JP4558397B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) that images a subject using nuclear magnetic resonance, and more particularly to an MRI apparatus that reduces artifacts due to periodic body movement such as respiratory movement of the subject. About.

  The MRI apparatus measures a nuclear magnetic resonance signal generated from atomic nuclei (usually protons) constituting an object by applying a high-frequency magnetic field pulse to the object placed in a static magnetic field. The spatial distribution of the distribution and the relaxation phenomenon of the excited state is imaged. Nuclear magnetic resonance signals are given different phase encodings depending on the gradient magnetic field, and a single image can be reconstructed by performing a Fourier transform.

  If the subject moves during the creation of this single MR image, a large artifact is generated in the image, which degrades the image quality. Such body motion artifacts (body motion artifacts) should be given a predetermined phase encoding amount to a predetermined measurement point, but in a state where the phase encoding amount is applied to another measurement point by the movement of the subject. This occurs because of the Fourier transform. As a cause of the body motion artifact, an important thing is a body motion artifact due to breathing.

  As a method of removing body motion artifacts due to respiratory motion, 1) monitor the position of the subject at the time of measurement, and use only the echo signal measured when the detected subject position is within a predetermined body motion range. Reconstruction method (respiration gating method), 2) Monitoring the position of the subject at the time of measurement, and shifting the imaging slice position according to the deviation of the detected position from the reference position (slice tracking method), etc. Has been proposed (Patent Document 1). As a method of monitoring the position of the subject, there is a method using a respiration sensor attached to the abdominal wall or chest wall of the subject, but an additional echo not using phase encoding is measured as a navigator echo, and the position is determined from the projection data. The method of detecting is widespread.

  Among the above-mentioned conventional body motion artifact removal methods, the breath gating method has a problem that the measurement time becomes long because there are many signals that are not used for image reconstruction. In addition to increasing the number, it has also been proposed to control the arrangement in the k-space, that is, how to give the phase encoding, according to the subject position at the time of signal measurement (Patent Document 2). However, even in this case, it takes a considerable time to acquire all the phase encoded signals.

  In addition, as a problem common to 1) and 2), navigator echo measurement must be performed before the measurement for obtaining the subject image (this measurement is distinguished from navigator echo measurement). Time is extended. In the slice tracking method, it is necessary to calculate a subject position using a navigator echo, and then to determine an imaging cross section from the calculated subject position, and dead time (cannot be measured) for these calculations. (Time) may shorten the data acquisition time in the main measurement or shift the acquisition timing.

  Furthermore, due to the respiratory movement of the subject, deformation in the nearby organ will occur, but since these deformations are not considered in the conventional slice tracking method, if the organ with deformation is the imaging target, There is also a problem that a good image cannot be obtained.

On the other hand, as another body motion artifact suppression method, there is also a method of monitoring the position of the subject at the time of measurement and controlling the method of applying phase encoding based on the correspondence table between the body motion histogram and the phase encoding (ROPE method). Non-Patent Document 1). In this method, phase encoding control is performed so that the signal measured when the subject is at a high frequency position is in the low frequency region of k-space, so that the body motion artifacts in the entire visual field can be suppressed. For example, there is a possibility that a low phase encoding amount may be assigned to a signal measured at a position where the deformation from the reference shape is large, and the deformation problem cannot be solved. Further, in the ROPE method, since the slice position is fixed, there is a possibility that the target region is out of the field of view and cannot be imaged.
JP 2000-157508 JP Japanese National Patent Publication No. 9-508050 J. Computer Assisted Tomography 9 (4): 835-838,1985

  The present invention can effectively reduce body motion artifacts, perform navigator echo measurement for body motion monitoring at a low frequency, and can secure a substantially long data measurement time in the main measurement, and is used for an image. The purpose is to increase the number of data. It is another object of the present invention to provide an MRI apparatus capable of minimizing the influence of deformation of a target portion accompanying body movement and obtaining a good image.

  The MRI apparatus of the present invention that solves the above problems applies a high-frequency magnetic field pulse and a gradient magnetic field pulse to a subject according to a predetermined pulse sequence, measures a nuclear magnetic resonance signal generated by the subject, and takes an image of the subject Imaging means, and control means for controlling the imaging means, wherein the control means stores the periodic body motion of the subject as a body motion function or body motion curve, and a book for imaging the subject. The body movement position is detected from the navigator echo acquired prior to the measurement, the body movement position at the time of the main measurement is predicted from the detected position and the body movement function or the body movement curve, and the body movement position is estimated based on the predicted body movement position. Means for determining imaging parameters in the measurement.

Specifically, the imaging parameter is an imaging slice position and a phase encoding amount, or an imaging slice position and a phase encoding amount.
Preferably, in the MRI apparatus of the present invention, the control means acquires the navigator echo at a time other than the data acquisition time, and based on the body motion information newly detected by the navigator echo, the current body motion function or body motion curve. Update.
In the MRI apparatus of the present invention, the control means includes a body motion function or a body motion curve representing periodic body motions in a plurality of directions, and predicts body motion positions in a plurality of directions at the time of actual measurement. Can do.

  Furthermore, the MRI apparatus of the present invention includes imaging means for applying a high-frequency magnetic field pulse and a gradient magnetic field pulse to a subject according to a predetermined pulse sequence, measuring a nuclear magnetic resonance signal generated by the subject, and capturing an image of the subject. Control means for controlling the imaging means, the control means storing a table representing a relationship between a displacement amount of the periodic body movement of the subject and a shift amount of the slice position, and a subject. The displacement amount of the body movement is detected from the navigator echo acquired immediately before the actual measurement to be imaged, the shift amount of the slice position corresponding to the detected displacement amount is obtained from the table, and the subsequent measurement is obtained. Means for applying the shift amount.

  According to the MRI apparatus of the present invention, the position of the subject at the time of the main measurement is predicted based on the previously determined body movement information, and the imaging parameters corresponding to the position are calculated and determined, whereby the body movement monitor In addition to reducing the frequency of the navigator echo sequence (navigator sequence) for the camera, it is not necessary to calculate the shooting parameters each time the main measurement is performed continuously. This measurement can be performed without any occurrence.

  By determining the slice position as an imaging parameter, the slice position can be shifted in accordance with the shift of the subject position due to body movement, and the influence of body movement can be eliminated. In addition, by controlling the phase encoding amount, even if there is deformation due to body movement, the data at the position where a large amount of deformation has occurred is placed in the high-frequency region of k-space to minimize the effects of deformation. can do.

In addition, the navigator sequence is executed using the time not used for the main measurement in ECG synchronized imaging, and the body motion function is updated using the position information obtained thereby, so that the body motion function and the actual body motion are Can be dealt with in real time to shifts in the body movement cycle and body movement artifacts can be suppressed with high accuracy.
By performing body motion in a plurality of directions, for example, in a three-dimensional direction, body motion artifacts can be suppressed with higher accuracy.
Further, by storing a table representing the relationship between the body motion displacement amount and the slice position shift amount in advance, it is possible to eliminate the shift in the main measurement time associated with the calculation.

Embodiments of the MRI apparatus of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied.
This MRI apparatus includes a magnet 102 that generates a static magnetic field in a space in which a subject 101 is inserted, a gradient magnetic field coil 103 that generates a gradient magnetic field in this space, and an RF coil that generates a high-frequency magnetic field in the imaging region of the subject. 104, an RF probe 105 for detecting a nuclear magnetic resonance (MR) signal generated by the subject 101, and a bed 112 for inserting the subject 101 into the static magnetic field space.

The gradient magnetic field coil 103 is composed of gradient magnetic field coils in three directions (X, Y, Z) orthogonal to each other, and generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 109, respectively. Depending on how the gradient magnetic field is applied, the imaging cross section of the subject can be determined and position information can be given to the MR signal.
The RF coil 104 generates a high-frequency magnetic field according to the signal from the RF transmission unit 110. The signal of the RF probe 105 is detected by the signal detection unit 106, processed by the signal processing unit 107, and converted into an image signal by calculation. The image is displayed on the display unit 108.

  The gradient magnetic field power supply 109, the RF transmission unit 110, and the signal detection unit 106 are controlled by the control unit 111. The control time chart is generally called a pulse sequence, and various pulse sequences (imaging sequences) determined by the imaging method are stored in advance in a storage unit (not shown) as a program. In addition to such a storage unit, the control unit 111 includes an input device (user interface: UI) for selecting an imaging sequence and inputting imaging parameters and the like.

  In the MRI apparatus of the present invention, a sequence (navigator sequence) for generating and acquiring a navigator echo as a body motion monitor can be executed independently of the imaging sequence or in combination with the selected imaging sequence. The navigator sequence is an echo (navigator echo) that excites a region of interest (such as the diaphragm) to be monitored locally using a high-frequency magnetic field and a selective gradient magnetic field, and does not add a phase encoding gradient magnetic field from this local excitation region. This is the sequence to get. For example, the navigator sequence can be appropriately executed by the user in accordance with the selection of the imaging sequence. However, the navigator sequence is executed under the control of the control unit 111 as a predetermined sequence combined with the imaging sequence.

  The signal processing unit 107 (control unit 111) performs body movement of the subject based on a series of navigator echoes obtained discretely by executing a navigator sequence for a certain period of time, in addition to the above-described operations such as image reconstruction. The body motion function or body motion curve to be expressed is created and stored in the storage unit, and the body motion position at the time of measurement is predicted from the subject position and the body motion curve monitored in the subsequent measurement, and based on the prediction result The slice position and / or phase encoding amount of the imaging sequence is determined in advance, and measurement is performed with the slice position and / or phase encoding amount determined at the time of measurement.

  Hereinafter, as a first aspect of the present invention, an embodiment of respiratory motion artifact suppression using the MRI apparatus configured as described above will be described. In the first aspect of the present invention, the MRI apparatus (signal processing unit) predicts the body movement position at the time of the main measurement based on the body movement function, and determines the slice position using the predicted body movement position.

  FIG. 2 shows an embodiment thereof. In the present embodiment, first, a function representing the respiratory motion of the subject (hereinafter referred to as a body motion approximation function or an approximation function) is obtained prior to the main measurement (step 21). For this reason, a sequence (navigator sequence) that continuously measures navigator echoes is executed for a period of one or more cycles of respiratory motion, and a series of navigator echoes is measured (step 211). The navigator sequence selects and excites a region that most reflects respiratory motion, for example, a cross section including the diaphragm, and measures an echo signal without applying phase encoding. For example, if one respiratory cycle is about 4 seconds and the navigator sequence is executed every 200 ms, 20 navigator echoes can be measured. This is shown in FIG. In FIG. 3, the horizontal axis represents time, and the vertical axis represents the amount of displacement of the diaphragm being monitored, indicating that 20 navigator sequences 301 are executed in one cycle 302 of body movement.

  A projection image is created by one-dimensional Fourier transform of the obtained echo signal in the frequency encoding direction, and diaphragm positions 303 and 304 are monitored by using a technique such as detecting an edge from the signal intensity profile of the projection image. Are obtained (step 212). The position information may be obtained as coordinates on the image, or may be obtained as a shift amount with respect to a predetermined reference position. Next, a body motion approximation function 305 that approximates the respiratory cycle is calculated using the obtained detection position and the time at which the position was obtained (step 213). As a method for obtaining the approximate function, a known method such as a least square method can be used, and it is preferable that the function is a polynomial including a periodic function such as sin and cos and includes a first-order term reflecting a drift component. In this embodiment, the function of Formula 1 is used as the body motion approximation function.

  Here, the time t is, for example, 0 when the first navigator sequence is executed, and increases every 200 ms if the interval for executing the navigator sequence is 200 ms. Alternatively, the start of the navigator sequence is started in synchronization with the R wave of the electrocardiogram, the R wave detection time is set to 0, the first navigator sequence execution time is S (predetermined delay time), and the second and subsequent times are S + n. It is good also as * 200 [ms] (n is 2, 3, 4 ...). The body motion approximation function thus obtained is stored in the storage unit of the control unit 111.

  Next, the main measurement is started (step 22). This measurement is a measurement for obtaining an image of a target region of the subject, and an arbitrary imaging sequence is selected according to the target tissue or image. FIG. 4 shows an embodiment in which this measurement is performed in synchronization with an electrocardiogram waveform (R wave).

  In the present embodiment, in order to predict the body movement position of the subject at the time of the main measurement prior to the main measurement 402, the navigator sequence 401 is first executed (step 221). The navigator sequence 401 is the same as the navigator sequence (FIG. 3, 301) executed to obtain the function 305 representing respiratory motion, and measures an echo signal from a cross-section including a region to be monitored without applying phase encoding. .

  The position at the time of echo measurement is obtained from the obtained echo in the same manner as in step 212, and the detected position is applied to the body motion approximation function 305 (fitting). That is, the time corresponding to the position 403 detected from the navigator sequence 401 is obtained within the integrated motion cycle of the body motion approximation function 305. In this case, since there are two points 403 ′ and 403 ″ on the body motion approximation function 305 corresponding to one detection position 403, two or more navigator sequences are executed, and the body motion approximation function 305 is detected. In practice, the respiration amplitudes are not all the same, so as shown in Fig. 4, it is applied to the body motion approximation function 305 using the position 406 detected in the plural navigator sequences. If it is determined that the points 403 ′ to 406 ′ on the body motion approximation function coincide with the points 403 ′ to 406 ′ on the body motion approximation function from the positions 403 to 406 detected by executing the navigator sequence 401 four times, the measurement time T of the detected position 406 The position (displacement) in the main measurement 402 is predicted by associating the point 406 ′ with the time t (step 222), that is, the time T from the first R wave is substituted into the body motion approximation function 305, One R wave and R to be the gate signal for starting this measurement The displacement Ta at the start of the main measurement 402 can be calculated by using the interval Ta (Tb) between and the delay time Δt from the R wave to the start of the main measurement, which serves as a reference for association. The time may be a clear time other than the time of the first R wave.

  If the amount of body movement displacement of the subject in the main measurement 402 can be predicted, the imaging section (slice position) is shifted in accordance with the amount of displacement (step 223). Specifically, the shift of the slice position gives an offset value corresponding to the shift amount of the slice position to the frequency of the excitation pulse (RF pulse) in the pulse sequence of this measurement. Also, an offset amount is given to the phase encoding gradient magnetic field according to the predicted amount of body movement displacement. This measurement is performed with this updated pulse sequence (step 224). As a result, it is possible to obtain an image that surely includes the target portion and greatly reduces the influence of body movement. Moreover, after the navigation echo measurement that is performed prior to the main measurement, by the time the main measurement is started, the position prediction and the calculation of the photographing cross-sectional position have been completed, so the dead time for calculation is unnecessary, The main measurement corresponding to the displacement amount can be performed at an arbitrary timing. When acquiring image data of multiple time phases within one cardiac cycle, it is also possible to calculate the slice position by predicting the amount of body movement displacement for each time phase and shift the slice position continuously. is there.

  Next, another embodiment of the present invention will be described with reference to FIG. The MRI apparatus of this embodiment has a function of correcting a deviation between the displacement amount predicted from the body motion approximation function 305 and the displacement amount at the time of actual measurement. Other than that is the same as that of the above-mentioned embodiment. In the measurement over a relatively long time, there is a possibility that the respiratory cycle changes with the passage of time, and the actual displacement amount deviates from the displacement amount predicted from the body motion approximation function 305. In the present embodiment, in order to detect such a deviation, a navigator sequence is executed using a time other than the data acquisition time to detect a real-time displacement amount, and the currently used body motion approximation function 305 and The displacement detected in real time is displayed on the display unit.

  Also in the present embodiment, a body motion approximation function that approximates the respiratory motion is obtained by executing a navigator sequence of one respiratory cycle or more (step 501), and the position detected from the navigator echo is determined at the start of this measurement. 2 and predicting the amount of displacement at the time of actual measurement and determining the imaging section at that time (steps 502 and 503) is the same as in the embodiment of FIG. 2, and these steps 501 to 504 are steps of FIG. This corresponds to 21 (211 to 213) and step 22 (221 to 224). The body motion approximation function used to predict the amount of displacement during the actual measurement is displayed on the monitor as a curve. Thereafter, when this measurement sequence is repeated, the navigator sequence is executed using a time other than the data acquisition time (step 505). For example, in the example illustrated in FIG. 6, the navigator sequence 601 is executed a plurality of times by using a waiting time between one main measurement sequence and the next main measurement sequence.

  The amount of displacement of the subject (part to be monitored) at the time of execution is detected from the navigator echo measured in each navigator sequence 601, and is displayed on the body motion approximation function displayed on the monitor (step 506). This is shown in FIG. In the figure, a curve 701 represents a body movement approximation function, and points indicate positions detected by navigator echoes. The user can confirm that there is a deviation between the body movement approximation function used to predict the displacement amount and the actual displacement amount from the display on the monitor. Whether to update the function 701 itself is selected (step 507).

  If it is determined that the deviation exceeds the allowable range, it is further determined whether this deviation is a certain amount of deviation (primary deviation amount) in the horizontal axis direction or the vertical axis direction, or whether the cycle itself is changing. (Step 508). If it is determined that there is no change in the cycle itself, for example, a reset button 702 is pressed. Thus, the displacement amount newly obtained in step 506 is reapplied to the current body motion approximation function, and the displacement amount in the subsequent main measurement and slice position determination are performed (step 509). That is, the body motion approximation function is applied by shifting the time corresponding to the primary deviation amount. On the other hand, when it is determined that the cycle itself is shortened or extended, the displacement amount cannot be predicted using the current body motion approximation function, and the update button is pressed. As a result, the process returns to step 501 to obtain a new approximation function, which is used as a body motion approximation function in the subsequent main measurement.

  In the above explanation, it is assumed that the deviation is merely a certain amount of deviation or due to a change in the cycle itself, and different processing is performed accordingly, but when it is determined that the deviation is caused regardless of the type of deviation. Alternatively, the body motion approximation function may be updated.

  Further, in the above description, the case where the resetting or updating of the body motion approximation function is performed manually according to the user's judgment has been described, but it is also possible for the control unit to judge and automatically carry out. In this case as well, it may be determined whether the shift is a primary shift or a change in the cycle itself, and if a shift occurs, the body motion approximation function may be updated uniformly. Specifically, the navigator sequence is executed, and the difference between the detected position obtained at that time and the position (corresponding position) obtained by substituting the time when the navigator sequence is executed into the body motion approximation function is obtained, When this difference exceeds a predetermined threshold, the data measured immediately before the navigator sequence is discarded, the body motion approximation function is reset, and the discarded data is measured again. If the difference between the position detected by the navigator sequence and the position obtained from the body motion approximation function exceeds the threshold value, the data acquisition is stopped and the body motion approximation function is updated.

  As described above, according to the present embodiment, even when the main measurement time is relatively long, the deviation between the body motion approximation function and the actual body motion over time is corrected and the slice position is determined with high accuracy. An image with suppressed body movement can be obtained. In addition, since the calculation for correcting the deviation is executed only when the deviation is equal to or greater than the threshold value, the time for calculating the slice position can be minimized.

  Next, as another embodiment, an embodiment in which three-dimensional body movement detection is performed will be described. As described above, there is a diaphragm as a part that reflects respiratory motion most. In the embodiment of FIG. 2, only the movement amount of the diaphragm in the direction parallel to the body axis is monitored. ), In reality, not only in a direction parallel to the body axis, but also in a direction perpendicular thereto, a displacement accompanying respiratory motion occurs. Therefore, in the present embodiment, as shown in FIG. 8, navigator echoes are acquired from three directions A, B, and C orthogonal to each other for a part 801 for which movement is to be detected, and a body motion approximation function in three directions is created.

  The step of creating the body motion approximation function is the same as that in the embodiment of FIG. 2, and a navigator sequence as shown in FIG. 3 is executed for one or more periods of body motion in each of the three directions A, B, and C. Then, the obtained detection position is fitted to a predetermined function. In this measurement, as shown in FIG. 9, the navigator sequences 901, 904, 907 in the A direction, the navigator sequences 902, 905, 908 in the B direction, and the navigator sequences 903, 906, 909 in the C direction are executed cyclically, for example. Then, the position in each direction (910, 913, 916 for the A direction, 911, 914, 917 for the B direction, and 912, 915, 918 for the C direction) is detected from the navigation echo obtained in each case. The detected position in each direction is applied to body motion approximation functions 921, 922, and 923 in the same direction, and the position of the target part at the time of the main measurement is calculated.

The calculation (prediction) of the position is the same as that in the embodiment of FIG. 2, and the time point serving as the reference, for example, the time T from the time of detection of the first R wave is substituted into the body motion approximation function, The displacement amount at the start of the main measurement 920 can be calculated using the interval Ta with the R wave serving as the gate signal and the delay time Δt from the R wave to the start of the main measurement. Also for the main measurement executed after that, the displacement amount at the time of measurement is similarly calculated using Tb instead of the interval Ta.
If the body movement displacement amount is calculated in the three directions as described above, the slice position for the main measurement is determined based on these displacement amounts. For example, an offset value corresponding to the shift amount of the slice position is given to the frequency of the RF pulse, and an offset amount corresponding to the body motion displacement amount is given to the phase encoding gradient magnetic field.

  According to the present embodiment, the body motion approximation function is obtained for each of the three directions, and the slice position of the main measurement is determined from these approximate functions and the positions detected for the three directions. Therefore, the detection frequency in each direction is 1 / Although it decreases to 3, the accuracy of correcting the slice position is improved. Although the calculation of the shift and inclination of the three-dimensional space is complicated, it can be performed in advance, so that there is no delay in the main measurement accompanying the calculation.

  As described above, as the first aspect of the present invention, the MRI apparatus having the function of predicting the displacement amount during the main measurement based on the body motion approximation function and correcting the slice position in the main measurement has been described. Here, the case where the periodic body movement of the subject is stored as a function has been described. However, the body movement curve (displacement-time curve) is stored, and the amount of displacement at the time of the main measurement is calculated from the detected position. It is also possible to predict.

  Furthermore, after executing the navigator sequence prior to the actual measurement, not only the body motion function or body motion curve may be obtained, but the relationship between the displacement and the slice position may be tabulated in advance. That is, as shown in FIG. 10, the amplitude (+ s to −s) of the body motion function is calculated from the body motion function 1003 as a change amount in pixel units, and the shift amount of the slice position corresponding to this change amount is set as a table 1005. Remember. In this measurement, the navigator echo is acquired immediately before to detect the displacement amount, and the slice position (shift amount) corresponding to the detection position is determined with reference to the table. Thus, the main measurement sequence 1001 can be executed without causing a delay at the slice position determined by the execution of the immediately preceding navigator sequence 1000.

  As body motion information, it is preferable to obtain a body motion function or a body motion curve by executing a navigator sequence. However, in the present invention, body motion information can be obtained by other methods (for example, a body motion sensor). It is. As shown in FIG. 10, when it is sufficient to know the amount of change in body movement in advance, the user may input the amplitude of body movement instead of obtaining it from the body movement function. If the subject does not move and the position does not change during the measurement, the body movement can be corrected most simply.

  Next, as a second aspect of the present invention, an MRI apparatus having a function of predicting a displacement amount at the time of actual measurement from body motion information measured in advance and determining a phase encoding amount based on the displacement amount will be described. Here, the configuration of the apparatus is the same as in FIG. 1, but as a function of the control unit 111 for controlling the pulse sequence, the phase encoding amount of the echo signal measured at that time according to the position (displacement amount) at the time of the main measurement, that is, It is characterized by having a function for controlling data arrangement in the data space.

  FIG. 11 and FIG. 12 show an embodiment applied to electrocardiogram synchronous imaging. In this embodiment, first, a body motion approximation function representing the respiratory motion of the subject is obtained prior to the main measurement. This step is the same as step 21 (212 to 213) in FIG. 2, and a navigator sequence is continuously executed for a time longer than one cycle of respiratory motion (step 1101), a series of navigator echoes are measured, The amount of displacement of the monitor part (for example, the diaphragm) when the navigator sequence is executed is obtained. As a result, a curve 1202 in which the displacement amount is plotted with respect to the time axis is obtained. This curve is fitted to a predetermined function and stored as a body motion approximation function (step 1102). At the same time, a body movement reference position 1201 is set. As the reference position for body movement, for example, a position with the least movement (a position where the slope of the curve is flat, for example, the end of expiration) or a high frequency in the body movement cycle is selected.

  Next, the main measurement is started in synchronization with the electrocardiogram waveform (R wave) 1203 (step 1103). In the embodiment shown in FIG. 12, a case where three main measurement sequences 1204, 1214, and 1224 are executed to acquire one piece of image reconstruction data (k-space data 1230) is illustrated. The sequences 1204, 1214, and 1224 are started after a predetermined delay time Td from the R wave. Then, the navigator sequence 1205 is executed using the first delay time (step 1104), and the body movement displacement 1206 at that time is detected from the obtained navigation echo (step 1105). At this time, at least two navigation echoes are acquired, and the detected plurality of body movement positions 1206 are fitted to a pre-stored body movement function 1202, and body movement positions 1207 at the time of execution of the main measurement 1204, 1214, 1224, 1217 and 1227 are predicted (step 1106). Next, the predicted body movement positions 1207, 1217, and 1227 are compared with a preset reference position 1201, and in this measurement sequence (1224 in the illustrated example) executed when the difference is the smallest body movement position Then, the phase encoding amount is determined so that the data 1228 acquired at that time is arranged in the low frequency region of the k space (step 1107). In this measurement sequence (1214 in the example shown) executed when the predicted body motion position and the reference position have the largest difference, the data 1118 acquired at that time is placed in the high frequency region of k-space. The phase encoding amount is determined as follows.

  The result of assigning the phase encoding amount according to the predicted body movement position is shown in the data space 1230 of FIG. Here, the data 1228 acquired in the main measurement sequence 1224 is arranged in the low frequency region A of the k space 1230, the data 1208 acquired in the main measurement sequence 1204 is arranged in the intermediate frequency region B, and the main measurement sequence is arranged in the high frequency region C. Data 1218 acquired in 1214 is arranged.

  When the phase encoding amount is determined in this way, the measurement is continued while changing the slice position according to a known slice tracking method (step 1108). That is, the main measurement sequence 1204 is executed by shifting the slice position by the difference between the position 1207 at the time of main measurement predicted by the body movement approximation function 1202 and the detection position 1206 by the immediately preceding navigator sequence and the reference position 1209. Similarly, the navigator sequence 1215 is executed after the next R wave detection (step 1104), and the position 1217 ′ at the time of execution of the main measurement sequence 1214 is predicted from the position 1216 detected at that time. In the main measurement sequence 1214, The slice position is shifted by the difference between this position 1217 ′ and the reference position 1201. Similarly, the measurement sequence 1224 is executed by shifting the slice based on the detected position to the navigator sequence 1225 executed immediately before (step 1106).

  As described above, according to the present embodiment, the phase encoding amount is controlled based on the difference between the slice position and the reference position at the time of measurement, and the difference between the k-space low frequency data for determining the contrast of the image and the reference position is small. Since the data is used, even when the tissue (organ) is deformed with the change of the slice position, it is difficult to be affected by the deformation, and an image with further reduced body motion artifacts can be obtained.

  In the above embodiment, the case where the slice position is controlled based on the body movement positions 1217 ′ and 1227 ′ detected by the navigator sequence executed in combination with this measurement sequence has been described, but the body movement position 1207 predicted first is described. , 1217, and 1227 can be used to predetermine slice positions when executing each main measurement sequence. That is, it may be executed in combination with the first mode. Such an embodiment is shown in FIG.

  13, the same elements as those in FIG. 12 are denoted by the same reference numerals. This embodiment is different from the embodiment of FIG. 12 in that the positions 1207, 1217, and 1227 at the time of main measurement predicted from the position detected by the navigator sequence 1205 executed immediately before the main measurement sequence 1204 and the body motion approximation function 1202 are used. Is used to determine the phase encoding amount and to determine the slice position. In this embodiment, since it is not necessary to execute the navigator sequences 1215 and 1225 before the main measurement sequence, the main measurement can be performed from the cardiac phase immediately after the R wave, and any delay time can be adjusted. Data can be acquired during the cardiac phase.

  For example, when an image for each cardiac phase is obtained in cardiac imaging, the number of temporal phases can be increased. Such an embodiment is shown in FIG. 14, the same elements as those in FIGS. 12 and 13 are denoted by the same reference numerals. In this embodiment, a plurality of (3 in the figure) cardiac time phases are measured in one cardiac cycle (between R and R), and the position 1206 detected by the navigator sequence 1206 executed prior to the main measurement is approximated to the body motion. From the function 1202, positions 151 to 159 in the measurement 141 to 149 of each cardiac cycle are predicted. The difference between the predicted position and the reference position 1201 in the body motion is calculated, and the phase encoding amount corresponding to the difference from the reference position is determined for each time phase measurement. That is, as shown in data spaces 181 to 183 in FIG. 14, in time phase 1 (data space 181), the acquired data 167 of measurement 147 having the smallest difference from the reference position is arranged in the low frequency region A of k space, and The phase encoding amount is determined so that the acquired data 164 of the measurement 144 having the largest difference from the position is arranged in the high frequency region C. Similarly, the phase encoding amount is determined for time phase 2 and time phase 3.

  Next, each cardiac cycle measurement 141 to 149 is sequentially executed, and at this time, the slice position is shifted by an amount corresponding to the difference between the predicted positions 151 to 159 and the reference position 1201 in the measurement. Thus, when a plurality of time-phase cardiac images are taken without stopping breathing, it is possible to obtain an image in which the influence of deformation due to the respiratory motion of the heart is suppressed while performing slice position correction with high accuracy.

  In the above, the second aspect of the present invention has been described, and here also the case where the periodic body motion of the subject is stored as a function has been described. However, the body motion curve (displacement-time curve) is stored and this curve is stored. It is also possible to predict the displacement amount at the time of actual measurement from the detected position.

  According to the present invention, the body movement position in the main measurement is predicted based on the body movement information obtained in advance, and the slice position and the phase encoding at the time of the main measurement are determined accordingly. This measurement can be performed without dead time. Further, by controlling the phase encoding amount in conjunction with the shift of the slice position, it is possible to reduce the influence of deformation accompanying body movement as much as possible, and to obtain an image in which body movement artifacts are further suppressed.

The figure which shows the whole outline | summary of the MRI apparatus with which this invention is applied The figure which shows one Embodiment of operation | movement of the MRI apparatus by the 1st aspect of this invention The figure which shows the navigator sequence and body motion approximation function in embodiment of FIG. The figure which shows this measurement in embodiment of FIG. The figure which shows other embodiment of operation | movement of the MRI apparatus by the 1st aspect of this invention. The figure which shows this measurement in embodiment of FIG. The figure which shows the example of a display of the display part in embodiment of FIG. Diagram explaining 3D body motion detection The figure which shows this measurement in another embodiment The figure which shows another embodiment The figure which shows one Embodiment of operation | movement of the MRI apparatus by the 2nd aspect of this invention. The figure which shows this measurement in embodiment of FIG. The figure which shows this measurement in another embodiment The figure which shows this measurement in another embodiment

Explanation of symbols

101 ... subject, 102 ... static magnetic field magnet, 103 ... gradient coil, 104 ... high frequency coil, 105 ... high frequency probe, 107 ... signal processing unit, 108 ... display 110, a control unit.

Claims (6)

An imaging means for applying a high-frequency magnetic field pulse and a gradient magnetic field pulse to a subject according to a predetermined pulse sequence, measuring a nuclear magnetic resonance signal generated by the subject, and taking an image of the subject, and a control for controlling the imaging means A magnetic resonance imaging apparatus comprising:
The control means includes
Prior to a plurality of main measurements in which the periodic body motion of the subject is stored as a body motion function or a body motion curve representing a temporal change in the subject position, and the subject is imaged in synchronization with each R wave. obtained from a plurality of navigator echoes plurality of body motion position respectively detected, and a plurality of body movement positions issued該検, and the body movement function or body motion curve, time from the R wave to the time of each main measurement , Means for predicting body movement positions at the time of each main measurement, and determining imaging parameters in the main measurement based on the predicted body movement positions,
A magnetic resonance imaging apparatus comprising: An imaging means for applying a high-frequency magnetic field pulse and a gradient magnetic field pulse to a subject according to a predetermined pulse sequence, measuring a nuclear magnetic resonance signal generated by the subject, and taking an image of the subject, and a control for controlling the imaging means A magnetic resonance imaging apparatus comprising:
The control means includes
Prior to a plurality of main measurements in which the periodic body motion of the subject is stored as a body motion function or a body motion curve representing a temporal change in the subject position, and the subject is imaged in synchronization with each R wave. obtained from a plurality of navigator echoes plurality of body motion position respectively detected, and a plurality of body movement positions issued該検, and the body movement function or body motion curve, time from the R wave to the time of each main measurement , Means for predicting the body movement position at the time of each main measurement, and determining the imaging slice position and / or phase encoding amount in the main measurement based on the predicted body movement position,
A magnetic resonance imaging apparatus comprising:   When there are a plurality of predicted body movement positions, the control means performs phase encoding in a k-space high frequency region at a body movement position having a large difference from a preset reference position, and a body movement having a small difference from the reference position. The magnetic resonance imaging apparatus according to claim 2, wherein the position is a phase encoding of a k-space low frequency region. The control means acquires a navigator echo at a time other than the data acquisition time, and updates a current body motion function or body motion curve based on body motion information newly detected by the navigator echo. Item 4. The magnetic resonance imaging apparatus according to any one of Items 1 to 3 . The control means includes a body motion function representing a periodic body motion in a plurality of directions or a body motion function,
The magnetic resonance imaging apparatus according to claim 1, wherein body movement positions in a plurality of directions at the time of the main measurement are predicted. The magnetic resonance imaging apparatus according to claim 1 or 2,
The control means includes
Means for storing a table indicating the relationship between the amount of displacement of the periodic body motion of the subject and the amount of shift of the slice position, and the amount of displacement of the body motion from the navigator echo acquired immediately before the main measurement for imaging the subject. Magnetic resonance imaging comprising: means for detecting and obtaining a shift amount of a slice position corresponding to the detected displacement amount from the table, and applying the obtained shift amount to the main measurement to be subsequently executed apparatus.

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