JP2010233907A - Magnetic resonance imaging apparatus and sensitivity correcting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To remove an artifact caused by displacement of a subject to be examined and a sensitivity correction error caused by the difference between the imaging conditions. <P>SOLUTION: A sensitivity image of each element receiving coil and a sensitivity image of a second receiving coil are obtained under an almost same imaging condition, and the sensitivity image of each element receiving coil and the sensitivity image of the second receiving coil are used to obtain the sensitivity distribution of the first receiving coil, and the sensitivity of the image of the first receiving coil is corrected by using the sensitivity distribution. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a technique for correcting image sensitivity unevenness caused by a non-uniform distribution of sensitivity of a receiving coil in nuclear magnetic resonance imaging (hereinafter referred to as “MRI”).

  MRI equipment measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals generated by nuclear spins that make up the body of a subject, especially human tissue, and two-dimensionally describes the shape and function of the head, abdomen, limbs, etc. Or three-dimensional imaging device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In this MRI apparatus, as a receiving coil for detecting an NMR signal generated from a subject, a high-sensitivity receiving coil called a “multiple receiving coil” or a “phased array coil” has been widely used in recent years. Various things have been proposed accordingly. A multiple receiver coil is composed of a plurality of relatively high-sensitivity element receiver coils, and a new image is synthesized by synthesizing an image for each element receiver coil obtained by Fourier transforming the NMR signal from each element receiver coil. A composite image is acquired (hereinafter referred to as “MAC composition”). According to this, an image with a high signal-to-noise ratio is acquired for each element receiving coil, and composite image information with a high signal-to-noise ratio is acquired over a wide range using an image with a high signal-to-noise ratio for each element receiving coil. can do.

On the other hand, since a plurality of images are combined into a single combined image, when image information with poor sensitivity uniformity is combined, the sensitivity uniformity of the combined image also deteriorates. For this reason, when performing image synthesis, sensitivity correction is performed using a reception coil with good uniformity, for example, a body coil built in the MRI apparatus body. Conventionally, as described in Patent Document 1, in order to perform this sensitivity correction, sensitivity measurement is performed in advance prior to the main measurement. With this sensitivity measurement, three-dimensional sensitivity image data is acquired using multiple receiving coils and body coils, respectively, and the signal intensity ratio (S _MAC / S), which is a divided value of the signal intensity S _MAC and S _WB of each sensitivity image data. based on the _WB), 3-dimensional sensitivity distribution information of multiple receive coils is estimated. Then, sensitivity distribution information of the same cross section as that of the main measurement image is extracted from the obtained three-dimensional sensitivity distribution information of the multiple receiving coils, and the sensitivity unevenness of the composite image is corrected using this.

  On the other hand, the sensitivity distribution information of the multiple receiving coil is estimated by post-processing from the image itself obtained from the NMR signal received by the multiple receiving coil, and the sensitivity distribution information of the composite image is estimated using the obtained sensitivity distribution information of the multiple receiving coil. A method of correcting uniformity is also used. For example, by performing smoothing processing on the image data for each element reception coil obtained by each element reception coil constituting the multiple reception coil, image data of low frequency components for each element reception coil is created, There is a method of synthesizing these and substituting them as sensitivity distribution information (Patent Document 2).

Japanese Patent Laid-Open No. 08-056928 Japanese Patent Laid-Open No. 2001-161657

Based on the signal intensity values S _MAC and S _WB of the respective sensitivity image data obtained from multiple receiver coils and body coils by conventional sensitivity measurement, the three-dimensional of the multiple receiver coils is based on the divided value (S _MAC / S _WB ) In the method of estimating the sensitivity distribution information and correcting the sensitivity distribution of the composite image, the sensitivity image data is collected between the collection of the sensitivity image data using the multiple receiving coil and the collection of the sensitivity image data using the body coil in sensitivity measurement. There remains a problem that misregistration may occur in the two-sensitivity image data due to the movement of the specimen or the like.

  In addition, even if the above-mentioned positional deviation does not occur during the collection of sensitivity image data using multiple receiving coils and body coils in sensitivity measurement, the subject as described above between the sensitivity measurement and the main measurement. There is a possibility that a positional deviation occurs between the three-dimensional sensitivity distribution information and the main measurement image due to the movement of the image. These still have the problem of causing artifacts and poor sensitivity correction in the composite image after sensitivity correction.

  Further, in many cases, imaging conditions are different between the main measurement and the sensitivity measurement. In sensitivity measurement, as described above, the purpose is to acquire sensitivity distribution information of multiple receiving coils, and therefore it is common to set imaging conditions that suppress tissue contrast as much as possible. For example, when a pulse sequence of the gradient echo method is used, increasing the flip angle leads to T1 enhancement, so the flip angle is made as small as possible. On the other hand, since the purpose of this measurement is for image diagnosis, the contrast between tissues is extremely important. Therefore, it is common to set imaging conditions that make the contrast between tissues stand out. For example, an IR (inversion recovery) pulse is applied as a pre-pulse in order to enhance the inter-tissue contrast between white matter and gray matter. The difference in the imaging conditions between the main measurement and the sensitivity measurement causes a difference in spatial nonuniformity of the RF power of the transmission coil and a difference in reception sensitivity of the multiple reception coils in both measurements. As a result, even if sensitivity correction is performed using sensitivity image data acquired by sensitivity measurement, a sensitivity correction failure may occur due to the influence of sensitivity distribution differences resulting from these differences. For this reason, even if the sensitivity correction is performed correctly under certain imaging conditions, there remains a problem that if the imaging conditions are changed, the sensitivity correction is not performed correctly, and sensitivity unevenness may occur even after the sensitivity correction.

  On the other hand, in the method of correcting the sensitivity nonuniformity using the sensitivity distribution information of the multiple receiving coil estimated by post-processing from the image data itself by the multiple receiving coil, the estimation accuracy of the sensitivity distribution information of the multiple receiving coil is low. Since the correction becomes insufficient, there remains a problem that the uniformity of the finally obtained image after sensitivity correction cannot be sufficiently obtained.

  For these reasons, a sensitivity correction technique free from artifacts caused by positional deviation of the subject and sensitivity correction failures due to differences in imaging conditions is extremely important.

  Therefore, the present invention has been made in view of the above-described problems, and an object of the present invention is to eliminate sensitivity correction defects due to artifacts caused by displacement of the subject and differences in imaging conditions.

  In order to achieve the above object, the present invention is configured as follows. That is, the sensitivity image of each element receiving coil and the sensitivity image of the second receiving coil are acquired under substantially the same imaging conditions, and using the sensitivity image of each element receiving coil and the sensitivity image of the second receiving coil, The sensitivity distribution of the first receiving coil is obtained, and the sensitivity of the image of the first receiving coil is corrected using the sensitivity distribution.

  Specifically, the MRI apparatus of the present invention includes a first receiving coil formed by combining a plurality of element receiving coils, and a second coil having a wider sensitivity range than the first receiving coil. A reception unit that receives a nuclear magnetic resonance signal from the subject, a measurement control unit that controls measurement of the nuclear magnetic resonance signal from the subject based on a predetermined pulse sequence, and a nuclear magnetic field An arithmetic processing unit that acquires an image of the subject using the resonance signal, and the measurement control unit controls the acquisition of the sensitivity image for each element receiving coil and the sensitivity image of the second receiving coil, and performs arithmetic processing. The unit obtains the sensitivity distribution of the first receiving coil by using the sensitivity image for each element receiving coil and the sensitivity image of the second receiving coil, and uses the sensitivity distribution to obtain the image of the first receiving coil. When correcting the sensitivity, the measurement control section The sensitivity images of the two receiving coils are acquired under substantially the same imaging conditions.

  The sensitivity correction method of the present invention includes a sensitivity image acquisition step of acquiring a sensitivity image for each element reception coil and a sensitivity image of the second reception coil under substantially the same imaging conditions, and a sensitivity image for each element reception coil. A sensitivity distribution obtaining step for obtaining a sensitivity distribution of the first receiving coil using the sensitivity image of the second receiving coil, and a sensitivity correcting step for correcting the sensitivity of the image of the first receiving coil using the sensitivity distribution; It is characterized by having.

  According to the MRI apparatus and the sensitivity correction supplement method of the present invention, it becomes possible to remove the sensitivity correction failure due to the artifact due to the displacement of the subject and the difference in the imaging conditions. As a result, unevenness in sensitivity of the image is reduced, and a composite image with a high signal-to-noise ratio can be acquired.

1 is a schematic overall configuration diagram of a magnetic resonance imaging apparatus to which the present invention is applied. The figure which shows a part of signal detection part of the receiving system to which this invention is applied. The figure which shows the example of arrangement | positioning of a body coil and a multiple RF coil. The figure which shows the sequence of a gradient echo method. The figure which shows the sequence of a fast spin echo method. FIG. 6 is a diagram showing a flow of sensitivity correction processing according to the present invention corresponding to claim 1; The figure which shows the measurement space of a parallel imaging method. The figure which shows the phantom image which reconfigure | reconstructed the aliasing data and sensitivity data of a parallel imaging method. FIG. 5 is a diagram showing a flow of sensitivity correction processing according to the present invention corresponding to claim 2.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses an NMR phenomenon to obtain an image of a subject 101, and as shown in FIG. 1, a static magnetic field generating magnet 102, a gradient magnetic field coil 103, a gradient magnetic field power source 109, a transmission coil 104, An RF transmission unit 110, a reception coil 105 and a signal detection unit 106, a signal processing unit 107, a measurement control unit 111, an overall control unit 108, a display / operation unit 113, and a subject 101 are mounted on the subject. And a bed 112 for taking the specimen 101 into and out of the static magnetic field generating magnet 102.

  The static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method and in the body axis direction in the horizontal magnetic field method. A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.

  The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z, which are the coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field power source 109 that drives it. To be supplied with current. Specifically, the gradient magnetic field power source 109 of each gradient magnetic field coil is driven according to a command from a measurement control unit 111 described later, and supplies a current to each gradient magnetic field coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three axial directions of X, Y, and Z. At the time of imaging, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding gradient magnetic field pulse (Gp) and a frequency encoding gradient magnetic field pulse (Gf) are applied in the direction, and position information in each direction is encoded in the echo signal.

  The transmission coil 104 is a coil that irradiates the subject 101 with a high-frequency magnetic field pulse (hereinafter referred to as an RF pulse), and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, nuclear magnetic resonance is induced in the nuclear spins of the atoms constituting the living tissue of the subject 101. Specifically, the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111 described later, amplitude-modulates a high-frequency pulse, and amplifies and supplies the high-frequency pulse to the transmission coil 104 disposed in the vicinity of the subject 101 By doing so, the subject 101 is irradiated with the RF pulse. In the present invention, the transmission coil 104 is provided with a body coil that combines transmission of RF pulses and reception of NMR signals. This body coil is built in the MRI apparatus main body.

  The reception coil 105 is a coil that receives an NMR signal (echo signal) emitted by the NMR phenomenon of the nuclear spin constituting the biological tissue of the subject 101, and is connected to the signal detection unit 106 to receive the received echo signal. The data is sent to the detection unit 106. The signal detection unit 106 performs processing for detecting an echo signal received by the reception coil 105. Specifically, an echo signal of the response of the subject 101 induced by the RF pulse irradiated from the transmission coil 104 is received by the receiving coil 105 disposed in the vicinity of the subject 101, and a measurement control unit 111 described later. Signal detector 106 amplifies the received echo signal, divides the signal into two orthogonal signals by quadrature detection, samples each of the signals by a predetermined number (e.g., 128, 256, 512, etc.) The data is converted into a digital quantity by / D conversion and sent to a signal processing unit 107 described later. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data. In the present invention, the reception coil 105 uses a reception coil called a “multiple reception coil” or a “phased array coil” formed by combining a plurality of element reception coils. The multiple receiving coil is a receiving-only coil that expands the imaging field of view while maintaining the high sensitivity of the relatively high-sensitivity element receiving coil and increases the sensitivity over a wide range.

  The measurement control unit 111 mainly transmits various commands for data collection necessary for the reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106. It is a control part which controls these. Specifically, the measurement control unit 111 operates under the control of the overall control unit 108 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 based on a predetermined pulse sequence. The application of the RF pulse and the gradient magnetic field pulse to the subject 101 and the detection of the echo signal from the subject 101 are repeatedly executed to collect echo data necessary for reconstruction of the image of the subject 101.

  The overall control unit 108 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit having a CPU and a memory, an optical disk, a magnetic disk, etc. And a storage unit. Specifically, the measurement control unit 111 is controlled to execute echo data collection, and when echo data is input from the signal processing unit 107, the arithmetic processing unit performs signal processing, image reconstruction by Fourier transform, and the like. The processing is executed, and the resulting image of the subject 101 is displayed on the display / operation unit 108 described later and recorded in the storage unit. In particular, the arithmetic processing unit performs image synthesis and sensitivity correction according to the present invention.

  The display / operation unit 113 is a display that displays a tomographic image of the subject 101, and an operation unit such as a trackball or a mouse and a keyboard that inputs various control information of the MRI apparatus and control information of processing performed by the overall control unit And consist of This operation unit is arranged close to the display, and the operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display.

  In FIG. 1, the transmission coil 104 and the gradient magnetic field coil 103 on the transmission side face the subject 101 in the static magnetic field space of the static magnetic field generating magnet 102 into which the subject 101 is inserted, in the case of the vertical magnetic field method. If the horizontal magnetic field method is used, the subject 101 is installed so as to surround it. The receiving coil 105 on the receiving side is installed so as to face or surround the subject 101.

  The radionuclide to be imaged by the current MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as a clinically popular one. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

(Details of receiving coil and signal detector)
Next, details of the receiving coil and the signal detector according to the present invention will be described with reference to FIG. FIG. 2 shows a part of the signal detectors of the body coil and the multiple receiving coil. In the example shown in FIG. 2, the multiple receiving coil 203 is configured by connecting four element receiving coils 2011 to 2014 to preamplifiers 2021 to 2024, respectively. The body coil 209 is configured by connecting one transmission / reception coil 204 to a switch circuit 205 and a preamplifier 208. Since the body coil 209 also functions as an RF pulse irradiation function, the switch circuit 205 switches between functioning as a transmission unit and functioning as a reception unit. When body coil 209 functions as a transmission unit, transmission / reception coil 204 is electrically connected to RF transmission unit 110 including modulator 207 and high-frequency amplifier 208 by switch circuit 205. When body coil 209 functions as a receiving unit, transmission / reception coil 204 is electrically connected to preamplifier 206 by switch circuit 205.

  The reception signal selector 210 has a function of selectively connecting the element reception coil that has received the echo signal to the signal detection unit 106. The reception signal selector 210 corresponds to high-speed switching between the body coil 209 and the multiple reception coil 203.

  The signal detection unit 106 includes four A / D conversion / quadrature detection circuits 2121 to 2124 arranged in parallel. The echo data detected by the A / D conversion / orthogonal detection circuits 2121 to 2124 is sent to the signal processing unit 107. The signal processing unit 107 performs Fourier transform, filtering, synthesis operation, and the like. These various processes performed by the signal processing unit 107 are preliminarily incorporated in the storage unit as programs, and the CPU reads them into the memory and executes them as necessary.

An arrangement example of the body coil 209 and the multiple reception coil 203 is shown in FIG. FIG. 3 is a schematic diagram of a cross section perpendicular to the body axis direction of the subject.
The surface coils 2011 to 2014, which are each element receiving coil, constituting the multiple receiving coil 203, are arranged around the Z axis, for example, around the cross section including the region of interest of the subject 101. Further, a transmission / reception coil 204 of the body coil 209 is disposed outside each of the surface coils 2011 to 2014. The subject 101 is irradiated with a high-frequency pulse by the transmission / reception coil 204. When the echo signal from the subject 101 is received by the transmission / reception coil 204, the signal from each part of the cross section including the region of interest can be received uniformly although it is a low signal. On the other hand, when receiving by the multiple receiving coil 203, although it is spatially non-uniform in sensitivity, each surface coil 2011-2014 is configured to be able to receive an echo signal from a cross section including the region of interest as a high signal. .

Next, an imaging method using the MRI apparatus having the above configuration will be described.
FIG. 4 is a diagram showing an example of a pulse sequence of the gradient echo (GrE) method. RF, Gs, Gp, Gr, and Echo represent an RF pulse, a slice gradient magnetic field, a phase encoding gradient magnetic field, a read gradient magnetic field, and an echo signal, respectively. In the gradient echo method, the measurement control unit 111 applies an RF pulse 401 together with the slice gradient magnetic field pulse 402, and excites nuclear spins in a specific region of the subject to generate transverse magnetization. Thereafter, the phase encoding gradient magnetic field pulse 403 is applied, and then the readout gradient magnetic field pulse 404 is applied, and the echo signal 405 is measured. The time from the application of the RF pulse 401 to the measurement of the echo signal 405 (echo time) TE, and the interval (repetition time) TR of the RF pulse 401 are parameters that determine the tissue contrast in the image. It is set in advance in consideration.

  FIG. 5 is a diagram showing an example of a pulse sequence of the fast spin echo (FSE) method. The meaning of each symbol is the same as in FIG. In this fast spin echo method, the measurement control unit 111 applies a 90 ° RF pulse 501 together with the slice gradient magnetic field pulse 5031 to excite nuclear spins in a specific region of the subject to generate transverse magnetization. Thereafter, 180 ° RF pulses 5021 to 5023 are applied together with slice gradient magnetic field pulses 5032 to 5034 in order to refocus the phase dispersion of the transverse magnetization due to non-uniformity of the static magnetic field or the like. Further, phase encoding gradient magnetic field pulses 5041 to 5043 are applied, and then readout gradient magnetic field pulses 5051 to 5053 are applied to measure echo signals 5061 to 5063 (the second time is 5064 to 5066).

The gradient echo method generates transverse magnetization by applying a single RF pulse 401 and measures one echo signal, while the fast spin echo method uses one 90 ° RF pulse 501 to generate transverse magnetization. Upon occurrence, a plurality of echo signals are measured. The time from the application of the 90 ° RF pulse 501 to the acquisition of the echo signal 5062 closest to the low frequency region of the K space (effective echo time) TE _eff , and the interval (repetition time) TR of the 90 ° RF pulse 501 are: This parameter determines the tissue contrast in the image, and is set in advance in consideration of the target tissue and the like.

(First embodiment)
Next, a first embodiment of the MRI apparatus and sensitivity correction method of the present invention will be described. In the present embodiment, sensitivity correction of multiple receiving coils is performed using the main measurement data measured in the main measurement. Preferably, the echo signal is measured by switching between the body coil and the multiple receiving coil, and the imaging conditions of the body coil and the multiple receiving coil are made the same as much as possible. Hereinafter, the present embodiment will be described in detail based on FIG.

  In this embodiment, sensitivity measurement is not performed before the main measurement, but sensitivity measurement is also performed in the main measurement. Alternatively, this measurement also serves as sensitivity measurement. Therefore, in this embodiment, the conventional prior sensitivity measurement is not performed, and only the main measurement is performed. Here, the main measurement is a measurement performed under an imaging condition in which contrast is conspicuous for the purpose of acquiring a diagnostic image, instead of using an imaging condition in which contrast is difficult to obtain for the purpose of acquiring a sensitivity image. The imaging conditions of the sensitivity images of the body coil and the multiple receiving coil are made the same as much as possible so that there is no difference other than the system adjustment value. The system adjustment values are values for optimizing the system such as transmission gain, reception gain, resonance frequency, etc., and acquiring a good image. The system adjustment value needs to be optimized for each of the body coil and the multiple receiving coil. Normally, tuning measurement is performed prior to this measurement, and the system adjustment value is based on the tuning measurement result. Is set. Imaging conditions for the same sensitivity measurement include RF pulse excitation angle (flip angle), repetition time TR and echo time TE, slice position, slice thickness, imaging field of view FOV, and the like.

  Further, in the present embodiment, in the main measurement, the main measurement image and the sensitivity distribution image of the multiple reception coil are acquired substantially simultaneously by switching the body coil and the multiple reception coil under the control of the measurement control unit 111. To do. The arithmetic processing unit corrects the sensitivity of the main measurement image acquired by the multiple receiving coil with the sensitivity distribution image of the multiple receiving coil. As a result, it is possible to remove artifacts due to the displacement of the subject and poor sensitivity correction due to differences in imaging conditions. In addition, it is not necessary to acquire a three-dimensional sensitivity image according to the prior art.

  Various methods can be considered for switching between the body coil and the multiple receiving coil. As the switching timing is made faster and the switching cycle is shortened, the positional deviation of the subject is reduced between the main measurement data using the body coil and the main measurement data using the multiple receiving coil. Furthermore, images having exactly the same tissue contrast are acquired under the imaging conditions for both measurement. In other words, the measurement control unit 111 switches the receiving coil at the timing of maintaining substantially the same tissue contrast using the same imaging condition, and controls imaging with both coils, which is a problem of the prior art. Then, artifacts and sensitivity correction defects due to differences in imaging conditions are removed.

  As a specific example of the switching method of both receiving coils, in the gradient echo pulse sequence used in the example of FIG. 4, it is optimal to switch the receiving coil for each TR, which is one of the parameters for determining the tissue contrast. That is, the measurement control unit 111 controls the reception signal selector 210 in FIG. 2 for each TR to switch both reception coils and alternately measure the respective echo signals. However, the present invention is not limited to this, every time the excitation slice changes, every time the magnetic field strength of the phase encoding gradient magnetic field changes, every time the integration for improving the signal-to-noise ratio is repeated for the number of integrations, or a combination of these It is also possible to switch the receiving coil at the timing.

  In addition, in the example of the pulse sequence of the fast spin echo method shown in FIG. 5, in the odd echoes such as the first echo 5061 and the third echo 5063 other than the pattern of switching the reception coil every TR, Measure the signal, measure the echo signal with the body coil in the even echo such as the second echo 5062, etc., measure the echo signal with the body coil at the odd repetition echo at the next repetition, and receive multiple signals at the even echo It is also possible to measure echo signals with a coil and repeat these alternately. Alternatively, the receiving coil may have a reverse pattern. That is, for each measurement of the echo signal, the measurement control unit 111 controls the reception signal selector 210 in FIG. 2 to switch both reception coils to control the measurement of each echo signal.

(Sensitivity correction processing flow)
Next, a processing flow of sensitivity correction processing according to the present embodiment will be described with reference to FIG. The ellipse in FIG. 6 represents processing, and the rectangle represents processing results and data. This processing flow is stored in advance in a storage unit such as a magnetic disk as a program, and is executed by the CPU reading it into the memory and executing it as necessary. Hereinafter, each step will be described in detail.

  First, the measurement control unit 111 performs main measurement data (K space data) 6011 to 6013 using multiple reception coils and a main coil using body coils under substantially the same imaging conditions while switching the reception coils in the main measurement. Measurement data (K space data) 612 is measured. For example, as described above, when a pulse sequence of the gradient echo method is used, the measurement control unit 111 controls the measurement of each main measurement data by switching the reception coil for each TR. Or when using the pulse sequence of a fast spin echo method, a receiving coil is switched for every echo signal, and measurement of each main measurement data is controlled.

  Next, the arithmetic processing unit acquires main measurement images 6031 to 6033 by performing Fourier transform 602 on the main measurement data 6011 to 6013. Further, by combining these images 604, a post-synthesis main measurement image 605 is obtained.

  On the other hand, the arithmetic processing unit performs the following processing to acquire the sensitivity distribution image 618 using the data in the low frequency region of the main measurement data (K space data) measured by the body coil and the multiple receiving coil. Thereby, by using the low frequency region data, it is possible to acquire a smoothed sensitivity distribution image in which the contrast between tissues is suppressed.

  First, the arithmetic processing unit performs processing 606 for extracting low-frequency region data from the main measurement data 6011 to 6013, and acquires sensitivity image data 6071 to 6073. Next, sensitivity images 6091 to 6093 are obtained by performing Fourier transform 608 on the sensitivity image data 6071 to 6073. Next, the sensitivity images 6091 to 6093 are subjected to an image composition process 610 to obtain a composited sensitivity image 611.

  In addition, the arithmetic processing unit performs processing 613 for extracting low-frequency region data from the main measurement data 612 of the body coil, and acquires reference image data 614. Next, a reference image 616 is obtained by performing Fourier transform 615 on the reference image data 614.

  The post-combination sensitivity image 611 represents the sensitivity distribution of the multiple receiving coils determined by the positional relationship and sensitivity of each coil of the multiple receiving coils. Reference image 616 represents the sensitivity distribution of the body coil. The arithmetic processing unit obtains the sensitivity distribution image 618 of the multiple receiving coil by performing sensitivity distribution calculation processing 617 using these sensitivity distributions. In the sensitivity distribution calculation process 617, the sensitivity distribution image 618 of the multiple receiving coil is obtained by the following equation.

C ＝ S _MAC / S _TR (1)
Here, C represents a sensitivity distribution image 618 of the multiple receiving coil, S _MAC represents a post-synthesis sensitivity image 611, and S _TR represents a reference image. The sensitivity distribution calculation process 617 includes a smoothing process, a process for removing background noise, a smoothing process for smoothing the boundary between the tissue and the background, and a post-synthesis main measurement image in addition to the calculation of the expression (1). An enlargement process for making the image size the same as that of 605 is included, and the arithmetic processing unit performs these processes together with the process of equation (1) to obtain the sensitivity distribution image 618.

  Finally, the arithmetic processing unit obtains the post-sensitivity-corrected main measurement image 620 by performing sensitivity correction processing 619 on the post-synthesis main-measurement image 605 using the sensitivity distribution image 618. In the sensitivity correction processing 619, the main measurement image 620 after sensitivity correction is obtained by the following equation.

I _FINAL = I _MAC / C (2)
Here, I _FINAL is a main measurement image 620 after sensitivity correction, I _MAC is a main measurement image 605 after synthesis, and C is a sensitivity distribution image 618.
The above is the outline of the processing flow of the present embodiment.

  As described above, according to the MRI apparatus and the sensitivity correction method of the present embodiment, substantially the same with respect to the post-synthesis main measurement image obtained by the multiple receiving coils and having a high signal-to-noise ratio but uneven sensitivity. Sensitivity correction is performed using a post-synthesis sensitivity image having a tissue contrast and reduced positional displacement of the subject, so that a synthesized image with a high signal-to-noise ratio and no sensitivity unevenness can be acquired. That is, it is possible to perform sensitivity correction by removing artifacts based on the positional deviation of the subject in the prior art and sensitivity correction failures due to differences in imaging conditions between sensitivity measurement and main measurement.

(Second embodiment)
Next, a second embodiment of the MRI apparatus and sensitivity correction method of the present invention will be described. In the present embodiment , the parallel imaging method is used in combination, and in the measurement using at least one of the measurement using the multiple receiving coil and the measurement using the body coil, the measurement of a part of the K space data is thinned out. Reduce measurement time. Hereinafter, parts different from the first embodiment will be described with reference to FIG. 9, and description of the same parts will be omitted.

  First, the parallel imaging method will be briefly described. The parallel imaging method is a method of shortening the imaging time by thinning out the data measurement in the phase encoding direction using multiple receiving coils. Specifically, by using the fact that the sensitivity distribution of multiple receiving coils is spatially different from each other, using K-space data in which data measurement in the phase encoding direction is thinned, aliasing occurs on the image. A calculation based on the parallel imaging method is performed using the sensitivity distribution information of each element receiving coil constituting the multiple receiving coil, and one image without aliasing is acquired as an image of the multiple receiving coil. Moreover, the maximum ratio which thins out phase encoding can be increased, so that the number of the element receiving coils which comprise a multiple receiving coil increases. For example, when the receiving coil is N, the phase encoding can be reduced to 1 / N at the maximum. In this way, the phase encoding is thinned out at a constant rate to reduce the number of repetitions of the phase encoding, and the imaging time can be shortened.

  The parallel imaging methods include the SENSE method and SMASH method described below with reference to FIGS. The SENSE method is a method in which a folded image for each element receiving coil is developed and synthesized using the sensitivity distribution information for each element receiving coil in the image space. The SMASH method is not measured in the K space. In this method, data is obtained by interpolation using sensitivity distribution information of element receiving coils. The present embodiment is equally applicable to any method. The parallel imaging method and the present embodiment will be described below based on the SENSE method.

  Since the present invention does not perform sensitivity measurement before the actual measurement, when applying the parallel imaging method in the present embodiment, it is necessary to acquire the sensitivity distribution of each element reception coil constituting the multiple reception coil in the present measurement. . Therefore, the K space data of the body coil and the multiple receiving coil defined by the phase encode gradient magnetic field and the read gradient magnetic field is thinned out in the high frequency region and densely filled in the low frequency region. A technique of thinning out only such a high frequency region and simultaneously acquiring a folded image and a sensitivity distribution image is described in Patent Document 2, and this embodiment uses this technique.

  FIG. 7 is a diagram showing the K space when measured by the parallel imaging method. The white portion of the data indicates the phase encoding to be measured, and the gray portion indicates the phase encoding that is not measured. The phase encoding step is different between the region a702 and the region b703 of the K space 701. In the region a702 occupying the low frequency region in the phase encoding direction, echo signals are densely measured in the phase encoding direction (ky), and in the region b703 occupying the high frequency region, echo signals are sparsely measured in the phase encoding direction. The acquired K space data is divided into folded image data 704 and sensed image data 705. In the area a702, the phase encoding used only for the sensitivity image data 705 and the phase encoding used together with the sensitivity image data 705 and the folded image data 704 are alternately repeated.

  FIG. 8 shows a folded image 803 reconstructed from the folded image data 704 obtained by the parallel imaging method, and a sensitivity image 804 reconstructed from the sensitivity image data 705. The folded image 803 shown in FIG. 8 is an image in which folding is generated by reducing the matrix and reducing the FOV in the phase encoding direction by the amount of phase encoding thinned out. Such a folded image is acquired for each element receiving coil. On the other hand, the sensitivity image 804 is a blurred image whose resolution is lowered by the smaller number of phase encodes, but the FOV in the phase encode direction is the same size without being reduced. The folded image 803 for each element receiving coil is subjected to folding removal and image synthesis using the sensitivity distribution image for each element receiving coil obtained using the sensitivity image 804 for each element receiving coil.

  As described above, by acquiring the folded image data 704 and the sensitivity image data 705 from the main measurement data, it is possible to shorten the imaging time and realize a parallel imaging method with excellent real-time characteristics.

  Next, a processing flow of sensitivity correction processing according to the present embodiment will be described with reference to FIG. FIG. 9 is a diagram illustrating a processing flow of sensitivity correction processing according to the present embodiment, to which the parallel imaging method is applied. This processing flow is stored in advance in a storage unit such as a magnetic disk as a program, and is executed by the CPU reading it into the memory and executing it as necessary. Hereinafter, each step will be described in detail.

  First, the measurement control unit 111 controls measurement of main measurement data (K space data) 9011 to 9013 using multiple receiving coils and main measurement data (K space data) 910 using a body coil in the main measurement. . Here, the main measurement data 9011 to 9013 using the multiple receiving coils and the main measurement data 910 using the body coil are thinned out in the high frequency region in the phase encoding direction of the K space as shown in FIG. Thus, the phase encode data is acquired densely in the low frequency region.

  Next, the arithmetic processing unit acquires folded image data 9031 to 9033 by performing folded image data extraction processing 902 from the main measurement data 9011 to 9013. Further, these are subjected to Fourier transform 904 to obtain folded images 9051 to 9053.

  On the other hand, the arithmetic processing unit acquires sensitivity distribution images 9161 to 9163 and sensitivity distribution images 923 for each element reception coil using the data of the low frequency region in the acquired main measurement data (K space data). The following processing is performed. Thereby, by using the low frequency region data, it is possible to acquire a smoothed sensitivity distribution image in which the contrast between tissues is suppressed.

  First, the arithmetic processing unit performs processing 906 for extracting low frequency region data from the main measurement data 9011 to 9013, respectively, and obtains sensitivity data 9071 to 9073. Next, sensitivity images 9091 to 9093 are acquired by performing Fourier transform 908 on the sensitivity data 9071 to 9073. Next, the sensitivity images 9091 to 9093 are subjected to the image composition processing 920 to obtain a combined sensitivity image 921.

  In addition, the arithmetic processing unit performs processing 911 for extracting low-frequency region data from the body coil main measurement data 910 to obtain reference image data 912. Next, the reference image 914 is acquired by performing Fourier transform 913 on the reference image data 912.

  Next, the arithmetic processing unit performs sensitivity distribution calculation processing 915 using the sensitivity images 9091 to 9093 and the reference image 914, thereby acquiring sensitivity distribution images 9161 to 9163 for each element receiving coil. In the sensitivity distribution calculation process 915, sensitivity distribution images 9161 to 9163 for each element receiving coil are obtained by the following equations.

C _CH = S _CH / S _TR (3)
Here, C _CH represents sensitivity distribution images 9161 to 9163 for each element receiving coil, S _CH represents sensitivity images 9091 to 9093 for each element receiving coil, and S _TR represents a reference image 914. In addition to the calculation of equation (1), the sensitivity distribution calculation process 915 includes a smoothing process, a process for removing background noise, a smoothing process for smoothing the boundary between the tissue and the background, a main measurement image 919 and An enlargement process for making the image sizes the same is included, and the arithmetic processing unit performs these processes together with the process of equation (1) to obtain sensitivity distribution images 9161 to 9163.

  Next, the arithmetic processing unit uses the sensitivity distribution images 9161 to 9163 and the aliased images 9051 to 9053 for each element receiving coil to perform matrix creation processing 917 and inverse matrix calculation that realize aliasing removal and image synthesis of the parallel imaging method. By performing the process 918, a post-combination main measurement image 919 is acquired.

  Next, the arithmetic processing unit obtains a sensitivity distribution image 923 by performing sensitivity distribution calculation processing 922 using the combined sensitivity image 921 and the reference image 914. The process here is the same as the sensitivity distribution calculation 617 process, and the expression (1) is used to calculate the sensitivity distribution image 923.

Finally, a sensitivity-corrected main measurement image 925 is obtained by performing sensitivity correction processing 924 on the post-synthesis main measurement image 919 obtained by the parallel imaging method, using the sensitivity distribution image 923. In the sensitivity correction 924, the main measurement image 925 after sensitivity correction is acquired using the calculation formula shown by the formula (2).
The above is the outline of the processing flow of the present embodiment.

  In the description of the present embodiment, the measurement of the main measurement data of the body coil is also described in the case of thinning in the high frequency region in the phase encoding direction of the K space and densely filling in the low frequency region. If the imaging conditions are such that the sensitivity image of each element receiving coil of the coil and the reference image of the body coil have the same contrast, the data measurement of the high frequency region is omitted when measuring the main measurement data of the body coil. Only measurement may be performed.

  As described above, according to the MRI apparatus and the sensitivity correction method of the present embodiment, by using the parallel imaging method together, the increase in time for measuring the sensitivity image is suppressed as much as possible, and the signal-to-noise ratio is high. A composite image with no sensitivity unevenness can be acquired.

(Third embodiment)
Next, a third embodiment of the MRI apparatus and sensitivity correction method of the present invention will be described. In the present embodiment, the data amount measured by the body coil is made smaller than the data amount measured by the multiple receiving coil. Thereby, extension of measurement time is suppressed. Hereinafter, only portions different from the above-described embodiments will be described based on FIGS. 6 and 9, and description of the same portions will be omitted.

There are a plurality of methods for reducing the amount of data measured by the body coil to be less than the amount of data measured by the multiple receiving coil, and any of them may be adopted.
As a first example, the number of times of main measurement in the body coil is made smaller than the number of times of main measurement in the multiple receiving coils. For example, when the number of integrations is set to 2 or more, data collection for the number of integrations is required for the main measurement with multiple receiving coils, but only data collection for the number of integrations is required for the main measurement with the body coil. It is possible to suppress the extension of the measurement time by performing such measures.

Further, as a second example, in order to acquire the sensitivity distribution image 618 or the sensitivity distribution image 923 as the main measurement data 612 in FIG. 6 or the main measurement data 910 in FIG. 9 acquired in the main measurement with the body coil, respectively. Collect only a sufficiently low frequency region. On the other hand, the main measurement data 6011 to 6013 in FIG. 6 or the main measurement data 9011 to 9013 in FIG. 9 acquired by the main measurement with the multiple receiving coils includes data in the high frequency region as well as the low frequency region. Then, the arithmetic processing unit acquires each image so that the matrix size of the reference image of the body coil is the same as the matrix size of the sensitivity image of the element coil. When the matrix sizes are different, the arithmetic processing unit obtains the sensitivity distribution image 618 or 923 after enlarging or reducing the matrix size of one of the images so as to match the matrix size of the other image. .

  This second method is effective for a pulse sequence such as the gradient echo method shown in FIG. 4 in which one echo signal is acquired at one repetition time TR. This is because only one echo signal corresponding to one phase encoding in the K space is measured in one repetition time TR, so that the main measurement data 6011 to 6013 using a multiple receiving coil and the main measurement data 612 using a body coil are used. This is because it can be easily controlled to contain the same tissue contrast. In other words, it is easy to measure an echo signal using the same repetition time TR and the same echo time TE at each phase encoding position in the K space of each reception coil.

  On the other hand, in a pulse sequence such as the fast spin echo method shown in FIG. 5 in which a plurality of echo signals are acquired in one repetition time TR, the main measurement data 6011 to 6013 using a multiple receiving coil and a body coil are used. In order for the main measurement data 612 to contain exactly the same tissue contrast, the following control is necessary. First, in the case of main measurement data 6011 to 6013 using multiple receiving coils, the K space data acquired in one repetition time TR is all data belonging to the low frequency region or all data belonging to the high frequency region. . In the case of the main measurement data 612 using the body coil, all the K space data acquired in one repetition time TR is data belonging to the low frequency region.

  However, as a result of such measurement control, in the K space data of each coil, many adjacent data are acquired with different TEs. Since transverse magnetization has a relaxation phenomenon called T2 attenuation, a large difference occurs in signal values when acquired with different TEs. This difference in signal value appears as a step in the K space. This step appears as an artifact in an image obtained by Fourier transforming the main measurement data. For this reason, in order to remove the artifacts caused by T2 attenuation, a further advanced correction technique is required. For example, the main measurement data is multiplied by an appropriate weighting factor in accordance with the phase encoding in the phase encoding direction so that the level difference is not caused by aligning the signal intensity of the main measurement data using multiple receiving coils. Can do.

  A third example is to use the first method or the second method in combination when the parallel imaging method described in the second embodiment is used. In particular, when the second method is used in combination, the measurement data using the body coil is measured only in the low frequency region of the K space, and the amount of data measured by the body coil is measured by multiple receiving coils. The amount of data to be reduced can be reduced. The details of using the parallel imaging method have been described in detail in the description of the second embodiment, and are omitted here.

  By using any of the above examples, the amount of data acquired by the main measurement with the body coil can be made smaller than the amount of data acquired by the main measurement with the multiple receiving coil.

  As described above, according to the MRI apparatus and the sensitivity correction method of the present embodiment, by reducing the amount of data measured in the main measurement with the body coil than the amount of data measured in the main measurement with the multiple receiving coil, By extending the measurement time as much as possible, it is possible to acquire an image with a high signal-to-noise ratio and no sensitivity unevenness.

  The above is description of each embodiment of the MRI apparatus and sensitivity correction method of this invention. However, the MRI apparatus and the sensitivity correction method of the present invention are not limited to the contents disclosed in the description of the above-described embodiment, and may take other forms based on the gist of the present invention.

  For example, in the description of each of the above-described embodiments, the example in which the body coil built in the MRI apparatus main body is used as the receiving coil with uniform sensitivity has been described. However, the essence of the present invention uses the body coil. The present invention is not limited to this, and if there is another receiving coil with uniform sensitivity, the same effect as described in the present invention can be obtained by using it. For example, if there is no body coil built into the MRI main unit, use multiple coil coils and a coil channel that can capture images without relatively uneven sensitivity in the multiple receiving coil as a coil with uniform sensitivity. The obtained sensitivity correction can be realized.

  101 subject, 102 static magnetic field generating magnet, 103 gradient magnetic field coil, 104 transmission coil, 105 reception coil, 106 signal detection unit, 107 signal processing unit, 108 overall control unit, 109 gradient magnetic field power supply, 110 RF transmission unit, 111 measurement Control unit, 112 beds, 203 multiple RF coils, 2011-2014 RF receiver coils that make up multiple RF coils, 204 RF transceiver coils that make up body coils, 208 high-frequency amplifiers, 209 body coils, 211 signal detectors, 401 RF pulses , 402 slice encoding gradient magnetic field pulse, 403 phase encoding gradient magnetic field pulse, 404 readout gradient magnetic field pulse, 405 echo signal, 501 90 ° RF pulse, 5021-5023 180 ° RF pulse, 5031-5034 slice encoding gradient magnetic field pulse, 5041- 5043 Phase encoding gradient magnetic field pulse, 5051-5053 Reading gradient magnetic field pulse, 5061-5066 Echo signal, 701 K space

Claims (11)

A first receiving coil comprising a combination of a plurality of element receiving coils and a second coil having a sensitivity range wider than that of the first receiving coil, and receiving a nuclear magnetic resonance signal from a subject. A receiving unit to
Based on a predetermined pulse sequence, using the receiving unit, a measurement control unit that controls measurement of a nuclear magnetic resonance signal from the subject; and
An arithmetic processing unit that acquires an image of the subject using the nuclear magnetic resonance signal;
With
The measurement control unit controls acquisition of a sensitivity image for each element receiving coil and a sensitivity image of the second receiving coil,
The arithmetic processing unit obtains a sensitivity distribution of the first receiving coil using a sensitivity image for each element receiving coil and a sensitivity image of the second receiving coil, and uses the sensitivity distribution to A magnetic resonance imaging apparatus for correcting sensitivity of an image of one receiving coil,
The magnetic resonance imaging apparatus, wherein the measurement control unit obtains a sensitivity image for each element reception coil and a sensitivity image of the second reception coil under substantially the same imaging conditions. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the measurement control unit captures a sensitivity image of the second receiving coil while capturing an image of the first receiving coil. The magnetic resonance imaging apparatus according to claim 2.
The measurement control unit measures a nuclear magnetic resonance signal using the first receiving coil and measures a nuclear magnetic resonance signal using the second receiving coil at each repetition time (TR) of the pulse sequence. And a magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 2.
The measurement control unit performs the first control at least one time each time the excitation slice changes, the magnetic field strength of the phase encoding gradient magnetic field changes, and the integration for improving the signal-to-noise ratio is repeated for the number of integrations. A magnetic resonance imaging apparatus that switches between measurement of a nuclear magnetic resonance signal using a reception coil and measurement of a nuclear magnetic resonance signal using the second reception coil. The magnetic resonance imaging apparatus according to claim 1.
The measurement control unit makes at least the excitation angle of the RF pulse, the repetition time (TR) of the pulse sequence, and the echo time (TE) the same, and the sensitivity image for each element reception coil and the second A magnetic resonance imaging apparatus that controls imaging of a sensitivity image of a receiving coil. The magnetic resonance imaging apparatus according to claim 1.
The arithmetic processing unit obtains a sensitivity distribution of the first receiving coil by using at least a part of an image nuclear magnetic resonance signal measured using the first receiving coil. Imaging device. The magnetic resonance imaging apparatus according to claim 1.
The measurement control unit controls the measurement using the first receiving coil so as to thin out the measurement of a part of the K space data,
The magnetic resonance imaging apparatus, wherein the arithmetic processing means performs an operation based on a parallel imaging method on the partially thinned K space data to obtain an image of the first receiving coil. The magnetic resonance imaging apparatus according to claim 1.
The measurement control unit controls the number of integrations in measurement using the second reception coil to be smaller than the number of integrations in measurement using the first reception coil. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the measurement control unit uses only the low frequency region of the K space as the K space data measured using the second receiving coil. A magnetic resonance imaging apparatus having a first receiving coil formed by combining a plurality of element receiving coils and a second coil having a wider sensitivity range than the first receiving coil operates, and the first receiving coil operates. A sensitivity correction method for correcting the sensitivity of an image of a receiving coil,
A sensitivity image acquisition step of acquiring a sensitivity image for each element reception coil and a sensitivity image of the second reception coil under substantially the same imaging conditions;
A sensitivity distribution acquisition step for obtaining a sensitivity distribution of the first receiving coil using a sensitivity image for each element receiving coil and a sensitivity image of the second receiving coil;
A sensitivity correction step of correcting the sensitivity of the image of the first receiving coil using the sensitivity distribution;
A sensitivity correction method characterized by comprising: The sensitivity correction method according to claim 10,
In the sensitivity image acquisition step, a sensitivity image of the second reception coil is captured while a sensitivity image of each element reception coil is captured.

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