JP2007301295A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus accurately connecting respective images of a plurality of continuous sites of a subject. <P>SOLUTION: At first, this magnetic resonance imaging apparatus performs positioning imaging 701 and also acquires a navigator echo in the positioning imaging. The acquired navigator echo is defined as a reference for detecting a breath-holding level in the measurement. After acquiring the positioning images of the whole station, this MRI apparatus performs full-scale imaging. Then, the heart and the diaphragm are largely deviated in the leg direction of the subject relative to an imaging slice 721, but the slice position 712 is at the center of the magnetic field, so that an image wherein the heart is deviated to the end of the FOV (Field Of View) is acquired. This MRI apparatus acquires the navigator echo at the breath-holding time before starting the imaging (an acquisition position 713), monitors a displacement 711 by comparing the navigator echo with the navigator echo acquired in the positioning, and performs the imaging after moving the bed 412 by just an amount equivalent to the displacement 711. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus that measures nuclear magnetic resonance signals from hydrogen, phosphorus, and the like in a subject and visualizes nuclear density distribution, relaxation time distribution, and the like.

  In the magnetic resonance imaging apparatus (MRI apparatus), in recent years, a method of capturing the whole body (whole body MRI) while moving the bed on which the subject is placed continuously or stepwise is being performed in the clinical field.

  This has been reported to be an effective method for whole body screening examinations, and it has been shown that screening examinations that have been performed with multiple modalities such as CT, bone scintigraphy, PET, etc., can be performed only with an MRI apparatus. ing.

  Non-patent document 1 shows an example of shooting while moving the bed stepwise (multi-station shooting). For example, images from the head to the lower limb are imaged in 6 steps (6 stations), and an imaging surface is set at each station (for example, a COR cross section). The bed moves between shots and the bed is stationary at each position during the shot. When imaging is performed in the order of stations, if the imaging cross section is a part (chest or abdomen) that is affected by respiratory motion of the subject, breath-hold imaging is performed to suppress artifacts due to respiratory motion.

  When images of 6 stations from the head to the lower limb are acquired in this way, all of them are connected to create one image. If necessary, after moving the shooting position in the AP direction, images of 6 stations are acquired again. For example, when images for six stations are acquired at a certain cross-sectional position, photographing is repeated by sequentially changing the cross-sectional position with another cross-section and further with another cross-section, and an image for a necessary region is acquired. Then, the images of the six stations acquired at the respective cross-sectional positions are connected to form one image.

  Here, the problem is when imaging the chest and abdomen where the subject is breathing. As described above, when photographing the chest and abdomen, breath-hold photographing is performed in order to suppress respiratory motion artifacts. That is, an image is acquired by breath holding at each of the imaging section of the chest and the imaging section of the abdomen, and the two images are overlapped by about several centimeters to join together to create one image.

  Since whole-body images in multi-station imaging are taken with a somewhat large FOV (Field of View), the distortion of the images occurs at the end of the FOV due to the effect of magnetic field inhomogeneity. Therefore, even if there is no part that is not completely captured in the image, an inaccurate combined image in which the boundary portion of the station is distorted can be obtained. In order to avoid such a situation, the FOVs are usually photographed with some overlap between the stations, and are finally connected.

  Patent Document 1 discloses a technique for determining and correcting phase correction and frequency correction using navigator signals in order to reduce motion artifacts in the MRI apparatus.

JP 9-508050 gazette Whole-Body MRI: A Simple Approach using Automatic Tab1e Movement and Dedicated, Post-processing, ISMRM10 (2002), M Bock et al

  However, in whole body MRI, a correct combined image can be obtained if the breathing levels match between images of the chest and abdomen of the subject, but if the breath-holding levels are significantly different from each other, the images are stitched together. However, a slice position shift (misregistration) occurs at the boundary portion of the station, and there is a missing portion that is not reflected in the image, resulting in an inaccurate combined image.

  Patent Document 1 does not recognize the occurrence of image distortion or the like in the image boundary portion when the breath-holding level of the subject is different between adjacent images in whole-body MRI, and a technique for acquiring an appropriate image Is not considered.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of accurately combining respective images of a plurality of consecutive parts of a subject.

  The magnetic resonance imaging apparatus of the present invention controls the shadowing means of the subject, the moving means for moving the subject, the means for reconstructing images of a plurality of imaging regions of the subject, and the movement and stop of the moving means And when the body movement of the subject is detected between adjacent regions of the subject, the body of the subject is detected. In accordance with the amount of movement, imaging is performed by moving the moving means so that the target region of the subject becomes the center of a predetermined imaging field of view.

  The present invention realizes a magnetic resonance imaging apparatus that does not cause deviation between slices (misregistration) even when the subject's breath-holding level is deviated, and does not deviate the region to be imaged at the end of the FOV. can do.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. 1, the MRI apparatus generates a magnet 402 that generates a static magnetic field around a subject 401, a gradient magnetic field coil 403 that generates a gradient magnetic field, an RF coil 404 that generates a high-frequency magnetic field, and a subject 401. And an RF probe 405 for detecting an MR signal.

  The gradient magnetic field coil 403 is configured by gradient magnetic field coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in accordance with a signal from the gradient magnetic field power supply 409. The RF coil 404 generates a high-frequency magnetic field according to the signal from the RF transmitter 410. The signal of the RF probe 405 is detected by the signal detection unit 406, processed by the signal processing unit 407, converted into an image signal, and displayed on the display unit 408.

  The gradient magnetic field power supply 409, the RF transmission unit 410, and the signal detection unit 406 are controlled by the control unit 411, and the control time chart is generally called a pulse sequence. The bed 412 is for the subject to lie down and is driven by the bed driving unit 413.

  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, and 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.

  Next, whole body MRI will be described. As shown in FIG. 2, images of the subject from the head to the lower limb are imaged in 6 steps (6 stations), and imaging surfaces 101 to 106 are set in each station (the example shown in FIG. 2 is COR). cross section). The bed 412 moves between shootings, and the bed 412 is stationary at each position during shooting. In the order of imaging in the order of the imaging planes 101 to 106, if the imaging cross section is a part (chest 102 or abdomen 103) that is affected by the respiratory motion of the subject, breath holding imaging is performed to suppress artifacts due to respiratory motion. Do.

  When images of 6 stations from the head to the lower limb are acquired, all of them are connected to create one image. If necessary, after moving the shooting position in the AP direction, images of 6 stations are acquired again.

  For example, as shown in FIG. 3, when images for 6 stations are acquired at a certain cross-sectional position 201, imaging is repeated by sequentially changing the cross-sectional position to another cross-sectional position 202 and another cross-sectional position 203, and a necessary area. Get an image of the minute. Then, the images of the six stations acquired at the respective cross-sectional positions 201, 202, and 230 are connected to form one image.

  Here, as described above, for example, even if the respiratory levels are different between images of the chest and abdomen of the subject, an accurate combined image is obtained in the present invention. The principle will be explained.

  FIG. 4 is an explanatory diagram of the first embodiment of the present invention. The example shown in FIG. 4 is a case where, in multi-station imaging, there is an imaging slice in the chest that is affected by the respiratory motion of the subject, and imaging is performed so that the heart is at the center of the FOV. It is.

  First, positioning imaging is performed (701, FIG. 4A). At the time of this positioning photographing, navigator echoes are also acquired at a timing as close as possible, such as immediately before or after image acquisition, rather than acquiring only an image. Reference numeral 702 denotes a photographing slice position, and reference numeral 703 denotes a navigation acquisition position. The broken line indicates the magnetic field center.

  The navigator echo acquired here is used as a reference for detecting the breath holding level (respiration displacement) at the time of the main measurement. Once the positioning images for all stations have been acquired, the actual shooting is performed.

  Before the start of imaging, the subject holds his / her breath (710, FIG. 4B). However, depending on the level of the subject's breath holding, the portion to be imaged may deviate greatly from when the positioning image is acquired. In the example shown in FIG. 4, the heart and the diaphragm are greatly displaced in the foot direction of the subject with respect to the imaging slice 712 (corresponding to the inspiration period).

  However, since the slice position 712 is at the center of the magnetic field, if an image is taken in this state, an image in which the heart approaches the end of the FOV is acquired.

  Therefore, in the present invention, a navigator echo is acquired even at the time of breath holding before the start of imaging (navigation acquisition position 713), and the navigator echo acquired at the navigation acquisition position 713 and the navigator echo acquired at the navigation acquisition position 703 at the time of positioning. And the positional deviation amount 711 at the time of breath holding is monitored.

  From the monitor result, before the start of photographing, the bed 412 is moved by an amount corresponding to the positional deviation amount 711 (when photographing 720, (C) in FIG. 4, bed moving amount 721, photographing slice 722), and photographing is performed.

  FIG. 5 shows a procedure flow of the main photographing 710 and 720. In FIG. 5, the operator performs only the breath holding instruction (step 801) and the start of photographing (step 802), and the rest is the same as normal photographing. The subject who has received the instruction to hold his / her breath starts holding his / her breath at an arbitrary breathing level (step 803).

  When the breath holding can be confirmed, photographing is started (step 802). A navigator sequence is executed immediately after the start of imaging, and navigator echo is obtained (step 804). The acquired navigator echo is calculated based on the navigator echo acquired at the time of capturing the positioning image (step 805). The time required from the start of the navigator sequence to the calculation of the positional deviation amount is as short as about 50-60 ms. Subsequently, the bed position is adjusted by an amount corresponding to the obtained positional deviation amount (step 806).

  Here, the bed position is controlled as shown in FIG. That is, in FIG. 6, the positional deviation amount 1402 calculated by the navigator echo in the image reconstruction unit 407 is transmitted to the CPU 413 for bed control. When the CPU 413 receives the displacement 1402, the CPU 413 transmits a drive start signal 1404 to the motor 1407 for driving the bed 412, and the bed 412 starts moving.

  The moving distance of the bed 412 is counted by a rotary encoder 1408 connected to the bed 412 and transmitted to the CPU 413 as a pulse signal 1405. For example, when the bed 412 moves 1 mm, the rotary encoder 1408 performs high-precision control such as transmitting 300 pulses to the CPU 413.

  First, when the number of pulses corresponding to the pulse signal 1405 corresponds to the positional deviation amount 1402 sent from the image reconstruction processing unit 407, the CPU 413 uses the same system as the motor start (stop) signal 1404 to perform the motor of the bed 412. A stop signal is transmitted to 1407. By such control, it is possible to reflect the amount of displacement due to respiratory motion detected by the navigator echo in the amount of bed movement.

  The amount of bed movement is 5 to 6 cm when the respiration level is greatly different, and the required time is about 2 to 3 seconds. As soon as the bed 412 stops, the main photographing is started (step 807).

  Even if the subject holds his / her breath at an arbitrary breathing level by the control as described above, the imaging target region does not shift in the FOV, and when the images are connected with an image acquired at an adjacent station, Misregistration does not occur.

  In addition, it is possible to prevent a situation in which an imaging region enters at the end of the FOV where image distortion greatly occurs. In addition, since the subject can hold his / her breath at an arbitrary breathing level, training for adjusting the breath-holding level which is usually performed before the start of the examination is unnecessary.

  When the present invention is not applied, the overlap amount between adjacent images is usually about 10% of the FOV. However, in consideration of the positional displacement due to respiratory motion, it is necessary to add the respiratory motion displacement to 10% of the FOV. The respiratory movement displacement is 10 to 20 mm for breathing at rest, and 50 mm or more for deep breathing.

  For example, the overlap amount when FOV = 400 mm is 50 mm to 90 mm obtained by adding respiratory motion displacement to 40 mm, which is 10% of 400 mm, and the imaging efficiency is greatly reduced.

  On the other hand, according to the present invention, since an additional overlap due to respiratory motion is unnecessary, it is possible to improve imaging efficiency.

  7 to 10 are examples in which the present invention is not applied, and are comparative examples with the present invention.

  FIGS. 7 and 8 show a case where a COR cross section is taken in multi-station imaging, and images are acquired with breath-holding in the chest imaging sections 501 and 601 and the abdominal imaging sections 502 and 602 (see FIG. 7). (A), (A) in FIG. 8), the two images are overlapped by several centimeters and joined together 503, 603 (FIG. 7 (B), FIG. 8 (B)) to create one image 504 and 604 (FIG. 7C, FIG. 8C).

  As shown in FIG. 7, if the respiratory levels match between the images of the chest and abdomen, a correct combined image 504 can be acquired. However, as shown in FIG. 8A, when the chest 602 inhales a lot compared to the abdomen 602 and the breathing levels are different from each other, when the images are joined together, Therefore, a slice position shift (misregistration) occurs. As a result, there is an inaccurate combined image such as a portion that is not shown in the image ((C) in FIG. 8).

  Further, when the present invention is not applied, the whole-body image in multi-station imaging is captured with a somewhat large FOV, so that the distortion of the image occurs due to the influence of the magnetic field nonuniformity at the end of the FOV. Therefore, even if there is no part that is not completely captured in the image, an inaccurate combined image in which the boundary portion of the station is distorted can be obtained.

  FIG. 9 and FIG. 10 are examples in which the chest station 1201 and the abdominal station 1211 are respectively taken by breath-holding and connected. In FIG. 9, the chest station 1201 and the abdominal station 1211 have the same respiration level. Here, it is assumed that the areas to be overlapped are set as 1202 and 1212 and photographed ((A) in FIG. 9). At this time, even if image distortion is greatly generated in the areas 1204 and 1214 at the end of the FOV, the overlap areas 1202 and 1212 include areas 1203 and 1213 in which image distortion does not occur. In addition, since the region 1213 supplements the region 1204 and the region 1203 supplements the region 1204, no image distortion occurs in the region 1222 of the image 1221 after joining (FIG. 9 ( B)).

  On the other hand, as shown in FIG. 10, it is assumed that the breath-holding level when photographing the chest station 1301 is significantly different from that when photographing the abdominal station 1311 ((A) of FIG. 10). In this case, even if the overlap areas are set as 1302 and 1312, the areas 1304 and 1314 having large image distortion can be used for joining both images. As a result, the connected image 1321 is an image in which the boundary portion 1322 of the station is distorted ((B) in FIG. 10).

  On the other hand, as in the embodiment of the present invention, the misalignment amount at the time of breath holding is monitored using the navigator echo, and the bed 412 is moved by a distance corresponding to the monitored misalignment amount, and photographing is performed. If this is performed, the boundary portion of the image of the connected abdomen and chest does not become a distorted image, and an accurate combined image can be obtained.

Next, a second embodiment of the present invention will be described.
Although the first embodiment of the present invention is an example of photographing in the COR section, the same control is possible in other sections. The second embodiment of the present invention is an example in the case of photographing with a TRS cross section.

  FIG. 11 is an explanatory diagram of the second embodiment of the present invention. The data after imaging is usually reformatted into a whole body image of the COR cross section. The basic procedure is the same as in the first embodiment.

  (A) of FIG. 11 shows the positioning time 901, and navigator echoes are acquired as close as possible to the positioning image and image acquisition (imaging slice 902, navigation acquisition position 903). Although the positioning image shown in FIG. 11 is a TRS cross section, other cross sections such as a COR cross section may be used.

  The navigator echo acquired at the time of positioning 901 is used as a reference when detecting the breath holding level (respiration displacement) at the time of the main measurement, as in the first embodiment. When the positioning images for all the stations have been acquired, the breath is held, and then the main photographing is performed ((B) in FIG. 11, when the breath holding is started 910). At that time, a navigator echo is acquired immediately before the start of imaging (navigation acquisition position 913), and the navigator echo acquired at this position 913 is compared with the navigator echo acquired at the acquisition position 903, thereby shifting the position at the time of breath holding. The quantity 911 (photograph slice 912) is monitored.

  From the monitor result of the positional deviation amount 911, before the start of photographing, the bed is moved by an amount corresponding to the positional deviation amount 911 ((C) in FIG. 11, moving amount 921, photographing slice 922), and photographing is performed (at the time of photographing). 920).

  Here, in the first and second embodiments, when the bed is moved at the time of photographing, it is also possible to display the amount and direction of bed movement on the display unit 408 or a separately provided monitor. That is, the position set before photographing is displayed as shown in FIG. 12A, and the amount and direction of movement of the bed after movement are displayed on the screen as shown in FIG.

  In FIG. 12, a line 1502 indicating the center of the magnetic field, an imaging slice position 1503, a bed and a subject 1504, and a portion 1505 indicating the amount and direction of movement of the bed are displayed in a window 1501. When taking a picture after detecting the respiratory motion of the subject 1504 during the measurement and moving the bed, the display contents are updated as shown in FIG. Specifically, the bed, the subject, the numerical value of the movement amount, and the movement direction are updated to appropriate amounts while the magnetic field center and the imaging slice fixed to the magnetic field center are left as they are.

  By using the first and second embodiments of the present invention described above and acquiring whole-body images having a plurality of contrasts such as T1 and T2, it can be used as an image for a screening test. In addition, in the case of whole body diffusion-weighted image (DWI) imaging, which has been rapidly spreading in recent years for tests such as tumor detection and benign / malignant discrimination, body motion (respiratory motion) is very large in addition to misregistration. Affect. The present invention is also effective for such shooting.

  Here, in the first and second embodiments described above, as shown in FIG. 5, the operator started imaging after confirming that the subject stopped breathing. It is also possible to recognize and start shooting.

  The third embodiment of the present invention is an example of automatically recognizing a breath holding state and starting photographing. Other configurations and operations are the same as those in the first embodiment and the second embodiment, and thus detailed description thereof is omitted.

  FIG. 13 is an operation flowchart of the third embodiment of the present invention. In FIG. 13, the operator issues a breath-holding instruction to the subject and starts imaging (step 1001). When imaging is started, only a navigator sequence for monitoring respiratory movement displacement is executed at regular intervals (at intervals of 200 to 300 ms) (step 1002). When several respiratory movement displacements are obtained (step 1003), it is confirmed whether the patient is breathing or in a breath-hold state (step 1004).

  As shown in FIG. 14, the method for confirming whether the patient is breathing or in the breath-holding state is based on a preset threshold value (about 5 mm) for the amount of change corresponding to the number of respiratory reciprocal displacements (three points in the example of FIG. 14). It is done by comparing with. For example, if the displacement amount exceeds the threshold as in the displacement position 1101, it is determined that breathing is in progress (step 1102), and if the displacement amount is within the threshold as in the displacement position 1103, it is determined that breath holding has started. (Step 1104).

  If it is determined that the breath is being held, the bed position is adjusted in accordance with the breath holding level (step 1005), and the actual photographing is executed (step 1006).

  By such control, the operation performed by the operator is simplified to only once before the start of photographing, and the same effect as in the first embodiment can be obtained.

  Note that the breath holding detection of the subject can also use a separate sensor for detecting the respiratory motion of the subject.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is explanatory drawing of whole body MRI. It is explanatory drawing of whole body MRI. It is explanatory drawing of the 1st Embodiment of this invention. It is a procedure flowchart of the main imaging | photography in the 1st Embodiment of this invention. It is explanatory drawing of control of the bed position in the 1st Embodiment of this invention. It is explanatory drawing of the example when not applying this invention. It is explanatory drawing of the example when not applying this invention. It is explanatory drawing of the example when not applying this invention. It is explanatory drawing of the example when not applying this invention. It is explanatory drawing of the 2nd Embodiment of this invention. It is a figure which shows the example of a display of a bed moving amount and a moving direction at the time of moving a bed at the time of imaging | photography. It is an operation | movement flowchart of the 3rd Embodiment of this invention. It is explanatory drawing of the 3rd Embodiment of this invention.

Explanation of symbols

101-106 Imaging surface 401 Subject 402 Static magnetic field magnet 403 Gradient magnetic field coil 404 RF coil 405 RF probe 406 Signal detection unit 407 Signal processing unit 408 Display unit 409 Gradient magnetic field power supply 410 RF transmission unit 411 Control unit 412 Bed 413 Bed control unit

Claims (5)

Imaging means for imaging the subject by nuclear magnetic resonance, moving means for moving the subject to move the imaging target area, means for reconstructing images of a plurality of areas of the subject imaged by the imaging means, A magnetic resonance imaging apparatus comprising: a control unit that controls movement and stop of the moving unit and images a plurality of adjacent regions of the subject by the imaging unit;
A body motion detecting means for detecting a body motion of the subject between adjacent areas;
The control means moves the moving means according to the amount of body movement of the subject detected by the body movement detecting means so that the target site of the subject is at the center of a predetermined imaging field of view. Magnetic resonance imaging device.   The magnetic resonance imaging apparatus according to claim 1, wherein the body movement detecting unit detects the body movement using a navigation echo signal acquired by the imaging unit.   3. The magnetic resonance imaging apparatus according to claim 2, wherein the control unit detects when the subject is breath-holding based on the navigation echo signal, and images the subject when the breath-hold is detected. Resonance imaging device.   The magnetic resonance imaging apparatus according to claim 1, wherein imaging regions adjacent to each other of the subject have overlapping portions.   In the magnetic resonance imaging apparatus according to any one of claims 1, 2, 3, and 4, when the moving unit is moved so that the target site of the subject is the center of a predetermined field of view, A magnetic resonance imaging apparatus comprising display means for displaying a moving amount and a moving direction of the moving means.

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