JP4685496B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus that obtains a tomographic image of a desired part of a subject using a nuclear magnetic resonance phenomenon, and in particular, magnetic resonance that performs whole body imaging (whole body MRI) while moving a bed on which the subject lies. The present invention relates to an imaging apparatus.

  The subject of MRI imaging that is currently widely used in clinical practice is the main constituent substance of the subject, proton. By imaging the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon in the excited state, the form or function of the human head, abdomen, and extremities is photographed two-dimensionally or three-dimensionally.

  Next, an MRI imaging method will be described. Different phase encodings are given depending on the gradient magnetic field, and echo signals obtained by the respective phase encodings are detected. As the number of phase encodings, values such as 128, 256, and 512 are usually selected per image. Each echo signal is usually obtained as a time series consisting of 128, 256, 512, and 1024 sampling data. These data are two-dimensionally Fourier transformed to create one MR image.

  In addition, when an image is obtained with an MRI apparatus, an image having various tissue contrasts can be obtained by changing parameters such as TE and TR or performing image calculation. In clinical practice, an image in which MR signals due to fat are suppressed is often required.

  Typical methods for obtaining an image with suppressed fat include (1) a method by frequency selective excitation, (2) a method by inversion excitation, and (3) a method by image calculation.

  The method (1) requires that the uniformity of the static magnetic field generated by the magnet is extremely high in the space around the subject. Further, in the method (2), the uniformity of the static magnetic field is not required, but the signal value of the tissue having a T1 value similar to that of fat as well as the signal due to fat is suppressed. In addition, the overall SNR also decreases.

  A typical method of (3) is the Dixon method, which is described in detail in Non-Patent Document 1.

  In the 2-Point Dixon (2PD) method, imaging is performed twice by changing the TE in a gradient echo (GE) sequence. In the first imaging, the TE is set to an integral multiple of 2τ. In the second imaging, TE is set longer by τ than in the first imaging. At the time of the first photographing, the water signal and the fat signal are in phase. On the other hand, the water signal and the fat signal are in opposite phases during the second imaging.

  If the magnitudes of the water signal and fat signal at the (x, y) coordinate position of the image are W (x, y) and F (x, y), respectively, the first and second image signals. S1 (x, y) and S2 (x, y) are respectively expressed as follows.

S1 (x, y) = W (x, y) + F (x, y) --- (1)
S2 (x, y) = W (x, y) -F (x, y) --- (2)
Therefore, a water image is obtained from S1 (x, y) + S2 (x, y) = 2W (x, y) as an added image, and S1 (x, y) −S2 (x, y) = 2F ( A fat image is obtained from x, y).

  In the MRI apparatus, it is ideal that the static magnetic field generated by the magnet is uniform in the space around the subject. However, when the generated magnetic field of the magnet is distorted, the generated static magnetic field itself may have nonuniformity.

  In addition, when the subject is inserted into the imaging space, nonuniformity may occur in the static magnetic field due to different magnetic susceptibility for each part of the subject. The non-uniformity of the static magnetic field in the field of view of MRI (FIELD OF VIEW: FOV) changes the frequency of the MR signal and causes image quality deterioration such as displacement and flow in the obtained image.

  In addition, since the phase of an image changes due to non-uniformity of a static magnetic field, a correct result cannot be obtained when performing a complex operation between images. When the static magnetic field is nonuniform, the above formulas (1) and (2) are expressed as follows.

S1 (x, y) = (W (x, y) + F (x, y)) exp (iα (x, y)) --- (3)
S2 (x, y) = (W (x, y) −F (x, y)) exp (i (α (x, y) + α ′ (x, y))) −−− (4)
Α (x, y) and α ′ (x, y) in equation (4) are both components of phase rotation due to inhomogeneous static magnetic field and depend on the position. Α (x, y) occurs at time 2τ × n (= TE), α ′ (x, y) occurs at time τ, and α (x, y) becomes α ′ (x , Y) is 2n times.

  As described above, when there is non-uniform static magnetic field, a phase difference occurs between the water signal at the first imaging and the water signal at the second imaging, and the water signal and the fat signal are separated by simple addition / subtraction. Can not do it.

  Therefore, auto shimming is performed to directly correct non-uniformity of the static magnetic field in the FOV by using an additional coil (shim coil), or post-processing is performed on the image to correct the influence of the non-uniformity of the static magnetic field.

  The latter method, that is, a method in which a signal phase correction process is added to the Dixon method using a static magnetic field inhomogeneous distribution map is referred to as a 3-Point Dixon (3PD) method. In the 3PD method, imaging is performed three times by changing TE. The first and second imaging are the same as in the 2PD method. In the third imaging, TE is further extended by τ (2τ from the first imaging) than the second imaging. The signal S3 (x, y) at the time of the third imaging is expressed as follows.

S3 (x, y) = (W (x, y) + F (x, y)) exp (iα (x, y) + 2α ′ (x, y)) −−− (5)
At the time of the first photographing, the water signal and the fat signal have the same phase, and the phase is α. Similarly, the water signal and the fat signal are in opposite phases during the second imaging. The phase value of the water signal is α + α ′. At the time of the third imaging, the water signal and the fat signal are in the same phase again, and the phase value is α + 2α ′. Since the water signal and the fat signal have the same phase at the first and third imaging, the phase of S3 (x, y) / S1 (x, y) is obtained as in the following equation (6). The amount of phase rotation due to non-uniform static magnetic field can be obtained.

arg (S3 (x, y) / S1 (x, y)) = 2α '(x, y) --- (6)
However, arg () means obtaining the phase.

  The value of the above equation (6) is obtained for all (x, y), subjected to an unwrapping process for removing around the principal value, and divided by 2 to obtain a phase rotation amount α ′ (x, y) due to static magnetic field inhomogeneity Get.

  The following equations (7) and (8) are executed using the obtained α ′ (x, y).

S1 ′ (x, y) = S1 (x, y) exp (−i2nα ′ (x, y)) (7)
S2 ′ (x, y) = S2 (x, y) exp (−i (2n + 1) α ′ (x, y)) −−− (8)
When the above equations (7) and (8) are calculated, a water image is obtained from S1 ′ (x, y) + S2 ′ (x, y) = 2W (x, y) as an added image, and S1 as a subtracted image. A fat image is obtained from '(x, y) -S2' (x, y) = 2F (x, y).

  There is also a method of obtaining a water / fat separated image by obtaining a phase rotation amount due to non-uniform static magnetic field from only two signals obtained by TE and TE + τ. This 2PD method with static magnetic field correction is described in Non-Patent Document 2.

  When the phase rotation amount due to the static magnetic field inhomogeneity is obtained in the 3PD method or the 2PD method with static magnetic field correction, it is necessary to perform a process called unwrapping or rewinding as described above.

  The unwrap processing method is also described in Patent Document 1 and Patent Document 2, but is also described in Non-Patent Document 2, Non-Patent Document 3, and Non-Patent Document 4 in addition to them.

  Since this unwrapping (rewinding) process is easily affected by noise, it requires a complicated and long calculation time, such as making a mask to remove the influence of noise.

JP 2002-306441 A JP 2002-306445 A W. Thomas Dixon et al., Simple Proton Spectroscopic Imaging, RADIOLOGY, Vol. 153, 189-194 (1984) Bernard D. Cooms et al., Two-Point Dixon Technique for Water-Fat Signal Decomposition with B0 Inhomogeneity Correction, Magnetic Resonance in Medicine, Vol. 38, 884-889 (1997) M. Peter and X. Hu, Direct Calculation of Wrap-Free Phase Image, Proceedings of Annual Meeting of the Society of Magnetic Resonance in Medicine (= SMRM), No. 721, 1993 Jerzy Szumowski et al., Phase Unwrapping in the Three-point Dixon Method for Fat Suppression MR Imaging, Radiology, Vol. 192, 555-561 (1994)

  By the way, in recent years, so-called whole-body MRI, in which whole-body imaging is performed while moving a bed, is becoming popular in clinical practice. Whole body MRI is promising for whole body screening tests for tumor metastasis. There is also a demand for obtaining a fat-suppressed image in this whole body MRI. This is because the contrast of the tumor increases by suppressing the fat signal.

  FIG. 6 is an imaging process flowchart when water / fat separation imaging is applied to whole body MRI.

  As shown in step S601 in FIG. 6, imaging is performed at a certain subject position, and image data necessary for water / fat separation processing is acquired. In step S602, a static magnetic field inhomogeneous distribution map of the photographed slice is created based on the data obtained in step S601. Subsequently, in step S603, the phase correction of the image data obtained in step S601 is performed using the static magnetic field inhomogeneous distribution diagram obtained in step S602.

  Next, in step S604, addition / subtraction processing of the image data phase-corrected in step S603 is performed to obtain a water / fat separated image. In step S605, the bed is moved to the next subject imaging position, and the process returns to step S601. By repeating steps S601 to S605 for the necessary slices, a whole body water / fat separation image can be obtained.

  Steps S602 to S604 after shooting are complex and time-consuming image processing. In particular, as described above, the process of creating the static magnetic field inhomogeneous distribution map in step S602 is a very complicated and time-consuming process, but the bed cannot be moved while these processes are being performed. For this reason, it is very time consuming to take a water / fat separated image of the whole body.

  In order to prevent this, water / fat separation processing may be performed in parallel with imaging after moving the bed, but in that case, the memory and arithmetic unit for the image being processed before moving the bed, the bed moving It is necessary to separately have a memory and an arithmetic unit for data to be captured later, and a large-capacity memory and an additional arithmetic unit are required, which increases the cost of the MRI apparatus.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of shortening the imaging time for water / fat separation imaging or imaging sequence that requires correction data for each slice similarly in whole body MRI. is there.

In order to achieve the above object, the present invention is configured as follows.
(1) Static magnetic field generating means, transport means for transporting a subject within a static magnetic field space generated by the static magnetic field generating means, magnetic field applying means for applying a gradient magnetic field and a high frequency magnetic field to the subject, and subject Receiving means for receiving a nuclear magnetic resonance signal from the signal, signal processing calculation means for reconstructing a tomographic image of the subject based on the nuclear magnetic resonance signal received by the receiving means, the static magnetic field generating means, and the conveying means And a control means for controlling the operation of the magnetic field applying means and the receiving means, wherein the control means moves the transport means so that the receiving means from each of the plurality of regions of the subject to receive a magnetic resonance signal, said signal processing means, based on a signal received by said receiving means, the static magnetic field inhomogeneity distribution de in one region of the plurality of regions It calculates the data, using the static magnetic field inhomogeneity distribution data in the one region, to reconstruct a tomographic image by correcting the image data in other areas of the plurality of regions.

(2) In the above (1), the signal processing calculation means calculates a difference between the static magnetic field inhomogeneous distribution data in the one region and the static magnetic field distribution data in the other region of the subject, It is assumed that the static magnetic field inhomogeneous distribution data in the region is calculated, and the tomographic image is reconstructed by correcting the image data in the other region of the subject using the calculated static magnetic field inhomogeneous distribution data .

(3) above (1) according to any of (2), the tomographic image is assumed Ru water-fat separation images der.

(4) above (1), (2) according to any of (3), said conveying means, after stopping a predetermined time in the stop position corresponding to each of said plurality of regions to move.

(5) above (1), (2) according to any of (3), said conveying means is continuously moved relative to the position corresponding to each of said plurality of regions.

  According to the present invention, when a subject is moved and a plurality of slice images are imaged, and when an imaging method that requires complicated correction processing is performed, the processing time is significantly extended without cost. Can be prevented.

  That is, when water / fat separation imaging is applied to whole body MRI, imaging time can be shortened without increasing costs.

  Embodiments of the present invention will be described below with reference to the drawings.

  A first embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 is a schematic configuration diagram of an MRI apparatus (magnetic resonance imaging apparatus) to which the present invention is applied.

  In FIG. 1, the MRI apparatus includes a magnet 102 that generates a static magnetic field around a subject 101, a gradient magnetic field coil 103 that generates a gradient magnetic field in a space where the static magnetic field is generated, and a high-frequency magnetic field in this space region. And an RF probe 105 for detecting an MR signal generated by the subject 101.

  The magnet 102 is provided around the subject 101 and generates a uniform static magnetic field around the subject 101 in a direction perpendicular or parallel to the body axis.

  The gradient magnetic field coil 103 is composed of gradient magnetic field coils in three directions of x, y, and z, and each generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 109.

  The RF coil 104 generates a high-frequency magnetic field in response to a signal from the RF transmitter 110 and excites atomic nuclei constituting the imaging cross section of the subject 101 to cause nuclear magnetic resonance.

  The RF probe 105 receives an NMR signal that is an echo signal by nuclear magnetic resonance of the biological tissue of the subject 101. The NMR signal received by the RF probe 105 is detected by the signal detection unit 106, processed by the signal processing unit 107, and further converted into an image signal by calculation. The converted image is displayed on the display unit 108.

  The gradient magnetic field power supply 109, the RF transmitter 110, and the signal detector 106 are controlled by the controller 111, and the control time chart is generally called a pulse sequence. The bed 112 is for the subject to lie down.

  The first embodiment of the present invention uses the configuration of the MRI apparatus described above, performs imaging of a part of the subject 101 while the bed 112 is stopped, moves the bed 112 after the imaging is completed, and again This is applied to the so-called multi-station method in which the bed 112 is stopped and another part is imaged.

  FIG. 2 is an explanatory diagram showing a case where a COR section of a subject is imaged by the multi-station method.

  In FIGS. 1 and 2, the MRI apparatus moves the bed 112, thereby moving the subject 101 to a first stop position (hereinafter, referred to as a first station) 301 and a distance 303 from the first station 301. To the second stop position (hereinafter referred to as the second station) 304 that has been moved, and to the third stop position (hereinafter referred to as the third station) 308 that has moved the bed 112 from the second station 304 by the distance 306. It can be stopped.

  First, in the first station 301, the slice 302 set on the head is photographed. In imaging at the first station 301, data necessary for water / fat separation processing is acquired, and a static magnetic field inhomogeneous distribution map necessary for water / fat separation processing for the slice 302 is calculated.

  The calculation of the static magnetic field inhomogeneous distribution map in the first station 301 includes an unwrapping process for removing a discontinuous jump and obtaining a smooth static magnetic field inhomogeneous distribution map.

  Subsequently, the subject 101 is moved, and the slice 305 is imaged at the second station 304. Since the photographing position of the slice 305 photographed at the second station 304 is the same as the photographing position of the slice 302 photographed at the first station 301 in the apparatus coordinate system, the static magnetic field inhomogeneity caused by the MRI apparatus is changed. do not do. Hereinafter, the static magnetic field inhomogeneity caused by the MRI apparatus will be described.

  FIG. 3 is a schematic diagram showing the operation of whole body MRI.

  In FIG. 3, the subject 101 shows a state at the first imaging position. After imaging at this position, the bed 112 is moved by the distance 201, the subject 101 is moved to the imaging position 202, and the next imaging is performed.

  At this time, the imaging position of the subject 101 moves by a distance 201. However, in the apparatus coordinate system such as the magnet 102, the imaging area 203 is substantially fixed, so that the static magnetic field inhomogeneity caused by the MRI apparatus is almost unchanged. do not do.

  Returning to FIG. 2, the static magnetic field inhomogeneity caused by the MRI apparatus does not change as described above. Therefore, the water / fat separation process for the slice 305 uses the static magnetic field inhomogeneous distribution map created by the process for the slice 302. be able to.

  When the static magnetic field inhomogeneous distribution map created by the processing for the slice 302 is used as it is for the processing for the slice 305, the creation time of the static magnetic field inhomogeneous distribution map for the slice 305 becomes unnecessary.

  Further, when creating a static magnetic field inhomogeneous distribution map for the slice 305, it is necessary to remove the above-described discontinuous jump and perform an unwrap process to obtain a smooth static magnetic field inhomogeneous distribution map. In this case, the difference between the static magnetic field inhomogeneous distribution map for the unwrapped slice 302 and the static magnetic field distribution chart measured before the creation of the slice 305 is calculated, and the unwrap processing is performed on the difference. I do. And by returning to the said static magnetic field distribution map about the difference which performed the unwrap process, the statically processed static magnetic field inhomogeneous distribution map with respect to the slice 305 can be obtained.

  In this way, if the unwrap process is performed only on the difference, the processing time can be significantly reduced as compared with the case where the unwrap process is performed on all the static magnetic field distribution diagrams.

  For the slice 305, when the static magnetic field inhomogeneous distribution map created for the slice 302 is used as it is and the bed 112 is further moved and the slice 307 is imaged at the third station 308, a new static magnetic field non-uniformity is obtained. There is also a method of recreating a static magnetic field inhomogeneous distribution map every several stations, such as recreating a uniform distribution map.

  Also in this case, when creating a static magnetic field inhomogeneous distribution map for the slice 307, the calculation time can be shortened by performing the unwrap process only on the difference as described above.

  In addition, it may be difficult to create a static magnetic field inhomogeneous distribution map for a slice of a region in which a subject such as a leg is divided into a plurality of regions, but artifacts may occur. Generation of artifacts can be suppressed by creating a static magnetic field inhomogeneous distribution map of an undivided region and using this.

  According to the first embodiment of the present invention configured as described above, the whole field MRI imaging is performed by the multi-station method, and particularly when the water / fat separation imaging is performed, the static magnetic field that has significantly extended the imaging time. By omitting part of the calculation of the non-uniform distribution map and shortening the image processing time at each station, it is possible to quickly move to the next station and reduce the time required for whole body imaging. it can. Here, in the examination result by the inventor of the present application, the time required to create the static magnetic field inhomogeneous distribution map for the matrix size 512 × 512 image is shortened to about 1/3 of the conventional technique when the difference is calculated. It was possible.

  In addition, it is possible to suppress the occurrence of artifacts in the static magnetic field inhomogeneous distribution map for a slice in which a subject such as a leg is divided into a plurality of regions.

  Next, a second embodiment of the present invention will be described with reference to the drawings.

  The configuration of the MRI apparatus to which the second embodiment of the present invention is applied is the same as the configuration shown in FIG.

  In the second embodiment, the present invention is applied to so-called multi-slice imaging, in which the number of slices captured at each station of the multi-station method described in the first embodiment is made plural.

  FIG. 4 is an explanatory diagram showing a case where the subject 101 is imaged by the multi-slice method of multi-slice in the second embodiment.

  1 and 4, the MRI apparatus can move the subject 101 to the first station 410 and the second station 411 moved by a distance 405 from the first station 410 by moving the bed 112.

  First, in the first station 410, four slices 401 to 404 are photographed. When performing water / fat separation photography, a static magnetic field non-uniform distribution map is required for each of the four slices. Conventionally, magnetostatic lift non-uniform distribution maps have been calculated for each of the four slices.

  In the second embodiment, first, a static magnetic field nonuniform distribution map is created for one of four slices.

  For example, a static magnetic field nonuniform distribution map is calculated for the slice 403. The slice for creating the static magnetic field inhomogeneous distribution map first is preferably a slice with a large proportion of the area in which the subject appears in the captured image.

  This is because the static magnetic field inhomogeneous value cannot be calculated in a region where the signal from the subject is low or absent, and the static magnetic field inhomogeneous value in such a region has a certain signal value of the subject, This is because the static magnetic field inhomogeneity is an interpolation value or extrapolation value from an area where calculation is possible.

  Subsequently, a static magnetic field inhomogeneous distribution map of adjacent slices, that is, slices 402 and 404 is created. At this time, as described above, unwrap processing is performed only on the difference, and static magnetic field inhomogeneous distribution maps of the smoothly processed slices 402 and 404 are created.

  As a result, the calculation time for creating the static magnetic field inhomogeneous distribution map for the slices 402 and 404 can be shortened compared to the calculation time for creating the static magnetic field inhomogeneous distribution map for the slice 403.

  Subsequently, the static magnetic field inhomogeneous distribution map of the slice 401 is created. At this time, the difference between the static magnetic field inhomogeneous distribution map of the slice 402 and the static magnetic field distribution chart measured prior to the creation of the slice 401 is calculated. Only by performing the unwrapping process, the calculation time can be shortened.

  When the slice interval of the multi-slice is small, the same static magnetic field non-uniform distribution map can be used over several slices. For example, when the slice interval between the slices 401 to 404 is small, the static magnetic field inhomogeneous distribution map of the slice 403 can be used as it is for the slices 401, 402, and 404. In this case, the calculation time of the static magnetic field inhomogeneous distribution map for three slices can be saved.

  Alternatively, the static magnetic field inhomogeneous distribution map of slice 403 can be used as it is for slice 402 and slice 404, and only the static magnetic field inhomogeneous distribution map of slice 401 can be recalculated.

  At this time, the static magnetic field inhomogeneous distribution map of the slice 401 is unwrapped only for the difference from the static magnetic field inhomogeneous distribution map of the slice 403. According to this method, the calculation time of the static magnetic field inhomogeneous distribution map of the slice 401 can be shortened, and the creation time of the static magnetic field inhomogeneous distribution map of the slice 402 and the slice 404 becomes unnecessary.

  Next, the subject 101 is moved, and the slices 406 to 409 are imaged at the second station 411. Since the imaging positions of the slices 406 to 409 are the same as the imaging positions of the slices 401 to 404 captured at the first station 410 in the apparatus coordinate system, the static magnetic field inhomogeneous distribution map of the slices 401 to 404 is used. can do.

  When the static magnetic field inhomogeneous distribution maps of the slices 401 to 404 are used as they are, the creation time of the static magnetic field inhomogeneous distribution maps for the slices 406 to 409 is not required.

  Also, when creating a static magnetic field inhomogeneous distribution map for slices 406 to 409, unwrap processing is performed only on the difference from the static magnetic field inhomogeneous distribution map of slices 401 to 404, thereby significantly increasing the processing time. It can be shortened.

  Moreover, the calculation time can be shortened by recreating the static magnetic field non-uniform distribution map every several stations.

  In addition, it is possible to suppress the occurrence of artifacts in the static magnetic field inhomogeneous distribution map for a slice in which a subject such as a leg is divided into a plurality of regions.

  Also in the second embodiment configured as described above, the same effect as in the first embodiment can be obtained.

  Next, a third embodiment of the present invention will be described with reference to the drawings.

  The configuration of the MRI apparatus to which the third embodiment of the present invention is applied is the same as the configuration shown in FIG.

  In the third embodiment, the present invention is applied to a so-called moving bed method in which shooting is performed while a bed is continuously moved.

  FIG. 5 is an explanatory diagram showing a case where a TRS cross section of a subject is imaged by the moving bed method in the third embodiment.

  In FIGS. 1 and 5, the MRI apparatus continuously performs imaging of the slice 503 while moving the subject 501 in the direction indicated by the arrow 502 by moving the bed 112. The position of the slice 503 in the apparatus coordinate system is fixed, but the relative position changes because the subject 501 moves continuously.

  First, a slice 503 is imaged at a certain subject position, a static magnetic field nonuniform distribution map is created, and water / fat separation processing is performed using the static magnetic field nonuniform distribution map, thereby creating a water / fat separated image. The position of the subject 501 at this time is set as an imaging position 504. After the water / fat separation image is created at the photographing position 504, photographing is started again.

  Since the bed 112 is continuously moving, the subject 501 is at a position different from the first imaging position 504 when imaging is resumed. The position of the subject 501 at this time is a second imaging position 505. The slice 503 is photographed at the second photographing position 505, and water / fat separation processing is performed.

  At this time, in either the first imaging position 504 or the second imaging position 505, since the fixed slice 503 is imaged in the apparatus coordinate system, the static magnetic field inhomogeneity caused by the MRI apparatus does not change.

  For this reason, what was calculated | required in the 1st imaging position 504 can be used as it is, without producing a static magnetic field inhomogeneous distribution map anew about the 2nd imaging position 505. FIG. As a result, the calculation time of the static magnetic field inhomogeneous distribution map is omitted, and the processing time of the living body related to the water / fat separation processing is greatly shortened.

  In the moving bed method, the distance between the first shooting position 504 and the second shooting position 505 depends on the shooting time including the water / fat separation process and the moving speed of the bed. That is, if the shooting time is long, the moving speed of the bed needs to be lowered.

  However, according to the third embodiment, the time required for the water / fat separation process is shortened as compared with the prior art, so that the moving speed of the bed can be increased. Thereafter, it is possible to obtain an image for the whole body by repeating the third imaging position, the fourth imaging position, the fifth imaging position,... And the imaging time for the whole body.

  In the moving bed method, the moving direction of the bed is often not changed. That is, when photographing the whole body, the movement direction is either from the head to the foot or from the foot to the head. When the slice to be imaged first is the head or the foot, it is considered that the ratio of the area where the subject is captured in the captured image is smaller than that of the other areas.

  Since the value of the static magnetic field inhomogeneity cannot be calculated in a region where there is no signal from the subject, it is not preferable to use the static magnetic field inhomogeneous distribution map created from the image of the head or foot for the whole body. In view of this, water / fat separation imaging is performed in advance in a region where the ratio of the subject such as the abdomen is large, and a static magnetic field non-uniform distribution map is created. By using such a static magnetic field inhomogeneous distribution map for water / fat separation processing of other parts, the processing accuracy can be improved.

  Alternatively, the static magnetic field inhomogeneous distribution map created first may not be used for the whole body as it is, but the static magnetic field inhomogeneous distribution map may be recreated every few slices. When a TRS cross section is imaged by the moving bed method, for example, a static magnetic field inhomogeneous distribution map is regenerated every 5 slices. According to this method, compared to the case where the static magnetic field inhomogeneous distribution map created first is used for the whole body as it is, the distance between the site where the static magnetic field inhomogeneous distribution map was created and the imaging part is closer, so the processing accuracy is improved To do.

  Also in this case, when re-creating the static magnetic field inhomogeneous distribution map, the calculation time can be shortened by performing unwrap processing only on the difference from the static magnetic field inhomogeneous distribution map used in the previous slice. Can do.

  In the case of this method, the processing time is extended as compared with other slices, and the bed moving distance during this period is increased. For this reason, equivalent image accuracy can be obtained by reducing the moving speed of the bed during creation of the static magnetic field nonuniform distribution map.

  Furthermore, even in this case, if water / fat separation imaging is performed in a region where the ratio of the region in which the subject is captured in the captured image, such as the abdomen, is preliminarily created, It can be used for photographing a static magnetic field non-uniform distribution map of a slice of a head or a foot to be photographed.

  Also in the third embodiment of the present invention configured as described above, the same effect as in the first embodiment can be obtained.

  In the above first to third embodiments, the case where the present invention is applied to the creation of a static magnetic field inhomogeneous distribution map in water / fat separation processing has been described. The present invention can be applied to a combination of an imaging sequence that requires a total body MRI and whole body MRI.

  An example of the imaging sequence is echo planar imaging.

  In echo planar imaging, data for correcting signal phase disturbance due to eddy current is acquired for each slice. Acquisition of this correction data is performed by executing a correction data acquisition sequence prior to photographing a slice.

  The correction data acquisition sequence is not executed for all slices acquired by whole body MRI, but is acquired in advance or executed every several slices, thereby shortening the imaging time.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is explanatory drawing which shows the case where the COR cross section of a subject is image | photographed with the multi-station method of the MRI apparatus by the 1st Embodiment of this invention. It is a schematic diagram which shows operation | movement of the whole body MRI to which this invention is applied. It is explanatory drawing which shows the case where a test object is image | photographed by the multi-station method made into the multi-slice of the MRI apparatus by the 2nd Embodiment of this invention. It is explanatory drawing which shows the case where the TRS cross section of a test object is image | photographed by the moving bed method of the MRI apparatus by the 3rd Embodiment of this invention. It is a flowchart at the time of applying water and fat separation imaging to whole body MRI of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Subject 102 Static magnetic field magnet 103 Gradient magnetic field coil 104 RF coil 105 RF probe 106 Signal detection part 107 Signal processing part 108 Display part 109 Gradient magnetic field power supply 110 RF transmission part 111 Control part 112 Bed

Claims (5)

A static magnetic field generating means, a transport means for transporting a subject into a static magnetic field space generated by the static magnetic field generating means, a magnetic field applying means for applying a gradient magnetic field and a high frequency magnetic field to the subject, and a nucleus from the subject A receiving means for receiving a magnetic resonance signal, a signal processing calculation means for reconstructing a tomographic image of the subject based on the nuclear magnetic resonance signal received by the receiving means, the static magnetic field generating means, the conveying means, and the magnetic field application Control means for controlling the operation of the means and the receiving means ,
The control means moves the transport means to receive a nuclear magnetic resonance signal from each of a plurality of regions of the subject by the receiving means,
Said signal processing means, based on a signal received by said receiving means, and calculating the static magnetic field inhomogeneity distribution data in an area of the plurality of regions, the static magnetic field inhomogeneity distribution data in the one region A magnetic resonance imaging apparatus characterized in that a tomographic image is reconstructed by correcting image data in another region of the plurality of regions. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the signal processing calculation unit calculates a difference between the static magnetic field inhomogeneous distribution data in the one region and the static magnetic field distribution data in another region of the subject. Magnetism characterized in that static magnetic field inhomogeneous distribution data is calculated in other regions, and the tomographic image is reconstructed by correcting image data in other regions of the subject using the calculated static magnetic field inhomogeneous distribution data. Resonance imaging device. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the tomographic image is a water / fat separation image . 4. The magnetic resonance imaging apparatus according to claim 1, wherein the transport unit is moved to a stop position corresponding to each of the plurality of regions after being stopped for a certain period of time. Imaging device. 4. The magnetic resonance imaging apparatus according to claim 1, wherein the transport unit continuously moves with respect to a position corresponding to each of the plurality of regions. 5. .

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