic resonance (NMR) to the radio frequency (RF) coil 7 under the control of the RF sequencer 5b. Are subjected to various signal processing to form an echo signal.

  In addition to the controller 6 described above, the control / arithmetic unit includes a multiplexer 11 that receives a digital echo signal formed by the receiver 8R. The multiplexer 11 selectively switches the output path between the controller side and the reconstruction unit side in response to a control signal from the controller 6. Further, on one output side of the multiplexer 11, a reconstruction unit 12 that performs image reconstruction by a Fourier transform method, a storage unit 13 that stores the reconstructed image data, a display device 14 that displays an image, and an input And a container 15. As described above, the controller 6 incorporates a computer and controls the operation contents and operation timing of the entire system.

  Next, the operation of this embodiment will be described. Here, it is assumed that the FSE (Fast SE) method is adopted as the pulse sequence.

  The controller 6 performs the process of FIG. This process includes two types of pre-scans A and B. First, the controller 6 causes the sequencer 5 to execute the pre-scan A of the pulse sequence shown in FIG. 3 with the switch path of the multiplexer 11 switched to the controller 6 side, and inputs an echo signal obtained as a result (FIG. 3). 2 step S1).

  In this prescan A, first, a high-frequency excitation pulse RFex (application phase φ = 0 °, flip angle θ = α (90 ° in this case)) is applied together with the slicing gradient magnetic field pulse Gss. After the elapse of time τ / 2, the first high-frequency inversion pulse RFre1 (application phase φ = 90 °, flip angle θ = β1 (here, 180 °)) is applied together with the slicing gradient magnetic field pulse Gss. The echo signal E (1) is read out while applying the read gradient magnetic field pulse Gro after τ time from the application of the first high-frequency excitation pulse RFex. Thereafter, the second and subsequent high frequency inversion pulses RFre2, RFre3,... Are applied together with the slicing gradient magnetic field pulse Gss every τ time from the application of the first high frequency inversion pulse RFre1, and echo signals E (2), E (3 ), ... are similarly read out. In this print scan A, as shown in the figure, the phase encoding gradient magnetic field pulse Gpe is always zero.

  In this pudding spin A, if there is a phase shift in the nuclear spin due to some cause, the echo signals E (2), E (3),. Are divided into two echo components, namely, a main echo component Emain (2) (Emain (3),...) And a sub-echo component Esub (2) (Esub (3),...) (See FIG. 3). When there is no cause for such phase shift, the phase arg {Emain (n)} of the main echo component Emain (n) and the phase arg {Esub (n)} of the sub-echo component Esub (n) are equal (n = 2). 3, ...).

Next, the controller 6 causes the sequencer 5 to execute the second pre-scan B shown in FIG. 4, and inputs the echo signal obtained as a result (step S2 in FIG. 2).

  Also in this pre-scan B, first, a high-frequency excitation pulse RFex (application phase φ = 0 °, flip angle θ = α (here 90 °)) is applied together with the slicing gradient magnetic field pulse Gss. After the lapse of τ / 2, a high-frequency inversion pulse RFre1 (application phase φ = 90 °, flip angle θ = α (180 ° in this case)) is applied together with the slicing gradient magnetic field pulse Gss. Then, the echo signal E (1) is read out while applying the read gradient magnetic field pulse Gro after τ time from the application of the first high-frequency excitation pulse RFex. Thereafter, the second and subsequent high frequency inversion pulses RFre2, RFre3,... Are applied together with the slice gradient magnetic field pulses Gss every τ time from the application of the first high frequency inversion pulse RFre1, and echo signals E (2), E ( 3), ... are read in the same way.

  In this pre-scan B, the application phase φ of the even-numbered high-frequency inversion pulses RFre2, RFre4,... Is rotated by 180 ° further from that of the odd-numbered high-frequency inversion pulses RFre3, REre5,. The value (φ = 270 °) is set. Further, the phase encoding gradient magnetic field pulse Gpe is always zero as in the prescan A.

  When there is a phase shift in the nuclear spin due to some cause, each of the echo signals E (2), E (3),. The main echo component Emain (2) (Emain (3),...) And sub-echo component Esub (2) (Esub (3),...) (See FIG. 3), and even-numbered high frequency inversion pulses. Since the application phase φ of RFre2, RFre4,... Is rotated by 180 °, only the phase arg {Esub (n)} of the sub-echo components Esub (2), Esub (3),. Rotate 180 ° with respect to the corresponding phase.

The reason for this will be described with reference to FIG. As shown in the figure, it is assumed that the phase of the _nth echo signal is θ _n , the phase of the (n + 1) th and n + 2th high frequency inversion pulses are φ _{n + 1} and φ _{n + 2} , respectively. Phase _{θ n + 2, SE, STE} (stimulatedecho) component of the phase theta _{n + 2, STE} of SE (spin echo) components generated by the n + 2 th RF inversion pulse,

It is represented by

In the case of FIG. 3, φ _n = 90 °, θ _n = 90 °, θ _{n + 2, SE} = 90 °, θθ _{n + 2, STE} = 90 ° for all integers n of 1 or more. Therefore, the phases of Emain (n) and Esub (n), which are a combination of a plurality of echo paths on the phase diagram, are also equal, and each is 90 °.

  In the case of FIG.

When n = 2m (m is an integer of 1 or more)

Since the STE component generated from φ _{n + 1} and φ _{n + 2} from the n-th main echo component Emain (n) is Esub (n + 2), the sub-echo in FIG. It can be seen that the phase of the component Esub (n) is 270 °, that is, 180 ° inverted from the state of FIG.

  FIG. 8 shows only the phase correspondence for a limited number of paths that occur between adjacent high-frequency inversion pulses. However, the regularity shown in FIG. 38 described above also applies to the phase correspondence of echo signals. Therefore, the phase of the sub-echo component of prescan B is rotated by 180 ° with respect to that of prescan A.

  In this embodiment, the combination of the application phases of the CPMG pulse series is basically changed. However, the present invention is not limited to this, and the combination of the application phases of the high frequency pulses of the pre-scans A and B is the above (*). Any combination of phases may be used as long as the equation is satisfied and the main echo component Emain (n) or the sub-echo component Esub (n) has a phase difference of 180 ° between the two prescans.

Again, referring back to FIG. As a result of the pre-scan, if the echo signal obtained by pre-scan A is Ea (n) and the echo signal obtained by pre-scan B is Eb (n) (n = 2, 3,...)

The relationship is established. Therefore, the controller 6 performs an averaging process between the echo signals obtained in the pre-scans A and B (step S3 in FIG. 2). That means
[Equation 10]
{Ea (n) + Eb (n)} / 2 (where n = 1, 2,...)
Perform the operation. The concept of this averaging operation is shown in the schematic diagrams of FIGS. 5 (a), 5 (b) and FIG. As shown in these figures, in the second and subsequent echo signals E (2), E (3),..., The sub-echo components Esub (2), Esub (3),. Since they differ from each other by 180 ° in B, they are canceled appropriately, and only the main echo component Emain (n) (n = 1, 2,...) Is extracted.

In FIG. 6B,
[Equation 11]
{Ea (n) -Eb (n)} / 2
A state is schematically shown in which only the sub-echo component Esub (n) (n = 2, 3,...) Is extracted when the above calculation is performed.

  Further, the controller 6 calculates pure phase shifts φmain (1) and φmain (2) and positional shifts pmain (1) and pmain (2) of the peak of the main echo component based on the first and second echo signals. (FIG. 2, step S4).

Then, as in the conventional case,
[Equation 12]
φmain (1) −φmain (2)
And [Equation 13]
pmain (1) -pmain (2)
Based on this value, the correction amount Δφ of the application phase φ of the radio frequency pulse RF or the correction amount ΔGss of the gradient magnetic field pulse Gss for the high frequency pulse RF and the correction amount ΔGro of the read direction gradient magnetic field pulse Gro at the time of the main scan are calculated (same as above). Figure step S5).

  Finally, the controller 6 switches the switch path of the multiplexer 11 to the reconstruction unit 12 side, and causes the sequencer 5 to execute the main scan based on the pulse sequence shown in FIG. 7 (step S6 in FIG. 2).

  In the main scan, as shown in the figure, a pulse sequence reflecting the above correction calculation result is used. That is, the application phase φ of the high-frequency excitation pulse RFex first applied together with the slice gradient magnetic field pulse Gss is a value of φ = Δφ, or the polarity-inverted pulse applied after the slice gradient magnetic field pulse Gss. The intensity of the slice direction gradient magnetic field pulse Gss for preventing phase is a value corrected by ΔGss. At the same time, the intensity of the read-direction gradient magnetic field pulse Gro applied in parallel with the slice-direction gradient magnetic field pulse Gss for preventing the dephasing is a value corrected by ΔGro.

  After the elapse of time τ / 2 from the first excitation, the first radio frequency inversion pulse RFre1 (application phase φ = 90 °, flip angle θ = β1 (here 180 °)) is applied together with the slicing gradient magnetic field pulse Gss. Next, a phase-encoding gradient magnetic field pulse Gpe whose intensity (pulse area) is adjusted is applied every time this sequence is executed, and the time τ has elapsed since the first selective excitation. Then, the read gradient magnetic field pulse Gro is applied, and in parallel with this, the first echo signal E (1) is collected.

  In the same manner, high-frequency inversion pulses RFre2, RFre3,... (Application phase φ = 90 °, flip angles θ = β2, β3,... (180 ° in this case) are sequentially applied together with the slicing gradient magnetic field pulse Gss. The echo signals E (2), E (3),... Associated with the phase inversion of the spins are sequentially collected, and after each echo is collected in FIG. Is applied as a rewinding pulse, thereby preventing the generation of pseudo echoes during the main scan.

  The echo signal E (n) collected by the main scan is sequentially received by the receiver 8R and converted into echo data. This processing includes quadrature detection and A / D conversion of the echo signal E (n). The echo data is arranged in the reconstruction unit 12 on a memory forming a k (Fourier) space in accordance with the encoding amount, and reconstructed into an MR image by a Fourier transform method.

  As described above, in the present embodiment, the sub-echo component of the second and subsequent echo signals (stimulated echo component when referring only to the second echo signal) that could not be removed by the conventional method is used. Since it can be almost completely removed and only the main echo component can be accurately extracted, the correction accuracy for the pulse sequence of the main scan is remarkably improved, and there is an advantage that a stable main scan can be performed.

  Note that the method of calculating the correction amounts Δφ, ΔGss, and ΔGro in the present embodiment is not limited to the above-described method, and, for example, as apparent from the equation (1), for example, other than the first and second, that is, the third The difference between the phase difference and the echo peak position of the even-numbered and odd-numbered main echo components after the first may be used for calculating the correction amounts Δφ, ΔGss, and ΔGro. Further, the echo data obtained by the prescan is subjected to a one-dimensional Fourier transform in the kr direction to obtain a phase distribution curve in the readout direction, and the slope and intercept of the curve, that is, the primary component in the readout direction of the phase distribution curve and A zero-order component may be obtained, and the position deviation and phase deviation of the echo peak may be obtained based on the first-order component and the zero-order component.

(Second Embodiment)
A second embodiment according to the present invention will be described with reference to FIGS. In addition, in the following embodiments including this embodiment, the same or equivalent components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted or simplified.

  The MRI apparatus according to this embodiment is the same as that described above, and the controller 6 executes a series of processes shown in FIG.

  First, the controller 6 sequentially instructs the sequencer 5 to execute two types of pre-scans C and D related to the FSE method with the switch path of the multiplexer 11 switched to the controller 6 side, and inputs the collected echo signals. (Steps S11 and S12 in FIG. 9). FIG. 10 shows the pulse sequence for the first prescan C, and FIG. 11 shows the subsequent prescan D, respectively.

  In the first pre-scan C, one high frequency excitation pulse RFex (applied phase φ = 0 °, flip angle θ = α (90 ° in this case)) followed by a plurality of high frequency inversion pulses RFre1, RFre2,. = 90 °, flip angles θ = β1, β2,... (180 ° in this example) are sequentially applied together with the slice gradient magnetic field pulse Gss, and each echo signal Ec (n) is applied together with the application of the gradient magnetic field pulse Gro in the readout direction. Is collected. The phase encoding gradient magnetic field pulse Gpe is set to have the same intensity change as the shot when collecting the data line including the center of the k space in the main scan. As a result, the high-frequency pulse and the gradient magnetic field pulse in the pre-scan C are set to have the same waveform as that of the shot when the data line including the center of the k space in the main scan is collected (this point is the conventional and first). Different from the pre-scanning pulse sequence according to the embodiment).

  Assuming that there is a phase shift in the nuclear spin due to some cause, the echo signals E (2), E (3), etc. other than the first echo signal E (1) are It is divided into two echo components, that is, a main echo component Emain (2) (Emain (3),...) And a sub echo component Esub (2) (Esub (3),...) (See FIG. 10).

  Subsequent prescan D is also followed by one high-frequency excitation pulse RFex (applied phase φ = 0 °, flip angle θ = α (here 90 °)) followed by a plurality of high-frequency inversion pulses RFre1, RFre2,. = 90 ° or 270 °, flip angle θ = β1, β2,... (180 ° in this case) are sequentially applied together with the slice gradient magnetic field pulse Gss, and each echo signal Ed is applied together with the application of the gradient magnetic field pulse Gro in the readout direction. (n) is collected. The phase encoding gradient magnetic field pulse Gpe is set to have the same intensity change as the shot when collecting the data line including the center of the k space in the main scan. As a result, the gradient magnetic field pulse in the pre-scan D is set to the same waveform as the shot when collecting the data line including the center of the k space in the main scan (this point is the same as in the conventional and first embodiments). This is different from the pre-scan pulse sequence according to FIG. However, for the high frequency pulse, similarly to the pre-scan B in the first embodiment described above, the application phase of the even-numbered high frequency inversion pulses RFre2, RFre4,... Is further rotated by 180 ° and φ = 270 ° is set. Has been.

  Again, if there is a phase shift in the nuclear spin due to some cause, each of the echo signals E (2), E (3),... Other than the first echo signal E (1) has two echo components, That is, it is divided into a main echo component Emain (2) (Emain (3),...) And a sub-echo component Esub (2) (Esub (3),...) (See FIG. 11), and even-numbered high frequency inversion pulses RFre2,. Since the application phase φ of RFre4,... Is rotated by 180 °, only the phase arg {Esub (n)} of even-numbered sub-echo components Esub (2), Esub (3),. Rotate 180 ° to that of time.

Now, let Ec (n) be the echo signal obtained by prescan C, and Ed (n) be the echo signal obtained by prescan D. In both pre-scans, the position corresponding to the effective echo time, that is, the echo signal excluding the n _eff echo signal E (n _eff ) (the third echo signal E (3) in the examples of FIGS. 10 and 11) Is phase encoded. Therefore, the echo signals that can be used for the correction amount calculation during the main scan are only the positions corresponding to the effective echo time, that is, the third echo signals Ec (3) and Ed (3).

Then the controller 6, the third echo signal main echo component Emain of E (3) corresponding to the n _eff-th echo signal E (n _eff) of (3) and the sub echo component Esub (3),

based on,

It calculates | requires by calculating (step S13 of FIG. 9). FIG. 12 shows the main echo component Emain (3) and the sub-echo component Esub (3) extracted by this calculation. Among the present embodiment of the third echo signal E (3) corresponding to the n _eff-th echo signal E (n _eff), since the sub echo signal Esub (3) between the phase difference 180 °, the above As described above, the main echo signal Emain (3) is extracted by the addition average, and the sub echo signal Esub (3) is extracted by the subtraction average.

  Further, based on these echo data, the controller 6 shifts the peak phase shift φmain (3) and position shift pmain (3) of the main echo component of the third echo signal E (3) and the phase shift of the peak of the sub-echo component. φsub (3) and displacement psub (3) are calculated (step S14 in FIG. 9).

Next, as in the conventional method, the phase difference between the same n-th main echo component and sub-echo component [Equation 16]
φmain (n _eff ) −φsub (n _eff )
And the difference in peak position [Equation 17]
pmain (n _eff ) -psub (n _eff )
Value, ie, here

Based on this value, the correction amount Δφ of the application phase φ of the radio frequency pulse RF or the correction amount ΔGss of the gradient magnetic field pulse Gss for the high frequency pulse RF and the correction amount ΔGro of the read direction gradient magnetic field pulse Gro at the time of the main scan are calculated (same as above). FIG. Step S15). Note that the polarity of the calculation result for the correction is inverted depending on whether the n _eff is an odd number or an even number. Therefore, the order of subtraction is unified so that the polarity is always the same.

  Finally, the controller 6 switches the switch path of the multiplexer 11 to the reconstruction unit 12 side, and causes the sequencer 5 to execute the main scan based on the pulse sequence shown in FIG. 12 (step S16 in FIG. 9).

  In the main scan, as shown in the figure, a pulse sequence reflecting the above correction calculation result is used. That is, the application phase φ of the high-frequency excitation pulse RFex (flip angle θ = α (90 ° in this case) first applied with the slice gradient magnetic field pulse Gss is a value of φ = Δφ, or this slice gradient The intensity of the slice-direction gradient magnetic field pulse Gss for preventing the phase reversal, which has been reversed in polarity, applied after the magnetic field pulse Gss is a value corrected by ΔGss. The intensity of the read direction gradient magnetic field pulse Gro applied in parallel with Gss is a value corrected by ΔGro.

  7, echo signals E (1), E (2),... Associated with the phase inversion of nuclear spins are sequentially collected using a plurality of high frequency inversion pulses RFre1, RFre2,. The echo signals E (n) collected by the main scan are sequentially input to the reconstruction unit 12 as echo data via the receiver 8R and reconstructed into an MR image by the Fourier transform method.

  As described above, according to the present embodiment, in addition to the advantages of the first embodiment described above, among the phase shift components caused by the gradient magnetic field pulses in the phase encoding direction, spatially uniform phase shift components, that is, The zero-order component can be suitably corrected, and the advantage that the correction accuracy for the pulse sequence of the main scan is further improved can be obtained.

  Note that the calculation methods of the correction amounts Δφ, ΔGss, and ΔGro in the present embodiment are not limited to those described above. For example, one-dimensional Fourier transform is performed in the kr direction on the echo data obtained by prescan, and the readout direction The phase distribution curve is obtained, and the slope and intercept of the curve, that is, the first-order component and the zero-order component in the readout direction of the phase distribution curve are obtained, and the position of the echo peak is shifted based on the first-order component and the zero-order component. The phase shift may be obtained.

(Modification of the second embodiment)
A modification of the second embodiment will be described with reference to FIGS. In this modification, the GRASE method is applied in place of the FSE method described above, and the sequencer 5 can command the pulse sequence of the GRASE method in both pre-scan and main scan in response to a command from the controller 6. It is like that.

  The controller 6 executes a series of processes shown in FIG.

  First, the controller 6 sequentially instructs the sequencer 5 to execute two types of pre-scans E and F related to the GRASE method with the switch path of the multiplexer 11 switched to the controller 6 side, and inputs the collected echo signals. (Steps S21 and S22 in FIG. 14). FIG. 15 shows the pulse sequence for the first prescan E, and FIG. 16 shows the pulse sequence for the subsequent prescan F.

  The pulse sequences of these two types of pre-scans E and F use one high-frequency excitation pulse RFex and a plurality of high-frequency inversion pulses RFre1, RFre2,..., Which are the same as those in FIGS. Further, the polarity of the readout gradient magnetic field pulse Gro is inverted three times between the high frequency inversion pulses to collect echo signals Ee (m, n) and Ef (m, n), respectively. The phase encoding gradient magnetic field pulse Gpe at this time is set to the same intensity change as that of the shot when collecting the data line including the center of the k space in the main scan.

  In both prescans E and F, if there is a phase shift in the nuclear spin due to some cause, the echoes other than the first echo signals E (1,1) to E (1,3) are the same as described above. Each of the signals E (2,1), E (2,2), (2,3), E (3,1),... Has two echo components, that is, the main echo component Emain (2,1) (Emain (2,2), Emain (2,3), Emain (3,1), ...) and the sub-echo component Esub (2,1) (Esub (2,2), Esub (2,3), Esub (3 , 1),... (An example of which is shown in FIGS. 15 to 17 for the echo signal E (3, 2)). In addition, in the later prescan F, the application phase φ of the even-numbered high frequency inversion pulses RFre2, RFre4,... Is rotated by 180 ° and φ = 270 °. Only the phase rotates 180 ° relative to that during prescan E.

Now, let Ee (n, m) be the echo signal obtained by prescan E, and Ef (n, m) be the echo signal obtained by prescan F. In both prescans, the position corresponding to the effective echo time, that is, the n _eff -th echo signal E (n _eff , m ′) (in the example of FIGS. 15 and 16, the signal E (3, The echo signals other than 2)) are phase encoded. Therefore, the echo signal that can be used for the correction amount calculation at the time of the main scan is only the position corresponding to the effective echo time, that is, the signal E (3, 2) in the third echo signal.

Next, the controller 6 here converts the main echo component Emain (3.2) and the sub-echo component Esub (3,2) of the echo signal E (3,2),

It calculates | requires by calculating (FIG.14 step S23). FIG. 17 shows the main echo component Emain (3,2) and the sub-echo component Esub (3,2) extracted by this calculation.

  Further, based on these echo data, the controller 6 determines the phase shift φmain (3,2) and position shift pmain (3,2) of the main echo component peak of the echo signal E (3,2) and the peak of the sub-echo component. The phase shift φsub (3,2) and the position shift psub (3,2) are calculated (step S24 in FIG. 14).

Next, similarly to the conventional method, the value of the phase difference between the same n-th main echo component and sub-echo component, that is, here,

Based on this value, the correction amount Δφ of the application phase φ of the radio frequency pulse RF or the correction amount ΔGss of the gradient magnetic field pulse Gss for the high frequency pulse RF and the correction amount ΔGro of the read direction gradient magnetic field pulse Gro at the time of the main scan are calculated (same as above). Figure step S25).

  Finally, the controller 6 switches the switch path of the multiplexer 11 to the reconstruction unit 12 side, and causes the sequencer 5 to execute the main scan based on the pulse sequence shown in FIG. 18 (step S26 in FIG. 14). In this scan, as shown in the figure, the application phase φ of the radio frequency excitation pulse RFex first applied together with the slice gradient magnetic field pulse Gss is a value of φ = Δφ, or this slice gradient magnetic field The intensity of the slice-direction gradient magnetic field pulse Gss applied after the pulse Gss for preventing phase inversion with polarity inversion is a value corrected by ΔGss. At the same time, the intensity of the read-direction gradient magnetic field pulse Gro applied in parallel with the slice-direction gradient magnetic field pulse Gss for preventing the dephasing is a value corrected by ΔGro.

  As described above, according to the present embodiment, the advantages of the first and second embodiments described above can be obtained. Furthermore, unlike the FSE method, the GRASE method does not change the waveform shape of the phase encoding gradient magnetic field pulse so much between the high-frequency inversion pulses, so that the phase error generation pattern based on the phase encoding becomes regular. Artifacts can be effectively reduced by correcting ΔGss and ΔGro indicated by diagonal lines.

(Third embodiment)
Furthermore, an MRI apparatus according to the third embodiment will be described with reference to FIGS. In this embodiment, two types of pre-scans G and H are used, and the phase encode amount is applied to each echo at the time of the scan in the same manner as in the main scan. Here, a case where the pulse sequence is the GRASE method will be described.

  This MRI apparatus has the same configuration as that of each of the embodiments described above, and the controller 6 executes the processing shown in FIG.

  The controller 6 switches the switch path of the multiplexer 11 to the controller 6 side, sequentially instructs the sequencer 5 to execute two types of pre-scans G and H according to the GRASE method, and inputs the collected echo signals (FIG. 19). Steps S31 and S32).

  The first prescan G shown in FIG. 20 and the next prescan H shown in FIG. 21 are both different from the previous ones, and the phase encoding gradient magnetic field pulse Gpe is applied to each echo Eg (n, The phase encoding amount applied to m) or Eh (n, m) is changed for each shot. This phase encoding is applied in order to collect two-dimensional echo data in an unadjusted state corresponding to the entire surface or part of the k space. The application phase of the high-frequency pulses RFex, RFre1, RFre2,... Is changed in the same manner as in the case of the two types of prescans A and B in the first embodiment described above. For this reason, two-dimensional echo data is collected in two sets, that is, Eg (n, m) or Eh (n, m) by changing the combination of applied phases of the high-frequency pulse.

In the prescans G and H, as an example, the number of high frequency inversion pulses of GRASE method = 5, the number of gradient echoes between adjacent high frequency inversion pulses = 3, and the total number of echoes per shot = 15 Is shown. In the drawing, only the gradient magnetic field waveform when the matrix of 2 shots and the phase encoding direction of the image = 30 is shown because of display, but the number of shots is the n _eff -th corresponding to the effective echo time. The number of shots that can stably measure the primary phase change in the phase encoding direction of the screen with only the echo signal, for example, 8 shots or more (when this is expressed by data in k space, data including the center position of k space) It is desirable that the number of adjacent echo data lines centering on the line is 8 or more).

  Now, if there is a phase shift in the nuclear spin due to some cause, each of the echo signals E (2, m), E (3, m), etc. other than the first echo signal E (1, m) It is divided into two echo components, that is, main echo component Emain (2, m) (Emain (3, m),...) And sub-echo component Esub (2, m) (Esub (3, m),. , See echo signals Eg (3,2) and Eh (3,2) in FIGS. Moreover, in the later prescan H, the application phase φ of the even-numbered high-frequency inversion pulses RFre2, RFre4,... Is rotated by 180 °, so that the even-numbered sub-echo component Esub (2, m), Only the phase of Esub (4, m),...

Similarly to the above-described embodiments, as shown in FIG. 22, the two-dimensional echo signal obtained by the first prescan G is Eg (n, m), and the echo signal obtained by the next prescan H is obtained. Is Eh (n, m), the two-dimensional main echo component Emain (n, m) on the k plane and the two-dimensional sub-echo component Esub (n, m) are

Extracted in

For this reason, if the position corresponding to the effective echo time in both pre-scans, that is, the n _effth echo signal E (n, m) is, for example, E (3,2),

Thus, the main echo component Emain (3,2) and the sub-echo component Esub (3,2) are extracted. (FIG. 19, step S33). The extracted echo components Emain (3,2) and Esub (3,2) are illustrated in FIGS. 22 (c) and 22 (d), respectively. Of the two sets of two-dimensional echo signal E corresponds to the n _eff-th (3,2), sub echo signal Esub (3,2) because the phase of each other different 180 ° (one in FIG. 21 by a dotted line As shown above, the main echo signal Emain (3,2) is extracted by addition averaging, and the sub-echo signal Esub (3,2) is extracted by subtraction averaging.

  Further, the controller 6 uses the two-dimensional echo data Emain (3,2) and Esub (3,2) on the k plane to generate the main echo of the (3,2) th echo signal E (3,2). The phase shift φmain (3.2) and the position shift pmain (3,2) of the component peak, and the phase shift φsub (3,2) and the position shift psub (3,2) of the peak of the sub-echo component are calculated (step in FIG. 19). S34). Here, the positional deviations pmain (3,2) and psub (3,2) are both two-dimensional vector quantities.

Next, based on the equations (1) and (2), the phase difference between the same (3,2) -th main echo component and sub-echo component [Equation 23]
φmain (3,2) −φsub (3,2),
And the difference between the peak position vectors [Equation 24]
pmain (3,2) -psub (3,2)
Based on this value, the correction amount Δφ of the application phase φ of the high-frequency pulse RF, the correction amount ΔGss of the slicing gradient magnetic field pulse Gss, the correction amount ΔGro of the read gradient magnetic field pulse Gro, and the phase encoding based on this value The correction amount ΔGpe of the gradient magnetic field pulse Gpe for operation is calculated (step S35 in the figure). Note that the polarity of the calculation result for the correction is inverted depending on whether the n _eff is an odd number or an even number. Therefore, the order of subtraction is unified so that the polarity is always the same.

  Finally, the controller 6 switches the switch path of the multiplexer 11 to the reconstruction unit 12 side, and causes the sequencer 5 to execute the main scan based on the pulse sequence shown in FIG. 23 (step S36 in FIG. 19).

  In the main scan, as shown in the figure, a pulse sequence reflecting the above correction calculation result is used. That is, the application phase φ of the high-frequency excitation pulse RFex (flip angle θ = α (90 ° in this case) first applied together with the slice gradient magnetic field pulse Gss is a value of φ = Δφ, or this slice gradient The intensity of the slice-inverted gradient magnetic field pulse Gss applied after the magnetic field pulse Gss is corrected by ΔGss, and at the same time, the slice-direction gradient magnetic field pulse for preventing the dephase is applied. The intensity of the read gradient magnetic field pulse Gro applied in parallel with Gss is a value corrected by ΔGro, and the intensity of the phase encoding gradient magnetic field pulse Gpe is a value corrected by ΔGpe.

  Hereinafter, as in the above-described embodiment, echo signals E (n, m) accompanying the phase inversion of nuclear spins are sequentially collected using a plurality of high-frequency inversion pulses RFre1, RFre2,... And reconstructed into MR images. The

  As described above, according to the present embodiment, in addition to the advantages of the first and second embodiments, the primary spatial distribution in the phase encoding direction among the phase shift components caused by the gradient magnetic field pulses in the phase encoding direction is obtained. It is possible to suitably correct even the components possessed, and it is possible to obtain the advantage that the correction accuracy for the pulse sequence of the main scan is remarkably improved and a high-quality MR image can be provided.

  Note that the method of calculating the correction amounts Δφ and ΔG (ΔGss, ΔGro, ΔGpe) in the present embodiment is not limited to the above-described method. For example, a two-dimensional image is once reconstructed from two-dimensional echo data obtained by pre-scanning, and the slope and intercept of the phase distribution curve in the readout direction and phase encoding direction, that is, the primary in the phase readout direction and phase encoding direction. The component and the 0th order component are obtained. Then, the positional deviation and phase deviation of the echo peak may be obtained based on the primary component and the zeroth component, and the correction amounts Δφ and ΔG of the main scan sequence may be calculated.

  Each of the above embodiments has been described with a two-dimensional scan. However, a three-dimensional scan or a scan with a higher dimension can be similarly corrected by increasing the vector dimension.

(Fourth embodiment)
Further, an MRI apparatus according to the fourth embodiment will be described with reference to FIGS. In this embodiment, two types of pre-scanning are the same as before, but the characteristics of pre-scanning are taken into consideration, and an imaging target that matches the characteristics is used during pre-scanning.

  The MRI apparatus to be used has the same configuration as that of the first embodiment, and the controller 6 executes a series of processes shown in FIGS. FIG. 24 shows processing during pre-scanning, and FIG. 25 shows processing during main scanning.

  The purpose of the pre-scan is to measure the phase error between the main echo component and the sub-echo component due to hardware imperfections such as gradient magnetic field coils. For this reason, the subject (imaging target) at the time of pre-scanning does not have to be the same as the subject at the time of actual scanning for obtaining an actual MR image. Therefore, in this embodiment, a phantom is used instead of the actual subject at the time of pre-scanning. As the phantom, in order to ensure measurement accuracy, it is desirable that the signal value distribution is as uniform as possible. For example, a phantom in which an acrylic resin container is filled with an aqueous copper sulfate solution is preferable.

  After such a phantom is installed in the diagnostic space of the magnet 1, the controller 6 starts the pre-scan process of FIG. 24 by giving a command to the controller 6.

First, in step S41 in FIG. 24, the controller 6 determines resolution and slice thickness, which are imaging conditions for the two types of prescans G and H. The prescans G and H are the same as those described in the third embodiment, and use a pulse sequence according to the GRASE method. Next, in step S42, two types of pre-scans G and H are sequentially executed according to the determined imaging conditions. Next, at step S43, it carried out in the same manner as in processing in the case of the third embodiment, the n _eff-th (= (n, m) th) main echo component of the echo signal E corresponding to the (n, m) Emain (n, m), sub-echo component Esub (n, m),

Extract based on the following formula.

  Further, the process proceeds to step S44, where the zero-order and first-order components φmain (n, m), pmain () of the spatial distribution of the phase error of each of the main echo component Emain (n, m) and sub-echo component Esub (n, m) are obtained. n, m), φsub (n, m), and psub (n, m) are obtained. In step 45, the phase difference between the main echo component and the sub echo component based on the equations (1) and (2) and the difference between the peak position vectors are calculated. In step 46, these pieces of difference information are stored in the storage unit 13, for example, as correction data together with the pre-scan spatial resolution and slice thickness information.

  When the main scan is instructed, the controller 6 starts the process of FIG. First, in step S51 in FIG. 25, resolution and slice thickness, which are imaging conditions for the main scan, are determined by a method such as referring to information provided by an operator. Next, the process proceeds to step S52, and the prescan G and H imaging conditions (resolution, slice thickness), the phase difference between the main echo component and the sub echo component, and the difference between the peak position vectors, which are stored in advance during prescan, are used as correction data. read out.

  In step S53, the correction amounts Δφ and ΔG are calculated based on the read-out correction data in the same manner as described above. At this time, the imaging conditions of the main scan and the spatial resolution in each direction of the image are compared with those at the time of the pre-scan, and the correction amounts Δφ and ΔG are obtained according to the ratio. Further, in step S54, the sequencer 5 is instructed to execute the main scan by a pulse sequence reflecting the correction amounts Δφ and ΔG.

  By performing pre-scanning using a phantom having an ideal signal value distribution in this way, correction data with increased accuracy can be obtained. The imaging target moves during pre-scanning, and the accuracy of the phase error correction data does not decrease, and is not affected by changes in the signal from the imaging target. A scan can be performed. For the patient, pre-scanning for collecting phase error correction data is not necessary, so that only the main scan is required, the patient's restraint time is reduced, and diagnosis is facilitated.

  In this embodiment, when the phase error distribution in the phase encoding direction is so small that it can be ignored, pre-scanning with phase encoding zero is performed as in pre-scans A and B in the first embodiment, and the phase encoding direction is not set. It is also possible to measure only the phase error distribution in this direction and to perform correction based on this phase error information. If necessary, as seen in the pre-scans C and D of the second embodiment, a pre-scan for obtaining a phase error distribution including a zero-order phase error component due to a gradient magnetic field in the phase encoding direction is performed. You may implement.

(Modification of the fourth embodiment)
An example in which the fourth embodiment is further modified will be described with reference to FIGS. The MRI apparatus according to this modified embodiment collects the phase error characteristics of each physical channel of the gradient magnetic field in advance by two types of prescans G and H (the same prescan as in the fourth embodiment), and performs the main scan. In some cases, the correction amounts Δφ and ΔG of the pulse sequence are calculated from imaging conditions related to the gradient magnetic field such as the cross-sectional direction, resolution, and slice thickness, and the main scan is performed. As a result, as will be described later, it is possible to obtain the sequence correction amounts Δφ and ΔG at the time of imaging a cross section in an arbitrary direction without performing the same pre-scan in the cross-sectional direction as the main scan.

  The controller 6 performs the processes of FIGS. 26 and 27 instead of the process of FIG. 24 described above and the process of FIG. 25 described above.

  The process of FIG. 26 includes pre-scanning, and shows a calculation procedure of each element (displayed by the array α) of the correction gradient magnetic field coefficient matrices Kp, ro, Kp, pe, Kp, ss. The process of FIG. 27 shows a calculation procedure of each element (indicated by the array β) of the correction high-frequency phase coefficient vectors Kφ, ro, Kφ, pe, Kφ, ss.

Further, the correspondence relationship between each element of the correction gradient magnetic field coefficient matrix Kp, ro, Kp, pe, Kp, ss and each element of the array α is expressed by the following equation (9).

The correspondence relationship between the elements of the correction high-frequency phase coefficient vectors Kφ, ro, Kφ, pe, Kφ, ss and the elements of the array β is expressed by the following equation (10).

The correspondence relationship between the correction gradient magnetic field coefficient matrix Kp, ro, Kp, pe, Kp, ss and the correction gradient magnetic field ΔG is expressed by the following equation (11).

The correspondence relationship between the correction high-frequency phase coefficient vectors Kφ, ro, Kφ, pe, Kφ, ss and the correction amount Δφ of the applied phase of the high-frequency pulse is expressed by the following equation (12).

Further, the definition of the matrix R representing the rotation in the imaging section direction is shown in Expression (13), and each element of the correction gradient magnetic field ΔG is shown in Expression (14).

  Also, M and imgCh are the spatial frequency magnitudes in the imaging channel imgCh direction (RO, PE, and SS directions related to the pulse sequence), and M and phCh are physical channel phCh directions (X, Y, and Z gradient magnetic fields). Assuming that the spatial frequency is in the direction of the physical channel related to the coil, it is assumed that these and the rotation matrix R indicating the direction of the imaging section are coupled by the above-described equation (13).

As shown in equations (9) and (11),
[Formula 32]
α [imgCh] [phCh1] [phCh2]
Is a coefficient indicating how much the primary phase distribution in the phCh2 direction is corrected when imgCh is made to correspond to phCh1. T in the equation (11) is the time width of the gradient magnetic field for correction, and F and imgCh in the equation are the actual values for the gradient magnetic field of the intensity required to obtain the reference imaging condition for the subscript imaging channel imgCh. The ratio of gradient magnetic field strengths of the scans in the above equation, and Mo and imgCh in the formula are the magnitudes of the spatial frequencies used as a reference in the imgCh direction.

As shown in equations (10) and (12),
[Equation 33]
β [imgCh] [phCh]
Is a coefficient indicating how much the zero-order phase distribution is corrected when the RO, PE, and SS imaging channels imgCh related to the pulse sequence are associated with the physical channel phCh. F and imgCh in the equation (12) are the same values as in the equation (11).

  Next, a calculation procedure of each element of the correction gradient magnetic field coefficient matrix shown in FIG. First, which element of the matrix is to be obtained is selected (step S61 in the figure). Next, a cross-sectional direction related to the element is selected (step S62). In order to obtain a certain element of the matrix, it is necessary to change several imaging conditions and obtain and compare spin phase shift distributions in the respective imaging conditions.

  Therefore, after initial setting to the variable i = 0 for counting the number of acquisitions (step S63), imaging conditions such as spatial resolution and slice thickness are determined so that the target matrix element can be extracted (step S64). The number of acquisitions can also be reduced if some matrix elements are clearly considered to be zero due to the characteristics of the gradient coil or sequence.

Next, the prescans G and H of FIG. 24 according to the fourth embodiment described above (a series of processes including steps S42 to S44 of FIG. 24) are performed (step S65 of FIG. 26). Next, from the two-dimensional vectors pmain and psub indicating the positional deviation of the main echo component and the sub echo component obtained by the pre-scan, the difference between the peak position vectors is calculated for each variable i.
[Formula 34]
It calculates from (DELTA) Pi = pmain-psub (step S66).

  Next, the controller 6 shifts the processing to step S67, determines whether or not the imaging condition remains, increments the variable i if there is still an imaging condition (i = i + 1), and returns to step S64. Thereby, the processing of steps S64 to S66 is executed once or a plurality of times, and the difference ΔPi between the peak position vectors is obtained under different (or one) imaging conditions.

If the determination in step S67 is NO and it is determined that all the prescans under the predetermined imaging conditions have been completed, then the controller 6 then selects the corresponding matrix element based on the imaging condition information and the peak position vector difference ΔPi. Correction amount [Equation 35]
Δα [imgCh] [phCh1] [phCh2]
Is determined (step S69). Using this correction amount Δα [imgCh] [phCh1] [phCh2], the corresponding matrix element

Is obtained (step S70). This series of processes is performed for all the elements α selected in advance in step S61 (step S71).

On the other hand, each element of the correction high-frequency phase coefficient vector shown in FIG. 27 is also calculated in the same flow as in the case of FIG. 26 described above (see steps S81 to 91 in FIG. 27). In step S81, the element β [imgCh] [phCh] for obtaining the correction high-frequency phase coefficient vector is selected in advance. In step S86, the phase difference between the main echo component and the sub-echo component is calculated based on the above-described prescans G and H and the subsequent processing calculation.
Δφi = φmain−φsub
Is calculated for each imaging condition. Further, in step S89, the correction amount of the corresponding vector element based on the imaging condition information and the phase difference Δφi [Equation 38]
Δβ [imgCh] [phCh]
In step S90, the corresponding vector element

Is required.

  The correction gradient magnetic field coefficient matrix and the correction high-frequency phase coefficient vector thus obtained are stored in the storage unit 13 or the internal memory. Then, during the main scan, the processing described with reference to FIG. 25 is executed. That is, the imaging conditions for the main scan are determined (step S51 in the figure), the stored correction gradient magnetic field coefficient matrix and correction high-frequency phase coefficient vector are read (step S52 in the figure), and the equations (11) and (12) ) To calculate the correction amount Δφ of the applied phase and the correction amount ΔG of the gradient magnetic field (step S53). A main scan of the pulse sequence taking these correction amounts Δφ and ΔG into consideration is executed (step S54).

  By processing in this way, the pre-scan in the same cross-sectional direction as the main scan becomes unnecessary, and a cross-section in the arbitrary direction can be imaged (main scan) without performing such pre-scan.

  The characteristics of the imaging technique according to this modification will be described in detail in comparison with the prior art. The elements that are corrected in the method described in US Pat. No. 5,378,985 and Japanese Patent Laid-Open No. 6-54827 described in the section of the prior art are Kp, ro, Kp, Of the elements of ss and the correction high-frequency phase coefficient vector, it may be considered to correspond to the elements of Kφ, ro, Kφ, ss. Since the separation of each element is not clear, it is necessary to perform the pre-scan immediately before the main scan and set the same cross-sectional direction, spatial resolution, and slice thickness as the main scan.

  On the other hand, in this modified embodiment, in addition to the conventional elements, the diagonal elements of Kp, pe of the correction gradient magnetic field coefficient matrix and the correction high-frequency phase coefficient vectors Kφ, pe, which cannot be corrected conventionally, are also included. It is corrected. In addition to this increase in the number of elements to be corrected, in this variation, the off-diagonal elements of all the correction gradient magnetic field coefficient matrices can be measured and corrected independently, so that the imaging cross-sectional directions of the main scan and the pre-scan are different. Even if they are different, the correction amount is calculated. In addition, by correcting the phase of the high-frequency pulse and the gradient magnetic field, most elements of the matrix and vector are corrected. Therefore, even with an MRI apparatus having the same gradient coil, the MRI apparatus according to this modification can reconstruct a high-quality MR image that is much more stable than the conventional one.

  Note that the calculation models such as the above-described equations (11) and (12) can be appropriately changed according to the characteristics of the gradient magnetic field system.

(Fifth embodiment)
Further, an MRI apparatus according to the fifth embodiment will be described with reference to FIGS. Unlike the previous embodiment, this embodiment does not correct the application phase of the high-frequency pulse in the pulse sequence and the waveform of the gradient magnetic field in the main scan, but includes a phase including a second-order or higher-order phase distribution error. An MR image in which the error is accurately corrected is obtained.

  The hardware of the MRI apparatus has the same configuration as the conventional one, and the controller 6 executes a series of processes combining the pre-scan and the main scan shown in FIG. Here, the case where the FSE method is used is illustrated.

  First, the controller 6 causes the sequencer 5 to sequentially execute two types of pre-scans A and B with the multiplexer 11 switched to the controller side (step S91 in the figure). The pre-scans A and B are set to be the same as those described in the first embodiment.

Then, the main echo component Emain (n) and the sub echo component Esub (n) are obtained from the echo signals Ea (n) and Eb (n) obtained in the prescans A and B in the same manner as described above.

And the absolute value | {Ea (n) + Eb (n)} / 2 | and | {Ea (n) −Eb (n)} / 2 | are calculated, and the absolute value is calculated as the main echo component Emain. (n) is set as amplitude information EPmain and EPsub of the sub-echo component Esub (n) (step S92; see FIG. 32).

  Next, the controller 6 switches the multiplexer 11 to the reconstruction unit side, and causes the sequencer 5 to sequentially execute two types of main scans I and J (step S93). The pulse sequences of the main scans I and J are shown in FIGS. As shown in these figures, except for the pattern of the applied phase of the high frequency pulse, it is the same as the normal FSE method, and the applied phase of the high frequency pulse and the gradient magnetic field waveform are corrected as in the above-described embodiment. Not. The applied phase pattern of the high frequency pulse in the first main scan I is the same as that in the normal FSE method, but in the subsequent main scan J, the applied phase φ is extra 180 ° for the even numbered high frequency inversion pulse. The value is set to the value rotated to. That is, φ = 270 °, and the pattern of the applied phase is the same as that in the prescan B described above, for example.

  Next, the controller 6 instructs the reconstruction unit 11 to automatically perform the processing from step S94.

In response to this command, the reconstruction unit 11 uses the echo data Ei (n) and Ej (n) obtained by the main scans I and J to use the two-dimensional main echo component Emain and sub-echo in the k space. The component Esub is extracted and separated (step S94). This process is the same as the method described above,

It is performed according to the following formula. Schematic diagrams of the separated main echo component Emain and sub-echo component Esub are shown in FIGS. The main echo component Emain is indicated by a solid line, and the sub echo component Esub is indicated by a dotted line.

  As is clear from the equations (1) and (2), in both the main echo component Emain and the sub-echo component Esub, the phase value is a certain value φ1 at the odd number and another value φ2 at the even number. Thus, the two values φ1 and φ2 are alternately repeated. In addition, the phase value of the odd-numbered echo signal in the main echo component Emain and the phase value of the even-numbered echo signal in the sub-echo component Esub component are almost the same in most cases considering the regularity of the gradient magnetic field.

  In accordance with this principle, as shown in FIG. 32, the reconstruction unit 11 displays odd-numbered echo data when the echo signal arranged at the center position of the k space is even-numbered, and conversely when the echo signal is odd-numbered. The even-numbered echo data is completely replaced for each echo block in the k-spaces of the main echo component Emain and the sub-echo component Esub to create new main echo component Emain 'and sub-echo component Esub' (step S95). As a result, in the new main echo component Emain ′ and sub-echo component Esub ′ in the new k space, there is almost no phase shift for each echo block, and the arrangement data in the k space is almost completely connected as shown in FIG. Become.

  However, since the main echo component Emain ′ and the sub-echo component Esub ′ at this stage are both arranged alternately for each echo block in the k space, the change graph Amain of the amplitude value in FIG. 32 in the ke direction. , Asub, there is a variation in amplitude value for each echo block.

  Therefore, the reconstruction unit 11 inputs the amplitude information EPmain and EPsub created based on the pre-scans A and B described above from the controller 6, and the amplitude of the main echo component Emain 'and the sub-echo component Esub' as amplitude correction data. Amplitude correction is performed for each echo block so that the value is constant for the entire k space (step S96). By this amplitude correction, two new sets of main echo component Emain ″ and sub echo component Esub ″ are created in the k space (see FIG. 32).

  Next, the reconstruction unit 11 individually reconstructs two sets of the main echo component Emain "and the sub-echo component Esub" by two-dimensional Fourier transform, and creates two real-time MR images MRmain and MRsub, respectively (step S97). ). In any case, since the echo data Emain "and Esub" in the k space have almost no phase shift, it is possible to almost avoid the occurrence of ghost artifacts in the reconstructed images MRmain and MRsub even if the adjustment is shifted.

The reconstruction unit 11 further performs a process of aligning the phases of the pixel signals at the same position in the two images (step S98). As this processing, for example, assuming that pixel values (complex data) at the same position in two images are IA (r) and IB (r) (see FIG. 33), the phase difference Δθ (r) is
[Formula 42]
Δθ (r) = arg {IB (r)} − arg {IA (r)}
Seeking
[Equation 43]
IB '(r) = IB (r) · exp {-jΔθ (r)}
Using the relationship, the final image I (r)
[Equation 44]
I (r) = {IA (r) + IB '(r)} / 2
Asking.

Or approximately

  Here, (IB (r) / IA (r)) * may be a conjugate complex number of (IB (r) / IA (r)), and the final image I (r) may be obtained for each pixel.

further,
[Equation 46]
I (r) = {| IA (r) | + | IB (r) |} / 2
The final image I (r) may be obtained for each pixel.

  When the signals at the same pixel position are thus aligned, finally, both MR images MRmain and MRsub are added for each pixel to create one final image MRimage (step S99).

  As described above, according to the present embodiment, at least two main scans are required for imaging, but the second or higher order that cannot be corrected in principle by correcting the application phase of the high-frequency pulse or the gradient magnetic field waveform. Even when a higher-order phase error distribution exists, the phase error distribution including such higher-order components can be reliably eliminated, and a stable high-quality MR image without image quality deterioration can be provided. In addition, since a method that does not use pre-scan phase information is employed, an MR imaging method that is resistant to the movement of the imaging target can be provided.

  Note that in the method according to the fifth embodiment described above, amplitude correction may not be necessary depending on the situation, so the pre-scans A and B are not necessarily essential.

  Further, by reducing the flip angle of the high-frequency inversion pulse, the amplitude ratio between the main echo component and the sub-echo component can be changed, and the fluctuation of the amplitude in the k space can be suppressed. In the case of the deformation method based on such a decrease in the flip angle, the pulse sequence of the main scan itself is not corrected or changed by the prescan information (as is the case with the fifth embodiment), so that the main scan takes longer than the scan corresponding to the prescan. It may be collected first.

  Furthermore, the method according to the fifth embodiment and its modification can be implemented in an imaging method using a plurality of high frequency inversion pulses found in a CPMG pulse sequence and a phase inversion CP pulse sequence, such as the GRASE method.

  However, in the case of the GRASE method, there is a case where the order of simply converting even-numbered or odd-numbered echo data of a plurality of echo blocks (echo arrangement sections) arranged in the phase encoding direction on the k space may not be maintained. . An example is shown in FIG. The two k-space data Emain and Esub shown in the figure are based on the GRASE method, the number of high-frequency inversion pulses is an odd number (= NRF = 3), the number of gradient echoes between high-frequency inversion pulses NGE = 3, and the echo data E (NRF, NGE). Thus, when the number of high frequency inversion pulses is an odd number, the echo data E (NRF, NGE) of the odd number (or even number) echo block between the high frequency inversion pulses corresponds to step S25 in FIG. It is only necessary to form two new k-space data Emain ′ and Esub ′ having the same phase by conversion in the processing. As described above, since the regularity of the phase shift is used, an artifact can be effectively reduced particularly in an imaging method such as the GRASE method in which the phase encode waveform does not change in shape between the high frequency inversion pulses. it can.

1 is a block diagram showing an outline of an MRI apparatus according to an embodiment of the present invention. 3 is a schematic flowchart showing processing of a controller in the first embodiment. 2 is a pulse sequence showing one prescan A of the first embodiment. 6 is a pulse sequence showing the other pre-scan B of the first embodiment. Explanatory drawing which shows isolation | separation extraction of the main echo component. Explanatory drawing which shows isolation | separation extraction of the main echo component and a sub echo component. The pulse sequence of the main scan in 1st Embodiment. The figure for demonstrating 180 degree phase rotation of a subecho signal. The schematic flowchart which shows the process of the controller in 2nd Embodiment. The pulse sequence which shows one prescan C of 2nd Embodiment. The pulse sequence which shows the other prescan D of 2nd Embodiment. Explanatory drawing which shows isolation | separation extraction of the main echo component and a sub echo component. The pulse sequence of the main scan in 2nd Embodiment. The schematic flowchart which shows the process of the controller in the modification of 2nd Embodiment. The pulse sequence which shows one pre-scan E in the modification of 2nd Embodiment. The pulse sequence which shows the other prescan F in the modification of 2nd Embodiment. Explanatory drawing which shows isolation | separation extraction of the main echo component and a sub echo component. The pulse sequence of this scan in the modification of 2nd Embodiment. The schematic flowchart which shows the process of the controller in 3rd Embodiment. The pulse sequence which shows one prescan G of 3rd Embodiment. The pulse sequence which shows the other prescan H of 3rd Embodiment. Explanatory drawing which shows isolation | separation extraction of the main echo component and a sub echo component. The pulse sequence of the main scan in 3rd Embodiment. 10 is a schematic flowchart illustrating processing at the time of pre-scanning by a controller according to a fourth embodiment. 10 is a schematic flowchart illustrating processing during a main scan of a controller according to a fourth embodiment. 14 is a schematic flowchart illustrating a calculation process of a correction gradient magnetic field coefficient matrix at the time of pre-scanning in a modification of the fourth embodiment. 14 is a schematic flowchart showing a calculation process of a correction high-frequency phase coefficient vector at the time of pre-scanning in a modification of the fourth embodiment. 10 is a schematic flowchart showing image acquisition processing according to the fifth embodiment. The pulse sequence which shows one main scan I of 5th Embodiment. The pulse sequence which shows the other main scan J of 5th Embodiment. Explanatory drawing which shows isolation | separation extraction of the main echo component in a two-dimensional k space, and a subecho component. The figure explaining the production | generation process from the main echo component on two 2-dimensional k space, a sub echo component to one real space image. The figure for demonstrating the phase alignment of pixels. The schematic diagram of the other example which shows conversion of echo data. The pulse sequence which shows an example of the conventional FSE method. The pulse sequence which shows an example of the conventional GRASE method. The schematic diagram explaining the application phase (phi) of a high frequency pulse. The figure of an example of a phase dispersion | distribution diagram. A pre-scan pulse sequence according to one of the conventional methods. A pulse sequence of the main scan according to one of the conventional methods. 6 is a schematic flowchart of processing from a pre-scan to a main scan according to a conventional method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnet 2 Static magnetic field power supply 3x-3y Gradient magnetic field coil 5 Sequencer 6 Controller 7 High frequency coil 8T, 8R Transmitter, receiver 11 Multiplexer 12 Reconfiguration unit 13 Storage unit

Claims (11)

Before the main scan for obtaining the MR image of the subject, a first pulse sequence having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses and the application phase of the plurality of high-frequency inversion pulses set as a reference phase value Pre-scan, and the application phase of the even-numbered high frequency inversion pulse having a high frequency excitation pulse and a plurality of high frequency inversion pulses, and a phase difference of 180 ° with respect to the reference phase value. Scanning means for performing each second pre-scan of the pulse sequence set to a value having,
The main scan based on a first echo data group comprising a plurality of echo signals obtained by the first prescan and a second echo data group comprising a plurality of echo signals obtained by the second prescan. Correction means for correcting in advance the pulse sequence of
The gradient magnetic field for phase encoding of the first and second pre-scan pulse sequences is always zero,
The correction means adds or subtracts the corresponding echo data of the first echo data group and the second echo data group to each other to add or subtract the main echo assembly including the main echo component and the main echo component. Means for separating / extracting the sub-echo assembly comprising the echo components of the main scan, and means for automatically correcting the pulse sequence of the main scan based on the correction data obtained based on the main and sub-echo assemblies. An MRI apparatus characterized by Before the main scan for obtaining the MR image of the subject, a first pulse sequence having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses and the application phase of the plurality of high-frequency inversion pulses set as a reference phase value Pre-scan, and the application phase of the even-numbered high frequency inversion pulse having a high frequency excitation pulse and a plurality of high frequency inversion pulses, and a phase difference of 180 ° with respect to the reference phase value. Scanning means for performing each second pre-scan of the pulse sequence set to a value having,
The main scan based on a first echo data group comprising a plurality of echo signals obtained by the first prescan and a second echo data group comprising a plurality of echo signals obtained by the second prescan. Correction means for correcting in advance the pulse sequence of
The gradient magnetic field for phase encoding of the first and second pre-scan pulse sequences is always zero,
The correction means adds or subtracts the corresponding echo data of the first echo data group and the second echo data group to each other to add or subtract the main echo assembly including the main echo component and the main echo component. Means for separating / extracting sub-echo aggregates composed of echo components, and phase shifts of echo peaks of odd-numbered and even-numbered echo data in each of the main and sub-echo aggregates, and An MRI apparatus comprising: means for calculating at least one of positional deviation; and means for automatically correcting the pulse sequence of the main scan based on correction data obtained from the calculated value. Before the main scan for obtaining the MR image of the subject, a first pulse sequence having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses and the application phase of the plurality of high-frequency inversion pulses set as a reference phase value Pre-scan, and the application phase of the even-numbered high frequency inversion pulse having a high frequency excitation pulse and a plurality of high frequency inversion pulses, and a phase difference of 180 ° with respect to the reference phase value. Scanning means for performing each second pre-scan of the pulse sequence set to a value having,
The main scan based on a first echo data group comprising a plurality of echo signals obtained by the first prescan and a second echo data group comprising a plurality of echo signals obtained by the second prescan. Correction means for correcting in advance the pulse sequence of
The gradient magnetic field for phase encoding of the first and second pre-scan pulse sequences is always zero,
The correction means adds or subtracts the corresponding echo data of the first echo data group and the second echo data group to each other to add or subtract the main echo assembly including the main echo component and the main echo component. A means for separating / extracting a sub-echo assembly comprising the echo components of at least one of the zero-order or first-order phase distributions of each of the main and sub-echo assemblies. An MRI apparatus comprising: means; and means for automatically correcting the pulse sequence of the main scan based on correction data obtained according to the comparison result. 4. The MRI apparatus according to claim 1, wherein the first and second pre-scan pulse sequences and the main scan pulse sequence are a CPMG pulse sequence or a phase-inverted CP pulse sequence. 5. 4. The MRI apparatus according to claim 1, wherein the first and second pre-scan pulse sequences and the main scan pulse sequence are pulse sequences according to a GRASE method or a high-speed SE method. 4. The correction data is at least one of a correction value of an application phase of a high-frequency excitation pulse of the pulse sequence of the main scan, a correction value of a slice gradient magnetic field, and a correction value of a read gradient magnetic field. The MRI apparatus according to 1. The MRI apparatus according to claim 1, wherein the correction data is a correction value of an application phase of the high-frequency excitation pulse, a correction value of the slicing gradient magnetic field, and a correction value of the lead-like gradient magnetic field. Before the main scan for obtaining the MR image of the subject, a first pulse sequence having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses and the application phase of the plurality of high-frequency inversion pulses set as a reference phase value Pre-scan, and the application phase of the even-numbered high frequency inversion pulse having a high frequency excitation pulse and a plurality of high frequency inversion pulses, and a phase difference of 180 ° with respect to the reference phase value. Scanning means for performing each second pre-scan of the pulse sequence set to a value having,
The main scan based on a first echo data group comprising a plurality of echo signals obtained by the first prescan and a second echo data group comprising a plurality of echo signals obtained by the second prescan. Correction means for correcting in advance the pulse sequence of
Said first, phase-encoding gradient field pulse sequence of the second pre-scanning, the part or all of the echo data acquisition time of shots and the same phase-encoding gradient magnetic field waveform in the k-space in the scan Yes,
The correction means relates to echo data corresponding to an effective echo time by adding or subtracting echo data corresponding to an effective echo time on the time axis in the first echo data group and the second echo data group. Means for separating and extracting a main echo assembly composed of main echo components and a sub-echo assembly composed of echo components other than the main echo component, and an effective echo time corresponding to the main echo assembly and the sub echo assembly; A means for calculating at least one of a phase shift and a position shift of an echo peak of the echo data; and a means for automatically correcting the pulse sequence of the main scan with correction data obtained from the calculated value. MRI apparatus according to. Before the main scan for obtaining the MR image of the subject, a first pulse sequence having a high-frequency excitation pulse and a plurality of high-frequency inversion pulses and the application phase of the plurality of high-frequency inversion pulses set as a reference phase value Pre-scan, and the application phase of the even-numbered high frequency inversion pulse having a high frequency excitation pulse and a plurality of high frequency inversion pulses, and a phase difference of 180 ° with respect to the reference phase value. Scanning means for performing each second pre-scan of the pulse sequence set to a value having,
The main scan based on a first echo data group comprising a plurality of echo signals obtained by the first prescan and a second echo data group comprising a plurality of echo signals obtained by the second prescan. Correction means for correcting in advance the pulse sequence of
Said first, phase-encoding gradient field pulse sequence of the second pre-scanning, the part or all of the echo data acquisition time of shots and the same phase-encoding gradient magnetic field waveform in the k-space in the scan Yes,
The correction means adds or subtracts the echo data of the first echo data group and the second echo data group to each other, and a main echo aggregate including a two-dimensional main echo component in k space and the main echo Means for separating / extracting a two-dimensional sub-echo assembly in k-space consisting of echo components other than the components, and a phase shift of an echo peak of the desired main echo component from the main echo assembly and the sub-echo assembly And a means for calculating at least one of a two-dimensional positional deviation vector and an echo peak phase deviation and a two-dimensional positional deviation vector of the desired sub-echo component, and a pulse sequence of the main scan using correction data obtained from the calculated value An MRI apparatus comprising means for automatically correcting The correction means obtains information related to a specific phase error for the combination of the imaging channel and the physical channel of the gradient magnetic field coil based on the first and second echo data groups; The MRI apparatus according to claim 1, 2 or 3, further comprising means for obtaining correction data of a pulse sequence of the main scan from phase error-related information and imaging conditions related to a gradient magnetic field. The MRI apparatus according to claim 10, wherein the imaging condition includes an imaging section direction, resolution, and slice thickness specified at the time of the main scan.

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