JP6087543B2 - Magnetic resonance imaging system - Google Patents

Description

  Embodiments described herein relate generally to a magnetic resonance imaging apparatus.

  In magnetic resonance imaging, the nuclear spin of a subject placed in a static magnetic field is magnetically excited with an RF (Radio Frequency) signal of Larmor frequency, and an image is reconstructed from the magnetic resonance signal generated by this excitation. This is an imaging method.

  As a motion correction method in the field of magnetic resonance imaging, there is an imaging method called PROPELLER (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction). In this imaging method, k-space data is filled in a non-orthogonal shape by rotating a rectangular rectangular area.

James G. Pipe, “Motion correction with PROPELLER MRI: Application to Head Motion and Free-breathing Cardiac Imaging”, MRM, 42, 1999, p.963-969

  The problem to be solved by the present invention is to provide a magnetic resonance imaging apparatus capable of improving the accuracy of motion correction.

  The magnetic resonance imaging apparatus according to the embodiment includes a detection unit and a correction unit. The detection unit weight-adds the k-space data for a plurality of channels filled in a non-orthogonal manner by rotating the rectangular area between the channels for each rectangular area, and the data for each rectangular area after the weighted addition is obtained. Based on this, the amount of movement of the imaging region during imaging is detected. The correction unit corrects k-space data of each channel using the motion amount.

FIG. 1 is a diagram illustrating a configuration of an MRI (Magnetic Resonance Imaging) apparatus according to the present embodiment. FIG. 2 is a diagram illustrating an example of a receiving coil according to the present embodiment. FIG. 3 is a diagram for explaining an imaging method in the present embodiment. FIG. 4 is a diagram showing a uniform sensitivity distribution in the rotation direction, compared with the present embodiment. FIG. 5 is a diagram illustrating an example of a head T1-weighted image in the present embodiment. FIG. 6 is a diagram showing a procedure for motion amount detection in the present embodiment. FIG. 7 is a diagram for explaining a specific example in the present embodiment. FIG. 8 is a diagram for explaining a specific example of motion amount detection in the present embodiment. FIG. 9 is a diagram for explaining the sensitivity distribution in the present embodiment. FIG. 10 is a diagram illustrating a rotation direction of k-space data in the present embodiment. FIG. 11 is a diagram for explaining a shift in the rotation direction of k-space data in the present embodiment.

  Hereinafter, a magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus” as appropriate) according to an embodiment will be described with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of an MRI apparatus 100 according to the present embodiment. The subject P is not included in the MRI apparatus 100. The static magnetic field magnet 1 is formed in a hollow cylindrical shape and generates a uniform static magnetic field in an internal space. The static magnetic field magnet 1 is, for example, a permanent magnet or a superconducting magnet. The gradient coil 2 is formed in a hollow cylindrical shape and generates a gradient magnetic field in the internal space. Specifically, the gradient magnetic field coil 2 is disposed inside the static magnetic field magnet 1 and receives a power supply from the gradient magnetic field amplifier 3 to generate a gradient magnetic field. The gradient magnetic field amplifier 3 supplies electric power to the gradient magnetic field coil 2 in accordance with a control signal transmitted from the sequence control unit 10.

  The bed 4 includes a top plate 4a on which the subject P is placed, and the top plate 4a is inserted into the cavity of the gradient magnetic field coil 2 serving as an imaging port in a state where the subject P is placed. Usually, the bed 4 is installed such that the longitudinal direction is parallel to the central axis of the static magnetic field magnet 1. The couch controller 5 drives the couch 4 to move the couchtop 4a in the longitudinal direction and the vertical direction.

  The transmission coil 6 generates a high frequency magnetic field. Specifically, the transmission coil 6 is arranged inside the gradient magnetic field coil 2 and receives a supply of RF pulses from the transmission unit 7 to generate a high-frequency magnetic field. The transmission unit 7 transmits an RF pulse corresponding to the Larmor frequency to the transmission coil 6 in accordance with the control signal transmitted from the sequence control unit 10.

  The receiving coil 8 receives a magnetic resonance signal (hereinafter referred to as “MR (Magnetic Resonance) signal” as appropriate). Specifically, the receiving coil 8 is disposed inside the gradient coil 2 and receives an MR signal radiated from the subject P due to the influence of the high-frequency magnetic field. The receiving coil 8 outputs the received MR signal to the receiving unit 9. In this embodiment, the receiving coil 8 is used for explanation, but a transmitting / receiving coil having both the function of the transmitting coil and the function of the receiving coil may be used.

  The receiving unit 9 generates MR signal data based on the MR signal output from the receiving coil 8 in accordance with the control signal sent from the sequence control unit 10. Specifically, the receiving unit 9 generates MR signal data by digitally converting the MR signal output from the receiving coil 8, and sends the generated MR signal data to the computer system 20 via the sequence control unit 10. Send. The receiving unit 9 may be provided on the gantry device side including the static magnetic field magnet 1, the gradient magnetic field coil 2, and the like.

  The sequence control unit 10 controls the gradient magnetic field amplifier 3, the transmission unit 7, and the reception unit 9. Specifically, the sequence control unit 10 transmits a control signal based on the pulse sequence execution data transmitted from the computer system 20 to the gradient magnetic field amplifier 3, the transmission unit 7, and the reception unit 9.

  The computer system 20 includes an interface unit 21, an image reconstruction unit 22, a storage unit 23, an input unit 24, a display unit 25, and a control unit 26. The interface unit 21 is connected to the sequence control unit 10 and controls input / output of data transmitted / received between the sequence control unit 10 and the computer system 20. The image reconstruction unit 22 reconstructs image data from the MR signal data transmitted from the sequence control unit 10 and stores the reconstructed image data in the storage unit 23.

  The storage unit 23 stores parameter values set as parameters included in the imaging conditions, image data stored by the image reconstruction unit 22, and other data used in the MRI apparatus 100. For example, the storage unit 23 is a semiconductor memory element such as a RAM (Random Access Memory) or a flash memory, a hard disk, an optical disk, or the like.

  The input unit 24 receives various instructions for editing imaging conditions, imaging instructions, and the like from the operator. For example, the input unit 24 receives a parameter value setting instruction or the like for a parameter included in the imaging condition. For example, the input unit 24 is a mouse, a keyboard, or the like. The display unit 25 displays an imaging condition editing screen, an image, and the like.

  The control unit 26 comprehensively controls the MRI apparatus 100 by controlling each unit described above. For example, when the editing of the imaging condition is received from the operator, the control unit 26 generates pulse sequence execution data based on the received imaging condition, and transmits the generated pulse sequence execution data to the sequence control unit 10. For example, the control unit 26 is an integrated circuit such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA), or an electronic circuit such as a central processing unit (CPU) or a micro processing unit (MPU). As shown in FIG. 1, the control unit 26 according to the present embodiment includes a collection unit 26a, a motion amount detection unit 26b, and a motion amount correction unit 26c. Each of these parts will be described in detail later.

  FIG. 2 is a diagram illustrating an example of the receiving coil 8 according to the present embodiment. 2A is a side view showing the arrangement of the eight coil elements E1 to E8 built in the receiving coil 8, and FIG. 2B is a front view. As shown to (A) and (B), the eight coil elements E1-E8 are arrange | positioned so that a head may be made to wrap around along the head surface of the subject P. FIG. Of the eight coil elements E1 to E8, five coil elements E1 to E5 are arranged at the upper part, and three coil elements E6 to E8 are arranged at the lower part.

  Now, the MRI apparatus 100 according to the present embodiment uses the receiving coil 8 to image the head of the subject P by an imaging method called PROPELLER or the like.

  FIG. 3 is a diagram for explaining an imaging method in the present embodiment. In the present embodiment, the MRI apparatus 100 collects data points in the k space in a non-orthogonal manner and fills the k space. Specifically, the MRI apparatus 100 fills the data points in the k space by repeating the collection by changing the rotation angle of the rectangular rectangular area (hereinafter referred to as “blade” as appropriate). In principle, the MRI apparatus 100 sequentially changes the intensity ratio of gradient magnetic field pulses (for example, the intensity ratio of two orthogonal axes among three gradient magnetic fields) during imaging, thereby changing the rotation angle of the blade. Repeat the changed collection. In FIG. 3A, a two-dimensional k-space is illustrated, white circles indicate Cartesian data, and black circles are filled non-orthogonally by the imaging method of the present embodiment. Data in the blade is shown. Further, as shown in (B), the MRI apparatus 100 repeats the collection while changing the rotation angle of the blade from 0 ° to about 180 °. Note that (B) shows an example in which the number of blades is “6”.

  Here, as shown in FIG. 3B, the vicinity of the center of each blade (also corresponding to the vicinity of the center of the k space) is a common area collected many times. For this reason, the MRI apparatus 100 detects the amount of movement (for example, the amount of rigid body rotation and the amount of parallel movement) of the imaging region during imaging by comparing the data of the common area between the blades. The MRI apparatus 100 corrects the motion of each blade data (hereinafter referred to as “blade data” as appropriate) using the detected motion amount, and then obtains orthogonal data (FIG. 3 (A) in FIG. 3). ) Is estimated, and the image in which the imaging region is depicted is reconstructed using the estimated orthogonal data. This estimation is referred to as “gridding” or the like.

  The movement amount detection will be further described. The rigid body rotation amount in the real space corresponds to a shift amount in the rotation direction when the data of the common region in the k space is compared between the blades. Therefore, the MRI apparatus 100 calculates the correlation in the rotation direction between the data in the common area, and calculates the shift amount (rotation angle) in the rotation direction. In this case, since the first-order phase shift occurs in the k-space data due to the parallel movement of the imaging region, the MRI apparatus 100 rotates after taking the absolute value (ABS (absolute value)) of the k-space data. Calculate direction correlation.

  For example, when imaging is performed with the receiving coil 8 having a plurality of coil elements as illustrated in FIG. 2, the MRI apparatus 100 generally takes the absolute value of the MR signal output from each coil element and then performs each blade. This is simply added between the coil elements, and the data of the common area among the blade data after the simple addition is compared between the blades.

  Now, according to the principle of motion amount detection described above, it is assumed that the sensitivity distribution of the coil after addition, which is simply added between the coil elements, is uniform in the rotation direction in real space. This is because when the imaging region rotates in real space and the sensitivity distribution of the coil is uniform in the rotation direction, the correlation in the rotation direction in the k space is maintained. FIG. 4 is a diagram showing a uniform sensitivity distribution in the rotation direction, which is contrasted with the sensitivity distribution in the present embodiment.

  However, when the luminance distribution of the imaging region approaches rotational symmetry, it is difficult to detect the rigid body rotation amount with the above-described method. This is because when the luminance distribution of the imaging region is rotationally symmetric (when the signal change is small in the rotational direction), the luminance distribution of the data in the common area in k-space is also rotationally symmetric, so This is because it is difficult to find a point where the correlation of the maximum is. FIG. 5 is a diagram showing an example of the head T1-weighted image in the present embodiment. As shown in FIG. 5, for example, in the case of an axial cross-sectional image of the head T1-weighted image, detection of the rigid body rotation amount becomes difficult.

  Therefore, the MRI apparatus 100 according to the present embodiment dares to give the coil sensitivity distribution non-uniformity in the rotation direction when the target image has a characteristic that the luminance distribution of the imaging region is rotationally symmetric. This makes it possible to detect the amount of rigid body rotation. In the following, an embodiment in which the sensitivity distribution has non-uniformity in the rotation direction will be described.

  FIG. 6 is a diagram illustrating a processing procedure of motion amount detection in the present embodiment. As shown in FIG. 6, first, the collection unit 26a controls the sequence control unit 10 to start imaging (step S101). Subsequently, the collection unit 26a collects k-space data for a plurality of channels for each blade by using the reception coil 8 having a plurality of coil elements (step S102).

  The reason why the “coil (element)” and the “channel” are used properly is that the MR signal obtained by each coil (element) may be channel-combined into data. In the following description, “coil (element)” and “channel” are described without distinction. Both are differences in whether or not channels are combined, and the embodiments described below can be applied to both.

  Next, the motion amount detection unit 26b weights and adds blade data for a plurality of channels between the channels for each blade, and detects the motion amount of the imaging region during imaging based on the data for each blade after the weighted addition. To do. At this time, the motion amount detection unit 26b performs weighted addition so that the signal value of the data after weighted addition has a change in the rotation direction in the k space.

  That is, as described above, generally, the blade data for a plurality of channels is simply added between the channels for each blade, and the motion amount detection unit 26b adds the blade data for the plurality of channels between the channels for each blade. Weighted addition is performed (step S103). In the case of simple addition, in the real space, the sensitivity distribution of the coil after addition is uniform in the rotation direction that is the direction of movement of the imaging region. On the other hand, in the case of weighted addition, the sensitivity distribution of the coil after addition forms a pattern according to the position of the coil element in the real space, and has contrast in the rotation direction. In the data synthesis by this weighted addition, it does not matter whether it is complex data or absolute value data.

  Subsequently, the motion amount detection unit 26b converts the data in the vicinity of the center of the k space common to each blade among the data for each blade after the weighted addition from the orthogonal coordinate to the polar coordinate (step S104). That is, since the synthesized data is orthogonal coordinates, the motion amount detection unit 26b first converts it into polar coordinates. Data used for detecting the amount of motion is data inside a circle near the center of the k-space.

Then, the motion amount detection unit 26b calculates the correlation between the combined data by the following equation (1) (step S105), and sets the rotation angle θ at which the correlation coefficient is the highest in the rotation direction of the k space. The shift amount is calculated (step S106).

  The motion amount correction unit 26c estimates the orthogonal data from all the blade data after performing motion correction (shift processing) of each blade data using the motion amount calculated by the motion amount detection unit 26b. Using the estimated orthogonal data, an image in which the imaging region is depicted is reconstructed.

  FIG. 7 is a diagram for explaining a specific example in the present embodiment. The above-described motion amount detection processing procedure will be described again using a specific example. As shown in FIG. 7, in this specific example, coil elements A, B, C, and D and channel data A, B, C, and D will be described as examples. Hereinafter, a case where the MR signal received by the coil element A is used as it is for the channel data A (the same applies to B, C, and D) will be described.

  As shown in FIG. 7, the coil elements A and B are arranged to face each other, and the coil elements C and D are arranged to face each other. For example, assuming the head coil 8 illustrated in FIG. 2, the coil elements A and B correspond to, for example, the coil elements E2 to E4 and E6 to E8, and the coil elements C and D include, for example, It can be considered to correspond to the coil elements E1 · E8 and E5 · E6.

  FIG. 8 is a diagram for explaining a specific example of motion amount detection in the present embodiment. As shown in FIG. 8, k-space data for each blade exists in each of the channels A, B, C, and D by the collection by the collection unit 26a. In FIG. 8, only two blades 1 and 2 are shown for convenience of explanation.

  Then, for example, the motion amount detection unit 26b sets the weighting coefficients of the channels A and B to “0” (indicated by a dotted line in FIG. 8), sets the weighting coefficient of the channel C to “1”, and sets the weighting coefficient of the channel D to “−. 1 ”, weighting addition is performed for each blade, and individual data for each channel is converted into synthesized data. At this time, the motion amount detection unit 26b according to the present embodiment performs weighted addition by combining the weight coefficients “1” and “−1” of opposite signs so that the signs are switched around the characteristic structure of the imaging region. .

  FIG. 9 is a diagram for explaining the sensitivity distribution in the present embodiment. For example, as shown in FIG. 9A, when attention is paid to the coil element C, it is considered that the sensitivity in a region near the coil element C is high and the sensitivity in a far region is low. When combined with the sensitivity of the opposing coil element D, the sensitivity distribution is as shown by the solid line in (B). Here, for example, when the data of the coil element D is weighted with the opposite sign, the sensitivity distribution obtained by the combination of the coil elements C and D is considered to be a sensitivity distribution as shown by the dotted line in (B). Since the polarities between the channels are reversed, a sensitivity distribution that is not uniform in the rotation direction is formed in the real space, and data corresponding to this is obtained also in the k space. The point where the sign of the sensitivity distribution is switched is the “zero cross point”.

  In this embodiment, weighting is performed so that the “zero cross point” is in the vicinity of the characteristic structure of the imaging region. For example, when an axial cross-sectional image of the head T1-weighted image is considered, as shown in (C), the structure around the region 9b that separates the left and right hemispheres is more anatomically characteristic than the region 9a. Have When the head rotates, it is considered that the correlation of the rotation direction in the k space can be detected more easily by changing the sign of the signal in the region 9b. In addition, in this way, detection is possible not only when the head and the head coil 8 rotate together, but also when the head rotates independently within the head coil 8. It becomes possible. Thus, in this embodiment, by performing weighted addition using blade data of only the coil elements C and D, a sensitivity distribution that is not uniform in the rotation direction as shown in (D) is formed.

  Returning to FIG. 8, the motion amount detection unit 26b takes an absolute value for the combined data for each blade after weighted addition in order to avoid the influence of parallel movement, and then the data near the center of the k-space common to each blade. Convert from Cartesian coordinates to polar coordinates. Subsequently, the motion amount detection unit 26b calculates the correlation in the rotation direction of the k space between the blade 1 and the blade 2, and shifts the rotation angle θ with the highest correlation coefficient in the rotation direction of the k space. Calculate as a quantity. This shift amount corresponds to the amount of rigid body rotation between the blade 1 and the blade 2, and the movement of the blade 2 can be corrected using this amount of rigid body rotation.

  FIG. 10 is a diagram illustrating the rotation direction of the k-space data in the present embodiment, and FIG. 11 is a diagram for explaining the shift in the rotation direction of the k-space data in the present embodiment. As shown in FIG. 10, let the rotation direction of k space be (phi). As shown in FIG. 11A, when the signal value of the data in the common area in k-space is rotationally symmetric (having no change in the rotational direction), it is difficult to detect the correlation in the rotational direction of k-space. . On the other hand, as shown in FIG. 11B, when a sensitivity distribution that is not uniform in the rotation direction is formed in the real space and corresponding data is obtained in the k space, the correlation in the rotation direction of the k space is obtained. Can be detected.

  In this way, the motion amount detection unit 26b calculates the shift amount in the rotation direction of the k space similarly for the other blades, and obtains the rigid body rotation amount for each blade. Thereafter, the motion amount correction unit 26c performs motion correction (shift processing) on each blade data using the rigid body rotation amount calculated for each blade by the motion amount detection unit 26b, and then performs orthogonal correction from all blade data. Data is estimated, and an image in which an imaging region is depicted is reconstructed using the estimated orthogonal data.

  In the above-described embodiment, the example in which the channel weighting coefficient is determined in advance has been described, but the embodiment is not limited thereto. For example, the motion amount detection unit 26b evaluates the complexity of the change in the rotation direction in the k-space for the signal value of the data after weighted addition by performing a weighted addition trial, and evaluates that the complexity is relatively high. Weighting addition may be performed using the weighting factor.

  For example, the motion amount detection unit 26b defines in advance a “channel synthesis matrix” in which weighting coefficients are defined. Then, the motion amount detection unit 26b performs a weighted addition trial calculation for a certain blade to generate composite data, and evaluates the degree of complexity of the change in the rotation direction in the k space using an index such as variance. The motion amount detection unit 26b may select a weighting factor that increases the index from the “channel synthesis matrix”.

  As described above, according to the present embodiment, for example, even when the luminance distribution of the imaging region approaches rotational symmetry, the rigid body rotation amount can be detected, and the accuracy of motion correction can be improved. As a result, the amount of motion of the subject being imaged can be detected to provide a good diagnostic image.

(Other embodiments)
Note that the embodiment is not limited to the above-described embodiment.

  In the above-described embodiment, an example is described in which an imaging method called PROPELLER or the like is assumed, and k-space data for a plurality of channels is weighted and added for each blade to detect the rigid body rotation amount of the imaging region. The embodiment is not limited to this. For example, the parallel movement amount in the direction perpendicular to the projection direction can be detected in the projection space by weighting and adding k-space data for a plurality of channels.

  In the embodiment described above, an axial cross-sectional image of the head T1-weighted image is assumed as the target image, but the embodiment is not limited to this. In the case of other imaging regions, other cross-sectional images, images of other imaging types, and the like, the embodiment described above is used when the luminance distribution of the imaging region is rotationally symmetric, as in the above-described embodiment. The same can be applied.

  In the above-described embodiment, the two-dimensional k-space has been described as an example using FIG. 3 or the like, but the embodiment is not limited to this. The same applies to a three-dimensional k-space.

  According to the magnetic resonance imaging apparatus of at least one embodiment described above, the accuracy of motion correction can be improved.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

100 MRI apparatus 26 control unit 26a collection unit 26b motion amount detection unit 26c motion amount correction unit

Claims (8)

The k-space data for a plurality of channels filled non-orthogonally by rotating the rectangular area is weighted and added between channels for each rectangular area, and imaging is performed based on the data for each rectangular area after the weighted addition. A detection unit for detecting the amount of movement of the imaging region in
A magnetic resonance imaging apparatus comprising: a correction unit that corrects k-space data of each channel using the amount of motion.   The magnetic resonance imaging apparatus according to claim 1, wherein the detection unit performs weighted addition so that a signal value of the data after weighted addition has a change in a rotation direction in the k space.   The magnetic resonance imaging apparatus according to claim 2, wherein the detection unit performs the weighted addition by combining weighting factors having opposite signs.   4. The magnetic resonance imaging apparatus according to claim 3, wherein the detection unit performs the weighted addition in combination with weighting factors of opposite signs so that the signs are changed in the vicinity of a characteristic structure of the imaging region.   The detection unit evaluates the complexity of the change in the rotation direction in the k-space for the signal value of the data after weighted addition by performing a trial calculation of the weighted addition, and the weight evaluated as having a relatively high degree of complexity The magnetic resonance imaging apparatus according to claim 1, wherein weighted addition is performed using a coefficient.   The detection unit converts the data near the center of the k-space common to each rectangular area among the data for each rectangular area after the weighted addition, calculates the correlation between the rectangular areas after converting from orthogonal coordinates to polar coordinates, and k-space The magnetic resonance imaging apparatus according to claim 1, wherein a movement amount of the imaging region is detected by calculating a shift amount in a rotation direction of the magnetic resonance imaging apparatus.   The magnetic resonance imaging according to any one of claims 1 to 6, further comprising a reconstruction unit that reconstructs an image in which the imaging region is rendered using the corrected k-space data. apparatus. K-space data for a plurality of channels filled in a non-orthogonal manner, and k-space data for a plurality of channels including data of a common region in k-space are weighted and added between channels, and the weighted addition data is used. A detection unit for detecting the amount of movement of the imaging part during imaging;
A magnetic resonance imaging apparatus comprising: a correction unit that corrects k-space data of each channel using the amount of motion.