Image generation device, image generation method, and image generation program

Abstract

To generate a magnetic resonance image with improved image quality.SOLUTION: An image generation device 1 includes: an acquisition unit for acquiring MR data collected in a plurality of lead-out directions including a first lead-out direction and a second lead-out direction intersecting with the first lead-out direction, and a plurality of sensitivity maps corresponding to a plurality of coil elements used for collecting the MR data; a developed data generation unit for generating developed data in which a fold-back is developed in a 1D image space by executing 1D Fourier transformation along the lead-out directions in each lead-out direction using the MR data in each lead-out direction and the plurality of sensitivity maps; a complementation data generation unit for generating complementation data in which data is complemented for the MR data by executing 1D reverse Fourier transformation along the lead-out directions for the developed data in each lead-out direction; and an image generation unit for generating an MR image on the basis of the complementation data.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments disclosed in this specification and the drawings relate to an image generating device, an image generating method, and an image generating program. [Background technology]

[0002] A common issue in imaging with magnetic resonance imaging (MRI) devices is image aliasing, which is caused by an insufficient sampling rate for the received magnetic resonance (MR) signals (in other words, an insufficient field of view, which corresponds to the reciprocal of the sampling interval). Techniques to address this issue include setting a low-pass filter that matches the sampling rate and oversampling the MR signals.

[0003] However, in scans with variable readout directions, such as radial scans, when the low-pass filter used in this technology is applied to MR signals, the amount of signal filtered by the low-pass filter varies depending on the readout direction. As a result, the consistency of MR data arranged in k-space between different readout directions is lost. This causes a problem of noise, such as line-like artifacts called streaks, in MR images generated by scans with variable readout directions, resulting in degradation of image quality. While sufficient oversampling is required to eliminate image quality degradation, sufficient oversampling cannot be achieved due to performance limitations of MRI systems (e.g., sampling period, data volume, etc.). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-115967 Summary of the Invention [Problem to be solved by the invention]

[0005] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to generate magnetic resonance images with improved image quality. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described below can also be positioned as other problems. [Means for solving the problem]

[0006] An image generating device according to an embodiment includes an acquisition unit, a decompressed data generation unit, a complementary data generation unit, and an image generation unit. The acquisition unit acquires magnetic resonance data collected in multiple readout directions, including a first readout direction and a second readout direction intersecting the first readout direction, and multiple sensitivity maps corresponding to multiple coil elements used to collect the magnetic resonance data. The decompressed data generation unit generates decompressed data in which folding is unfolded in a one-dimensional image space by performing a one-dimensional Fourier transform along the readout direction using the magnetic resonance data and the sensitivity map for each readout direction. The complementary data generation unit generates complementary data in which data is complemented for the magnetic resonance data by performing a one-dimensional inverse Fourier transform along the readout direction on the decompressed data for each readout direction. The image generation unit generates a magnetic resonance image based on the complementary data. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a block diagram illustrating an example of an image generating apparatus according to an embodiment. [Figure 2] FIG. 2 is a block diagram showing an example of a magnetic resonance imaging apparatus according to an embodiment. [Figure 3]FIG. 3 is a diagram showing an example of the configuration of a receiving circuit according to the embodiment. [Figure 4] FIG. 4 is a diagram showing an example of a trajectory in k-space relating to readout of MR signals in a main scan according to the embodiment. [Figure 5] FIG. 5 is a flowchart showing an example of a procedure of an image generation process according to the embodiment. [Figure 6] FIG. 6 is a diagram showing an example of MR data in the readout direction and sampling points in k-space according to the embodiment. [Figure 7] FIG. 7 is a diagram showing an example of a one-dimensional image in a one-dimensional image space before being developed by a plurality of sensitivity maps in the readout direction ROD according to the embodiment. [Figure 8] FIG. 8 is a diagram showing an example of unfolded data obtained by performing a folding unfolding process using a plurality of sensitivity maps according to the embodiment. [Figure 9] 9 is a diagram showing an example of complementary data generated from the MR data shown in FIG. 6 according to the embodiment. [Figure 10] FIG. 10 is a diagram showing an example of an MR image generated from MR data by an existing reconstruction method as a comparative example, and an MR image generated based on complementary data by image generation processing in this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of an image generating device, an image generating method, and an image generating program will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an example of an image generating device 1. The image generating device 1 is installed, for example, in a modality equipped with various functions of the image generating device 1, or in a server in a hospital. Note that the various functions of the image generating device 1 may also be installed in a server of a medical image management system (hereinafter referred to as PACS (Picture Archiving and Communication Systems)) or a server of a hospital information system (hereinafter referred to as HIS (Hospital Information System)).

[0009] The modality is, for example, a medical image diagnostic device related to MRI, such as a magnetic resonance imaging (MRI) device, a PET (Positron Emission Tomography)-MRI device, or a SPECT (Single Photon Emission Computed Tomography)-MRI device. Hereinafter, for the sake of specificity, it is assumed that the image generating device 1 is mounted on the MRI device. In this case, the MRI device has various functions in the processing circuitry 15.

[0010] (Embodiment) 2 is a diagram showing an example of an MRI apparatus 100 according to this embodiment. As shown in FIG. 2, the MRI apparatus 100 includes a static magnetic field magnet 101, a gradient magnetic field coil 103, a gradient magnetic field power supply 105, a bed 107, 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 (imaging control unit) 121, a system control circuit (system control unit) 123, a memory 13, an input interface 127, a display 129, a communication interface 11, and a processing circuit 15. Note that the image generating device 1 may further include the input interface 127 and the display 129 in addition to the communication interface 11, the memory 13, and the processing circuit 15.

[0011] The static magnetic field magnet 101 is a magnet formed in a hollow, approximately cylindrical shape. The static magnetic field magnet 101 generates a substantially uniform static magnetic field in the internal space. For example, a superconducting magnet or the like is used as the static magnetic field magnet 101.

[0012] The gradient magnetic field coil 103 is a hollow, approximately cylindrical coil and is disposed on the inner surface of the cylindrical cooling vessel. The gradient magnetic field coil 103 receives current individually from a gradient magnetic field power supply 105 to generate gradient magnetic fields whose magnetic field strength changes along the mutually orthogonal X, Y, and Z axes. The gradient magnetic fields of the X, Y, and Z axes generated by the gradient magnetic field coil 103 form, for example, a slice selection gradient magnetic field, a phase encoding gradient magnetic field, and a frequency encoding gradient magnetic field. The slice selection gradient magnetic field is used to arbitrarily determine an imaging cross section. The phase encoding gradient magnetic field is used to change the phase of a magnetic resonance signal (hereinafter referred to as an MR (Magnetic Resonance) signal) according to a spatial position. The frequency encoding gradient magnetic field is used to change the frequency of the MR signal according to a spatial position.

[0013] 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 .

[0014] The bed 107 is a device equipped with 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 a bed control circuit 109.

[0015] The bed control circuit 109 is a circuit that controls the bed 107. The bed control circuit 109 drives the bed 107 in response to instructions from the operator via the input / output interface 17, thereby moving the tabletop 1071 in the longitudinal direction, the up-down direction, and in some cases the left-right direction.

[0016] The transmission circuit 113 supplies radio frequency pulses modulated at the Larmor frequency to the transmission coil 115 under the control of the imaging control circuit 121. For example, the transmission circuit 113 includes an oscillator, a phase selection unit, a frequency conversion unit, an amplitude modulation unit, an RF amplifier, and the like. The oscillator generates an RF pulse at a resonance frequency specific to the target atomic nucleus in a static magnetic field. The phase selection unit selects the phase of the RF pulse generated by the oscillator. The frequency conversion unit converts the frequency of the RF pulse output from the phase selection unit. The amplitude modulation unit modulates the amplitude of the RF pulse output from the frequency conversion unit according to, for example, a sinc function. The RF amplifier amplifies the RF pulse output from the amplitude modulation unit and supplies it to the transmission coil 115.

[0017] The transmission coil 115 is an RF (Radio Frequency) coil arranged inside the gradient magnetic field coil 103. In response to the output from the transmission circuit 113, the transmission coil 115 generates an RF pulse corresponding to a high frequency magnetic field.

[0018] 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 a high frequency magnetic field. The receiving coil 117 outputs the received MR signals to a receiving circuit 119. The receiving coil 117 is, for example, a coil array having one or more, typically a plurality of coil elements. For the sake of concreteness, the receiving coil 117 will be described below as a coil array having a plurality of coil elements.

[0019] The receive coil 117 may be configured by a single coil element. Although the transmit coil 115 and receive coil 117 are shown as separate RF coils in Fig. 2, the transmit coil 115 and receive coil 117 may be implemented as an integrated transmit / receive coil. The transmit / receive coil corresponds to the imaging region of the subject P and is, for example, a local transmit / receive RF coil such as a head coil.

[0020] The receiving circuit 119 generates digital MR signals (hereinafter referred to as MR data) based on the MR signals output from the receiving coil 117 under the control of the imaging control circuit 121. Specifically, the receiving circuit 119 performs signal processing such as detection and filtering on the MR signals output from the receiving coil 117, and then performs analog-to-digital (A / D) conversion (hereinafter referred to as A / D conversion) on the data that has undergone the signal processing to generate MR data. The receiving circuit 119 outputs the generated MR data to the imaging control circuit 121. For example, MR data is generated for each coil element and output to the imaging control circuit 121 together with a tag that identifies the coil element.

[0021] 3 is a diagram showing an example of the receiving circuit 119. The receiving circuit 119 has, for example, a low pass filter 191 and an A / D converter 193. Note that in addition to the low pass filter 191 and the A / D converter 193, the receiving circuit 119 may also be equipped with various circuits, such as a detector, that correspond to the above signal processing.

[0022] A cutoff frequency set by the setting function 153 is input to the low-pass filter 191 via the imaging control circuit 121. That is, the pass band of the low-pass filter 191 is set by the setting function 153. Note that the cutoff frequency may be input directly to the low-pass filter 191 from the processing circuit 15. The low-pass filter 191 filters the MR signal using the input cutoff frequency.

[0023] The sampling rate set by the setting function 153 is input to the A / D converter 193 via the imaging control circuit 121. That is, the sampling interval in the low-pass filter 191 is set by the setting function 153. The A / D converter 193 samples the MR signal that has passed through the low-pass filter 191 at a sampling timing according to the sampling rate. As a result, the A / D converter 193 generates MR data.

[0024] The imaging control circuit 121 controls the gradient magnetic field power supply 105, the transmission circuitry 113, the reception circuitry 119, etc. in accordance with the imaging protocol output from the processing circuitry 15, and performs imaging of the subject P. The imaging protocol has a pulse sequence according to the type of examination. The imaging protocol defines 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 gradient magnetic field power supply 105 supplies the current to the gradient magnetic field coil 103, the magnitude and time width of the radio frequency pulse supplied to the transmission coil 115 by the transmission circuitry 113, the timing at which the radio frequency pulse is supplied to the transmission coil 115 by the transmission circuitry 113, the timing at which the MR signal is received by the reception coil 117, etc. The imaging control circuit 121 drives the gradient magnetic field power supply 105, the transmission circuitry 113, the reception circuitry 119, etc. to image the subject P, and then receives MR data from the reception circuit 119 and stores the received MR data in the memory 13.

[0025] The imaging control circuit 121 executes a pulse sequence for imaging to acquire MR data. The imaging executed in this embodiment corresponds to a scan for acquiring a plurality of MR signals corresponding to a plurality of readout directions including a first readout direction and a second readout direction intersecting the first readout direction.

[0026] Fig. 4 is a diagram showing an example of a trajectory kTra in k-space related to the readout of MR signals in the main scan. The main scan may be, for example, a two-dimensional radial acquisition R2D, a three-dimensional radial acquisition (stack-of-stars Sos, koosh-ball KB), a propeller (periodically rotated overlapping parallel lines with enhanced reconstruction) acquisition PRP, or another acquisition using, for example, two blades BL2, as shown in Fig. 3.

[0027] For the sake of concreteness, the following description will be given assuming that the main scan is a two-dimensional radial acquisition R2D. Hereinafter, the two-dimensional radial acquisition R2D will be simply referred to as radial acquisition. The number of readouts in the radial acquisition performed as the main scan on the subject P, i.e., the number of trajectories kTra in the readout direction, is set in advance prior to the main scan.

[0028] The imaging control circuit 121 collects MR data (hereinafter referred to as sensitivity data) related to the generation of each of a plurality of sensitivity maps by an arbitrary imaging technique. The plurality of sensitivity maps correspond to a plurality of coil elements in the receive coil 117 used to collect MR data, and correspond to a plurality of images showing the distribution of sensitivity of the coil elements. The sensitivity maps are expressed by complex number data. The collection of sensitivity data is executed by the imaging control circuit 121, for example, in a pre-scan including a locator scan prior to a radial acquisition R2D as a main scan. The imaging control circuit 121 is realized, for example, by a processor.

[0029] The term "processor" refers to circuits such as a CPU, a graphics processing unit (GPU), an application specific integrated circuit (ASIC), a programmable logic device (e.g., a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA)).

[0030] The system control circuit 123 has hardware resources such as a processor, a read-only memory (ROM), a random access memory (RAM), etc. (not shown), and uses a system control function to control the MRI apparatus 100. Specifically, the system control circuit 123 reads a system control program stored in the memory, expands it on the memory, and controls each circuit of the MRI apparatus 100 according to the expanded system control program.

[0031] For example, the system control circuit 123 reads out an imaging protocol from the memory 13 based on imaging conditions input by the operator via the input interface 127. The system control circuit 123 transmits the imaging protocol to the imaging control circuit 121 and controls imaging of the subject P. The system control circuit 123 is realized by, for example, a processor. The system control circuit 123 may be incorporated into the processing circuit 15. In this case, the system control function is executed by the processing circuit 15, and the processing circuit 15 functions as a substitute for the system control circuit 123. The processor that realizes the system control circuit 123 is similar to that described above, and therefore a description thereof will be omitted.

[0032] The memory 13 stores various programs related to the system control functions executed in the system control circuit 123, various imaging protocols, imaging conditions including a plurality of imaging parameters that define the imaging protocols, etc. The memory 13 also stores a specific function 151, a setting function 153, an acquisition function 155, a developed data generation function 157, a complement data generation function 159, and an image generation function 161 that are realized by the processing circuit 15 in the form of programs executable by a computer.

[0033] The memory 13 also stores MR images generated by the image generation function 161 and prescan images generated by a prescan such as a locator scan. The prescan images are, for example, positioning images (also referred to as locator images) for setting the imaging field of view (hereinafter referred to as FOV (Field of view)) in the main scan, and sensitivity maps used for generating (reconstructing) MR images in the main scan. For example, the memory 13 stores a plurality of sensitivity maps respectively corresponding to a plurality of coil elements in the receive coil 117. The memory 13 also stores the FOV set in the locator image.

[0034] The memory 13 stores the sampling rate (also referred to as sampling frequency) or sampling interval used in the A / D converter 193 for the main scan that is executed after the prescan. The memory 13 also stores the cutoff frequency used in the low-pass filter 191 for the main scan. The memory 13 stores MR data related to the main scan and an algorithm for reconstructing an MR image based on the MR data.

[0035] The memory 13 may store various data received via the communication interface 11. For example, the memory 13 stores information about an examination order for the subject P (such as a region to be imaged, a purpose of the examination, etc.) received from an information processing system in a medical institution, such as a Radiology Information System (RIS).

[0036] The memory 13 is realized by, for example, a semiconductor memory element such as a ROM, a RAM, or a flash memory, a hard disk drive (HDD), a solid state drive (SSD), an optical disk, etc. The memory 13 may also be realized by a drive device that reads and writes various information from and to a portable storage medium such as a CD (Compact Disc)-ROM drive, a DVD (Digital Versatile Disc) drive, or a flash memory.

[0037] The input interface 127 accepts various instructions (e.g., a power-on instruction) and information input from an operator. The input interface 127 may be realized by, for example, a trackball, a switch button, a mouse, a keyboard, a touchpad that performs input operations by touching the operation surface, a touchscreen that integrates a display screen and a touchpad, a non-contact input circuit using an optical sensor, and a voice input circuit. The input interface 127 is connected to the processing circuitry 15 and converts input operations received from the operator into electrical signals and outputs the signals to the processing circuitry 15. Note that, in this specification, the input interface 127 is not limited to those having physical operation components such as a mouse and a keyboard. For example, an electrical signal processing circuit that receives electrical signals corresponding to input operations from an external input device provided separately from the MRI apparatus 100 and outputs the electrical signals to a control circuit is also included as an example of the input interface 127.

[0038] The input interface 127 inputs an FOV in response to a user instruction for a locator image displayed on the display 129. Specifically, the input interface 127 inputs an FOV in response to a user instruction to set a range in the locator image displayed on the display 129. Furthermore, the input interface 127 inputs various imaging parameters for the main scan in response to a user instruction based on an examination order.

[0039] The display 129 displays various GUIs (Graphical User Interfaces), MR images generated by the processing circuitry 15, pre-scan images such as locator images, and the like under the control of the processing circuitry 15 or the system control circuitry 123. The display 129 also displays various information related to imaging parameters for the main scan and pre-scan, and image processing. The display 129 is realized by, for example, a CRT display, a liquid crystal display, an organic EL display, an LED display, a plasma display, or any other display or monitor known in the art.

[0040] The communication interface 11 performs data communication with, for example, an HIS, a PACS, etc. Any standard may be used for communication between the communication interface 11 and the hospital information system, and examples thereof include HL7 (Hearth Level 7), DICOM, or both. The communication interface 11 receives information about an examination order for a subject P (such as an imaging target region and an examination purpose) received from an information processing system in a medical institution, such as an RIS. Furthermore, if the MRI apparatus 100 is not equipped with the image generating apparatus 1, the communication interface 11 in the image generating apparatus 1 receives MR data from the MRI apparatus 100 or the like that images the subject P in an examination on the subject P. At this time, the received MR data is stored in the memory 13.

[0041] The processing circuit 15 is realized by, for example, the above-mentioned processor. The processing circuit 15 includes a specifying function 151, a setting function 153, an acquisition function 155, an uncompressed data generation function 157, a complementary data generation function 159, an image generation function 161, and the like. The processing circuits 15 that respectively realize the specifying function 151, the setting function 153, the acquisition function 155, the uncompressed data generation function 157, the complementary data generation function 159, and the image generation function 161 correspond to a specifying unit, a setting unit, an acquisition unit, an uncompressed data generation unit, a complementary data generation unit, and an image generation unit. Each function, such as the specifying function 151, the setting function 153, the acquisition function 155, the uncompressed data generation function 157, the complementary data generation function 159, and the image generation function 161, is stored in the memory 13 in the form of a computer-executable program. For example, the processing circuit 15 realizes the function corresponding to each program by reading and executing the program from the memory 13. In other words, the processing circuit 15 having read each program has functions such as a specific function 151, a setting function 153, an acquisition function 155, an expanded data generation function 157, a complementary data generation function 159, and an image generation function 161.

[0042] In the above description, an example has been described in which the "processor" reads and executes a program corresponding to each function from memory 13, but the embodiment is not limited to this. If the processor is, for example, a CPU, the processor realizes the function by reading and executing a program stored in memory 13. On the other hand, if the processor is an ASIC, instead of storing a program in memory 13, the function is directly incorporated into the processor circuit as a logic circuit. Note that each processor in this embodiment is not limited to being configured as a single circuit per processor, and multiple independent circuits may be combined to configure a single processor and realize its function. Furthermore, although the description has been given assuming that a single storage circuit stores a program corresponding to each processing function, multiple storage circuits may be distributed and the processing circuit 15 may read the corresponding program from each storage circuit.

[0043] The processing circuitry 15 uses the identification function 151 to identify a signal region related to the generation of multiple MR signals in a pre-scan image of the subject P. The signal region corresponds to a region in the imaging space where protons such as water molecules exist. The identification function 151 identifies, as the signal region, a region in the pre-scan image where the subject P is imaged (hereinafter referred to as the imaging region). Specifically, the identification function 151 identifies the imaging region corresponding to the region of the subject P by detecting the image of the subject P in the pre-scan image or by user designation on the pre-scan image.

[0044] For example, the identification function 151 identifies a signal region by applying region detection processing for detecting an imaging region to the prescan image. The region detection processing can use existing image recognition processing, such as edge detection in the prescan image, and therefore a detailed description thereof will be omitted. Furthermore, the identification function 151 identifies a signal region in the prescan image displayed on the display 129 in accordance with a region identification instruction input by the user via the input interface 127. The region identification instruction is, for example, an instruction to input a shape (e.g., a rectangle or an ellipse) for specifying a range in the prescan image.

[0045] The processing circuitry 15 sets a sampling rate according to an FOV input by a user (hereinafter referred to as user FOV) using the setting function 153. For example, the setting function 153 sets a sampling rate (sampling interval corresponding to oversampling) corresponding to oversampling by multiplying the reciprocal of the size of the FOV by a constant. The upper limit of the sampling rate (hereinafter referred to as upper limit rate) is set in advance based on the performance limit of the MRI apparatus 100. Therefore, the setting function 153 sets the sampling rate to be equal to or lower than the upper limit rate. The setting function 153 outputs the set sampling rate to the A / D converter 193 via the imaging control circuit 121. As a result, MR data is generated by sampling MR signals at the sampling rate set according to the imaging field of view.

[0046] The processing circuitry 15 sets a cutoff frequency for the passband of the MR signal in the low-pass filter 191 based on the identified signal region using the setting function 153. Specifically, the setting function 153 sets the cutoff frequency by further using the user FOV in the main scan, which is a scan of the subject P in multiple readout directions, and the strength of the gradient magnetic field in the main scan.

[0047] The processing circuitry 15 may set cutoff frequencies for the passbands of the plurality of MR signals corresponding to the plurality of readout directions for each readout direction, regardless of the signal region, based on the user FOV for the main scan of the subject P, using the setting function 153. The setting function 153 may also set cutoff frequencies for the passbands of the plurality of magnetic resonance signals corresponding to the plurality of readout directions by multiplying a frequency determined based on the user FOV for the main scan of the subject P and the strength of the gradient magnetic field in the main scan by a constant.

[0048] That is, the cutoff frequency is set by the setting function 153 so that the signal amount of each of the multiple MR signals is constant regardless of the multiple readout directions. In other words, the cutoff frequency is set by the setting function 153 so as to include a signal region related to the generation of the MR signal. At this time, the cutoff frequency is higher than, for example, the sampling rate, i.e., the sampling frequency. For these reasons, the MR signals to be sampled by the A / D converter 193 are generated by filtering the MR signals received by the receive coil 117 with the low-pass filter 191 using the cutoff frequency previously set by the setting function 153.

[0049] The processing circuitry 15 acquires, from the memory 13, MR data acquired in multiple readout directions, including a first readout direction and a second readout direction intersecting the first readout direction, and multiple sensitivity maps corresponding to multiple coil elements used to acquire the MR data, using the acquisition function 155. The acquired MR data is acquired in multiple readout directions, including the first readout direction and the second readout direction intersecting the first readout direction. The multiple sensitivity maps correspond to multiple coil elements in the receiver coil 117 involved in the MR data acquisition and indicate the distribution of sensitivity of the coil elements. If the MRI device 100 does not include the image generating device 1, the acquisition function 155 acquires the MR data and multiple sensitivity maps from various MRI devices via the communication interface 11 and a network.

[0050] The processing circuitry 15 uses the MR data for each of the multiple readout directions in the main scan and multiple sensitivity maps to perform a one-dimensional Fourier transform along the readout direction for each readout direction using the unfolded data generation function 157. As a result, the unfolded data generation function 157 generates unfolded data for each readout direction in which folding is unfolded in the one-dimensional image space. Specific processing performed by the unfolded data generation function 157 will be described in detail in the process of generating an MR image based on MR data collected using multiple readout directions, including a first readout direction and a second readout direction intersecting the first readout direction (hereinafter referred to as image generation processing).

[0051] The processing circuitry 15 uses the complementary data generating function 159 to perform a one-dimensional inverse Fourier transform along the readout direction on the unfolded data for each readout direction. As a result, the complementary data generating function 159 generates complementary data for each readout direction, in which data is complementary to the magnetic resonance data. The complementary data corresponds to k-space data obtained by oversampling the MR signals at a sampling rate higher than the sampling rate for the MR signals before A / D conversion in generating the MR data. That is, the complementary data corresponds to pseudo oversampled data. In other words, the complementary data corresponds to data obtained by interpolating new data obtained by oversampling the MR signals that are the source of the MR data, for the MR data acquired by the acquisition function 155. The specific processing performed by the complementary data generating function 159 will be described in detail in the image generation processing.

[0052] The processing circuitry 15 acquires MR data generated by a prescan on the subject P (hereinafter referred to as prescan data) from the receiving circuitry 119, arranges the data in k-space, and generates a prescan image based on the prescan data arranged in k-space using the image generation function 161. The image generation function 161 stores the generated prescan image in the memory 13.

[0053] For example, the processing circuitry 15 uses the image generation function 161 to acquire MR data generated by a scan related to the generation of a locator image from the receiving circuitry 119, arrange the data in k-space, and generate (reconstruct) the locator image based on the MR data arranged in k-space. Also, the image generation function 161 acquires sensitivity data generated by a scan related to the generation of a sensitivity map from the receiving circuitry 119, arranges the data in k-space, and generates (reconstructs) a sensitivity map based on the sensitivity data arranged in k-space. Since existing reconstruction methods can be used to generate the locator image, sensitivity map, etc., a description thereof will be omitted.

[0054] The processing circuitry 15 generates MR images based on the complementary data for all readout directions using the image generation function 161. The generation of MR images based on the complementary data can be achieved by a general reconstruction method. Examples of the general reconstruction method include a reconstruction method using NUFFT (Non-Uniform Fast Fourier Transform) and gridding. Since the general reconstruction method can be achieved by existing technology, a description thereof will be omitted.

[0055] The image generation process executed by the MRI apparatus 100 and the image generation apparatus 1 of this embodiment configured as described above will be described with reference to Fig. 5 to Fig. 9. Fig. 5 is a flowchart showing an example of the procedure of the image generation process. First, various processes performed prior to the execution of the image generation process will be described, and then the image generation process will be described.

[0056] The imaging control circuitry 121 executes a prescan on the subject P. The processing circuitry 15 generates a prescan image based on the prescan data using the image generation function 161. The image generation function 161 stores the generated prescan image, for example, a locator image and a coil sensitivity map, in the memory 13. The system control circuitry 123 displays the locator image on the display 129.

[0057] Input interface 127 inputs an FOV (hereinafter referred to as user FOV) in a locator image in response to a user instruction. The user FOV is stored in memory 13. Processing circuitry 15 sets a sampling frequency according to the user FOV using setting function 153, and outputs the set sampling frequency to A / D converter 193. For example, setting function 153 sets a sampling frequency corresponding to oversampling, in other words, a sampling interval, by multiplying the reciprocal of the size of the user FOV by a constant.

[0058] Processing circuitry 15 uses identification function 151 to perform area detection processing on the locator image and identify a signal area in the locator image. Note that identification of a signal area in the locator image is not limited to area detection processing. The signal area may be identified, for example, by a user instruction via input interface 127. At this time, system control circuit 123 displays the pre-scan image on display 129. That is, display 129 displays the locator image under the control of system control circuit 123. Input interface 127 receives an input instruction for identifying a signal area in the displayed locator image through a user operation. As a result, processing circuitry 15 uses identification function 151 to identify the signal area.

[0059] The processing circuitry 15 sets the cutoff frequency of the low-pass filter 191 to be applied to the MR signals received by the main scan using the setting function 153. Specifically, the setting function 153 sets a provisional cutoff frequency (hereinafter referred to as provisional cutoff frequency) based on the strength of the gradient magnetic field in the main scan and the user FOV. For example, the setting function 153 sets the provisional cutoff frequency using the band of the low-pass filter 191 in the following equation: User FOV [cm] = (2 × low-pass filter bandwidth [Hz]) / readout gradient magnetic field strength [Hz / cm] In the above equation, the readout gradient magnetic field strength corresponds to the strength of the gradient magnetic field in the main scan.

[0060] Next, the setting function 153 transforms the tentative cutoff frequency into image space and compares it with the identified signal region. Hereinafter, the tentative cutoff frequency transformed into image space will be referred to as the tentative cutoff position. The setting function 153 identifies the position (hereinafter referred to as the farthest position) farthest from the tentative cutoff position among the signal regions that fall outside the passband of the low-pass filter 191 defined by the tentative cutoff position in each of the multiple readout directions in the actual scan. The setting function 153 sets the frequency corresponding to the farthest position (hereinafter referred to as the farthest frequency) as the cutoff frequency. Note that the setting function 153 may set a frequency obtained by adding a predetermined margin frequency to the farthest frequency as the cutoff frequency.

[0061] That is, the setting function 153 sets the cutoff frequency so as to include a signal region related to the generation of MR signals. Setting the cutoff frequency corresponds to, for example, widening the passband of the MR signal based on the temporary cutoff frequency. Note that the cutoff frequency may be set to include a signal region related to the generation of MR signals by a user instruction via the input interface 127.

[0062] The setting function 153 may set a cutoff frequency for each readout direction depending on the angle of the readout direction with respect to the kx direction. The setting function 153 may also set the cutoff frequency by multiplying the provisional cutoff frequency by a constant according to the sequence of the actual scan. In these cases, the setting function 153 sets the cutoff frequency so that the signal intensity of each of the multiple MR signals received by the actual scan is constant regardless of the multiple readout directions in the actual scan. The setting function 153 outputs the set cutoff frequency to the low-pass filter 191.

[0063] The imaging control circuit 121 executes radial acquisition as a main scan for the subject P. Specifically, the imaging control circuit 121 executes imaging along one readout direction in the main scan. As a result, the receiving coil 117 receives MR signals in the readout direction. The MR signals received by the receiving coil 117 are output to the receiving circuit 119. At this time, the MR signals are detected by a detector.

[0064] The low-pass filter 191 filters the detected MR signal using a set cutoff frequency. The filtering of the MR signal by the low-pass filter 191 reduces unnecessary signals outside the user FOV. This improves the S / N ratio of the MR signal, etc. The filtered MR signal is output to the A / D converter 193.

[0065] The A / D converter 193 samples the MR signal output from the low-pass filter 191 using a set sampling frequency. As a result, the A / D converter 193 generates MR data. That is, the A / D converter 193 performs A / D conversion on the MR signal that has passed through the low-pass filter 191 at the sampling frequency to generate MR data. The MR data is stored in the memory 13 via the imaging control circuit 121. When the collection of MR signals has been completed for all readout directions (i.e., all trajectories kTra in the readout direction) set in advance in this scan, image generation processing is started.

[0066] (Image generation processing) (Step S501) The processing circuitry 15 acquires the MR data and the multiple sensitivity maps from the memory 13 using the acquisition function 155. When the image generating device 1 is not installed in the MRI device 100, the acquisition function 155 acquires the MR data and the multiple sensitivity maps generated by the MRI device from the MRI device via the communication interface 11 and the network.

[0067] 6 is a diagram showing an example of MR data MRD at sampling points SP along the readout direction ROD in the k-space ks. As shown in FIG. 6, the MR data MRD is acquired by a main scan along the readout direction ROD.

[0068] (Step S502) The processing circuitry 15 uses the unfolded data generation function 157 to perform a one-dimensional Fourier transform along the readout direction using multiple sensitivity maps for all coil elements and MR data for each readout direction. For example, the NUFFT is used as the one-dimensional Fourier transform. As a result, the unfolded data generation function 157 generates unfolded data in which folding is unfolded in a one-dimensional image space along the readout direction. Data in which the unfolded data for all readout directions in the main scan are arranged in order of the angle indicating the readout direction is called projection data or sinogram.

[0069] For example, the decompressed data generating function 157 generates decompressed data for each read-out direction according to the following equation (1): An algorithm for executing equation (1) is stored in the memory 13.

number

number

number

number

number

number

[0070] That is, the argument that gives the minimum value on the right side of equation (1)

number

number

[0071] In the L2 norm on the right side of equation (1),

number

number

number

number

number

[0072] The expansion data generation function 157 generates expansion data as variables so as to minimize the right side of the formula (1).

number

number

[0073] FIG. 7 shows a one-dimensional image 1DI in a one-dimensional image space before being unfolded using multiple sensitivity maps in the readout direction ROD. At this time, aliasing appears in the user FOV UF in the one-dimensional image 1DI. FIG. 8 shows unfolded data NWI after unfolding processing of the aliasing using multiple sensitivity maps has been performed in this step. The unfolded data NWI shown in FIG. 8 is generated by one-dimensional sensitivity encoding (hereinafter referred to as SENSE (SENSitivity Encoding)) of the one-dimensional image 1DI shown in FIG. 7, as shown in equation (1).

[0074] As shown in Fig. 8, the field of view EFOV in the unfolded data NWI is expanded (enlarged) compared to the user FOV UF by unfolding processing. That is, the unfolded data corresponds to one-dimensional image data included in an expanded FOV obtained by enlarging the FOV in the readout direction in a one-dimensional image space along the readout direction. In addition, the cutoff frequency is set to include a signal region related to the generation of MR signals. Therefore, as shown in Fig. 8, no data loss occurs in the folding unfolding. The expansion of the FOV in Fig. 8 compared to Fig. 7 corresponds to increasing the sampling rate, i.e., shortening the sampling interval (oversampling).

[0075] (Step S503) The processing circuit 15 uses the complementary data generation function 159 to perform a one-dimensional inverse Fourier transform along the readout direction on the developed data NWI for each readout direction. As a result, the complementary data generation function 159 generates complementary data in which data is complemented for the MR data. The following equation (2) shows a mathematical formula for generating complementary data by the one-dimensional inverse Fourier transform performed in this step. The algorithm for executing equation (2) is stored in the memory 13.

[0076]

number

number

number

number

number

[0077] The complementary data generation function 159 generates complementary data for each readout direction by calculation using formula (2).

number

[0078] FIG. 9 is a diagram showing an example of the complementary data CPD generated from the MR data MRD shown in FIG. 6. As shown in FIG. 9, the complementary data CPD is located at an oversampling point OSP along the readout direction ROD in the k-space ks. As shown in FIG. 9 in comparison with FIG. 6, the complementary data CPD shown in FIG. 9 corresponds to k-space data obtained by oversampling the MR signals at a sampling rate higher than the sampling rate for the MR signals before A / D conversion in generating the MR data MRD shown in FIG. 6. That is, the complementary data CPD corresponds to digital data obtained by sampling the MR signals at a sampling rate higher than the sampling rate used by the A / D converter 193. In other words, the complementary data CPD shown in FIG. 9 corresponds to data obtained by interpolating new data obtained by oversampling the MR signals that are the source of the MR data shown in FIG. 6.

[0079] (Step S504) The processing circuitry 15 generates MR images based on the complementary data for all readout directions in the main scan using the image generation function 161. Specifically, the MR images for the main scan are generated by applying a general reconstruction method to the complementary data arranged in the k-space ks according to the readout direction. The image generation function 161 stores the generated MR images in the memory 13. At this time, the system control circuitry 123 may display the generated MR images on the display 129. Furthermore, if the MRI apparatus 100 does not include the image generation device 1, the generated MR images are output to, for example, a PACS server or an HIS server via the communication interface 11 and a network.

[0080] According to the image generating device 1 of the embodiment described above, MR data collected in multiple readout directions including a first readout direction and a second readout direction intersecting the first readout direction, and multiple sensitivity maps corresponding to multiple coil elements used to collect the MR data are acquired, and a one-dimensional Fourier transform along the readout direction is performed for each readout direction using the MR data and the multiple sensitivity maps, thereby generating unfolded data in which folding is unfolded in one-dimensional image space, and a one-dimensional inverse Fourier transform along the readout direction is performed for each readout direction on the unfolded data, thereby generating complementary data in which data is complemented for the MR data, and an MR image is generated based on the complementary data.

[0081] The complementary data generated by the image generating device 1 in this embodiment corresponds to k-space data obtained by oversampling the MR signal (the MR signal received by the receiving coil 117 during the main scan) before A / D conversion in generating the MR data at a sampling rate higher than the sampling rate for the MR signal.

[0082] In addition, the MR data used in the image generating device 1 in this embodiment is generated by sampling magnetic resonance signals at a sampling rate set according to the user FOV, and the MR signals are generated by filtering the MR signals received by the receiving coil 117 using a low-pass filter with a cutoff frequency set in advance by the setting function 153.

[0083] Furthermore, the cutoff frequency used in the MRI apparatus 100 of this embodiment is higher than the sampling rate set according to the user FOV. For example, the cutoff frequency is set by the setting function 153 so that the signal amount of each of the multiple MR signals corresponding to the multiple readout directions is constant regardless of the multiple readout directions. Furthermore, the cutoff frequency is set by the setting function 153 so as to include a signal region related to the generation of the multiple MR signals corresponding to the multiple readout directions. From these facts, the MRI apparatus 100 according to this embodiment can set the cutoff frequency so as to include a signal region SA related to the generation of the MR signals. As a result, the MRI apparatus 100 can set the cutoff frequency so that the signal amount of each of the multiple MR signals corresponding to the multiple readout directions is constant regardless of the multiple readout directions.

[0084] As described above, according to the image generating device 1 of the embodiment, even if sufficient oversampling cannot be performed in a scan in which the readout direction changes due to performance limitations of the MRI device 100, it is possible to generate complementary data corresponding to oversampling that exceeds the performance limitations, and generate k-space data that is consistent in the k-space. As a result, according to the present image generating device 1, it is possible to obtain high-quality images with few streaks.

[0085] 10 shows an example of an MR image (1) generated by an existing reconstruction method using MR data that is not sufficiently oversampled as a comparative example, and an MR image (2) generated based on complementary data by the image generation process of this embodiment. As shown in FIG. 10, the MR image (2) generated by the image generation process of this embodiment has reduced streak artifacts and improved image quality compared to the MR image (1). Therefore, according to the image generation device 1 of this embodiment, as shown in FIG. 10, even if sufficient oversampling cannot be performed due to performance limitations of the MRI device 100, an MR image with improved image quality can be generated for the main scan.

[0086] When the technical idea of this embodiment is realized as an image generation method, the image generation method acquires MR data collected in multiple readout directions, including a first readout direction and a second readout direction intersecting the first readout direction, and multiple sensitivity maps corresponding to the multiple coil elements used to collect the MR data. Using the MR data for each readout direction and the multiple sensitivity maps, a one-dimensional Fourier transform is performed along each readout direction to generate unfolded data in which folding is unfolded in a one-dimensional image space. A one-dimensional inverse Fourier transform is performed along each readout direction on the unfolded data to generate complementary data in which data is complemented for the MR data. An MR image is generated based on the complementary data. The image generation process and effects of this image generation method are similar to those described in the embodiment, and therefore will not be described here.

[0087] When the technical idea of this embodiment is realized by an image generation program, the image generation program causes a computer to acquire MR data collected in multiple readout directions, including a first readout direction and a second readout direction intersecting the first readout direction, and multiple sensitivity maps corresponding to the multiple coil elements used to collect the MR data, perform a one-dimensional Fourier transform along the readout direction for each readout direction using the MR data and the multiple sensitivity maps for each readout direction, thereby generating unfolded data in which folding is unfolded in a one-dimensional image space, perform a one-dimensional inverse Fourier transform along the readout direction on the unfolded data for each readout direction, thereby generating complementary data in which data is complemented for the MR data, and generate an MR image based on the complementary data.

[0088] For example, the image generation process can be realized by installing an image generation program in a computer in a modality such as the MRI apparatus 100 or a PACS server, and expanding the program in memory. In this case, the program that can cause the computer to execute the method can also be stored and distributed on a storage medium such as a magnetic disk (hard disk, etc.), an optical disk (CD-ROM, DVD, etc.), or a semiconductor memory. The procedure and effect of the image generation process using the image generation program are the same as those in this embodiment, so a description thereof will be omitted.

[0089] According to at least one of the embodiments described above, it is possible to generate a magnetic resonance image with improved image quality. That is, according to at least one of the embodiments, even if sufficient oversampling cannot be performed in a scan in which the readout direction changes, it is possible to generate data (complementary data) that is consistent in k-space, and it is possible to obtain a high-quality MR image with few streaks.

[0090] Although several embodiments have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations of embodiments can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0091] 1. Image generation device 11 Communication Interface 13. Memory 15 Processing circuit 100 Magnetic resonance imaging device 101 Static Magnetic Field Magnet 103 Gradient magnetic field coil 105 Gradient magnetic field power supply 107 Sleeper 109 Bed control circuit 111 Bore 113 Transmitting circuit 115 Transmitting Coil 117 Receiving Coil 119 Receiving circuit 121 Imaging control circuit 123 System Control Circuit 127 input interface 129 Display 151 Specific Functions 153 Setting Function 155 Acquisition Function 157 Expanded data generation function 159 Complementary data generation function 161 Image generation function 191 Low-pass filter 193 A / D converter

Claims

1. an acquisition unit that acquires magnetic resonance data acquired in a plurality of readout directions including a first readout direction and a second readout direction intersecting the first readout direction, and a plurality of sensitivity maps corresponding to a plurality of coil elements used to acquire the magnetic resonance data; a decompressed data generating unit that generates, for each readout direction, decompressed data in which folding is decompressed in a one-dimensional image space using the magnetic resonance data and the sensitivity map for each readout direction; a complementary data generating unit that generates complementary data for each of the readout directions by performing a one-dimensional Fourier transform along the readout direction on each of the developed data; and an image generating unit that generates a magnetic resonance image based on the complementary data; An image generating device comprising:

2. the complementary data corresponds to k-space data obtained by oversampling the magnetic resonance signals at a sampling rate higher than the sampling rate for the magnetic resonance signals prior to analog-to-digital conversion in generating the magnetic resonance data; The image generating device of claim 1 .

3. the magnetic resonance data is generated by sampling magnetic resonance signals at a sampling rate set according to an imaging field of view; the magnetic resonance signals are generated by filtering the magnetic resonance signals received by the coil elements with a low-pass filter using a preset cutoff frequency; the cutoff frequency is higher than the sampling rate; 3. The image generating device according to claim 1 or 2.

4. the cutoff frequency is set to include a signal region related to the generation of a plurality of magnetic resonance signals corresponding to a plurality of readout directions; The image generating device of claim 3 .

5. The decompressed data generation unit calculating, for each readout direction, a product of estimated expanded data obtained by one-dimensional inverse Fourier transform along a readout direction of estimated magnetic resonance data estimated when the magnetic resonance data is collected at a sampling rate higher than a sampling rate for collecting the magnetic resonance data, and the sensitivity map; performing a one-dimensional Fourier transform of the product along the readout direction; generating, for each readout direction, the unfolded data in which folding is unfolded in a one-dimensional image space by performing optimization so as to reduce a difference between the result of the one-dimensional Fourier transform and the magnetic resonance data; The image generating device of claim 1 .

6. The decompressed data generation unit generating the unfolded data for each of the readout directions by optimizing the sum of the L2 norm of the difference and a regularization term; The image generating device according to claim 5 .

7. acquiring magnetic resonance data collected in a plurality of readout directions including a first readout direction and a second readout direction intersecting the first readout direction, and a plurality of sensitivity maps corresponding to a plurality of coil elements used to collect the magnetic resonance data; generating, for each readout direction, unfolded data in which folding is unfolded in a one-dimensional image space using the magnetic resonance data and the sensitivity map for each readout direction; performing a one-dimensional Fourier transform along the readout direction on each of the expanded data for each of the readout directions, thereby generating complementary data in which data is complemented for the magnetic resonance data for each of the readout directions; generating a magnetic resonance image based on each of the complementary data; An image generating method comprising:

8. On the computer, acquiring magnetic resonance data collected in a plurality of readout directions including a first readout direction and a second readout direction intersecting the first readout direction, and a plurality of sensitivity maps corresponding to a plurality of coil elements used to collect the magnetic resonance data; generating, for each readout direction, unfolded data in which folding is unfolded in a one-dimensional image space using the magnetic resonance data and the sensitivity map for each readout direction; performing a one-dimensional Fourier transform along the readout direction on each of the expanded data for each of the readout directions, thereby generating complementary data in which data is complemented for the magnetic resonance data for each of the readout directions; generating a magnetic resonance image based on the complementary data; An image generation program that achieves this.

Images