JP7201360B2 - Magnetic resonance imaging device - Google Patents

Description

Embodiments of the present invention relate to magnetic resonance imaging apparatus.

A technology of a magnetic resonance imaging (hereinafter also referred to as MRI (Magnetic Resonance Imaging)) apparatus that performs shimming for a static magnetic field (hereinafter referred to as static magnetic field shimming) based on a static magnetic field distribution is known. For example, the MRI apparatus optimizes the pre-pulse center frequency and the RF pulse center frequency for each slice by static magnetic field shimming in multi-slice imaging. However, it is difficult to find the optimum center frequency in a short period of time. In addition, the image quality of the magnetic resonance image may not improve depending on the non-uniformity of the static magnetic field in each of a plurality of slices in multi-slice imaging.

JP 2010-35991 A JP 2014-140472 A

The problem to be solved by the present invention is to improve the image quality of magnetic resonance images.

A magnetic resonance imaging apparatus according to an embodiment includes a determination unit, an acquisition unit, a configuration unit, and an adjustment unit. The determination unit determines the cross-sectional position from the three-dimensional static magnetic field distribution. The collection unit collects the resonance frequency distribution at the cross-sectional position. The configuration unit associates the resonance frequency distribution with the three-dimensional static magnetic field distribution. The adjuster adjusts the center frequency in the subsequent imaging based on the associated three-dimensional static magnetic field distribution.

FIG. 1 is a diagram showing the configuration of a magnetic resonance imaging apparatus according to this embodiment. FIG. 2 is a flow chart showing an operation example of the magnetic resonance imaging apparatus according to this embodiment. FIG. 3 is a diagram showing an example of a shimming collection range according to this embodiment. FIG. 4 is a diagram showing an example of determined cross-sectional positions according to the present embodiment. FIG. 5 is a diagram showing a positioning image range, a shimming acquisition range, and a CFA (Center Frequency Adjustment) acquisition range according to the present embodiment. FIG. 6A is a diagram showing an example of a CFA collection range according to this embodiment. FIG. 6B is a diagram showing an example of a CFA collection range to be noted according to this embodiment. FIG. 7 is a diagram showing an example of resonance frequency distribution at determined cross-sectional positions according to the present embodiment. FIG. 8 is a diagram showing an example of a histogram of the static magnetic field distribution at determined cross-sectional positions according to the present embodiment.

A magnetic resonance imaging apparatus according to this embodiment will be described below with reference to the drawings. In the following embodiments, it is assumed that parts denoted by the same reference numerals perform the same operations, and overlapping descriptions will be omitted as appropriate.
FIG. 1 is a diagram showing the configuration of a magnetic resonance imaging apparatus 1 according to this embodiment. As shown in FIG. 1, the magnetic resonance imaging apparatus 1 includes a static magnetic field magnet 100, a shim coil 101, a shim coil power supply 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 105, a bed 107, and a bed control circuit 109. , a transmission circuit 113 , a transmission coil 115 , a reception coil 117 , a reception circuit 119 , an imaging control circuit 121 , an interface 125 , a display 127 , a storage device 129 , and a processing circuit 131 . The bed control circuit 109, imaging control circuit 121, interface 125, display 127, storage device 129, and processing circuit 131 are connected for data transmission regardless of whether they are wired or wireless. Note that the subject P is not included in the magnetic resonance imaging apparatus 1 .

The static magnetic field magnet 100 is a hollow, substantially cylindrical magnet. The static magnetic field magnet 100 generates a substantially uniform static magnetic field in its internal space. As the static magnetic field magnet 100, for example, a superconducting magnet or the like is used. As shown in FIG. 1, the Z-axis direction is assumed to be the same as the direction of the static magnetic field. The Y-axis direction is the vertical direction, and the X-axis direction is the direction perpendicular to the Z-axis and the Y-axis.

The shim coil 101 generates a correction magnetic field for correcting second-order or higher-order components of non-uniformity of the static magnetic field generated by the static magnetic field magnet 100 . The shim coil 101 is joined to the outer peripheral surface of the gradient magnetic field coil 103 via an insulating layer (not shown). In general, the inhomogeneity of the static magnetic field includes 0th order components, 1st order components X ¹ , Y ¹ , Z ¹ , and 2nd order components X ² , Y ² , Z ² , XY, YZ, ZX, etc. is divided into In addition, the non-uniformity of the static magnetic field also includes higher-order components such as third-order components. A multi-order component corresponds to a component of higher order than the second order component.

To simplify the explanation below, the higher-order components are assumed to be second-order components. At this time, the shim coil 101 has a secondary shim structure. ^At this ^time , the ^shim coils 101 ^{are 5} It has a structure of one coil pattern. The five coil patterns in the shim coil 101 correspond to the secondary components ZX, XY, YZ, (Z ² −(X ² +Y ² )/2) of the inhomogeneity of the static magnetic field according to the current supplied from the shim coil power supply 102. , (X ² −Y ² ) are generated respectively. When performing static magnetic field shimming that takes into account multiple-order components of non-uniformity of the static magnetic field, the shim coil 101 has a coil pattern corresponding to the multiple-order components. Static magnetic field shimming related to this embodiment will be described later.

The shim coil power supply 102 is a power supply device that supplies current to the shim coil 101 under the control of the imaging control circuit 121 . Specifically, the shim coil power supply 102 supplies current to each of the five coil patterns in the shim coil 101 independently. That is, the shim coil power supply 102 supplies currents corresponding to the secondary shimming values determined by the static magnetic field shimming according to this embodiment to each of the five coil patterns in the shim coil 101 .

The gradient magnetic field coil 103 is a hollow, substantially cylindrical coil. The gradient magnetic field coil 103 is arranged inside the shim coil 101 . The gradient magnetic field coil 103 is formed by combining three coils corresponding to the mutually orthogonal X, Y, and Z axes. The three coils in the gradient magnetic field coil 103 are individually supplied with current from the gradient magnetic field power source 105 to generate gradient magnetic fields whose magnetic field strengths vary along the X, Y, and Z axes.

The X-, Y-, and Z-axis gradient magnetic fields generated by the gradient magnetic field coil 103 form, for example, a frequency-encoding gradient magnetic field (also referred to as a readout gradient magnetic field), a phase-encoding gradient magnetic field, and a slice-selecting gradient magnetic field. A slice selection gradient magnetic field is used to arbitrarily determine an imaging section. A phase-encoding gradient magnetic field is used to change the phase of a magnetic resonance (hereinafter referred to as MR (Magnetic Resonance)) signal according to a spatial position. A frequency-encoding gradient magnetic field is used to vary the frequency of the MR signal according to spatial location. In addition, the X-, Y-, and Z-axis gradient magnetic fields generated by the gradient magnetic field coil 103 are used as offsets for the primary shimming of the static magnetic field.

The gradient magnetic field power supply 105 is a power supply device that supplies current to the gradient magnetic field coil 103 under the control of the imaging control circuit 121 . Specifically, the gradient magnetic field coil corresponding to the X-axis is supplied with a current from the gradient magnetic field power supply 105 to generate a correction magnetic field having ^a magnetic field direction substantially similar to the X1 component of the inhomogeneity of the static magnetic field and a frequency encoding magnetic field. to generate a gradient magnetic field for Also, the gradient magnetic field coil corresponding to the Y-axis is supplied with a current from the gradient magnetic field power source 105 to generate a correction magnetic field having ^a magnetic field direction substantially similar to the Y1 component of the inhomogeneity of the static magnetic field and a phase-encoding gradient magnetic field. and occur. The gradient magnetic field coil corresponding to the Z axis is supplied with ^a current from the gradient magnetic field power supply 105 to generate a correction magnetic field having a magnetic field direction substantially similar to the Z1 component of the inhomogeneity of the static magnetic field and a slice selection gradient magnetic field. Occur. That is, the three gradient magnetic field coils corresponding to the X, Y, and Z axes are used not only to generate gradient magnetic fields for imaging, but also to correct the first-order component of static magnetic field inhomogeneity.

The bed 107 is an apparatus having a top plate 1071 on which the subject P is placed. The bed 107 inserts the top plate 1071 on which the subject P is placed into the bore 111 under the control of the bed control circuit 109 . The bed 107 is installed in the examination room, for example, so that its longitudinal direction is parallel to the central axis of the static magnetic field magnet 100 .

A bed control circuit 109 is a circuit that controls the bed 107 . The bed control circuit 109 drives the bed 107 according to an operator's instruction via the interface 125 to move the table top 1071 in the longitudinal direction, the vertical direction, and in some cases the horizontal direction.

Under the control of the imaging control circuit 121 , the transmission circuit 113 supplies the transmission coil 115 with high-frequency pulses modulated at the Larmor frequency.

The transmission coil 115 is an RF coil arranged inside the gradient magnetic field coil 103 . The transmission coil 115 generates an RF (Radio Frequency) pulse corresponding to a high frequency magnetic field according to the output from the transmission circuit 113 . The transmission coil 115 is, for example, a whole-body coil (hereinafter referred to as a WB (Whole Body) coil) having a plurality of coil elements. A WB coil may be used as a transmit/receive coil. Also, the transmission coil 115 may be a WB coil formed by one coil.

The receiving coil 117 is an RF coil arranged inside the gradient magnetic field coil 103 . The receiving coil 117 receives MR signals emitted from the subject P by the high frequency magnetic field. Receiving coil 117 outputs the received MR signal to receiving circuit 119 . The receiving coil 117 is, for example, a coil array having one or more, typically a plurality of coil elements. It should be noted that although transmit coil 115 and receive coil 117 are depicted as separate RF coils in FIG. 1, transmit coil 115 and receive coil 117 may be implemented as an integrated transmit and receive coil. The transmission/reception coil corresponds to the imaging target region of the subject P, and is, for example, a local transmission/reception RF coil such as a head coil.

The receiving circuit 119 generates a digital MR signal (hereinafter referred to as MR data) based on the MR signal output from the receiving coil 117 under the control of the imaging control circuit 121 . Specifically, the receiving circuit 119 performs various signal processing on the MR signal output from the receiving coil 117, and then performs analog/digital (A/D) processing on the data subjected to the various signal processing. to Digital)) Perform conversion. The receiving circuit 119 samples the A/D converted data. Thereby, the receiving circuit 119 generates MR data. The receiving circuit 119 outputs the generated MR data to the imaging control circuit 121 .

The imaging control circuit 121 controls the shim coil power supply 102, the gradient magnetic field power supply 105, the transmission circuit 113, the reception circuit 119, etc. according to the imaging protocol output from the processing circuit 131, and performs imaging of the subject P. FIG. The imaging protocol has various pulse sequences according to imaging target sites and examination contents. The imaging protocol includes the magnitude of the current supplied to the gradient magnetic field coil 103 by the gradient magnetic field power supply 105, the timing at which the current is supplied to the gradient magnetic field coil 103 by the gradient magnetic field power supply 105, and the current supplied to the transmission coil 115 by the transmission circuit 113. The magnitude and duration of the high-frequency pulse, the timing at which the transmission circuit 113 supplies the high-frequency pulse to the transmission coil 115, the timing at which the reception coil 117 receives the MR signal, and the like are defined.

The interface 125 includes a switch button, a mouse, a keyboard, a touch pad for performing input operations by touching the operation surface, and a touch panel in which the display screen and the touch pad are integrated to receive various instructions and information input from the operator. It is realized by a screen, a non-contact input circuit using an optical sensor, an audio input circuit, and the like. The interface 125 is connected to the processing circuit 131 and the like, converts an input operation received from the operator into an electric signal, and outputs the electric signal to the processing circuit 131 . In this specification, the interface is not limited to those having physical operation parts such as a mouse and a keyboard. For example, the interface 125 also includes an electrical signal processing circuit that receives an electrical signal corresponding to an input operation from an external input device provided separately from the device and outputs the electrical signal to the control circuit.
The interface 125 inputs an MR signal acquisition range (hereinafter referred to as a first acquisition range) for shimming imaging, which will be described later, according to an operator's instruction. The interface 125 inputs an MR signal acquisition range (hereinafter referred to as a second acquisition range) for multi-slice imaging when generating an MR image, which will be described later, to a positioning image (Locator) according to an operator's instruction. . It is assumed that the second collection range at least partially overlaps with the first collection range. Note that the first acquisition range may be the same imaging area as the second acquisition range.

The display 127 displays various MR images generated by the image generation function 1313 under the control of the system control function 1311 in the processing circuit 131, various information on imaging and image processing, and the like. Display 127 is, for example, a display device such as a CRT display, liquid crystal display, organic EL display, LED display, plasma display, or any other display or monitor known in the art.

The storage device 129 stores MR data filled in the k-space via the image generation function 1313, image data generated by the image generation function 1313, various imaging protocols, imaging conditions including a plurality of imaging parameters defining the imaging protocols, and the like. memorize The storage device 129 stores programs corresponding to various functions executed by the processing circuit 131 . The storage device 129 stores a program (hereinafter referred to as a calculation program) for calculating a zero-order shimming value, a first-order shimming value, and a second-order shimming value by static magnetic field shimming according to this embodiment.

The 0th order shimming value corresponds to the resonant frequency of water in each of the multiple slices in the acquisition area for multi-slice acquisition. That is, the 0th order shimming value relates to the slice-by-slice correction in the acquisition region for the 0th order component of the static magnetic field inhomogeneity. The first order shimming values are applied to the three gradient coils from the gradient power supply 105 to correct the static magnetic field inhomogeneity X1, Y1, ^Z1 components in ^each of the multiple slices for ^a multislice acquisition. corresponds to the current value supplied to each. That is, the first-order shimming value relates to the slice-by-slice correction in the acquisition region for the first-order component of the static magnetic field inhomogeneity. The secondary shimming values are the ZX, XY, YZ, (Z ² −(X ² +Y ² )/2) components, and It corresponds to the current value supplied to each of the five coil patterns in the shim coil 101 from the shim coil power supply 102 to correct the (X ² -Y ² ) component. That is, the second order shimming value relates to the correction over the entire collection area for the second order component of the static magnetic field inhomogeneity.

The storage device 129 is, for example, a RAM (Random Access Memory), a semiconductor memory device such as a flash memory, a hard disk drive, a solid state drive, an optical disk, or the like. Also, the storage device 129 may be a drive device or the like that reads and writes various information from/to a portable storage medium such as a CD-ROM drive, a DVD drive, or a flash memory.

The processing circuit 131 has a processor (not shown) as hardware resources, memories such as ROM (Read-Only Memory) and RAM, etc., and controls the magnetic resonance imaging apparatus 1 . The processing circuit 131 has a system control function 1311 , an image generation function 1313 , a static magnetic field shimming function 1315 , a determination function 1317 , an acquisition function 1319 , a calibration function 1321 and an adjustment function 1323 . Various functions performed by the system control function 1311, the image generation function 1313, the static magnetic field shimming function 1315, the determination function 1317, the acquisition function 1319, the calibration function 1321 and the adjustment function 1323 are stored in the form of programs executable by a computer. 129. The processing circuit 131 is a processor that reads programs corresponding to these various functions from the storage device 129 and implements the functions corresponding to each program by executing the read programs. In other words, the processing circuit 131 in a state where each program is read has a plurality of functions shown in the processing circuit 131 of FIG.

In FIG. 1, it is assumed that these various functions are realized by a single processor. may be realized. In other words, each function described above may be configured as a program, and one processing circuit may execute each program, or a specific function may be implemented in a dedicated independent program execution circuit. may Also, in FIG. 1, the single storage device 129 has been described as storing the programs corresponding to the respective processing functions. A configuration in which the corresponding program is read may also be used.

The term "processor" used in the above description includes, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an Application Specific Integrated Circuit (ASIC), a programmable logic device (for example, a simple Circuits such as a Simple Programmable Logic Device (SPLD), a Complex Programmable Logic Device (CPLD), and a Field Programmable Gate Array (FPGA).

The processor implements various functions by reading and executing programs stored in the storage device 129 . Instead of storing the program in the storage device 129, the program may be configured to be directly installed in the circuit of the processor. In this case, the processor implements its functions by reading and executing the program embedded in the circuit. The bed control circuit 109, the transmission circuit 113, the reception circuit 119, the imaging control circuit 121, and the like are similarly configured by electronic circuits such as the processor.

The processing circuit 131 controls the magnetic resonance imaging apparatus 1 with a system control function 1311 . Specifically, the processing circuit 131 reads the system control program stored in the storage device 129, develops it on the memory, and controls various circuits and various power supplies in the magnetic resonance imaging apparatus 1 according to the developed system control program. do. For example, the processing circuit 131 reads the imaging protocol from the storage device 129 based on the imaging conditions input by the operator via the interface 125 . Note that the processing circuit 131 may generate an imaging protocol based on imaging conditions. The processing circuit 131 transmits an imaging protocol to the imaging control circuit 121 and controls imaging of the subject P. FIG.

The processing circuit 131 fills the MR data along the readout direction of k-space by means of the image generation function 1313, for example according to the strength of the readout magnetic field gradient. The processing circuitry 131 generates an MR image by performing a Fourier transform on the MR data filled in k-space. The processing circuit 131 outputs the generated MR image to the display 127 and storage device 129 .
The above is a schematic description of the overall configuration of the magnetic resonance imaging apparatus 1 according to this embodiment.

Next, an operation example of the magnetic resonance imaging apparatus 1 according to this embodiment will be described with reference to the flowchart of FIG.
In step S201, a positioning scan is performed. Specifically, the imaging control circuit 121 executes a positioning scan according to an instruction from the processing circuit 131, for example. Processing circuitry 131 generates a positioning image using the MR signals acquired from the positioning scan. Processing circuitry 131 may output the generated positioning image to display 127 .
In step S202, the processing circuitry 131 executes the static magnetic field shimming process by executing the static magnetic field shimming function 135 to generate a three-dimensional estimated static magnetic field distribution, which is the estimated three-dimensional static magnetic field distribution. Static magnetic field shimming uses the static magnetic field distribution generated by the MR signals acquired by shimming imaging for the first acquisition range to determine the static magnetic field inhomogeneity in the second acquisition range for each slice for multi-slice imaging. This is a process for correcting to Inhomogeneities in the static magnetic field are caused by a subject P placed in the static magnetic field. Therefore, shimming imaging is performed with the subject P inserted into the bore 111 . The static magnetic field shimming process will be described later.
In step S203, the processing circuit 131 executes the determining function 1317 to determine the cross-sectional position where the static magnetic field distribution is most uniform from the three-dimensional estimated static magnetic field distribution after shimming in the second acquisition range.

In step S204, the processing circuit 131 performs CFA (Center Frequency Adjustment) measurement at the cross-sectional position by executing the collection function 1319 . By executing the acquisition function 1319, the processing circuit 131 acquires the resonance frequency distribution at the cross-sectional position in the CFA measurement.
In step S205, the processing circuit 131 calibrates the three-dimensional estimated static magnetic field distribution based on the collected resonance frequency distribution by executing the calibration function 1321. FIG. Specifically, by executing the calibration function 1321, the processing circuit 131 associates the resonance frequency distribution with the three-dimensional estimated static magnetic field distribution, and converts the three-dimensional estimated static magnetic field distribution, which is a relative value, into an absolute value. to
In step S206, by executing the adjustment function 1323, the processing circuit 131 adjusts the center frequency of the RF pulse in subsequent MR imaging such as the main scan based on the calibrated three-dimensional estimated static magnetic field distribution.
In step S207, for example, the imaging control circuit 121 executes a main scan using an RF pulse whose center frequency is adjusted according to the imaging section.
This concludes the operation example of the magnetic resonance imaging apparatus 1 according to the present embodiment.

Next, the details of the static magnetic field shimming process executed in step S202 of this embodiment will be described.
The imaging control circuit 121 performs shimming imaging on the subject P. FIG. The imaging control circuit 121 performs shimming imaging, for example, by multi-slice imaging using a double echo method using two different echo time intervals. It should be noted that shimming imaging may also be performed by other imaging techniques such as, for example, multi-slice imaging using the triple-echo technique using three different echo time intervals. Specifically, the imaging control circuit 121 controls the gradient magnetic field power supply 105, the transmission circuit 113, and the reception circuit 119 according to the double echo method. The imaging control circuit 121 acquires three-dimensional MR signals via the receiving coil 117 and the receiving circuit 119 by shimming imaging. That is, MR signals corresponding to two echo time intervals are acquired. Note that the center frequency of the RF pulse in shimming imaging is determined by resonance frequency distribution measurement performed before shimming imaging and before acquisition of MR signals for positioning images.

The processing circuit 131 uses a static magnetic field shimming function 1315 to generate a plurality of static magnetic field distributions respectively corresponding to a plurality of slices in the first acquisition range based on MR signals acquired by shimming imaging. Specifically, processing circuitry 131 generates two complex images, each corresponding to two echo time intervals, based on the MR signals in each of the plurality of slices in the first acquisition range. The processing circuit 131 performs a complex conjugate operation on one of the two complex images, and multiplies the complex image on which the complex conjugate operation has been performed and the other complex image on which the complex conjugate operation has not been performed. to calculate Processing circuitry 131 generates a phase contrast image using the calculated phase of the product.

Processing circuitry 131 uses at least one of the two complex images to generate an intensity image through static magnetic field shimming function 1315 . Processing circuitry 131 extracts a background region in the phase contrast image based on the intensity image. Processing circuitry 131 removes the background from the phase contrast image using the extracted background region. The processing circuit 131 performs phase unwrapping processing in consideration of phase continuity on the phase contrast image from which the background has been removed. The processing circuit 131 performs linear transformation using the echo interval corresponding to the difference between the two echo time intervals and the gyromagnetic ratio for the phase difference value of each of the plurality of pixels in the phase-unwrapped phase-contrast image. generates a two-dimensional static magnetic field distribution as frequency information. The processing circuit 131 combines a plurality of two-dimensional static magnetic field distributions to generate a three-dimensional static magnetic field distribution (hereinafter referred to as pre-shimming distribution).

Processing circuitry 131 identifies imaging positions, ie, multiple slices, in the second acquisition range in the first acquisition range. The processing circuit 131 generates a plurality of static magnetic field distributions respectively corresponding to a plurality of slices based on the pre-shimming distribution and the specified imaging position. Generating a plurality of static magnetic field distributions respectively corresponding to a plurality of slices in the second acquisition range corresponds to reformatting into a plurality of slices using the pre-shimming distribution, eg cross-sectional conversion processing. Note that a plurality of static magnetic field distributions corresponding to a plurality of slices may be stored in the storage device 129 by default according to imaging target regions, sex, age, and the like. At this time, shimming imaging becomes unnecessary.

The processing circuit 131 uses the plurality of cross-sectional positions in the second acquisition range and the pre-shimming distribution to perform a static magnetic field shimming function 1315 for each of a plurality of slices respectively corresponding to the plurality of cross-sectional positions in the second acquisition range. , to perform slice-by-slice static magnetic field shimming. Specifically, the processing circuit 131 uses the static magnetic field shimming function 1315 to read the calculation program from the storage device 129 and load it into its own memory. The processing circuitry 131 calculates a zero order shimming value, a first order shimming value and a second order shimming value for each of the plurality of slices in the second acquisition range by means of a calculation program. Processing circuitry 131 associates the calculated 0th order shimming value, 1st order shimming value and 2nd order shimming value with the slice corresponding to the position of the display plane. The basic formula of the static magnetic field shimming will be described below, and then the static magnetic field shimming for each slice will be described.
An example of a basic equation for static magnetic field shimming is shown in equation (1) below.

x, y, and z in equation (1) are three-dimensional positions in space. Specifically, x represents a position with the center of the static magnetic field (hereinafter referred to as the magnetic field center) in the horizontal direction (X-axis) as the origin. y represents the position with the magnetic field center as the origin in the vertical direction (Y-axis). z represents the position with the magnetic field center as the origin in the axial direction (Z-axis). The unit of x, y, and z is [m]. a0 in equation (1) is the _zero -order shimming value. a ₀ represents the negative value of the center frequency of the RF pulse. The unit of _a0 is [ppm]. a ₁ , a ₂ , and a ₃ in Equation (1) are primary shimming values.

Specifically, a ₁ , a ₂ , and a ₃ represent the amount of change in resonance frequency per unit length for each of the X, Y, and Z axes. The amount of change in resonance frequency per unit length corresponds to the gradient of the gradient magnetic field, that is, the current value applied to the gradient magnetic field coil 103 . The unit of a ₁ , a ₂ and a ₃ is [ppm/m]. b ₀ (x, y, z) in Equation (1) is the resonance frequency before static magnetic field shimming at position (x, y, z). In other words, b ₀ (x, y, z) is the resonance frequency obtained by converting the three-dimensional static magnetic field distribution corresponding to the distribution before shimming, that is, the non-uniformity of the static magnetic field is the distribution of the resonance frequency. corresponds to what is expressed as The unit of b ₀ (x, y, z) is [ppm]. b ₀ ′(x, y, z) is the difference value between the resonance frequency after shimming at the position (x, y, z) and the center frequency a ₀ of the RF pulse. The unit of b ₀ ′(x, y, z) is [ppm].

The smaller the difference value between the resonance frequency after shimming and the center frequency of the RF pulse, the more ideal the static magnetic field shimming condition is. Considering a set (hereinafter referred to as a position set S) of all the positions of a plurality of pixels (hereinafter referred to as foreground pixels) in the foreground region corresponding to the non-background region for an image showing the before-shimming distribution, the position set S is , for example, represented by the following equation (2).

In equation (2), i represents the serial number of the foreground pixels. N represents the total number of foreground pixels.
At this time, Equation (1) can be set for N pixels over all foreground pixels in the image of the pre-shimming distribution. Summarizing the N equations for all foreground pixels, the following equation (3) can be obtained.

In equation (3), vector b', matrix X, vector a, and vector b are

, the formula (3) is expressed as the following formula (4).

As described above, the smaller each element of the vector on the left side of Equation (1), that is, the left side of Equation (3) or (4), the more ideal the static magnetic field shimming. Therefore, the uniformity of the static magnetic field is defined as the magnitude of the vector b', and the cost function E for the vector a, which is a collection of the 0th-order shimming value and the 1st-order shimming value, is defined as Equation (5).

The matrix Ω in Equation (5) is a matrix for normalization by the importance and correlation of each element of the vector b'. For example, if the matrix Ω is the identity matrix, the cost function is a simple sum of squares of vector elements. Also, if the matrix Ω is the covariance matrix with respect to the vector b', the cost function is the square of the Mahalanobis distance. A vector a, which is a combination of the zero-order shimming value and the first-order shimming value that minimizes the cost function of expression (5), can be obtained as the following expression (6) by the least-squares method.

The static magnetic field shimming for each slice will be described below. For a second acquisition range in which per-slice static magnetic field shimming is performed, considering a position set _Sj of a plurality of foreground pixels for each slice of the second acquisition range, the position set _Sj is, for example, given by Equation (7) below: expressed.

In equation (7), j represents the slice serial number in the second acquisition range. Also, M in Equation (7) represents the number of slices in the second acquisition range. i in equation (7) represents the serial number of the foreground pixels. N _j represents the total number of foreground pixels in slice j.

For slice-by-slice static magnetic field shimming, equation (1) can be written for N _j foreground pixels for each slice j in the second acquisition range. At slice j, let vector b _j ', matrix X _j , vector a _j , vector b _j be

defined as Vector bj corresponds to all foreground pixels in the static magnetic field distribution corresponding to slice _j among the plurality of static magnetic field distributions related to the above-mentioned pre-shimming distribution. Summarizing the Nj equations for all foreground pixels in slice _j , the following equation (8) can be obtained.

Processing circuit 131 uses static magnetic field shimming function 1315 to define and solve the cost function for equation (8) in the same manner as equation (5). As a result, M vectors _aj , which are combinations of the 0th-order shimming value and the 1st-order shimming value, are calculated. That is, by performing shimming using the value of vector _aj for each slice j of the multi-slice, it is possible to acquire inspection images by static magnetic field shimming for each slice.

Next, Equation (9) shows a basic equation regarding secondary shimming using a shim coil capable of spatially applying a secondary correction magnetic field distribution.

x, y, z, a ₀ , a ₁ , a ₂ , a ₃ , b ₀ , and b ₀ ′ in formula (9) are defined in the same way as in formula (1). a ₄ , a ₅ , a ₆ , a ₇ and a ₈ are secondary shimming values. Specifically, a ₄ , a ₅ , a ₆ , a ₇ , and a ₈ represent spatially nonlinear resonance frequency variations. The amount of change in the spatially nonlinear resonance frequency corresponds to the current value applied to the shim coil 101 . The unit of a ₄ , a ₅ , a ₆ , a ₇ and a ₈ is [ppm/m ² ].

At this time, Equation (9) can be set for N for all foreground pixels in the three-dimensional static magnetic field distribution image, which can be summarized as Equation (10) below.

In formula (10), vector b′, vector a, vector b, matrix X, matrix X′, and matrix X″ are

, the formula (10) is expressed as the following formula (11).

Equation (11) has the same format as Equation (4) except that the sizes of matrix X'' and vector a' are different. A vector a′, which is a combination of values, primary shimming values and secondary shimming values, can be determined.

The static magnetic field shimming for this embodiment using the 0th, 1st and 2nd order shimming values described above is formulated. Unlike 0th-order shimming and 1st-order shimming, 2nd-order shimming takes time until the magnetic field stabilizes after the current is applied to the shim coil 101. Therefore, during multi-slice acquisition, the correction magnetic field is switched at high speed in units of slices. difficult. Therefore, in the static magnetic field shimming according to the present embodiment, on the premise that secondary shimming is performed in common for all slices in the acquisition region, the optimum correction amount for the 0th-order shimming and the 1st-order shimming is calculated for each slice. With the goal. Summarizing the above contents, the basic expression of the static magnetic field shimming related to this embodiment is the following expression (12).

Here, vector b′, matrix X′″, vector a″, and vector b in equation (12) are

と表せる。

can be expressed as

Equation (12) has the same form as Equation (4). For this reason, the processing circuit 131 uses the static magnetic field shimming function 1315 to obtain the 0th-order shimming value and the 1st-order shimming value for each slice in the acquisition region in the same way as the expressions (5) and (6), and A vector a'' can be obtained which is a combination with the quadratic shimming value of . Specifically, processing circuitry 131 defines a cost function for equation (12) in the same manner as for equation (5). The processing circuitry 131 may calculate a vector a'' that is a combination of the zeroth, first, and second order shimming values by the least-squares method that minimizes the cost function associated with equation (12).

Next, an example of the shimming collection range will be described with reference to FIG.
The shimming acquisition range is a range in which static magnetic field shimming is performed, and is the range described as the first collection range in the above description of the static magnetic field shimming process. In the example of FIG. 3, a shimming acquisition range 303 formed by 11 slices is determined for the positioning image 301 of the sagittal section. The shimming collection range 303 may be manually set by the operator, for example. Alternatively, the processing circuit 131 may set a range including a ROI (Region of Interest) in subsequent MR imaging as the shimming acquisition range 303 .
Execution of the static magnetic field shimming function 1315 causes the processing circuitry 131 to generate a three-dimensional estimated static magnetic field distribution in the shimming collection area 303 .

Next, the details of the cross-sectional position determination processing executed in step S203 will be described.
By executing the determination function 1317, the processing circuit 131 searches and determines the cross-sectional position where the static magnetic field distribution is most uniform in the three-dimensional space for the three-dimensional estimated static magnetic field distribution. As a search method, for example, a particle swarm optimization technique is used.

Conditions (parameters) for determining whether the static magnetic field distribution at the cross-sectional position is uniform include, for example, that the area of the cross section being searched in the shimming acquisition range 303 of the three-dimensional estimated static magnetic field distribution is equal to or greater than a threshold, and At least one of the SD (Standard Deviation) of the static magnetic field distribution in the cross section, the half width of the resonance frequency corresponding to the static magnetic field distribution, and the entropy of the static magnetic field distribution is searched for a cross section that is equal to or less than a threshold value, and the cross section The position should be determined as the cross-sectional position.
The condition that the area is equal to or greater than the threshold is that when the area of the cross section where the uniform static magnetic field distribution is obtained is extremely small, the static magnetic field distribution is locally uniform, but the entire shimming acquisition range 303 As can be seen, the contribution rate of the static magnetic field distribution in the cross section is low. Therefore, if the center frequency is determined from the resonance frequency distribution obtained by CFA measurement, which is collected based on the cross section and will be described later, image quality may deteriorate as a result.

Although the particle swarm optimization method is given as an example of the search method, the method is not limited to this, and a general optimization algorithm for searching for minimum values (local minimum values) may be used. As another search method, a cross-sectional position where the static magnetic field distribution is most uniform may be searched for the three-dimensional estimated static magnetic field distribution along each of the x-axis, the y-axis, and the z-axis.
That is, the cross section of the yz plane is searched along the x-axis direction, and the most uniform cross section position within the shimming collection range 303 is extracted as a cross section position candidate. Similarly, for other axes, a cross section on the zx plane is searched along the y-axis direction, a cross section on the xy plane is searched along the z-axis direction, and cross-section position candidates are extracted. Ultimately, the most uniform cross-sectional position should be determined as the cross-sectional position from among the three cross-sectional position candidates.

Further, the search range and cross-section conditions may be controlled according to the imaging target region. For example, since the cross-sectional area of the imaging cross-section differs between the abdomen and the head, the imaging target region and the threshold value of the corresponding condition are stored in advance as a table in the storage device 129, for example. The processing circuit 131 executing the determination function 1317 may determine the cross-sectional position using a threshold value corresponding to the imaging target region in the imaging range corresponding to the imaging target region.
Note that the processing circuit 131 that executes the determination function 1317 may determine the cross-sectional position from the pre-shimming distribution, which is the actually collected three-dimensional static magnetic field distribution, instead of the three-dimensional estimated static magnetic field distribution after shimming.

Next, an example of the determined cross-sectional position will be described with reference to FIG.
The right diagram in FIG. 4 shows a positioning image 400 in which the determined cross-sectional position 401 is displayed as a marker in the shimming acquisition range 303 shown in FIG. 3, and the left diagram in FIG. Here, it is assumed that the shimming acquisition range 303, which is the first acquisition range, and the imaging range, which is the second acquisition range, are the same range.
In the estimated static magnetic field distribution 410 shown in the left diagram of FIG. 4, regions of the same static magnetic field intensity are shown in the same pattern. As a result of searching for the cross-sectional position by the determination function 1317, it is assumed that an image with the most uniform static magnetic field distribution within the second acquisition range as shown in the left diagram of FIG. 4 is obtained at the cross-sectional position 401 indicated by the thick line.

Next, an example of the resonance frequency distribution obtained in step S204 will be described with reference to FIGS. 5 to 7. FIG.
FIG. 5 is a diagram showing the range of the positioning image 400, the shimming acquisition range 303, and the CFA acquisition range 501 when the cross-sectional position 401 shown in FIG. 4 is determined. The CFA acquisition range indicates the range in which the resonance frequency distribution for the subject P is acquired.
The right view of FIG. 5 shows a coronal cross section of the positioning image 400, the middle view of FIG. 5 shows an axial cross section of the positioning image 400, and the left view of FIG. shows a cross section.

A CFA acquisition range 501 is not within the range of the positioning image 400 like the shimming acquisition range 303, but indicates a range in which resonance frequencies are acquired at infinity on the extension of the plane formed by the cross-sectional position 401. FIG.
Therefore, it should be noted that the CFA acquisition range 501 extends to positions not imaged in the positioning image 400 . An example of a CFA collection area 501 of note will now be described with reference to FIGS. 6A and 6B.
FIG. 6A shows a case where the CFA acquisition range 501 fits within the area of the subject P displayed on the positioning image 400 . That is, there is no area of the subject P on the extension line of the CFA acquisition range 501 horizontally crossing the jaw portion of the subject P and outside the shimming acquisition range 303 . Therefore, the CFA collection area 501 shown in FIG. 6A is suitable.

On the other hand, FIG. 6B shows a case where the CFA acquisition range 501 does not fit within the area of the subject P displayed on the positioning image 400 and extends to the area of the subject P outside the shimming acquisition range 303 . That is, the lower body of the subject P exists outside the shimming acquisition range 303 on the extension line of the CFA acquisition range 501 extending along the body axis of the subject P. Therefore, when the resonance frequency distribution is generated based on the CFA acquisition range 501 shown in FIG. 6B, it is not suitable as the CFA acquisition range because the static magnetic field distribution in the area other than the shimming acquisition range has an effect.

As shown in FIG. 6B, the subject P is interrupted by the outer frame of the positioning image 400, and based on the determined cross-sectional position 401, when the CFA acquisition range 501 passes through the tissue of the subject P outside the shimming acquisition range 303, The processing circuitry 13 may warn the operator that the setting of the CFA collection range is not appropriate, such as by issuing an alert. Alternatively, by executing the determination function 1317, the processing circuit 131 may re-search the cross-sectional position 401 so that the CFA acquisition range 501 does not pass through a site other than the subject P displayed in the shimming acquisition range 303. .

FIG. 7 shows an example of a resonance frequency distribution at cross-sectional positions obtained by CFA measurement. The horizontal axis is frequency, and the vertical axis is received strength. At a cross-sectional position where the static magnetic field distribution is substantially uniform, the intensity of the resonance frequency corresponding to the dominant magnetic field intensity is high because the unevenness (variation) of the magnetic field intensity is small. That is, it is desirable to determine the center frequency based on the resonance frequency at which the resonance frequency distribution is steep.

Next, the calibration processing by the calibration function 1321 executed in step S205 will be described with reference to FIG.
FIG. 8 shows an example of a histogram of the static magnetic field distribution at cross-sectional positions. The horizontal axis indicates the magnetic field strength of the static magnetic field distribution, and the vertical axis indicates the frequency.

By executing the calibration function 1321, the processing circuit 131 associates the resonance frequency distribution at the cross-sectional position with the histogram of the static magnetic field distribution at the cross-sectional position. For example, by executing the calibration function 1321, the processing circuitry superimposes the graph of the resonance frequency distribution shown in FIG. 7 and the histogram shown in FIG. Note that the graph and histogram of the resonance frequency distribution may be superimposed on each other with reference to their respective peak values.
For example, when superimposing with peak values, the peak value of the histogram of the static magnetic field distribution at the cross-sectional position is the dominant magnetic field strength at the cross-sectional position. On the other hand, the peak value of the resonance frequency distribution at the cross-sectional position should be the resonance frequency corresponding to the dominant magnetic field strength, as described above. Therefore, by associating the peak value of the histogram with the peak value of the resonance frequency distribution, it is possible to grasp which magnetic field intensity corresponds to which resonance frequency at the cross-sectional position.

That is, since the static magnetic field distribution obtained by the static magnetic field shimming process is generated based on the phase difference image, the magnetic field strength difference in the static magnetic field distribution is a relative value. Therefore, it is difficult to adjust the center frequency only with the static magnetic field distribution. However, by executing the calibration function 1321, the processing circuit 131 associates a certain magnetic field intensity in the static magnetic field distribution with a resonance frequency based on the corresponding resonance frequency distribution, so that the intensity difference, which is a relative value, is converted to an absolute value. It can be converted into a frequency difference. As a result, since it is possible to convert which magnetic field intensity becomes which resonance frequency in the three-dimensional estimated static magnetic field distribution, the resonance frequency distribution can be calculated for any cross-sectional position.
Therefore, for example, when the imaging position in the imaging protocol within the first area is determined, the static magnetic field distribution in an arbitrary cross section can be known by referring to the calibrated three-dimensional estimated static magnetic field distribution. Therefore, by executing the adjustment function 1323, the processing circuit 131 can adjust the center frequency of the RF pulse based on the resonance frequency distribution corresponding to the static magnetic field distribution at the imaging position.

According to the present embodiment described above, the processing circuit 131 determines a cross-sectional position where the magnetic field distribution is uniform in the three-dimensional static magnetic field distribution, collects the resonance frequency distribution at the cross-sectional position, and uses the collected resonance frequency distribution to Calibrate the three-dimensional static magnetic field distribution. As a result, the center frequency of the RF pulse can be adjusted based on the resonance frequency corresponding to the calibrated three-dimensional static magnetic field distribution at the imaging target site during MR imaging in the subsequent imaging protocol.

Further, according to the magnetic resonance imaging apparatus 1 of the present embodiment, the display section is a section different from a plurality of sections corresponding to a plurality of slices, and three-dimensional data is obtained based on MR signals acquired by multi-slice imaging. is generated, and cross-section conversion processing using the position of the display cross-section is performed on the three-dimensional data to generate an MR image corresponding to the display cross-section, and the generated MR image can be displayed on the display 127. can.
That is, according to the magnetic resonance imaging apparatus 1, a resonance frequency corresponding to the magnetic field distribution of the display section is obtained in the calibrated static magnetic field distribution for any display section in the subsequent MR imaging.
According to at least one of the above-described embodiments, since the center frequency of the state in which the uniformity of the magnetic field is good can be selected for any cross section, the display cross section desired by the operator can have a high A high-quality MR image can be generated.

In this embodiment, regarding the inhomogeneity of the static magnetic field, second-order and higher-order components are also taken into consideration. need not be corrected. In this case, the magnetic resonance imaging apparatus 1 does not need to include the shim coil 101 and the shim coil power supply 102 .

In addition, each function according to the embodiment can also be realized by installing a program for executing the processing in a computer such as a workstation and deploying them on the memory. At this time, a program that can cause a computer to execute the method is stored in a storage medium such as a magnetic disk (hard disk, etc.), an optical disk (CD-ROM, DVD, Blu-ray (registered trademark) disk, etc.), a semiconductor memory, etc. It is also possible to distribute it as

While several embodiments of the invention have been described, these embodiments have been 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 modifications can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, as well as the scope of the invention described in the claims and equivalents thereof.

Reference Signs List 1 MRI apparatus 100 static magnetic field magnet 101 shim coil 102 shim coil power supply 103 gradient magnetic field coil 105 gradient magnetic field power supply 107 bed 109 bed control circuit DESCRIPTION OF SYMBOLS 111... Bore 113... Transmission circuit 115... Transmission coil 117... Reception coil 119... Reception circuit 121... Imaging control circuit 125... Interface 127... Display 129... Storage device 131... Processing circuit 1071... Top plate 1311... System control function 1313... Image generation function 1315... Static magnetic field shimming function 1317... Decision function 1319... Acquisition function 1321. ..Calibration function 1323..Adjustment function

Claims (7)

a determination unit that determines a cross-sectional position from a three-dimensional static magnetic field distribution;
a collection unit that collects a resonance frequency distribution that indicates the distribution of received signal strength with respect to frequency at the cross-sectional position;
a calibration unit that associates the resonance frequency distribution with the histogram of the three-dimensional static magnetic field distribution;
A magnetic resonance imaging apparatus comprising: an adjustment unit that adjusts a center frequency in subsequent imaging based on the associated three-dimensional static magnetic field distribution histogram . The determining unit determines a position of a cross section having an area equal to or greater than a first threshold value and having at least one of standard deviation, half width and entropy of the three-dimensional static magnetic field distribution equal to or less than a second threshold value as the cross-sectional position. 2. The magnetic resonance imaging apparatus of claim 1, wherein the determining is performed. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the determination unit determines the cross-sectional position from a search range corresponding to an imaging target region. 4. The magnetic field according to any one of claims 1 to 3 , wherein the adjustment unit adjusts the center frequency based on the three-dimensional static magnetic field distribution associated with an imaging position in subsequent imaging. Resonance imaging device. 5. The magnetic resonance imaging apparatus according to claim 1, wherein said three-dimensional static magnetic field distribution is an estimated static magnetic field distribution after shimming. 5. The magnetic resonance imaging apparatus according to claim 1, wherein said three-dimensional static magnetic field distribution is a collected static magnetic field distribution. The calibration unit associates the peak value of the resonance frequency distribution with the peak value of the histogram of the three-dimensional static magnetic field distribution,
The adjustment unit adjusts a center frequency in subsequent imaging based on the associated peak value of the three-dimensional static magnetic field distribution histogram.
The magnetic resonance imaging apparatus according to claim 1.

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