JP6050041B2 - Magnetic resonance imaging apparatus and FSE imaging method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) equipped with a pulse sequence based on a high-speed spin echo method, and the influence of an error magnetic field included in each echo continuously collected after one RF excitation. It relates to technology to be eliminated.

  In the MRI apparatus, various pulse sequences are used corresponding to the imaging target and the image type. Among these pulse sequences, a pulse sequence (FSE sequence) called a fast spin echo method (FSE method) is widely used because of its short imaging time and variety of image types that can be imaged.

  In the FSE method, after exciting a spin by a 90 ° RF pulse, a plurality of spin echoes are obtained by repeatedly inverting the excited spin using a plurality of 180 ° RF pulses. An echo group collected after one excitation is called an echo train (ETL), and the number of multi-shot excitations is called a shot number (Shot). The RF pulse for generating these echoes and the gradient magnetic field pulse for encoding the echoes require advanced and accurate control. The pulse sequence, which is software for realizing the FSE method, describes the ideal RF pulse and gradient magnetic field pulse intensity and timing. However, since an error occurs in the gradient magnetic field pulse and the RF pulse actually generated by hardware, it is difficult to realize an ideal pulse sequence in the static magnetic field space. For this reason, each echo includes a magnetic field error, and an artifact is generated in an image using the echo.

  Various improvement measures have been proposed for this problem. For example, in Non-Patent Document 1 and Patent Document 1, there is a method of acquiring reference data by pre-scanning and adjusting the pulse sequence using the result, thereby collecting echo data itself as close to ideal as possible. Proposed.

Japanese Patent Laid-Open No. 7-16544

R. Scott Hinks etc., “Fast Spin Echo Prescan for Artifact Reduction”, Soc. Magn. Reson. Abstract, 1995; 3: 634

  In the technique for adjusting the pulse sequence itself as described above, since uniform adjustment is performed on the assumption that a certain error has occurred in all echoes of the echo train, for example, there are different errors for each echo in the echo train. If it happens, we cannot respond. In addition, in order to perform an accurate adjustment, it is necessary to repeat the pre-scan, which causes an increase in imaging time.

  An object of the present invention is to avoid an error of each echo in an echo train as much as possible and reduce artifacts generated in a reconstructed image due to discontinuity of k-space data.

  In order to solve the above-mentioned problem, the present invention sets the order of phase encoding so that the echoes in the echo train of the FSE sequence are consecutive phase encodes every two adjacent echoes, and the echo train of the FSE sequence Are measured in the order of the phase encoding set.

  According to the present invention, an error of each echo in the echo train can be avoided as much as possible, and artifacts generated in a reconstructed image due to discontinuity of k-space data can be reduced.

The block diagram which shows the whole structure of the MRI apparatus with which this invention is applied The figure which shows the basic pulse sequence of FSE method performed with the MRI apparatus of this invention FIG. 4 is a diagram for explaining phase errors targeted by the MRI apparatus of the present invention, in which (a) shows a state where there is no phase error for each echo, (b) shows a state where a phase error occurs, The k-space data, the figure on the left shows the data after Fourier-transforming the k-space data in the frequency direction. It is a diagram showing the order of data filling to the k space of the first embodiment, (a) is the order of data filling to the k space (conventional example) in the case of the centric order of ETL = 6 and Shot = 8. (B) shows the order of data filling into the k-space when measuring every two adjacent echoes by successive phase encoding in the first embodiment. The pulse sequence shape of the FSE sequence which implement | achieves the centric order of 1st embodiment is shown. (a) (Centric) shows the application pattern of the phase encoding gradient magnetic field (Gp) corresponding to the conventional example of FIG. 4 (a), (b) (Mod.Centric) is the first of FIG. 4 (b) The application pattern of the phase encoding gradient magnetic field (Gp) corresponding to the case where the echoes adjacent to every two of the embodiments are measured by continuous phase encoding is shown. Functional block diagram of the first embodiment The flowchart which shows the process sequence of the arithmetic processing part (CPU) of 1st embodiment. It is an example showing the effect of the first embodiment, is a phase image obtained by Fourier transform of k-space data in the frequency encoding direction, (a) shows the case of the conventional example (Centric), (b) is the first. The case of one embodiment (Mod. Centric) is shown. It is a figure which shows the order of the data filling to k space in the case of the centric order of ETL = 6 and Shot = 8 of 2nd embodiment, (a) shows the echo train of positive rank phase encoding, (b) Indicates an echo train of reverse phase encoding. It is an example showing the effect of the second embodiment, is a phase image obtained by Fourier transform of k-space data in the frequency encoding direction, (a) shows a phase image in the case of positive-order phase encoding, (b) Indicates a phase image in the case of reverse phase encoding. The figure which shows the case of the complex addition in the image space of 2nd embodiment. This is a modification example in which every two adjacent echoes are measured by continuous phase encoding, which can be applied in both the first embodiment and the second embodiment, and (a) is an odd-numbered shot and an even-numbered shot. An example in which the encoding order of echoes adjacent to each other (that is, in a pair) is switched between two shots is shown. (B) is an example in which the phase encoding order in the pair is further switched between adjacent pairs. Indicates.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, an outline of an MRI apparatus to which the present invention is applied and a pulse sequence of a fast spin echo method executed by the MRI apparatus will be described with reference to FIG. 1 and FIG.

  The MRI apparatus shown in FIG. 1 mainly includes a static magnetic field generation magnetic circuit 2, a gradient magnetic field generation unit 3, a transmission unit 5, a reception unit 6, a signal processing unit 7, a sequencer 4, and central processing. A device (CPU) 8 and an operation unit (user interface unit) 9 are provided.

  The static magnetic field generating magnetic circuit 2 generates a uniform static magnetic field in the space in which the subject 1 is placed, and although not shown, a permanent magnet type, normal conducting type or superconducting type magnetic field generating means is arranged. . Depending on the direction of the static magnetic field, there are a horizontal magnetic field method parallel to the body axis of the subject and a vertical magnetic field method orthogonal to the body axis.

  The gradient magnetic field generating unit 3 includes a gradient magnetic field coil 31 wound in three axes of X, Y, and Z, and a gradient magnetic field power source 32 that drives each coil. By driving the gradient magnetic field power supply 32, gradient magnetic fields Gs, Gp, and Gf in arbitrary three axial directions can be applied to the subject 1. By applying this gradient magnetic field, a slice plane for the subject 1 can be set, and an echo signal is encoded to provide position information.

  The transmitter 5 emits a high-frequency signal to cause nuclear magnetic resonance to occur in the atomic nucleus constituting the biological tissue of the subject 1 by a high-frequency magnetic field pulse transmitted from the sequencer 4, and includes a high-frequency oscillator 51 and a modulation A high frequency amplifier 53, a high frequency amplifier 53, and a high frequency coil 54 on the transmission side, and the high frequency pulse output from the high frequency oscillator 51 is amplified by the high frequency amplifier 53 and then disposed in the vicinity of the subject 1 By supplying to the coil 54, the subject 1 is irradiated with electromagnetic waves (high-frequency signals).

  The receiving unit 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the biological tissue of the subject 1, and receives a high-frequency coil 61, an amplifier 62, and a quadrature detector 63 on the receiving side. And an A / D converter 64, and the receiving side high frequency wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave irradiated from the transmitting side high frequency coil 54 is disposed close to the subject 1. Detected by the coil 61, input to the A / D converter 64 via the amplifier 62 and the quadrature phase detector 63 and converted to a digital quantity, and further sampled by the quadrature phase detector 63 at the timing according to the command from the sequencer 4. The two collected data are sent to the signal processing unit 7.

The signal processing unit 7 performs image reconstruction calculation using the echo signal detected by the receiving unit 6 and displays an image. Processing such as Fourier transform, correction coefficient calculation, image reconstruction, etc. for the echo signal and the sequencer 4 A CPU (arithmetic processing unit) 8 that performs the above control, a memory unit, a data storage unit, and a display 75 are provided. The memory section is detected by a ROM (read-only memory) 71 that stores a program for performing image analysis processing and measurement over time, an invariant parameter used in the execution, and a measurement parameter obtained by the previous measurement and the reception section 6. It comprises an RAM (temporary writing / reading memory) 72 for temporarily storing an echo signal and an image used for setting the region of interest and storing parameters for setting the region of interest. The data storage unit records image data reconstructed by the CPU 8, and includes, for example, an optical disk 73 and a magnetic disk 74. The display 75 visualizes the image data read from the optical disk 73 or the magnetic disk 74 and displays it as a tomographic image.

  The sequencer (measurement control unit) 4 is a control means for applying a high-frequency magnetic field pulse and a gradient magnetic field pulse necessary for imaging according to a predetermined pulse sequence, operates under the control of the CPU 8, and performs various operations necessary for collecting MR image data. The command is sent to the transmission unit 5, the gradient magnetic field generation unit 3, and the reception unit 6. The operation unit 9 is a user interface unit for inputting control information of processing performed by the signal processing unit 7, and includes input means such as a trackball 91 and a keyboard 92, and display means. A display provided in the signal processing unit 7 can also serve as a display unit of the operation unit 9.

(Pulse sequence according to the present invention)
Next, a pulse sequence executed by the MRI apparatus will be described. There are various types of pulse sequences depending on the imaging method, and they are stored on the magnetic disk 74 as a program. When imaging, the parameters stored on the magnetic disk 74 and the parameters set by the user are read to the sequencer 4. To be executed. In this embodiment, a pulse sequence of the FSE method is executed as one of such pulse sequences.

  Fig. 2 shows the basic 2D pulse sequence of the FSE method. In this pulse sequence, as shown in the figure, an RF pulse 201 that excites a predetermined nuclear spin in a test object, usually a proton, is applied. The flip angle of the RF pulse 201 is 90 °, for example, and is hereinafter referred to as an excitation RF pulse or a 90 ° RF pulse. After applying the 90 ° RF pulse 201, an RF pulse 202 for inverting the excitation spin is applied at the timing of TE / 2 with respect to the echo time TE. This RF pulse 202 is called an inversion pulse or 180 ° RF pulse. The echo echo1 is collected when the time TE / 2 has elapsed after applying the 180 ° RF pulse. Hereinafter, 180 ° RF pulses are applied at the same time interval ITE (= TE), and echoes echo2, echo3,... Are measured between the 180 ° RF pulses. Note that in FIG. 2, three 180 ° RF pulses are typically shown, but the number of 180 ° RF pulses can be arbitrarily set within the transverse relaxation of the spin, and all phases after one excitation RF pulse. Encoding echoes can be measured (single shot), or excitation can be repeated and echoes of all phase encoding can be measured (multi-shot) with multiple excitations.

In the pulse sequence of FIG. 2, a gradient magnetic field Gs210 for selecting a slice is applied as the gradient magnetic field when the RF pulses 201 and 202 are applied, and a gradient magnetic field Gf230 in the frequency encoding direction is applied during echo measurement. Further, a gradient magnetic field Gp220 for phase-encoding each echo and a gradient magnetic field Gp 221 for rephasing are applied. In the case of 3D imaging, a gradient magnetic field for phase encoding and a gradient magnetic field for rephasing are also applied in the slice direction. In the radial scan that scans the k-space radially, the gradient magnetic field in the phase encoding direction and the gradient magnetic field in the frequency encoding direction are combined to perform echo encoding and reading.

  In addition, although not shown in FIG. 2, it is also possible to add a specific spin, for example, a pre-pulse for saturating the spin of adipose tissue or an IR pulse for inversion before the 90 ° RF pulse.

  K-space data is obtained by arranging echoes (sampling data) measured by executing such a pulse sequence in a memory space. For the sake of simplicity, the 2D pulse sequence shown in FIG. 2 will be described. The k-space is a two-dimensional plane with the horizontal axis as the frequency encoding direction and the vertical axis as the phase encoding direction. The phase encoding amount to be applied, that is, the strength of the applied phase encoding gradient magnetic field determines the arrangement order in the k space. In the pulse sequence shown in FIG. 2, the arrangement order is a centric order, and the phase encoding amount is increased by a predetermined increment in order from the first echo after the RF excitation. As a result, the echoes are sequentially arranged from the center of the k space toward a high spatial frequency region (hereinafter abbreviated as a high region). The MRI apparatus of the present invention is characterized in that the arrangement order of the k space and hence the application step of the gradient magnetic field applied to each echo of the echo train is adjusted.

  Each embodiment of the present invention will be described based on the configuration of the MRI apparatus described above and the pulse sequence executed by the apparatus.

<< First Embodiment >>
First, a first embodiment of the MRI apparatus and FSE imaging method of the present invention will be described. In the present embodiment, the order of phase encoding is set so that echoes adjacent to every two echoes in the echo train of the FSE sequence are phase encoded. Hereinafter, this embodiment will be described in detail.

(Error magnetic field)
First, the magnetic field error of the echo targeted by the MRI apparatus of this embodiment will be described.

In the pulse sequence shown in Fig. 2, each echo collected for every 180 ° RF pulse inversion has different measurement timing starting from 90 ° RF pulse application and the number of 180 ° RF pulse inversions are different. Each echo will contain a different error. This error includes an application amount error of gradient magnetic field pulses and RF pulses accumulated in a pulse sequence before measurement, an error caused by an unnecessary magnetic field due to an eddy current, and the like. Normally, the echo is controlled so that the intensity is maximum at the center of the frequency encoding gradient magnetic field, and in the k space where these echoes are placed, the center in the frequency encoding direction of the k space is shown in the left figure of Fig. 3 (a). Echo peaks are lined up. Converting such k-space data into phase information by Fourier transform to the frequency encoding direction, as shown in the right diagram of FIG. 3 (a), the phase becomes a uniform phase throughout the k-space.

However, if there is an error as described above in the echo, the position of the echo peak with respect to the frequency encoding gradient magnetic field is different, and the k-space data shows that the echo peak is shifted from the center in the frequency encoding direction as shown in Fig. 3 (b). Will contain an echo. When such k-space data is Fourier-transformed in the frequency encoding direction, the phase gradient differs depending on the echo, and discontinuity occurs between the echoes. The phase error φ of each echo can be expressed by the general formula (1).
[Equation 1]
φ = φ ₀ + φ ₁ r + φ ₂ r ² + ...
Wherein, r is the distance from the gradient center, .phi.0 does not depend on the position phase error (0-order errors), phi ₁ r is the phase error of the primary dependent on the position, φ ^₂ r ₂ is the phase of quadratic dependence on the position It is an error.

  Of these errors, the zero-order error, that is, the error that rides in an offset manner, is relatively easy to remove by adjusting the pulse sequence, but the error that occurs and accumulates in the second and subsequent echoes is only the adjustment of the pulse sequence. It cannot be solved easily.

  The present embodiment is intended to reduce errors by accumulating each echo, and for errors that differ depending on the echo, or errors that occur depending on the position of the imaging target in the imaging space, and control the order of echo placement in k-space. Thus, the same effect as in the state where the error is substantially removed is obtained.

(Echo placement order of this embodiment)
Next, when measuring echoes using the FSE sequence of the present embodiment, a method of measuring adjacent echoes by every two consecutive phase encodings will be specifically described with reference to FIGS. .

  The FSE sequence measures a plurality of echoes continuously, and at that time, the measurement order of echoes of each phase encoding can be arbitrarily set. A typical measurement order is a centric order of the measurement order from the center of the k-space toward the high frequency side, and a sequential order of the measurement order from one high frequency of the k-space to the other high frequency via the center. There is.

  FIG. 4 (a) shows a conventional example of the order of data filling into the k space in the case of centric order with ETL = 6 and Shot = 8. The numbers 1, 2, 3, ... indicate the echo number of the echo train. In the present embodiment, the centric order is obtained in which the data of the positive side region and the negative side region in the phase encoding direction of the k space (that is, one region and the other region sandwiching the zero phase encoding) are acquired by different shots. In the example of Fig. 4 (a), k space is divided into 6 x 2 = 12 segments in the phase encoding direction, one echo of each segment is measured in one shot, and each segment is shot for each shot. It is also an example of performing segment measurement to change the echo measured by.

  In the centric order, when two adjacent echoes are measured by continuous phase encoding in this embodiment, two adjacent echoes are paired out of echoes measured by one shot (excitation). Thus, the phase encoding of these echoes is made continuous. Accordingly, the number of divisions (number of segments) for dividing the k space in the phase encoding direction is half (that is, the same number as ETL). In the centric order of ETL = 6 and Shot = 8 in FIG. 4 (a), FIG. 4 (b) shows a case where adjacent echoes are measured by continuous phase encoding every two in this embodiment. In this case, of the six echoes to be measured, the first and second echoes are paired to make phase encoding continuous (phase encoding is 17 and 18 in the first shot), and the third and fourth Paired echoes to make phase encoding continuous (phase encoding is 10 and 9 in the first shot), and paired fifth and sixth echoes to make phase encoding continuous (phase encoding is 1 in the first shot) , 2). The number of segments is halved to 12/2 = 6.

  Further, FIG. 12 shows a modification in the case where the echoes adjacent to every two in this embodiment are measured by continuous phase encoding. FIG. 4 (a) is an example in which the encoding order of echoes adjacent to each other (that is, in pairs) is switched between odd-numbered shots and even-numbered shots in the FSE sequence. ) Shows an example in which the phase encoding order in the pair is switched between adjacent pairs in addition to the example of FIG. 4 (a). In other words, the example of FIG. 4B is an example in which the increase / decrease direction of the phase encoding in the pair is switched between adjacent pairs.

  Also in the sequential order, as in the centric order, continuous phase encoding can be given to every two adjacent echoes. In this way, a plurality of adjacent echo pairs are acquired every two in the k space within one echo train.

  When the ETL is an odd number, every second echo other than the last one echo is given a phase encoding that is continuous to adjacent echoes.

(FSE sequence shape of this embodiment)
FIG. 5 shows the pulse sequence shape of the FSE sequence that realizes the centric order shown in FIG. Fig. 5 (a) (Centric) is an application pattern of the phase encoding gradient magnetic field (Gp) corresponding to the conventional example of Fig. 4 (a), and Fig. 5 (b) (Mod. Centric) is shown in Fig. 4 ( This is an application pattern of a phase encoding gradient magnetic field (Gp) corresponding to a case where echoes adjacent to every two of the present embodiment in b) are measured by continuous phase encoding. As shown in FIG. 5, compared to the conventional example (FIG. 5A), in this embodiment (FIG. To be understood. Due to this effect, the error magnetic field can be canceled when the echoes adjacent to every two in this embodiment are measured by continuous phase encoding (FIG. 5 (b)). Note that the RF pulse (RF), slice gradient magnetic field (Gs), and frequency encoding gradient magnetic field (Gr) are common to the conventional example (FIG. 5 (a)) and the example of this embodiment (FIG. 5 (b)). It is.

(Description of each function)
Each function of the CPU 8 for realizing the FSE imaging method of the present embodiment will be described with reference to the functional block diagram of FIG. The CPU 8 of the first embodiment includes an FSE sequence setting unit 601, a phase encoding order setting unit 602, an FSE sequence generation unit 603, and an image reconstruction unit 604.

  The FSE sequence setting unit 601 calculates the ETL of the FSE sequence and the number of shots (Shot) based on the imaging conditions including the measurement order (sequential order or centric order). Then, the segment measurement is set so that the number of segments dividing the k-space in the phase encoding direction is the same as the ETL.

  The phase encoding order setting unit 602 is an echo measured in one shot (excitation) based on the measurement order set by the FSE sequence setting unit 601 and the calculated ETL (i.e., echo in the echo train). In addition, the phase encoding order is set so that every two adjacent echoes are continuous phase encoding. Specifically, two pairs are sequentially arranged from the first echo, and phase encoding continuous to each pair of echoes is set.

  The FSE sequence generation unit 603 generates FSE sequence control data based on the set measurement order, the calculated ETL and the number of shots, and the phase encoding of each echo in the set echo train, Notify the generated control data to the sequencer 4.

  The image reconstruction unit 604 fills the echo data of each phase encoding notified from the sequencer 4 into the corresponding phase encoding position in the k space, and the filling is completed (that is, the echo measurement is finished) The data is imaged by Fourier transform, and the image is displayed on the display 75.

(Description of processing flow)
Next, the processing flow of the present embodiment performed in cooperation with the above functional units will be described based on the flowchart shown in FIG. This processing flow is stored in advance on the magnetic disk 73 as a program, and is executed by the CPU 8 reading the program from the magnetic disk 73 and executing it. In this embodiment, the arrangement order in the k space is basically a centric order, and the arrangement order is controlled in consideration of the phase characteristics of each echo. Hereinafter, the processing contents of each processing step will be described in detail.

  In step S701, when the operator selects an FSE method pulse sequence as an imaging method via the operation unit 9 and inputs imaging conditions including the measurement order (sequential order or centric order), FSE The sequence setting unit 601 calculates the ETL and the number of shots of the FSE sequence, and sets the segment measurement of the same number of segments as the ETL.

  In step S702, the phase encoding order setting unit 602 performs phase encoding in which echoes adjacent to every two echoes in the echo train are consecutive based on the measurement order set in step S701 and the calculated ETL. The phase encoding order of each echo is set to

  In step S703, the FSE sequence generation unit 603, based on the measurement order set in step S701, the calculated ETL and the number of shots, and the phase encoding of each echo in the echo train set in step S702, Control data of the FSE sequence is generated, and the generated control data is notified to the sequencer 4.

  In step S704, the sequencer 4 uses the FSE sequence based on the control data of the FSE sequence generated in step S703, and measures each echo of echo train with the set phase encoding. Notify the image reconstruction unit 604.

In step S705, the image reconstruction unit 604 fills the k-space with the echo data of each phase encoding measured in step S704, images the k-space data by Fourier transform, and displays the image on the display 75. .
The above is the description of the processing flow of the present embodiment.

  As an example showing the effect of the present embodiment, FIG. 8 shows a phase image obtained by Fourier transforming the k-space data obtained in step S705 in the frequency encoding direction. FIG. 8A shows the case of the conventional example (Centric), and FIG. 8B shows the case of the present embodiment (Mod Centric). Each of the profiles along the straight line in the phase encoding direction at the position between the arrows is shown. In the conventional example of FIG. 8 (a), there is a large step in the phase for each segment area, but in the case of this embodiment (Mod Centric) of FIG. 8 (b), such a large phase between the segments. It is shown that there is no step. That is, it is understood that the influence of the error magnetic field can be offset in the phase image of the present embodiment.

  As described above, according to the present embodiment, when echoes in the echo train are measured using the FSE sequence, every two adjacent echoes are measured by continuous phase encoding. As a result, pulses having very close intensities as the phase encoding gradient magnetic field continue, and the error magnetic field can be canceled out. Therefore, an error of each echo in the echo train can be avoided as much as possible, and artifacts generated in the reconstructed image due to discontinuity of k-space data can be reduced.

<< in the second embodiment >>
Next, a second embodiment of the MRI apparatus and FSE imaging method of the present invention will be described. In this embodiment, the data obtained by measuring every two adjacent echoes by continuous phase encoding described in the first embodiment, and the data obtained by reversing the order of successive phase encodings. By performing complex addition, the influence of the error magnetic field is further reduced as compared with the first embodiment described above. Hereinafter, this embodiment will be described in detail.

(Echo placement order of this embodiment)
As shown in the phase image of FIG. 8, only by measuring every two adjacent echoes described in the first embodiment by continuous phase encoding, the phase of the even echo and the phase of the odd echo are calculated. There may be a small step between them. This is because the influence of the error magnetic field is slightly different between the even echo and the odd echo.

  In this embodiment, in order to eliminate such a phase step based on the influence of a slightly different error magnetic field between the even echo and the odd echo, in this embodiment, every two adjacent echoes are consecutively encoded in order of phase. The phase difference between the even echo and the odd echo is removed by complex addition of the measured data and the data measured by the reverse phase encoding in which the order of the successive phase encoding is reversed every two. To do.

The complex addition of data may be a complex addition of k-space data, or may be a complex addition of image data after imaging each k-space data. For example, as shown in the case of ETL = 6 and Shot = 8 centric order in FIG. 9, if [echo number (phase encoding)] is used, as shown in FIG. Train
Normal order echo train: [1 (18) -2 (17)]-[3 (10) -4 (9)]-[5 (2) -6 (1)]
As shown in Fig. 9 (b), the echo train of reverse phase encoding is
Reverse order echo train: [1 (17) -2 (18)]-[3 (9) -4 (10)]-[5 (1) -6 (2)]
It can be.

  In other words, echo data of the same phase encoding is the one measured evenly and the one measured oddly, and the way in which the phase difference is generated is reversed. The phase step is removed from the data. On the other hand, when performing complex addition of data in the image space, the phase of the artifact generated in the forward-order echo train image is opposite to the phase of the artifact generated in the reverse-order echo train image. Artifacts are removed from the image data.

  Also in this embodiment, any of the two modifications shown in FIG. 12 described in the first embodiment and measuring two adjacent echoes by continuous phase encoding can be applied. In other words, if the example of the phase encoding order shown in FIGS. 12 (a) and 12 (b) is the normal phase encoding, the reverse phase encoding may be obtained by reversing the phase encoding order.

(FSE sequence shape of this embodiment)
The FSE sequence of the present embodiment is basically the same as the pulse sequence shape shown in FIG. 5 (b) (Mod. Centric), and detailed description thereof is omitted, but the application pattern of the phase encoding gradient magnetic field is shown in FIG. (b) The applied pattern shown in (Mod.Centric) is for the forward echo train, and this applied pattern is reversed, and the applied pattern is changed between two adjacent phase encoding gradient magnetic fields. On the contrary (replacement), it becomes the thing for reverse order echo train.

(Description of each function)
Each function of the CPU 8 for realizing the FSE imaging method of the present embodiment will be described. The CPU 8 of the second embodiment has the same functional units as those of the first embodiment described above, but the processing content of each function is different. Hereinafter, only the parts different from the first embodiment will be described, and the description of the same parts will be omitted.

  Since the FSE sequence setting unit 601 is the same as the process described in the first embodiment, description thereof is omitted.

  Based on the measurement order (sequential order or centric order) set by the FSE sequence setting unit 601 and the calculated ETL, the phase encoding order setting unit 602 performs echo (that is, within echo train). The two echoes are paired so that every two adjacent echoes are made into continuous phase encoding. Then, the order of the phase encoding of each echo is set so as to perform the measurement that gives a continuous positive-order phase encoding to each pair of echoes and the measurement that gives a continuous reverse-order phase encoding to each pair of echoes. .

  The FSE sequence generation unit 603 measures the normal phase encoding and the reverse phase encoding based on the set measurement order, the calculated ETL and the number of shots, and the phase encoding of each echo in the set echo train. The control data of the FSE sequence for each measurement is generated, and the generated control data is notified to the sequencer 4.

  The image reconstruction unit 604 performs complex addition on the data measured by the normal phase encoding and the data measured by the reverse phase encoding. Specifically, the echo data of each phase encode measured each time is filled in the corresponding phase encode position in k space corresponding to each time, and when the filling is completed (that is, the echo measurement is completed). When performing complex addition of k-space data, the k-space data of each round is averaged. Then, the k-space data after the averaging is converted into an image by Fourier transform, and the image is displayed on the display 75. When complex addition is performed in the image space, the k-space data of each time is converted into an image by Fourier transform, and the addition average of the image data of each time is obtained, and the image after the addition average is displayed on the display 75.

(Description of processing flow)
Next, the processing flow of the second embodiment performed in cooperation with the above-described functions will be described. The processing flow of the second embodiment is the same as the processing flow of the first embodiment described above, but the processing content of each step is different. Hereinafter, only processing contents different from those of the first embodiment will be described, and description of the same processing contents will be omitted. However, in order to clarify that it is a processing step of the second embodiment, the step number is represented by adding “ ^CASE2 ”.

Step S701 ^CASE2 is the same as the processing in step S701 described above, and a description thereof will be omitted.

In step S702 ^CASE2 , each echo is set so that the processing in step S702 described above is set as measurement in forward phase encoding, and in addition, each pair of echoes is performed in continuous reverse phase encoding as measurement in reverse phase encoding. Set the phase encoding for.

In step S703 ^CASE2 , the FSE sequence generation unit 603 performs the measurement in the normal phase encoding and the reverse phase encoding set in step S702 ^CASE2 , the measurement order set in step S701 ^CASE2 , the calculated ETL and the number of shots. The control data of each FSE sequence is generated based on the phase encoding of each echo in the echo train in the measurement at, and the generated control data is notified to the sequencer 4.

In Step S704 ^CASE2 , the sequencer 4 performs measurement using the FSE sequence of the normal phase encoding and the reverse phase encoding based on the control data of the FSE sequence generated in Step S703 ^CASE2 , and displays the measured echo as an image. The reconfiguration unit 604 is notified.

In step S705 ^CASE2 , the image reconstruction unit 604 uses the echo data of each phase encoding measured by the positive phase encoding and the reverse phase encoding in step S703 ^CASE2 , respectively, for the k-space for the positive phase encoding and the reverse phase encoding. Fill the k-space. When complex addition is performed in the k-space, each k-space data is averaged, the k-space data after the averaging is Fourier transformed into an image, and the image is displayed on the display 75. Alternatively, when complex addition is performed in the image space, each k-space data is converted into an image by performing Fourier transform, an addition average of each image data is obtained, and the addition average image is displayed on the display 75.
The above is the description of the processing flow of the present embodiment.

As an example showing the effect of the present embodiment, the k-space data [Positive (Pos)] for normal phase encoding and the k-space data [Negative (Neg)] for negative phase encoding obtained in step S405 ^CASE2 are frequency-encoded. FIG. 10 shows a phase image obtained by Fourier transform in the direction. FIG. 10 (a) is a phase image in the case of forward order phase encoding, and FIG. 10 (b) is a phase image in the case of reverse order phase encoding. A profile along a straight line in the phase encoding direction at a position between the arrows is shown in FIG. 10 (c). It is understood that the method of changing a slight phase step in the phase encoding direction is completely opposite between the phase image in the case of forward phase encoding and the phase image in the case of reverse phase encoding. Therefore, it can be understood that if the k-space data (Pos) for forward-order phase encoding and the k-space data (Neg) for reverse-order phase encoding are complex-added, the slight remaining phase steps are canceled and removed. .

  Further, FIG. 11 shows the effect of this embodiment by complex addition in the image space. Fig. 11 (a) is an image obtained by Fourier transform of k-space data (Pos) for normal phase encoding, and Fig. 11 (b) shows k-space data (Neg) for reverse phase encoding. These are images obtained by Fourier transform, both of which are phantom images. In each case, artifacts are caused by slightly remaining phase steps. As shown in FIG. 11 (c), artifacts are removed from the averaged image of these image data (a) and (b).

  As described above, in the present embodiment, when echoes are measured using the FSE sequence, the data obtained by measuring consecutive echoes every two adjacent echoes and the sequence of successive phase encodings. The data obtained by reversing are added together. Thereby, it is possible to remove a slight step between the phase of the even-numbered echo and the odd-numbered echo phase due to the slightly different influence of the error magnetic field between the even-numbered echo and the odd-numbered echo. As a result, artifacts based on this slight phase difference can be removed, and the image quality can be further improved as compared with the case of the first embodiment described above.

  1 subject, 2 static magnetic field generation magnetic circuit, 3 gradient magnetic field generation unit, 4 sequencer, 5 transmission unit, 6 reception unit, 7 signal processing unit, 8 central processing unit (CPU), 9 operation unit (user interface unit)

Claims (12)

Based on a pulse sequence (FSE sequence) of the fast spin echo method, a measurement control unit that controls measurement of echoes from the subject,
An arithmetic processing unit that reconstructs an image of the subject using the echo;
Equipped with a,
The arithmetic processing unit includes a phase encoding order setting unit that sets an order of phase encoding so that echoes adjacent to every two echoes in the echo train of the FSE sequence are continuous phase encodes,
The phase encoding order setting unit switches the phase encoding order of adjacent echoes every two odd number shots and even number shots of the FSE sequence,
The said measurement control part measures each echo of the echo train of the said FSE sequence in the order of the respectively set phase encoding, The magnetic resonance imaging apparatus characterized by the above-mentioned. Based on a pulse sequence (FSE sequence) of the fast spin echo method, a measurement control unit that controls measurement of echoes from the subject,
An arithmetic processing unit that reconstructs an image of the subject using the echo;
With
The arithmetic processing unit includes a phase encoding order setting unit that sets an order of phase encoding so that echoes adjacent to every two echoes in the echo train of the FSE sequence are continuous phase encodes,
The said measurement control part measures each echo of the echo train of the said FSE sequence in the order of the phase encoding of a centric order, The magnetic resonance imaging apparatus characterized by the above-mentioned. Based on a pulse sequence (FSE sequence) of the fast spin echo method, a measurement control unit that controls segment measurement of echoes from the subject,
An arithmetic processing unit that reconstructs an image of the subject using the echo;
With
The arithmetic processing unit sets the phase encoding order so that echoes in the echo train of the FSE sequence are phase-encoded consecutively for every two adjacent echoes, and are arranged in different segments for every two. A phase encoding order setting unit for
The said measurement control part measures each echo of the echo train of the said FSE sequence in the order of the respectively set phase encoding, The magnetic resonance imaging apparatus characterized by the above-mentioned. The magnetic resonance imaging apparatus according to claim 3.
The phase encoding order setting unit switches the order of phase encoding of echoes adjacent to each other (that is, in pairs) between odd-numbered shots and even-numbered shots of the FSE sequence. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to claim 4.
The magnetic resonance imaging apparatus, wherein the phase encoding order setting unit interchanges the phase encoding order in a pair between adjacent pairs. The magnetic resonance imaging apparatus according to claim 3.
The said measurement control part makes each echo of the echo train of the said FSE sequence be a centric order, The magnetic resonance imaging apparatus characterized by the above-mentioned. The magnetic resonance imaging apparatus according to any one of claims 1 to 6 ,
The phase encoding order setting unit reverses the phase encoding order setting and setting the phase encoding order in which the echoes in the echo train of the FSE sequence are consecutive positive-order phase encodings every two adjacent echoes. And setting the phase encoding order for every two adjacent echoes to be consecutive reverse phase encoding,
The magnetic resonance imaging apparatus , wherein the arithmetic processing unit performs complex addition of the echo measured by the normal phase encoding and the echo measured by the reverse phase encoding . The magnetic resonance imaging apparatus according to any one of claims 1 to 7,
The phase encoding order setting unit reverses the phase encoding order setting and setting the phase encoding order in which the echoes in the echo train of the FSE sequence are consecutive positive-order phase encodings every two adjacent echoes. And setting the phase encoding order for every two adjacent echoes to be consecutive reverse phase encoding,
The arithmetic processing unit performs complex addition of an image reconstructed from echoes measured by the forward phase encoding and an image reconstructed from echoes measured by the reverse phase encoding Imaging device. The magnetic resonance imaging apparatus according to any one of claims 1 to 8 ,
The phase encode order setting unit, between echoes adjacent to every two (filled pairs), magnetic resonance imaging apparatus characterized that you change the order of phase encoding in the pair. A measurement control step for controlling the measurement of echo from the subject based on the pulse sequence (FSE sequence) of the fast spin echo method;
An arithmetic processing step for reconstructing an image of the subject using the echo; and
In an FSE imaging method in a magnetic resonance imaging apparatus comprising:
A phase encoding order setting step for setting the order of phase encoding so that every two echoes adjacent to echoes in the echo train of the FSE sequence are in continuous phase encoding;
In the phase encoding order setting step, the phase encoding order of adjacent echoes is changed every two between odd-numbered shots and even-numbered shots of the FSE sequence,
In the FSE imaging method, the measurement control step measures each echo of echo trains of the FSE sequence in a set phase encoding order . A measurement control step for controlling the measurement of echo from the subject based on the pulse sequence (FSE sequence) of the fast spin echo method;
An arithmetic processing step for reconstructing an image of the subject using the echo; and
In an FSE imaging method in a magnetic resonance imaging apparatus comprising:
A phase encoding order setting step for setting the order of phase encoding so that every two echoes adjacent to echoes in the echo train of the FSE sequence are in continuous phase encoding;
The FSE imaging method, wherein the measurement control step measures echoes of echo trains of the FSE sequence in the order of centric order phase encoding. A measurement control step for controlling the measurement of echo from the subject based on the pulse sequence (FSE sequence) of the fast spin echo method;
An arithmetic processing step for reconstructing an image of the subject using the echo; and
In an FSE imaging method in a magnetic resonance imaging apparatus comprising:
Phase encoding order setting step for setting the phase encoding order so that the echoes in the echo train of the FSE sequence are consecutive phase encodings every two adjacent echoes and are arranged in different segments every two. With
The FSE imaging method, wherein the measurement control step measures echoes of echo trains of the FSE sequence in the order of centric order phase encoding .

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