JP2011167559A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which enables an operator to take high quality images by an imaging method sensitive to static magnetic field uniformity while performing movement correction to keep the almost constant relation between a position of slice section and a position of imaging object site. <P>SOLUTION: A control part 107 detects a position in a static magnetic field of the diaphragm of a subject P and sets a size of the tolerance corresponding to a size of the variation of the detected positions. A reconstruction part 103 reconstructs the image based on data collected when a detected position is within the tolerance. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus (MRI apparatus) including an imaging method that employs data collected when a region such as a diaphragm is within a preset allowable range as data used for reconstruction.

  The magnetic resonance signal (navigator echo) that reflects the movement of the imaging target part due to respiration is acquired from the subject, and the data collected when the imaging target position falls within the preset tolerance range is used for reconstruction. There are known motion compensation methods.

  However, if this motion compensation method is adopted, not all of the collected data can be used for reconstruction, and the imaging time becomes long. For example, when the number of slices to be imaged is 70 and the matrix in the phase encoding direction is 128, all the data necessary for reconstruction is 70 × 128 = 8960 lines. Assuming that 20 lines of data can be acquired by collecting data for a period of 1 heartbeat, 8960/20 = 448 heartbeats are required to collect 8960 lines of data. When one heartbeat is 1 second, the minimum required imaging time is 448 seconds = about 7.5 minutes. However, when the above-described data selection is performed and the probability that the data is adopted is 50%, the actual imaging time is doubled to about 15 minutes.

  As described above, when the imaging time is long, the state of breathing of the subject, that is, the depth of breathing may change during imaging. For example, the depth of breathing changes due to the sleep of the subject. As a result, the position of the imaging target region changes greatly, and the period outside the allowable range becomes longer. For this reason, since the probability that the collected data is adopted for reconstruction is reduced, the imaging time is further extended. In the worst case, if the region to be imaged is always outside the allowable range, the reconstruction cannot proceed and the imaging cannot be terminated.

  In order to avoid such a situation, a method of improving the data collection efficiency by correcting the position of the allowable range has been considered (for example, see Patent Document 1).

  However, when this motion compensation method is employed, the location where data is actually collected does not move spatially, so data relating to different cross sections of the imaging target region is collected before and after the correction of the allowable range. FIG. 10 is a diagram illustrating a state in which the position of the imaging target part with respect to the slice position changes before and after the correction of the allowable range. FIG. 10A shows the state before the allowable range is corrected, and FIG. 10B shows the state after the allowable range is corrected. Since the data collected in this way will be used for reconstruction, the resulting image will be blurred due to body movements despite compensation. .

  Furthermore, as a method for dealing with such a problem, there is a method of moving the position of the slice section of imaging in accordance with the movement of the imaging target part each time. By combining this method with the above-described method for correcting the permissible range, the relationship between the slice cross-section position and the imaging target part position can be kept substantially constant. However, the position in the Z-axis direction with respect to the slice cross-sectional device greatly moves. This is shown in FIG. FIG. 11A shows a state before the slice position is corrected, and FIG. 11B shows a state after the slice position is corrected.

JP 2004-202043 A

  As described above, conventionally, in cardiac imaging, particularly coronary artery imaging, it is particularly important to suppress the fat tissue signal around the artery and improve contrast, and fat suppression is essential. In addition, a pulse sequence using steady state free precession (SSFP) with good contrast between the myocardium and blood is usually used as the imaging sequence. Well known to be sensitive to. In addition, there is an imaging method such as an echo planar imaging (EPI) method in which the uniformity of the static magnetic field in the imaging portion greatly affects the image quality.

  However, in the motion compensation method described above, since the slice position moves in the static magnetic field during imaging, the slice position cannot always be adjusted to the position where the static magnetic field intensity is uniform. For this reason, when the motion compensation method described above is applied to an imaging method that is sensitive to static magnetic field uniformity, the degree of fat suppression is reduced as compared with the case where data is continuously acquired in the vicinity of a position where the static magnetic field strength is uniform. Therefore, image quality degradation such as an increase in banding artifacts in the SSFP method is caused.

  The present invention has been made in consideration of such circumstances, and the purpose of the present invention is to compensate for motion so as to keep the relationship between the position of the slice cross section and the position of the imaging target portion substantially constant, An object of the present invention is to provide a magnetic resonance imaging apparatus capable of performing high-quality imaging by an imaging method sensitive to static magnetic field uniformity.

  The present invention relates to a collecting means for collecting data representing a magnetic resonance signal radiated from a subject placed in a static magnetic field, and a detection for detecting a position of the subject in the static magnetic field at a position detection portion. Means, setting means for setting the size of an allowable range in accordance with the size of variation of the position detected by the detecting means within a unit period, and collecting when the detected position is within the allowable range A magnetic resonance imaging apparatus including a reconstruction unit configured to reconstruct an image based on the data.

The figure which shows the structure of the magnetic resonance imaging apparatus (MRI apparatus) which concerns on one Embodiment of this invention. The figure which shows an example of a pulse sequence. The figure which shows the object area | region of the data collection for a motion detection. The figure which shows an example of the data which carried out the Fourier transformation of the data for motion detection. The figure which shows a mode that the change of the position of the diaphragm accompanying respiration appears in the data which carried out the Fourier transform of the data for a motion detection. The figure which plotted what carried out the Fourier transform of the data for motion detection with time change. The figure which shows the mode of the top-plate movement for compensating the change of the position of the heart. The figure which shows the mode of the change of the tolerance | permissible_range for respond | corresponding to the change of the stability of respiration. The figure which shows the example of a setting of the relationship between dispersion | distribution of a diaphragm position, and the breadth of an allowable range. The figure which shows a mode that the position of the imaging target site | part with respect to a slice position changes before and after correction of the tolerance | permissible_range by the conventional motion compensation method. The figure which shows a mode that the position of the Z-axis direction with respect to the apparatus of a slice cross section moves by the conventional motion compensation method.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus (MRI apparatus) according to the present embodiment. The MRI apparatus shown in FIG. 1 includes a static magnetic field magnet 1, a gradient magnetic field coil 2, a gradient magnetic field power supply 3, a bed 4, a bed control unit 5, a transmission RF coil 6, a transmission unit 7, a reception RF coil 8, a reception unit 9, and A computer system 10 is provided.

  The static magnetic field magnet 1 has a hollow cylindrical shape and generates a uniform static magnetic field in an internal space. As the static magnetic field magnet 1, for example, a permanent magnet, a superconducting magnet or the like is used.

  The gradient magnetic field coil 2 has a hollow cylindrical shape and is disposed inside the static magnetic field magnet 1. The gradient coil 2 is a combination of three coils corresponding to the X, Y, and Z axes orthogonal to each other. The gradient magnetic field coil 2 generates a gradient magnetic field in which the above three coils are individually supplied with electric current from the gradient magnetic field power supply 3 and the magnetic field intensity changes along the X, Y, and Z axes. The Z-axis direction is, for example, the same direction as the static magnetic field. The gradient magnetic fields of the X, Y, and Z axes correspond to, for example, the slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Ge, and the readout gradient magnetic field Gr, respectively. The slice selection gradient magnetic field Gs is used to arbitrarily determine an imaging section. The phase encoding gradient magnetic field Ge is used to change the phase of the magnetic resonance signal in accordance with the spatial position. The readout gradient magnetic field Gr is used for changing the frequency of the magnetic resonance signal in accordance with the spatial position.

  The subject P is inserted into the cavity (imaging port) of the gradient coil 2 while being placed on the top 41 of the bed 4. The top 41 of the bed 4 is driven by the bed control unit 5 and moves in the longitudinal direction and the vertical direction. Usually, the bed 4 is installed such that the longitudinal direction thereof is parallel to the central axis of the static magnetic field magnet 1.

  The transmission RF coil 6 is disposed inside the gradient magnetic field coil 2. The transmission RF coil 6 receives a high frequency pulse from the transmission unit 7 and generates a high frequency magnetic field.

  The transmission unit 7 includes an oscillation unit, a phase selection unit, a frequency conversion unit, an amplitude modulation unit, a high frequency power amplification unit, and the like. The oscillation unit generates a high-frequency signal having a resonance frequency unique to the target nucleus in the static magnetic field. The phase selection unit selects the phase of the high-frequency signal. The frequency conversion unit converts the frequency of the high-frequency signal output from the phase selection unit. The amplitude modulation unit modulates the amplitude of the high-frequency signal output from the frequency modulation unit, for example, according to a sync function. The high frequency power amplification unit amplifies the high frequency signal output from the amplitude modulation unit. As a result of the operation of each of these units, the transmission unit 7 transmits a high-frequency pulse corresponding to the Larmor frequency to the transmission RF coil 6.

  The reception RF coil 8 is disposed inside the gradient magnetic field coil 2. The reception RF coil 8 receives a magnetic resonance signal radiated from the subject due to the influence of the high frequency magnetic field. An output signal from the reception RF coil 8 is input to the reception unit 9.

  The receiving unit 9 generates magnetic resonance signal data based on the output signal from the reception RF coil 8.

  The computer system 10 includes an interface unit 101, a data collection unit 102, a reconstruction unit 103, a storage unit 104, a display unit 105, an input unit 106, and a control unit 107.

  The interface unit 101 is connected to the gradient magnetic field power source 3, the bed control unit 5, the transmission unit 7, the reception RF coil 8, the reception unit 9, and the like. The interface unit 101 inputs and outputs signals exchanged between these connected units and the computer system 10.

  The data collection unit 102 collects digital signals output from the reception unit 9 via the interface unit 101. The data collection unit 102 stores the collected digital signal, that is, magnetic resonance signal data in the storage unit 104.

  The reconstruction unit 103 performs post-processing, that is, reconstruction such as Fourier transform, on the magnetic resonance signal data stored in the storage unit 104, and obtains spectrum data or image data of the desired nuclear spin in the subject P. Ask.

  The storage unit 104 stores magnetic resonance signal data and spectrum data or image data for each patient.

  The display unit 105 displays various information such as spectrum data or image data under the control of the control unit 107. As the display unit 105, a display device such as a liquid crystal display can be used.

  The input unit 106 receives various commands and information input from the operator. As the input unit 106, a pointing device such as a mouse or a trackball, a selection device such as a mode change switch, or an input device such as a keyboard can be used as appropriate.

  The control unit 107 includes a CPU, a memory, and the like (not shown), and comprehensively controls the MRI apparatus according to the present embodiment. The control unit 107 compensates for the movement of the subject P in addition to the control function for realizing the well-known functions in the MRI apparatus, and keeps the relationship between the position of the slice cross-section and the position of the imaging target site substantially constant. For this purpose, a function for executing processing as described below is provided.

  Next, the operation of the MRI apparatus configured as described above will be described. Here, the case where the coronary artery is imaged using a real-time motion compensation method that estimates and compensates for the change in the position of the heart accompanying the change in the respiratory motion of the subject P from the motion of the diaphragm is taken as an example. The operation of the MRI apparatus will be described.

  FIG. 2 is a diagram showing an example of a pulse sequence.

  When imaging the coronary artery, the control unit 107 collects data for image reconstruction in a period PC corresponding to the diastole of the heart in synchronization with the R wave of the electrocardiogram signal acquired by an electrocardiograph (not shown). To the data collection unit 102. In the period PB prior to the period PC, the control unit 107 causes the transmission unit 7 to perform fat suppression excitation. Furthermore, in the period PA preceding the period PB, the control unit 107 causes the data collection unit 102 to collect data for motion detection.

  The target area for data collection for image reconstruction is assumed to be an area including the heart, which is an imaging target part, for example, as indicated by A1 in FIG. On the other hand, the target area for collecting data for motion detection is an area near the boundary between the liver and the lung as shown by A2 in FIG. When collecting data for motion detection, a gradient magnetic field pulse for reading is applied in the body axis direction (Z direction in FIG. 3). Thus, when the collected data for motion detection is Fourier transformed, data as shown in FIG. 4 is obtained. Based on the data shown in FIG. 4, the control unit 107 detects the position of the diaphragm as the boundary position between the liver and the lung.

  The position of the diaphragm reciprocates between the position at the time of expiration shown in FIG. 5 (a) and the position at the time of inhalation shown in FIG. 5 (b) in the short term. Therefore, the control unit 107 obtains the deviation amount of the detected diaphragm position with respect to the reference position, for example, using the position of exhalation as the reference position. When the deviation amount is within a predetermined allowable range (for example, 2.5 mm), the control unit 107 employs data collected in the immediately following period PC as raw data for image reconstruction, Otherwise, data collection is not performed in the PC immediately after, or the collected data is not adopted as raw data for image reconstruction.

  During imaging, the state of breathing of the subject P, that is, the depth of breathing, changes over time. The state of this change is shown in FIG. FIG. 6 is a plot of data obtained by performing Fourier transform on motion detection data with time. In the first half of FIG. 6, the position of the exhaled diaphragm is within the permissible range, but in the second half, the position of the exhaled diaphragm gradually changes to the head side and falls outside the permissible range. It has become. If the positional relationship between the center of the static magnetic field and the heart is in the state shown in FIG. 7A at the beginning of imaging, when the above phenomenon occurs, the positional relationship between the center of the static magnetic field and the heart is, for example, FIG. It has changed to the state shown in b). Such a phenomenon is caused by a change in the depth of respiration due to the sleep of the subject P.

  In order to compensate for such a shift of the heart position, the control unit 107 always measures the adoption rate of the data for image reconstruction collected in the recent fixed period. This can be realized, for example, by measuring the probability that the data obtained by collecting the data a certain number of times immediately before (for example, 50 times) is adopted as raw data for image reconstruction.

  Then, the control unit 107 returns the positional relationship between the center of the static magnetic field and the heart to the state shown in FIG. 7A when the measured data adoption rate falls below a predetermined value (for example, 50%). Therefore, the couch controller 5 is instructed to move the top board 41 as shown in FIG. The moving direction and the moving amount of the top plate 41 are determined based on, for example, the average value of the deviation amounts from the reference position collected during the recent fixed period. Note that it is desirable to move the top plate 41 at a timing when data collection is not performed. However, the data collected during the movement of the top board 41 may be invalidated by moving the top board 41 regardless of whether or not data collection is performed.

  Thus, the positional relationship among the static magnetic field center, the heart, and the slice position can be made substantially the same at the beginning of imaging shown in FIG. 7 (d) and after the top plate movement shown in FIG. 7 (e). As a result, high-quality imaging can be performed even when an imaging method sensitive to static magnetic field uniformity is applied.

  The control unit 107 measures the average position of the diaphragm over a predetermined number of times immediately before from the motion detection data, and when the deviation amount of the average position with respect to the center of the preset allowable range exceeds a certain amount, The plate 41 may be moved. Or the control part 107 always displays the data for a motion detection on the display part 105, and makes an operator determine the necessity of the movement of the top plate 41. FIG. The control unit 107 may wait for the operator to perform a movement instruction at the input unit 106 and move the top board 41 in accordance with the movement instruction.

  The above is an operation for compensating for a change in the respiratory state of the subject P over a relatively long period. The respiratory state of the subject P may vary in a relatively short period in addition to the change in the relatively long period as described above. Such a variation is caused by a large variation in the diaphragm position in exhaled air / inspired air, so that there is little risk that the imaging will not be terminated, but it will lead to an extended imaging time.

  Therefore, the control unit 107 measures the variance of the diaphragm position at a fixed number of times immediately before from the motion detection data, and corrects the allowable range according to this variance.

  For example, in the period P1 of FIG. 8, the respiration is relatively stable and the detected dispersion of the diaphragm position is small, whereas in the period P2, the dispersion increases compared to the period P1. In such a case, the control unit 107 changes to an allowable range R2 wider than the allowable range R1 at the time T1 when it is detected that the variance has increased. Thereby, it is possible to increase data collection efficiency and prevent the imaging time from being extended.

  In a state where breathing is stable, it is desirable to ensure the image quality by narrowing the allowable range. Therefore, even if the breathing becomes unstable once and the dispersion increases, the control unit 107 preferably narrows the allowable range when the breathing is improved again to the original stable state and the dispersion becomes small.

  The relationship between the dispersion of the diaphragm position and the width of the allowable range is set in advance. For example, as shown in FIG. 9A, it may be set in several steps discretely, or it may be set as a continuous function as shown in FIG. 9B. The input for evaluating the allowable range may be the probability of adopting data in addition to the dispersion of the diaphragm position.

  This embodiment can be variously modified as follows.

  The movement may be detected based on a part other than the diaphragm. For example, it is possible to detect the movement in the opposite direction based on the vertical movement of the abdomen.

  The site to be imaged is not limited to the heart.

  Only one of the movement control of the top plate 41 and the change of the allowable range may be performed.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

  DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... Gradient magnetic field coil, 3 ... Gradient magnetic field power supply, 4 ... Bed, 41 ... Top plate, 5 ... Bed control part, 6 ... Transmission RF coil, 7 ... Transmission part, 8 ... Reception RF coil, DESCRIPTION OF SYMBOLS 9 ... Reception part, 10 ... Computer system, 101 ... Interface part, 102 ... Data collection part, 103 ... Reconstruction part, 104 ... Memory | storage part, 105 ... Display part, 106 ... Input part, 107 ... Control part.

Claims (3)

A collecting means for collecting data representing a magnetic resonance signal emitted from an object placed in a static magnetic field;
Detecting means for detecting a position of the position detection portion of the subject in the static magnetic field;
Setting means for setting the size of the allowable range in accordance with the size of the variation in position detected by the detection means within a unit period;
A magnetic resonance imaging apparatus comprising: reconstruction means for reconstructing an image based on the data collected when the detected position is within the allowable range.   The magnetic resonance imaging apparatus according to claim 1, wherein the setting unit sets the size of the allowable range according to the variance of the detected positions.   The magnetic resonance imaging apparatus according to claim 1, wherein the setting unit sets the size of the allowable range according to a probability that the detected position is within the allowable range.

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