JP5285244B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus). In an excitation method in which a plurality of high-frequency magnetic field pulses are applied together with an oscillating gradient magnetic field (gradient magnetic field that reverses the polarity of the gradient magnetic field multiple times), It is related with the technique which corrects the influence of the.

  In imaging by an MRI apparatus, a high-frequency magnetic field pulse and a gradient magnetic field are used to excite molecules or atoms in an arbitrary plane having a predetermined thickness, and the generated echo signal is acquired to create an image. In order to generate an echo signal, the high-frequency magnetic field pulse and each gradient magnetic field pulse are applied according to a preset imaging sequence (pulse sequence).

  Examples of the pulse sequence include one that applies a plurality of high-frequency magnetic field pulses together with an oscillating gradient magnetic field. This pulse sequence is used in a two-dimensional selective excitation (spatial selective excitation) method for selectively exciting an arbitrary two-dimensional region (see, for example, Patent Document 1 and Non-Patent Document 1). The two-dimensional selective excitation is effective for, for example, processing such as exciting only a blood vessel site and tracing a bolus, or locally exciting only a water proton site while avoiding a fat site. In addition, since excitation can be performed while avoiding specific parts such as a part having a flow, a part to move, or a subcutaneous fat, an artifact of a flow or body movement can be suppressed. A pulse sequence in which a plurality of high-frequency magnetic field pulses are applied together with an oscillating gradient magnetic field is also used in a spectral-spatial ((spatial) frequency selective excitation) method that excites only a specific type of proton such as a water proton in an arbitrary plane. (For example, see Patent Document 2.)

  By using the gradient magnetic field pulse, an eddy current and a residual magnetic field are generated. The eddy current and the residual magnetic field are extra gradient magnetic fields applied to the center of each high frequency magnetic field pulse. By applying this extra gradient magnetic field, the phase of the nuclear spin changes, and a difference (shift) occurs between the irradiation phase of the high-frequency magnetic field pulse to be irradiated (hereinafter simply referred to as phase). In addition, when a plurality of high-frequency magnetic field pulses are applied together with the oscillating gradient magnetic field, the effects of these eddy currents and the residual magnetic field accumulate, so in such a pulse sequence, the amount of phase shift differs for each high-frequency magnetic field pulse. . For this reason, the nuclear spin of the target phase cannot be excited by each high-frequency magnetic field pulse, and the displacement of the excitation position and the distortion of the excitation occur, and as a result, the spatial selectivity and the frequency selectivity are lowered. Therefore, it is necessary to eliminate these effects. Regarding the influence of eddy currents, there is a technique for removing the influence by finely adjusting the gradient magnetic field (see, for example, Non-Patent Document 2).

JP 2001-95773 A JP-A-2005-87375 A K-Space Analysis of Small-Tip-Angle Excitation, J. Magn., 8l, 43-56 (1989) Calibration of echo-Planar 2D-Selective RF excitation pulses, OelhafenM, Magn ResonMed., 52 (5), l136-45 (2004)

  However, the effect of the residual magnetic field cannot be removed only by the method described in Non-Patent Document 2, that is, the adjustment of the gradient magnetic field. Further, it is desired to remove the influence of eddy current with higher accuracy.

  The present invention has been made in view of the above circumstances, and in imaging using a pulse sequence in which a plurality of high-frequency magnetic field pulses are applied together with an oscillating gradient magnetic field, the effects of eddy currents and residual magnetic fields due to the oscillating gradient magnetic field can be easily and accurately performed. An object of the present invention is to provide a technique for removing and obtaining good spatial selectivity and frequency selectivity.

  According to the present invention, in imaging using a pulse sequence in which a plurality of high-frequency magnetic field pulses are applied together with an oscillating gradient magnetic field, the phase shift of each RF pulse is detected and corrected from the excitation profile in the captured image or the phase of the received signal.

Specifically, a gradient magnetic field generating means for generating a gradient magnetic field in three orthogonal directions applied to an inspection object placed in a static magnetic field, and a high-frequency magnetic field generation means for generating a high-frequency magnetic field pulse for irradiating the inspection object When the polarity of the polarity inversion every multiple oscillating gradient magnetic field Ru by inverting the gradient, the high-frequency magnetic field pulse irradiation shines, the gradient magnetic field generating means so as to spatially selective excitation or frequency selective excitation of two-dimensional and It said control means for controlling the driving of the high frequency magnetic field generating means comprises a, due to the residual magnetic field and eddy currents due to the oscillating gradient magnetic field, the shift between the nuclear spins of the phase of excitation radio frequency magnetic field pulses of the phase, the frequency Provided is a magnetic resonance imaging apparatus comprising correction means for adjusting by correcting the phase of a magnetic field pulse.

  According to the present invention, in imaging using a pulse sequence in which a plurality of high-frequency magnetic field pulses are applied together with an oscillating gradient magnetic field, the effects of the eddy current and residual magnetic field due to the oscillating gradient magnetic field can be easily and accurately removed, and a good spatial selectivity can be obtained. , Frequency selectivity can be obtained.

<< First Embodiment >>
Hereinafter, a first embodiment to which the present invention is applied will be described with reference to the drawings.

  FIG. 1 is a block diagram of a typical MRI apparatus. As shown in the figure, the MRI apparatus 300 includes a magnet 302 that generates a static magnetic field in a space around a subject 301, a gradient magnetic field coil 303 that generates a gradient magnetic field in the space, and a high-frequency magnetic field pulse ( (Hereinafter referred to as “RF pulse”) and an RF probe 305 that detects an MR signal generated by the subject 301. The gradient magnetic field coil 303 is composed of coils that generate gradient magnetic fields in three directions of X, Y, and Z. Each of the gradient magnetic field coils 303 generates a gradient magnetic field according to a signal from the gradient magnetic field power supply 309, and an echo received by an RF probe 305 described later. Add location information to the signal. The RF coil 304 generates an RF pulse according to the signal from the RF transmitter 310. The RF probe 305 receives an echo signal. The echo signal is detected by the signal detection unit 306, signal processed by the signal processing unit 307, and converted into an image signal. The image signal is arithmetically processed by the image processing unit 313 and displayed on the display unit 308 as an image. The control unit 311 controls the operation of the entire MRI apparatus 300 in accordance with a program stored in a memory (not shown) or an operator instruction. In particular, in the program, a time chart for controlling operations of the gradient magnetic field power supply 309, the RF transmission unit 310, and the signal detection unit 306 is generally called a pulse sequence. The bed 312 is for the subject 301 to lie down.

  At present, the subject to be imaged by the MRI apparatus 300 is the main constituent substance of the subject, proton. The MRI apparatus 300 images the form or function of the human head, abdomen, extremities, etc. in a two-dimensional or three-dimensional manner by imaging the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon in the excited state. To do.

  In the MRI apparatus 300, different phase encoding is applied to the slice plane selected by the gradient magnetic field by the gradient magnetic field, and echo signals by the respective phase encoding are detected while applying the read gradient magnetic field. As the number of phase encodings to be given, values such as 128, 256, and 512 are usually selected per image. Each echo signal is usually obtained as a time-series signal composed of 128, 256, 512, and 1024 sampling data. These data are two-dimensionally Fourier transformed to create one MR image.

  In this embodiment, a two-dimensional selective excitation RF pulse for exciting a space two-dimensionally is used for the pulse sequence. FIG. 2 shows a pulse sequence used in this embodiment. This figure shows the timing of RF pulse irradiation, slice selective gradient magnetic field application, phase encode gradient magnetic field application, readout gradient magnetic field application, echo signal generation, and sampling (A / D) from the top.

  The two-dimensional selective excitation RF pulse 401 selectively applies an arbitrary two-dimensional shape by applying an oscillating gradient magnetic field 404, which is a gradient magnetic field for selecting a slice, a blip gradient magnetic field 405, and a plurality of RF pulses 403. This is a pulse to be excited. The amplitude modulation envelope 402 of the RF pulse 403 is basically a Fourier transform of the excitation shape in the blip gradient magnetic field 405 direction. Here, a case where the amplitude modulation envelope 402 of the RF pulse 403 has a sinc function shape in order to make the excitation shape in the direction of the desired blip gradient magnetic field 405 rectangular will be described as an example. The shape of the amplitude modulation envelope 402 is not limited to this. Further, each RF pulse constituting the plurality of RF pulses 403 is hereinafter referred to as a sub-pulse 403sub. Here, the oscillating gradient magnetic field 404 is applied in the z-axis direction, and the blip gradient magnetic field 405 is applied in the y-axis direction.

  In the pulse sequence of this embodiment, after applying the two-dimensional selective excitation RF pulse 401, a gradient magnetic field 406 is applied as a phase encoding gradient magnetic field in the y-axis direction, and after a predetermined time, a gradient magnetic field is applied as a readout gradient magnetic field in the x-axis direction. While being applied, the generated echo signal is detected and sampled for a predetermined time (A / D). The sampled signal is imaged by the image processing unit 313 through the signal processing unit 307.

  FIG. 3 is a diagram showing details of the two-dimensional selective excitation RF pulse 401. As shown in the figure, an eddy current 601 and a residual magnetic field 602 are generated in the applied oscillating gradient magnetic field 404. The phase of the nuclear spin is changed by an extra applied gradient magnetic field such as the eddy current 601 and the residual magnetic field 602, and a difference is generated from the phase of the RF pulse 403 to be irradiated. For example, since the residual magnetic field 602 is applied with a constant intensity with respect to time, the effect is accumulated, and the difference between the phase of the nuclear spin and the phase of the RF pulse 403 to be irradiated changes linearly for each sub-pulse 403 sub. This linear change is equivalent to changing the phase of the amplitude modulation envelope 402 linearly, and the excitation position in the Y direction determined by the amplitude modulation envelope 402 and the blip gradient magnetic field 405 is changed.

  In order to obtain good spatial selectivity, it is necessary to accurately match the phase of the nuclear spin with the phase of the RF pulse 403 to be irradiated. In the present embodiment, this is realized by controlling the phase on the RF pulse 403 side. That is, correction is made so that the phase of each sub-pulse 403 sub constituting the RF pulse 403 matches the phase of the nuclear spin shifted by the extra applied gradient magnetic field, and the influence of the eddy current 601 and the residual magnetic field 602 is eliminated.

  In this embodiment, pre-scanning is performed, and the difference between the phase of each sub-pulse 403 sub and the phase of the nuclear spin constituting the RF pulse 403 is calculated from the excitation profile on the obtained image, and this is calculated as the corresponding sub-pulse 403 sub. Are individually corrected as phase shift amounts, that is, correction values. The RF pulse 403 is irradiated with the phase after correction, and the main imaging by the selective excitation method is performed. The imaging sequence of the main imaging is not particularly limited, such as a spin echo sequence and a gradient echo sequence.

  Hereinafter, a procedure for correcting a shift between the phase of the RF pulse 403 and the phase of the nuclear spin caused by the zero-order component and the first-order component of the eddy current 601 and the residual magnetic field 602 in the pre-scan will be described. FIG. 4 is a flowchart of the phase correction process of the RF pulse 403 of this embodiment. The following processing is executed by the control unit 311 by controlling the operation of each unit in accordance with the pulse sequence and the program. In this embodiment, the phase shift of the RF pulse 403 due to the residual magnetic field 602 is corrected, then the shift due to the zero-order component of the eddy current 601 is corrected, and finally the shift due to the primary component of the eddy current 601 is corrected. .

  First, the phase shift of the RF pulse 403 caused by the residual magnetic field 602 is corrected. As described above, when the shape of the oscillating gradient magnetic field 404 is impaired by the residual magnetic field 602, a difference occurs between the phase of the nuclear spin and the phase of the sub-pulse 403sub to be irradiated. This appears as a displacement of the excitation position in the Y direction within the selected slice plane. The amount of phase shift between the sub-pulses 403 sub can be calculated from the amount of displacement of the excitation position in the Y direction. In the present embodiment, the phase of each sub-pulse 403sub is corrected using the calculated shift amount.

  In the case of the oscillating gradient magnetic field 404 in the Z direction, the gradient magnetic field strength of the residual magnetic field 602 is linearly generated in the Z direction. Therefore, the displacement amount of the excitation position in the Y direction due to the residual magnetic field 602 is also generated linearly in the Z direction. Therefore, the correction value of the phase of the RF pulse 403 by the residual magnetic field 602 is a function of the position in the Z direction (slice position). Therefore, in this embodiment, imaging is performed with two slice positions set, and the displacement amount of the excitation position in the Y direction corresponding to each slice position is calculated.

  The amount of displacement of the excitation position may be due to the residual magnetic field 602 or due to the eddy current 601. Here, it is necessary to extract the influence of only the residual magnetic field 602 and remove the influence of the eddy current 601. The polarity of the eddy current 601 is different between when the oscillating gradient magnetic field 404 is rotated forward and when it is reversed. However, the absolute amount applied is equal. Accordingly, since the eddy current 601 cancels between the forward rotation and the reversal, the nuclear spin caused by the eddy current 601 and the RF pulse 403 are between the RF pulses during the forward rotation and between the RF pulses during the reversal. The phase difference is equal. Therefore, imaging is performed by applying only the sub-pulse 403sub when the oscillating gradient magnetic field 404, which is a slice gradient magnetic field, is either forward or reverse in the RF pulse 403 in FIG. 2, and the excitation profile of the resulting image is obtained. By calculating the displacement amount of the excitation position from the above, it is possible to eliminate only the influence of the eddy current 601 and extract only the one caused by the residual magnetic field 602. Hereinafter, a case where only the sub-pulse 403sub in the case of normal rotation is applied will be described as an example.

  Here, Zex is an excitation size in the Z direction, Yex is an excitation size in the Y direction, and Z is a slice position. First, imaging in which only the sub-pulse 403sub is applied in the case of normal rotation is performed twice, when the excitation size Zex * Yex is constant and the slice position Z = Z1 is set, and when Z = Z2 is set [Step] 1-1]. Note that the slice positions Z1 and Z2 are slice positions whose phases are not shifted by 180 degrees or more between the sub-pulses 403sub constituting the RF pulse 403 based on the specifications of the MRI apparatus 300.

  FIG. 5 is a diagram for explaining the difference between the imaging result and the excitation profile at each slice position Zl, 0, Z2. Here, the slice position Z = 0 is a position where the gradient magnetic field is always 0 in the oscillating gradient magnetic field 404. In FIG. 5A, 201p, 200p, and 202p are line profiles (excitation profiles) in the Y direction at the same position in the X direction, respectively. Here, an excitation profile at X = 0 is shown as an example. The excitation position (field center) in the Y direction at the slice position Z = 0 is assumed to be Y = 0. That is, at the slice position Z = 0, the displacement amount of the excitation position in the Y direction is zero.

On the line profiles 201p and 202p of the imaging results 201 and 202 at the slice positions Zl and Z2, the amount of deviation from the center of the visual field of each excitation position 203 and 204 is detected [step 1-2]. These are deviation amounts of the excitation position from the center of the visual field in the Y direction due to the influence of the residual magnetic field 602 at each slice position. As described above, since the displacement amount of the excitation position in the Y direction is linear in the Z direction, for example, as shown in FIG. 5B, the horizontal axis is the slice position Z and the vertical axis is in the Y direction. by a technique such as determining the slope α shift amount of the excitation position and the from the graph the least squares method of the formula (e.g., representing the amount of deviation of the excitation position in the Y direction at an arbitrary slice position Z Y _{shift (Z),} the formula ( 1)) is determined.
Y _shift (Z) = αZ (1)

Using the obtained shift amount Y _shift (Z) at the slice position Z, a phase shift amount ΔPhr (Z) between the sub-pulses 403sub constituting the RF pulse 403 caused by the residual magnetic field 602 at the slice position Z is calculated [ Step 1-3]. Here, the phase shift amount ΔPhr (Z) between the sub-pulses 403 sub caused by the residual magnetic field 602 can be obtained by the following formula (2) using the _shift amount Y _shift (Z) obtained by the formula (1).
ΔPhr (Z) = − (Y _shift (Z)) / Δpp × 360 [deg] (2)
Here, Δpp is the folding interval 206 shown in FIG. When the application amount of the blip gradient magnetic field is S _Gz (Tesla / meter / second), the folding interval 206Δpp can be expressed as Δpp = 1 / (γS _Gz ). Equation (2) is determined by the function shape of the amplitude modulation envelope wave 402. For example, when the amplitude modulation envelope 402 is a sinc function shape and the number of sub-pulses 403 sub in the RF pulse 403 is N, Expression (2) is as follows.
ΔPhr (Z) = − (Y _shift (Z)) / ((N−1) × Yex / 2) × 360 [deg]

  Then, using the phase shift amount ΔPhr (Z) between the sub-pulses 403sub obtained in step 1-3, the phase of each sub-pulse 403sub is corrected [step 1-4]. That is, the correction value ΔPhr (Z) m for the phase of the mth sub-pulse 403sub is (m−1) * ΔPhr (Z) (m = 1, 2, 3,..., N). ΔPhr (Z) m is added to the phase of each sub-pulse 403sub to correct it.

  After correcting the phase shift due to the residual magnetic field 602, the phase shift of the RF pulse 403 due to the zero-order component of the eddy current 601 is corrected. As described above, the excitation position of the excitation profile is shifted in the Y direction due to the influence of the residual magnetic field 602. Other distortions in the excitation profile are due to the influence of the eddy current 601. In the present embodiment, as shown in FIG. 6, it appears as unnecessary excitation between excitations. Here, imaging is performed by applying all the RF pulses 403 as usual, and from the excitation profile of the image obtained as a result, the excitation signal intensity generated at a location that is not the position that should be originally excited, and the position that should be originally excited The ratio of the signal intensity of the excitation in the light is obtained, the phase shift amount between the subpulses 403sub is calculated from the value, and each subpulse 403sub is corrected.

  Specifically, first, imaging is performed by applying all the sub-pulses 403sub in the RF pulse 403 of FIG. 2 [step 2-1]. Imaging is performed with the excitation size Zex * Yex being constant and the slice position Z being set to zero. This is because the phase shift amount of the RF pulse 403 due to the zero-order component of the eddy current 601 does not depend on the position in the Z direction.

  Next, as in step 1-2 above, a line profile (excitation profile) in the Y direction at X = 0 of the imaging result is taken, and the excitation signal intensity and the excitation signal due to the influence of the eddy current at the position where excitation should be originally performed. The ratio with the intensity is calculated [Step 2-2].

The excitation profile at this time is shown in FIG. The position where the excitation due to the influence of the eddy current appears is determined by the function shape of the amplitude modulation envelope 402 and the number of sub-pulses 403 sub. Specifically, excitation, which corresponds to the read-out N / 2 artifact, appearing at the midpoint Y _M of the wrapping interval 206. _{Assuming that} the signal intensity at the position Y is S (Y), the ratio of the signal intensity S (0) of the excitation 501 at Y = 0, which is the position to be originally excited, to the signal intensity S (Y _M ) of the excitation 502 at YM. F = S (0) / S (Y _M ) is calculated.

  As an example, if the number of subpulses 403 sub is N and the amplitude modulation envelope 402 is a sinc function shape, excitation 502 due to the influence of eddy current 601 appears at the position of Y = Yex * (N−1) / 4. In this case, the signal intensity S (Y) of the excitation 501 at Y = 0, which is the position to be originally excited, and the signal intensity S (Y) of the excitation 502 at Y = Yex * (N−1) / 4 are measured. Then, the signal intensity ratio F = S (0) / S (Yex * (N−1) / 4) is calculated.

  Next, the necessity of correction is determined by comparing the calculated signal strength ratio F with a predetermined threshold [step 2-3]. That is, if the signal intensity ratio F is sufficiently large, it is determined that the excitation 502 due to the distortion of the excitation shape due to the eddy current 601 does not affect the excitation 501 at the position where it should be excited, and correction is unnecessary. For example, if the threshold value is 20 and the signal intensity ratio F is 20 or more, it is determined that the correction of the influence of the zero-order component of the eddy current 601 is not necessary, and the correction step of the influence of the primary component of the eddy current 601 described later is performed. Proceed to 3-1. On the other hand, if the signal intensity ratio F is less than 20, the process proceeds to step 2-4 to calculate the amount of phase shift. The threshold value (20 in this case) can be set to an arbitrary value depending on the purpose of use of the two-dimensional selective excitation RF pulse 401.

When the signal intensity ratio F is less than 20, the phase shift amount ΔPhe0 between the subpulses 403sub due to the 0th-order component of the eddy current 601 is calculated [step 2-4]. The absolute value ΔPhe0 of the phase shift amount between the sub-pulses 403sub is expressed by the following equation (3) using the signal intensity ratio F.
ΔPhe0 = arctan (F) / π × 360 [deg] (3)

  ΔPhe0 is the amount of phase shift between the sub-pulses 403sub. Therefore, the correction value ΔPhe0m for the phase of the mth subpulse 403sub is obtained by adding ΔPhe0 to the amount of deviation of the (m−1) th subpulse 403sub. Here, the phase shift amount ΔPhe0 due to the zeroth-order component of the eddy current 601 has positive and negative values alternately for each sub-pulse 403sub because the eddy current 601 is alternately generated as shown in FIG. Therefore, for example, if they appear in order of positive and negative, the amount of phase shift of the second subpulse 403sub from the first subpulse 403sub is ΔPhe0, and the amount of shift of the third subpulse 403sub from the first is ΔPhe0 (second Shift amount) + (− ΔPhe0) = 0, the shift amount from the first of the fourth sub-pulse 403sub is 0 (third shift amount) + ΔPhe0 = ΔPhe0, and the shift amount of the fifth sub-pulse 403sub from the first. ΔPhe0 (fourth shift amount) + (− ΔPhe0) = 0.

  Here, since it is unknown whether ΔPhe0 occurs in the order of positive or negative, first, a positive value is added as a correction value to the phase of the even-numbered (second, fourth,...) Sub-pulse 403sub. [Step 2-5]. Here, the same correction value ΔPhe0 is added to both sub-pulses 403sub. Thereafter, imaging is performed again under the same conditions as in Step 2-1, and the signal intensity ratio F is obtained from the distortion of the excitation profile in the same manner as in Steps 2-2 to 2-3 [Step 2-6].

  Then, the signal intensity ratio F is compared with a threshold value to determine whether the value adopted for the correction is acceptable [step 2-7]. In the present embodiment, for example, if the signal intensity ratio F is 20 or more, the value adopted in step 2-5 is set as the correction value. Note that 20 as the determination criterion can be set to an arbitrary value depending on the purpose of use of the two-dimensional selective excitation RF pulse 401, as described above.

  On the other hand, if it is less than 20, the negative value not used in step 2-5 is added as a correction value to the phase of the even-numbered (second, fourth,...) Sub-pulse 403sub [step 2-8]. As described above, the phase shift of the RF pulse 403 due to the zero-order component of the eddy current 601 is corrected. In the present embodiment, correction is performed so that the phase of the even-numbered sub-pulse 403sub matches the odd-numbered phase by adding to the phase of the even-numbered sub-pulse 403sub. However, conversely, the phase of the odd-numbered sub-pulse 403sub may be matched with the even-numbered phase by subtracting the correction value from the phase of the odd-numbered sub-pulse 403sub. In addition, either the positive or negative value of ΔPhe0 calculated by the equation (3) may be adopted first.

  After correcting the zero-order component of the eddy current 601, the phase shift of the RF pulse 403 due to the first-order component of the eddy current 601 is corrected. The procedure is basically the same as the correction of the zeroth-order component. However, the primary component of the eddy current 601 further changes linearly in the Z direction. Therefore, the correction value of the phase of the RF pulse 403 by the primary component of the eddy current 601 is a function of the position in the Z direction (slice position).

  Specifically, first, imaging is performed by applying all the sub-pulses 403sub in the RF pulse 403 of FIG. 2 [step 3-1]. Imaging is performed with the excitation size Zex * Yex being constant. However, when correcting the primary component, the slice position Z is set to Z3 (≠ 0). Note that the slice position Z3 is a slice position whose phase is not shifted by 180 degrees or more between the sub-pulses 403sub constituting the RF pulse 403 from the specifications of the MRI apparatus 300 and Zex.

Next, as in step 2-2, the excitation profile of the imaging result is taken, and the signal intensity S (Y _M ) of the excitation 502 at the position Y = Y _M due to the influence of the eddy current 601 and Y = the position that should be originally excited. The signal intensity S (0) of the excitation 501 at 0 is measured, and the signal intensity ratio F between them is calculated [step 3-2]. Here, it is assumed that the excitation 502 is generated at a position of Y _M = Yex * (N−1) / 4. Then, the signal intensity ratio F is compared with a predetermined threshold value to determine whether or not it is within an allowable range [step 3-3]. Here, as an example, if the threshold value is 60 and the signal intensity ratio F is 60 or more, it is assumed that the excitation at the position to be originally excited is not affected, it is determined that correction is unnecessary, and the process is terminated. The threshold (here, 60) can be set to an arbitrary value depending on the purpose of use of the two-dimensional selective excitation RF pulse 401.

On the other hand, when the signal intensity ratio F is less than 60, the amount of phase shift between the sub-pulses 403sub due to the primary component of the eddy current 601 is calculated [step 3-4]. Here, the absolute value ΔPhe1 (Z) of the phase shift amount between the sub-pulses 403sub at the slice position Z is obtained by the following equation (4) using the signal intensity ratio F.
ΔPhe1 (Z) = arctan (F) / π × Z / Z3 × 360 [deg] (4)

  Similar to the correction of the 0th-order component, two positive and negative values are obtained as the deviation amount. Which one should be adopted is determined by the same procedure as that for correcting the zeroth-order component. Here, first, a positive value among the values obtained by Expression (4) is added to the phase of the even-numbered sub-pulse 403sub [step 3-5]. Thereafter, imaging is performed again under the same conditions as in step 3-1, and the signal intensity ratio F is obtained from the distortion of the excitation profile in the same manner as in steps 3-2 to 3-3 [step 3-6].

  Then, like the 0th-order component, the signal intensity ratio F is compared with a predetermined threshold value to determine whether the value adopted for the correction is acceptable [step 3-7]. That is, here, for example, it is determined whether or not the signal intensity ratio F is 60 or more, and if it is 60 or more, the value adopted in step 3-5 is set as the correction value. On the other hand, if it is less than 60, a negative value not used in step 305 is added as a correction value to the phase of the even-numbered sub-pulse 403sub [step 3-8]. As described above, the phase shift of the RF pulse 403 due to the primary component of the eddy current 601 is corrected. In this case, similarly to the 0th-order component, the phase of the odd-numbered sub-pulse 403sub may be configured to be even-numbered. In addition, the correction value may be either positive or negative first.

  As described above, according to the present embodiment, in the pulse sequence in which a plurality of RF pulses are applied together with the oscillating gradient magnetic field, the deviation of the phase of the nuclear spin caused by the residual magnetic field and eddy current due to the oscillating gradient magnetic field from the phase of the RF pulse is Correction is performed by controlling the phase of the individual sub-pulses constituting the RF pulse. The shift between the phase of the sub pulse and the phase of the nuclear spin is calculated from the excitation position and distortion of the excitation profile of the imaging result.

  According to the present embodiment, it is possible to take measures against the two causes of the eddy current and the residual magnetic field with respect to the deviation of the phase of the nuclear spin from the RF pulse. Moreover, the distortion of the magnetic field by an apparatus can also be absorbed and a maintenance labor is not required. Furthermore, according to the method of the present embodiment, fine adjustment of the phase is easier than the method of controlling the phase by adjusting the magnetic field. Therefore, according to the present embodiment, the correction can be performed with high accuracy and easily, the influence of the residual magnetic field and the eddy current can be easily removed with high accuracy, and a good space selection ability can be obtained.

<< Second Embodiment >>
Next, a second embodiment to which the present invention is applied will be described. In the first embodiment, the phase shift amount of the RF pulse is detected from the excitation profile of the captured image in the pre-scan phase correction process. However, in this embodiment, the phase shift amount is obtained from the received signal (echo signal). Note that the MRI apparatus 300 of the present embodiment is basically the same as that of the first embodiment, and therefore details thereof will not be described here. Hereinafter, a description will be given focusing on the configuration different from the first embodiment. In the description, the same reference numerals are used for the same components as those in the first embodiment.

The echo signal to be measured is composed of a real part (Re) and an imaginary part (Im). The phase φ (t) of the echo signal F (t) at time t is expressed as tan ⁻¹ (Im (F (t))) / (Re (F (t))). tan ⁻¹ represents an arctangent function, Im (X) represents an imaginary part of the complex number X, and Re (X) represents a real part of the complex number X.

  FIG. 7 is a flowchart of the phase correction process of the RF pulse 403 of this embodiment. Also in this embodiment, as in the first embodiment, the difference between the phase of the nuclear spin generated by the residual magnetic field 602, the zero-order component of the eddy current 601 and the primary component of the eddy current 601 and the phase of the RF pulse 403 is detected. Correction is performed by controlling the phase on the RF pulse 403 side. These processes are executed by the control unit 311 by controlling the operation of each unit in accordance with the pulse sequence and the program.

  FIG. 8 shows a pulse sequence used in this embodiment. In the present embodiment, as in the first embodiment, the two-dimensional selective excitation RF pulse 401 is applied and the echo signal 801 is acquired. However, in this embodiment, the read gradient magnetic field 804 is applied in the blip gradient magnetic field direction so that the echo signal 801 can be separated for each sub-pulse 403 sub. Here, the case where the acquisition timing of the echo signal 801 is controlled and acquisition (A / D) 802 is separately performed for each sub-pulse 403 sub will be described as an example. Other configurations of the present embodiment are the same as the pulse sequence of the first embodiment. In the following, a case where there are five subpulses 403sub will be described as an example. Of course, the number of sub-pulses 403sub is not limited to this.

  In this embodiment, first, Z = 0 and Z = Z4 (≠ 0) and two slice positions are determined, set to the respective slice positions, the pulse sequence shown in FIG. 8 is executed, and the echo signal 801 is acquired. Then, using the phase at the peak of each echo signal at each slice position (maximum value of φ (t), hereinafter referred to as peak phase), the phase shift due to the residual magnetic field 602, the zero order of the eddy current 601 Correction is performed in the order of phase shift due to the component and phase shift due to the primary component of the eddy current 601.

  First, all the sub-pulses 403sub are applied to acquire (photograph) the echo signal 801. Here, the excitation size Zex * Yex is set constant, the slice position Z is set at two locations of Z = 0 and Z = Z4, and the pulse sequence shown in FIG. 8 is executed to obtain the echo signal 801 [Step. 4-1]. Note that the slice position Z4 is a slice position whose phase is not shifted by 180 degrees or more between the sub-pulses 403sub constituting the RF pulse 403 based on the specifications of the MRI apparatus 300.

  Next, the peak of the echo signal 801 to be examined for each subpulse 403sub is matched with the center of each sampling timing (A / D) [step 4-2]. Here, the received echo signal 801 is Fourier-transformed (FT) and discriminated from the phase map of the FT result. If the echo signal 801 is not received at the center of A / D, the echo signal 801 is corrected so that the peak of the echo signal 801 is at the center of A / D. The correction is performed by a known method (for example, see Patent Document 3 or Non-Patent Document 3). Note that the peak phase is detected from each of the five echo signals 801 acquired on each slice plane, and is stored in association with the slice plane and the irradiation order of the corresponding sub-pulse 403sub.

JP 09-248286 A Medical Electronics and Biotechnology, Vol. 35, no. 4, P372-P378 (1997)

  Next, the phase shift of the RF pulse 403 caused by the residual magnetic field 602 is corrected [Step 4-3]. The residual magnetic field 602 is generated linearly in the Z direction, and the influence (deviation) increases with time. In order to extract only the influence of the residual magnetic field 602, only the sub-pulse 403sub in the case where the oscillating gradient magnetic field 404 is either normal or inverted among the five echo signals 801 acquired by setting the slice position to Z4 is used here. The phase shift between the sub-pulses 403sub is calculated using the peak phase shift amount of the echo signal 801. Here, as an example, a case where the peak phase of the echo signal 801 based on the odd-numbered sub-pulse 403 sub (that is, the first, third, and fifth) is used will be described as an example.

  First, the shift amount θn of the peak phase of the n-th echo signal 801 from the peak phase of the first echo signal 801 is obtained. As described above, since the amount of peak phase shift changes approximately linearly with time, for example, as shown in FIG. 9, the horizontal axis represents the number n of the received echo signal 801, and the vertical axis represents the odd phase. The peak phase shift amount θn = βn + γ of the n-th echo signal 801 is obtained by a method such as obtaining a slope by a least square method from a graph plotting the peak phase 901 of the echo signal 801 (β and γ are constants).

Then, using the obtained deviation amount θn, a phase deviation amount ΔPhr (Z) between the sub-pulses 403sub constituting the RF pulse 403 caused by the residual magnetic field 602 at the slice position Z4 is calculated. The phase shift amount ΔPhr (Z) is expressed by the following equation (5).
ΔPhr (Z) = β [deg] × Z / Z4 (5)

  Then, the phase shift amount ΔPhr (Z) between the sub-pulses 403sub obtained by the equation (5) is used to correct the phase of each sub-pulse 403sub. Here, as in the first embodiment, the correction value ΔPhr (Z) m for the phase of the mth sub-pulse 403sub is (m−1) × ΔPhr (Z) (m = 1, 2, 3,... 5). ΔPhr (Z) m is added to the phase of each sub-pulse 403sub to correct it.

  By correcting the phase of each sub-pulse 403 sub in this way, the peak phase of the obtained odd-numbered echo signal 801 becomes constant as in 902. In the present embodiment, the peak phase shift is obtained using the echo signal 801 generated by the odd-numbered sub-pulse 403sub. However, you may obtain | require using the echo signal 801 by the even-numbered subpulse 403sub.

  Next, the phase shift of the RF pulse 403 due to the zero-order component of the eddy current 601 is corrected [step 4-4]. Here, using the peak phases of all the five echo signals 801 acquired by setting the slice plane Z = 0, the phase between sub-pulses 403 sub constituting the RF pulse 403 caused by the zero-order component of the eddy current 601 is obtained. Find the amount of deviation. The echo signal 801 obtained by setting the slice plane Z = 0 is used because the slice plane Z = 0 is not affected by the residual magnetic field 602, while the RF pulse 403 of the eddy current 601 is caused by the zero-order component. This is because the amount of phase shift does not depend on the position in the Z direction.

  First, a shift amount θe0 between the peak phase of each echo signal 801 acquired by setting the slice plane Z = 0 and the peak phase of the first echo signal 801 is obtained. Here, when the peak phase of each echo signal 801 is plotted with the number n of the received echo signal 801 on the horizontal axis and the peak phase on the vertical axis, the peaks of the odd-numbered and even-numbered echo signals 801 are plotted as shown in FIG. Each phase is constant. This is because the influence of the zero-order component of the eddy current 601 appears equally at the time of normal rotation and reversal of the oscillating gradient magnetic field 404, cancels out, and does not accumulate. Then, the difference between the peak phase of the odd-numbered echo signal 801 and the peak phase of the even-numbered echo signal 801 is detected as a peak phase shift amount θe0.

The peak phase shift amount of the echo signal 801 appears as it is in the phase shift amount ΔPhe0 between the sub-pulses 403sub constituting the RF pulse 403 due to the zero-order component of the eddy current 601. Therefore, it is expressed by Expression (6) using the obtained peak phase shift amount θe0.
ΔPhe0 = θe0 [deg] (6)

  Then, the obtained phase shift amount ΔPhe0 is added to the phase of the even-numbered sub-pulse 403sub. Or, conversely, the phase is subtracted from the phase of the odd-numbered sub-pulse 403sub. For example, when correction is performed by adding to the phase of the even-numbered sub-pulse 403 sub, the peak phase of each received echo signal 801 is changed from 1001 to 1002 in FIG. Therefore, since the phases are aligned between the sub-pulses 403 sub, the peak phases between the echo signals 801 are aligned. Thereby, the phase shift due to the zeroth-order component of the eddy current 601 of the RF pulse 403 is corrected.

  Next, the phase shift of the RF pulse 403 due to the primary component of the eddy current 601 is corrected [step 4-5]. Here, using the peak phases of all the five echo signals 801 obtained by setting the slice plane Z = Z4, the phase of each sub-pulse 403sub constituting the RF pulse 403 caused by the primary component of the eddy current 601 is obtained. Find the amount of deviation. In the slice plane Z = Z4, a phase shift is generated by adding the one caused by the residual magnetic field 602, the one caused by the zero-order component of the eddy current 601 and the one caused by the first-order component. Therefore, the phase shift caused by the primary component of the eddy current 601 can be obtained by removing the one caused by the residual magnetic field 602 and the one caused by the zero-order component of the eddy current 601.

  Here, when the peak phase of each echo signal 801 is plotted with the number n of the received echo signal 801 on the horizontal axis and the peak phase on the vertical axis, the result is as shown in FIG. As described above, each peak phase includes one caused by the residual magnetic field 602 and one caused by the zero-order component of the eddy current 601. First, subtract these. That is, from each peak phase, the shift amount θn of the peak phase of the nth echo signal 801 by the residual magnetic field 602 obtained in step 4-3 and the zeroth order of the eddy current 601 obtained in step 4-4. The peak phase shift amount θe0 due to the component is subtracted.

  The peak phase shift amount θe1 of the echo signal 801 obtained as a result of subtracting both shift amounts is the peak phase shift amount due to the primary component of the eddy current 601. Here, in this embodiment, since the shift amount of the peak phase due to the residual magnetic field 602 is obtained from the odd-numbered echo signal 801, the shift amount is generated at this time because the peak of the even-numbered echo signal 801 is generated. Phase 1101.

Since the primary component of the eddy current 601 changes linearly in the Z direction as described above, the phase shift amount ΔPhe1 (Z ) Is a function of the slice plane Z. Further, the peak phase shift amount of the echo signal 801 reflects the phase shift amount between the sub-pulses 403 sub as it is. Accordingly, the phase shift amount ΔPhe1 (Z) is expressed by Expression (7) using Z and the peak phase shift amount θe1 of the obtained echo signal 801.
ΔPhe1 (Z) = θe1 [deg] × Z / Z4 (7)

  The obtained phase shift amount ΔPhe1 (Z) is added to the phase of the even-numbered sub-pulse 403sub. Or, conversely, the phase is subtracted from the phase of the odd-numbered sub-pulse 403sub. Accordingly, for example, when correction is performed by adding to the phase of the even-numbered sub-pulse 403 sub, the peak phase of each received echo signal 801 is changed from 1101 to 1102 in FIG. 11. Therefore, the peak phase is constant between the echo signals 801, and the phase shift caused by the primary component of the eddy current 601 of the RF pulse 403 can be corrected.

  As described above, according to the present embodiment, in a pulse sequence in which a plurality of RF pulses are applied together with an oscillating gradient magnetic field, the phase of the nuclear spin caused by the residual magnetic field and eddy current due to the oscillating gradient magnetic field is different from the phase of the RF pulse. Deviations are corrected by controlling the phase of the individual subpulses that make up the RF pulse. The shift between the phase of the subpulse and the phase of the nuclear spin is calculated from the shift in the peak phase of the echo signal received corresponding to each subpulse. Therefore, as in the first embodiment, correction can be performed with high accuracy and easily, the influence of the residual magnetic field and eddy current can be removed with high accuracy and easily, and good spatial selectivity can be obtained.

  Furthermore, according to the present embodiment, signal acquisition as pre-scanning is only required twice, and it is not necessary to reconstruct the image, so that it is possible to reduce the time taken for the entire imaging.

  In this embodiment, the echo signal 802 by each sub-pulse 403sub is individually sampled (A / D). However, the present embodiment is not limited to this configuration. It is only necessary that the echo signal 801 can be separated for each sub-pulse. For example, FIG. 12 is another example of the pulse sequence of the present embodiment. As shown in this figure, the echo signals 1201 generated by all the sub-pulses 403sub may be sampled (A / D) 1202 at a time. In this case, the peak corresponding to the irradiated sub-pulse 403sub is detected from the sampled signal and set as the respective peak phase. In this configuration, correction for matching the peak of the received echo signal 1201 with the center of the sampling timing (A / D) may be performed only once. Therefore, the entire processing time is shorter than that in the above embodiment.

<< Third Embodiment >>
Next, a third embodiment to which the present invention is applied will be described. In each of the above embodiments, the case where a two-dimensional selective excitation RF pulse for selectively exciting an arbitrary two-dimensional region is applied has been described as an example. In this embodiment, a spatial frequency selective excitation RF pulse that excites only a specific type of proton is applied instead of the two-dimensional selective excitation RF pulse. The MRI apparatus of this embodiment is basically the same as that of each of the above embodiments, and will not be described here. Hereinafter, a description will be given focusing on the configuration different from the above embodiments.

FIG. 13 shows a spatial frequency selective excitation RF pulse 1402 used in the present embodiment. As shown in the figure, the spatial frequency selective excitation RF pulse 1402 of the present embodiment is applied to a specific type on a slice plane to be imaged by simultaneously applying a plurality of RF pulses 1403 and an oscillating gradient magnetic field 1404. Are selectively excited to collect echo signals in the corresponding frequency band. The frequency selection is determined by the shape of the amplitude modulation envelope of the RF pulse 1403 and the inter-pulse time τ1401. The pulse time τ1401 can be obtained by the following equation (8).
τ = 1 / (γ × δ × B ₀ ) / 2 (8)
Here, γ is the gyromagnetic ratio, δ is the chemical shift difference between water and fat, and B ₀ is the static magnetic field strength. For example, when δ = 3.5 ppm, γ = 42.57 × 10 ⁶ (Hz / T), B ₀ = 1.5 (T),
τ = 1 / (42.57 × 10 ⁶ × 3.5 × 1.5) / 2
= 2.24 (ms).

  FIG. 14 shows the distribution of spectral signals obtained by Fourier transforming the echo signals collected in this embodiment. In this figure, the horizontal axis represents frequency and the vertical axis represents signal intensity. A thick line represents a spectrum shifted from the apparatus distortion, and a thin line represents a desirable spectrum. The procedure for correcting the phase shift of the RF pulse 1403 caused by the residual magnetic field and the correction of the phase shift caused by the eddy current in the present embodiment is basically the same as in the first embodiment. However, the amount of phase shift of the RF pulse 1403 is detected by measuring a change in excitation intensity with respect to the frequency direction.

The amount of phase shift of the RF pulse 1403 due to the residual magnetic field 602 is obtained by measuring the displacement in the frequency direction of the peak position of the signal intensity of the spectrum signal of a specific type of proton. The peak position is displaced because the shape of the oscillating gradient magnetic field 404 is damaged by the residual magnetic field 602. For example, as in the first embodiment, the displacement of the peak position of the signal strength in the frequency direction at an arbitrary slice position Z is measured from the signal strength of water at the two slice positions Z1 and Z2. Then, a phase shift amount between the sub-pulses 403sub is calculated from the displacement amount. When the displacement amount W _shift (Z) is expressed as W _shift (Z) = αZ, the phase shift amount ΔPhr (Z) between the sub-pulses 403sub is calculated using the equation (2) of the first embodiment. it can. However, here, Y _shift (Z) is W _shift (Z). Then, (m−1) * ΔPhr (Z) is added to 403 sub of the m-th subpulse to correct it.

  In the present embodiment, the calculation of the signal intensity ratio F when calculating the phase shift amount of the RF pulse 1403 caused by the zero-order component of the eddy current 601 is performed between the signal intensities of the spectrum signals of different protons. . When calculating the phase shift amount of the RF pulse 1403 due to the zero-order component of the eddy current 601, as in the first embodiment, first, on the slice plane (Z = 0) that is not affected by the residual magnetic field 602. Set and execute the pulse sequence. Then, for example, the signal intensity ratio F between the signal intensity 1301 of water protons and the signal intensity 1302 of fat protons is obtained from the obtained result. If the signal intensity ratio F is equal to or smaller than a predetermined threshold, the phase shift amount ± ΔPhe0 between the sub-pulses 403sub is calculated by the following equation (3) as in the first embodiment, and the first embodiment Make corrections in the same way as.

  Further, as for the phase shift amount of the RF pulse 1403 due to the primary component of the eddy current 601, the slice position is set to Z = Z3 as in the first embodiment, and the pulse sequence is performed. Similarly, a signal intensity ratio F is obtained between signal intensities due to different protons. Then, if necessary, the phase deviation amount ± ΔPhe1 (Z) between the sub-pulses 1403sub is calculated by the same procedure as that of the first embodiment, using the equation (4), and the procedure similar to that of the first embodiment is calculated. Correct with.

  As described above, according to the present embodiment, in the spatial frequency selective excitation method using a pulse sequence in which a plurality of RF pulses are applied together with an oscillating gradient magnetic field, an RF pulse having a phase of a nuclear spin caused by a residual magnetic field and an eddy current The deviation from the phase of 1403 is corrected by controlling the phase of each sub-pulse constituting the RF pulse. In this embodiment, the shift between the phase of the sub-pulse and the phase of the nuclear spin is calculated from the frequency shift and intensity of the spectrum signal of a specific type of proton. Therefore, as in the above embodiments, the correction can be performed with high accuracy and easily, the influence of the residual magnetic field and the eddy current can be easily removed with high accuracy, and good frequency selectivity can be obtained.

  In the above embodiments, the case where the magnet 302 that generates the static magnetic field included in the MRI apparatus 300 is a permanent magnet has been described. For example, in the case of the MRl device 300 including a superconducting magnet that generates little residual magnetic field as the magnet 302 that generates a static magnetic field, the above-described procedure for correcting the phase shift caused by the residual magnetic field may be omitted. Thereby, the time required for all corrections can be further shortened.

  The correction of the phase of the RF pulse by the pre-scan in each of the above embodiments may be performed for each imaging depending on the magnitude of the influence of the residual magnetic field and eddy current, and the correction value is stored in a memory or the like. The same correction value may be used for a plurality of shootings.

It is a block diagram of the MRI apparatus of 1st embodiment. It is a figure which shows the pulse sequence of 1st embodiment. It is a figure which shows the detail of the two-dimensional selective excitation RF pulse of 1st embodiment. It is a flowchart of the phase correction process of RF pulse of 1st embodiment. It is a figure for demonstrating the shift | offset | difference of the imaging | photography result and excitation profile in each slice position of 1st embodiment. It is a figure which shows distortion of the excitation profile of 1st embodiment. It is a flowchart of the phase correction process of RF pulse of the second embodiment. It is a figure which shows the pulse sequence of 2nd embodiment. It is a figure which shows the change of the peak phase of the echo signal by the residual magnetic field of 2nd embodiment. It is a figure which shows the change of the peak phase of the echo signal by the eddy current zero order component of 2nd embodiment. It is a figure which shows the change of the peak phase of the echo signal by the eddy current primary component of 2nd embodiment. It is a figure which shows another example of the pulse sequence of 2nd embodiment. It is a figure which shows the detail of the spatial frequency selective excitation RF pulse of 3rd embodiment. It is a figure which shows distribution of the spectrum signal of 3rd embodiment.

Explanation of symbols

300: MRI apparatus, 301: subject, 302: magnet, 303: gradient magnetic field coil, 304: RF coil, 305: RF probe, 306: signal detection unit, 307: signal processing unit, 308: display unit, 309: gradient Magnetic field power supply, 310: RF transmission unit, 311: control unit, 312: bed, 313: image processing unit,

Claims (6)

A gradient magnetic field generating means for generating a gradient magnetic field in three orthogonal directions to be applied to an inspection object placed in a static magnetic field;
A high-frequency magnetic field generating means for generating a high-frequency magnetic field pulse for irradiating the inspection object;
The driving of the said high-frequency magnetic field pulse to the polarity reversal every multiple oscillating gradient magnetic field Ru by inverting the polarity of the gradient magnetic field irradiated with the two-dimensional of the gradient magnetic field generating means so as to spatially selective excitation and the high-frequency magnetic field generating means Control means for controlling;
Due to the residual magnetic field and eddy currents due to previous SL oscillating gradient magnetic field, and correcting means for adjusting by the deviation between the phase of the nuclear spins of phase with the radio frequency magnetic field pulses to excitation, to correct the phase of the radio frequency magnetic field pulses,
A magnetic resonance imaging apparatus comprising: A gradient magnetic field generating means for generating a gradient magnetic field in three orthogonal directions to be applied to an inspection object placed in a static magnetic field;
A high-frequency magnetic field generating means for generating a high-frequency magnetic field pulse for irradiating the inspection object;
Wherein refers to the high-frequency magnetic field pulse to the polarity reversal every multiple oscillating gradient magnetic field Ru by inverting the polarity of the gradient magnetic field irradiation, 2D of the gradient magnetic field generating means so as to spatially frequency selective excitation and the high-frequency magnetic field generating means Control means for controlling the driving of
Due to the residual magnetic field and eddy currents due to previous SL oscillating gradient magnetic field, and correcting means for adjusting by the deviation between the phase of the nuclear spins of phase with the radio frequency magnetic field pulses to excitation, to correct the phase of the radio frequency magnetic field pulses,
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1 or 2,
A calculation means for reconstructing an image related to the inspection object based on a magnetic resonance signal generated from the inspection object;
The magnetic resonance imaging apparatus, wherein the correction unit calculates the shift from the excitation profile of the reconstructed image and corrects the phase of the high-frequency magnetic field pulse. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus characterized in that the correction means calculates the deviation using phase information of a magnetic resonance signal acquired according to the control of the control means, and corrects the phase of the high-frequency magnetic field pulse. The magnetic resonance imaging apparatus according to claim 2,
The magnetic resonance imaging apparatus characterized in that the correction means calculates the deviation using a spectrum signal of a specific type of proton acquired according to the control of the control means, and corrects the phase of the high-frequency magnetic field pulse. A magnetic resonance imaging apparatus according to any one of claims 1 to 5,
The magnetic resonance imaging apparatus characterized in that the correction means calculates the shift due to the residual magnetic field and the shift due to the eddy current independently to correct the phase.

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