JP7291610B2 - MAGNETIC RESONANCE IMAGING DEVICE, IMAGE PROCESSING DEVICE, AND MAGNETIC RESONANCE IMAGING METHOD - Google Patents

Description

Embodiments disclosed in the present specification and drawings relate to a magnetic resonance imaging apparatus, an image processing apparatus, and a magnetic resonance imaging method.

A magnetic resonance imaging apparatus excites the nuclear spins of a subject placed in a static magnetic field with a high-frequency signal (RF (Radio Frequency) signal) of the Larmor frequency, and a magnetic resonance signal (MR (Magnetic Resonance) signal) to generate an image.

As is well known, the frequency of the MR signal emanating from the subject is proportional to the magnitude of the magnetic field. In a magnetic resonance imaging apparatus, a static magnetic field showing a spatially uniform magnitude and a gradient magnetic field showing a magnitude roughly proportional to the position from the magnetic field center are superimposed to produce a superimposed magnetic field showing different magnitudes depending on the position. is generating

In the magnetic resonance imaging apparatus, by applying this superimposed magnetic field to the subject, MR signals exhibiting different frequencies (or different phases) depending on the position of the tissue inside the subject are acquired. Then, the magnetic resonance imaging apparatus obtains the frequency (or phase) of the acquired MR signal and the signal intensity at that frequency (or phase) by reconstruction processing using, for example, inverse Fourier transform, etc., so that the subject Pixel values (that is, a reconstructed image of the subject) arranged for each position of the internal tissue are generated.

Reconstruction processing using inverse Fourier transform or the like assumes that the frequency of the MR signal changes in proportion to the position, that is, the frequency of the MR signal and the position are in a linear relationship.

On the other hand, the gradient magnetic field is generated by, for example, a cylindrical gradient coil. Normally, the gradient magnetic field changes linearly in the vicinity of the axial center of the gradient magnetic field coil (that is, in the vicinity of the magnetic field center), but exhibits nonlinear changes as the distance from the magnetic field center increases. Therefore, at a position distant from the magnetic field center in the reconstructed image, the linear relationship between the frequency and the position of the MR signal is no longer maintained, and pixel position deviation due to the nonlinearity of the gradient magnetic field, that is, image distortion occurs. will occur.
In order to suppress such image distortion, conventionally, a correction table for correcting the nonlinearity of the gradient magnetic field has been used.

On the other hand, in recent years, there has been an increasing demand for increasing the intensity of the gradient magnetic field and increasing the slew rate of the gradient magnetic field pulse. If these requirements are to be satisfied under physical restrictions such as the weight and size of the gradient coil and restrictions on development costs, the linear region of the gradient magnetic field must be narrowed to some extent as a characteristic of the gradient coil alone. The situation is becoming unavoidable. Therefore, instead of improving the characteristics of the gradient magnetic field coil alone, or in addition to improving the characteristics of the gradient magnetic field coil alone, there is a demand for a technique that ensures linearity over a wider range than before by correction using a correction table. It is

JP 2009-226199 A

An object to be solved by the embodiments disclosed in the specification and drawings is to appropriately correct image distortion caused by the nonlinearity of the gradient magnetic field generated by the gradient coil.

However, the problems to be solved by the embodiments disclosed in this specification and drawings are not limited to the above problems. A problem corresponding to each effect of each configuration shown in the embodiments described later can also be positioned as another problem to be solved by the embodiments disclosed in this specification and the like.

A magnetic resonance imaging apparatus according to one embodiment includes an imaging unit, a reconstruction unit, and a correction unit. The imaging unit includes at least a high-frequency coil and a gradient magnetic field coil, and detects magnetic resonance emitted from a subject in response to application of a high-frequency pulse output from the high-frequency coil and a gradient magnetic field formed by the gradient magnetic field coil. Collect signals. A reconstruction unit reconstructs the acquired magnetic resonance signals to generate a diagnostic image of the subject. The correction unit corrects the nonlinear characteristics of the gradient magnetic field to linear characteristics defined by the intensity of the gradient magnetic field at a position away from the magnetic field center of the gradient coil and the distance from the magnetic field center to the position. Distortion correction data is generated and used to correct the reconstructed diagnostic image.

1 is a configuration diagram showing an example of the overall configuration of a magnetic resonance imaging apparatus according to an embodiment; FIG. FIG. 2 is a block diagram particularly showing functions of a processing circuit in the magnetic resonance imaging apparatus; FIG. 1 is a first diagram for explaining a prior art related to correction of nonlinearity of a gradient magnetic field; FIG. 2 is a second diagram illustrating a conventional technique for correcting nonlinearity of a gradient magnetic field; 4 is a flowchart showing an operation example of the magnetic resonance imaging apparatus of the embodiment; 1 is a first diagram for explaining the operation concept of the magnetic resonance imaging apparatus of the embodiment; FIG. 4 is a flowchart showing another operation example of the magnetic resonance imaging apparatus according to the embodiment; FIG. 2 is a second diagram for explaining the operation concept of the magnetic resonance imaging apparatus of the embodiment; FIG. 4 is a diagram showing an example of a calibration phantom used in the magnetic resonance imaging apparatus of the embodiment; FIG. 7 is a view for explaining the operational concept of a magnetic resonance imaging apparatus according to the second embodiment; FIG. 11 is a diagram for explaining the operation concept of a magnetic resonance imaging apparatus according to the third embodiment; FIG. 11 is a diagram for explaining the operation concept of a magnetic resonance imaging apparatus according to the fourth embodiment; 2 is a block diagram showing an example of functions of the image processing apparatus according to the embodiment; FIG.

An embodiment of the present invention will be described below with reference to the accompanying drawings.
(First embodiment)
FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus 1 according to the first embodiment. The magnetic resonance imaging apparatus 1 includes a magnet pedestal 100, a control cabinet 300, a console 400, a bed 500, and the like.

The magnet mount 100 has a static magnetic field magnet 10, a gradient magnetic field coil 11, a WB (Whole Body) coil 12, etc., and these components are housed in a cylindrical housing. The bed 500 has a bed body 50 and a top board 51 . The magnetic resonance imaging apparatus 1 also has an RF coil 20 arranged close to the subject.

The control cabinet 300 includes a gradient magnetic field power supply 31 (31x for X axis, 31y for Y axis, 31z for Z axis), RF receiver 32, RF transmitter 33, and sequence controller .

The static magnetic field magnet 10 of the magnet pedestal 100 has a substantially cylindrical shape, and generates a static magnetic field in a bore (a space inside the cylinder of the static magnetic field magnet 10), which is an imaging region of a subject (for example, a patient). The static magnetic field magnet 10 incorporates a superconducting coil, and the superconducting coil is cooled to an extremely low temperature by liquid helium. The static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power supply (not shown) to the superconducting coil in the excitation mode, and then, when shifting to the persistent current mode, the static magnetic field power supply is separated. Once transferred to the persistent current mode, the static magnetic field magnet 10 continues to generate a large static magnetic field for a long time, for example, over one year. Note that the static magnetic field magnet 10 may be configured as a permanent magnet.

The gradient magnetic field coil 11 also has a substantially cylindrical shape and is fixed inside the static magnetic field magnet 10 . The gradient magnetic field coil 11 has a 3-channel structure. The gradient magnetic field coils of each channel of the gradient magnetic field coil 11 are supplied with currents from the gradient magnetic field power supplies (31x, 31y, 31z), respectively, and gradient magnetic fields are generated in the respective directions of the X-axis, Y-axis, and Z-axis. be.

The bed main body 50 of the bed 500 can move the top plate 51 in the vertical direction, and moves the subject placed on the top plate 51 to a predetermined height before imaging. After that, when imaging, the top plate 51 is moved horizontally to move the subject into the bore.

The WB coil 12 is fixed in a substantially cylindrical shape inside the gradient magnetic field coil 11 so as to surround the subject. The WB coil 12 transmits an RF pulse transmitted from the RF transmitter 33 toward the subject, and receives magnetic resonance signals (that is, MR signals) emitted from the subject by excitation of hydrogen nuclei.

The RF coil 20 receives MR signals emitted from the subject at a position close to the subject. The RF coil 20 is composed of, for example, multiple element coils. There are various types of RF coils 20, such as for the head, chest, spine, lower limbs, or whole body, depending on the imaging region of the subject. FIG. 1 illustrates the RF coil 20 for the chest. ing.

The RF transmitter 33 transmits RF pulses to the WB coil 12 based on instructions from the sequence controller 34 . On the other hand, the RF receiver 32 detects MR signals received by the WB coil 12 and the RF coil 20 and sends raw data obtained by digitizing the detected MR signals to the sequence controller 34 .

The sequence controller 34 scans the subject by driving the gradient magnetic field power supply 31 , the RF transmitter 33 and the RF receiver 32 under the control of the console 400 . Then, when the sequence controller 34 scans and receives raw data from the RF receiver 32 , it sends the raw data to the console 400 .

The sequence controller 34 has a processing circuit (not shown). This processing circuit is composed of, for example, a processor that executes a predetermined program, and hardware such as FPGA (Field Programmable Gate Array) and ASIC (Application Specific Integrated Circuit).
Console 400 is configured as a computer having processing circuitry 40 , memory circuitry 41 , display 42 and input device 43 .

The storage circuit 41 is a storage medium including ROM (Read Only Memory), RAM (Random Access Memory), and external storage devices such as HDD (Hard Disk Drive) and optical disc device. The storage circuit 41 stores various kinds of information and data, as well as various programs executed by the processor included in the processing circuit 40 .

The display 42 is a display device such as a liquid crystal display panel, plasma display panel, or organic EL panel. The input device 43 is, for example, a mouse, keyboard, trackball, touch panel, etc., and includes various devices for the operator to input various information and data.

The processing circuit 40 is, for example, a circuit including a CPU or a dedicated or general-purpose processor. The processor implements various functions described later by executing various programs stored in the storage circuit 41 . The processing circuit 40 may be configured by hardware such as FPGA (field programmable gate array) or ASIC (application specific integrated circuit). These hardware can also realize various functions described later. Also, the processing circuit 40 can realize various functions by combining software processing by a processor and a program and hardware processing.

The console 400 controls the entire magnetic resonance imaging apparatus 1 by these components. Specifically, it receives an imaging condition such as the type of pulse sequence, various information, or an instruction to start imaging by operating a mouse, keyboard, or the like (input device 43) by an operator such as a laboratory technician. The processing circuit 40 causes the sequence controller 34 to perform scanning based on the input imaging conditions, while reconstructing an image based on the raw data transmitted from the sequence controller 34, that is, the digitized MR signal. do. The reconstructed image is displayed on display 42 or stored in memory circuit 41 .

Note that, in the configuration of the magnetic resonance imaging apparatus 1 shown in FIG. 1, the components other than the console 400 (the control cabinet 300, the magnet pedestal 100, and the bed 500) constitute an imaging unit 600. FIG.

FIG. 2 particularly shows a functional block diagram of the processing circuit 40 in the magnetic resonance imaging apparatus 1. As shown in FIG. As shown in FIG. 2, the processing circuit 40 of the magnetic resonance imaging apparatus 1 includes an imaging condition setting function 401, a reconstruction function 402, a calibration function 403, a correction function 404, a calibration data generation function 405, and a distortion correction data generation function. Each function of 406 is realized. Each of these functions is realized, for example, by a processor included in the processing circuit 40 executing a predetermined program.

The imaging condition setting function 401 receives various imaging conditions set by the operator via the input device 43 and sets the received imaging conditions in the sequence controller 34 . The imaging conditions to be set include parameters related to the pulse sequence. Parameters related to the pulse sequence include parameters related to the RF pulse output from the WB coil 12 and parameters related to the gradient magnetic field pulse output from the gradient magnetic field coil 11 .

The imaging unit 600 applies RF pulses and gradient magnetic field pulses corresponding to the set imaging conditions to the subject. MR signals emitted from the subject in response to these applications are collected and sent to the reconstruction function 402 of the processing circuit 40 . The MR signal sent from the imaging unit 600 to the reconstruction function 402 is so-called k-space data.

A reconstruction function 402 performs reconstruction processing such as inverse Fourier transform on the MR signal to convert k-space data into real space data, thereby generating a diagnostic image of the subject.
The calibration function 403 uses the calibration data 411 stored in the storage circuit 41 to calibrate the diagnostic image. The processing of the calibration function 403 will be described below.

Calibration (or calibration) means that the size and/or position of the imaged object depicted in the diagnostic image matches the actual size and/or position of the imaged object. It is a process for adjusting the magnetic resonance imaging apparatus 1 .

For example, at any time before imaging a subject (for example, when the magnetic resonance imaging apparatus 1 is installed in a facility such as a hospital), a phantom of known size is imaged to obtain a calibration image. Then, the known size of the phantom is used as a reference value, and data that associates this reference value with the size of the phantom drawn in the calibration image is acquired as the calibration data 411 . In this case, the calibration data 411 is, for example, the ratio of the known phantom size X to the phantom size Y depicted in the calibration image. By using such calibration data 411, the tissue sizes and inter-tissue distances depicted in the reconstructed diagnostic image are converted into actual tissue sizes and inter-tissue distances of the subject. can be done.

A phantom used for calibration may be, for example, a cubic or cylindrical phantom filled with a homogeneous substance, or may be a phantom in which a plurality of small markers are arranged. In the former case, the outer dimensions of the phantom can serve as a reference for calibration. Also, in the latter case, the spacing between the markers can serve as a reference value for calibration.

Further, when the position of the outer edge of the phantom is set as the calibration position, the distance from the center of the magnetic field to the outer edge of the phantom can be used as the reference value for calibration. Alternatively, when the position of a specific marker among a plurality of markers in the phantom is set as the calibration position, the distance from the center of the magnetic field to the position of the specific marker can be used as the reference value for calibration.

The calibration method described above is a method of adjusting the reconstructed image using the calibration data 411, but a hardware calibration method is also conceivable. In the hardware calibration method, the gradient magnetic field intensity is adjusted using a calibration image obtained by imaging a phantom before imaging the subject (for example, when the magnetic resonance imaging apparatus 1 is installed). Here, the gradient magnetic field strength is adjusted, for example, by adjusting the value of the gradient magnetic field current supplied to the gradient magnetic field coil 11 from the gradient magnetic field power supplies (31x, 31y, 31z). For example, the actual size of the phantom and the distance between the markers are used as reference values, and these reference values match the size of the phantom drawn in the calibration image before calibration and the distance between the markers before calibration. Adjust the value of the gradient magnetic field current so that

Data used for adjusting the gradient magnetic field current is stored in the storage circuit 41 as the calibration data 411 . In imaging for obtaining a diagnostic image of the subject, the gradient magnetic field current reflecting the calibration data 411 is applied to the gradient magnetic field coil 11 via the imaging condition setting function 401 and the sequence controller 34 .

As in software calibration, when the position of the outer edge of the phantom is set as the calibration position, the distance from the center of the magnetic field to the outer edge of the phantom can be used as the reference value for calibration. Among them, when the position of the specific marker is taken as the calibration position, the distance from the center of the magnetic field to the position of the specific marker can be used as the reference value for calibration.

Such hardware calibration also allows the calibration positions of the actual outer edge of the phantom and the marker to be associated with the positions of the outer edge of the phantom and the marker depicted in the calibration image. Note that calibration may be performed by combining the above-described software calibration and hardware calibration.
The processing for acquiring the calibration data 411 and the processing for storing the calibration data 411 in the storage circuit 41 are performed by the calibration data generation function 405 shown in FIG.

In the magnetic resonance imaging apparatus 1 of this embodiment, the correction function 404 corrects the diagnostic image calibrated by the calibration function 403 using the distortion correction data 412 stored in the storage circuit 41 . Here, the distortion correction data 412 is data for correcting the nonlinear distortion of the gradient magnetic field so that the nonlinear characteristic of the gradient magnetic field becomes linear. In particular, in the magnetic resonance imaging apparatus 1 of the present embodiment, the nonlinear characteristics of the gradient magnetic field are linear characteristics based on the gradient magnetic field at the correction reference position set at a position away from the magnetic field center of the gradient coil 11. , are data for correcting the nonlinear distortion of the gradient magnetic field.
The distortion correction data 412 are generated by the distortion correction data generation function 406 and stored in the storage circuit 41 .

Before entering into a more specific description of the distortion correction data generation function 406 and the correction function 404 of the present embodiment, using FIGS. explain.

The graph shown in FIG. 3 is a graph whose origin is the position of the center of the magnetic field, and the horizontal axis of the graph indicates the position within the bore. Also, the vertical axis of the graph indicates the gradient magnetic field. In the example shown in FIG. 3, the horizontal axis of the graph is the position z in the Z direction, and the horizontal axis of the graph is the gradient magnetic field B(z) in the Z direction.

Let ω(z) denote the angular frequency of the MR signal caused only by the gradient magnetic field B(z), excluding the central angular frequency _ω0 of the MR signal caused by the static magnetic field _B0 . In addition, ω(z) can be expressed by the following (formula 1) and (formula 2).
ω(z)=γ・B(z) (Formula 1)
=γ・G(z)・z (Formula 2)

Here, γ is a constant called the gyromagnetic ratio. G(z) is the gradient of the gradient magnetic field B(z) in the application direction of the gradient magnetic field B(z) (that is, the rate of change of the gradient magnetic field B(z) with respect to z). In the graph of FIG. 3, the characteristic indicated by the curve labeled "Actual Gradient Magnetic Field Characteristic" corresponds to (Equation 1) or (Equation 2). As described above, since the angular frequency ω of the MR signal is proportional to the applied gradient magnetic field B(z), the vertical axis of the graph in FIG. 3 also corresponds to the angular frequency ω of the MR signal. .

Ideally, the gradient G(z) of the gradient magnetic field should exhibit a constant value over a wide range. It will be.

As shown in FIG. 3, although the gradient G(z) usually exhibits a substantially constant value near the magnetic field center, the gradient gradually decreases as the distance from the magnetic field center increases. , or the characteristic of the slope G(z) becomes non-linear.

Conventionally, distortion correction data, such as a distortion correction table, has been used to correct such nonlinear characteristics to linear characteristics. The conventional distortion correction table corrects the gradient G(z) of the gradient magnetic field B(z) so that the gradient G(z) at the magnetic field center of the gradient magnetic field B(z) is ₀ at any position in the z direction. be. The characteristic corrected by the distortion correction table can be represented by, for example, the following (Equation 3).
ω'(z)=γ·G ₀ ·z (Formula 3)
The characteristic represented by (Equation 3) corresponds to the straight line indicated by the solid line in the graph of FIG. Here, the characteristic represented by (Equation 3) is called a first ideal linear gradient magnetic field characteristic having a gradient _G0 of the gradient magnetic field at the magnetic field center.

On the other hand, in addition to correction using a distortion correction table, calibration using a phantom has also been conventionally performed. For example, the phantom is placed in the bore so that the center of the phantom coincides with the center of the magnetic field. Then, as described above, calibration is performed using the position of the outer edge of the phantom as the calibration position. This calibration provides calibration data that associates the angular frequency ω _C corresponding to the position of the outer edge of the phantom in the calibration image with the actual phantom calibration position z _C . Once the calibration data is obtained, the calibration data is applied to the reconstructed image to associate the position (or size) on the image with the position (or size) of the actual tissue of the subject. becomes possible.

By the way, such a calibration process is nothing other than associating the angular frequency ω''(z) of the MR signal obtained by imaging the subject with the position z by the following (Equation 4).
ω″(z)=γ· _GC ·z (Formula 4)
where G _C is the gradient defined by the gradient B(z) at the calibration position z _C , more precisely the gradient B(z _C ) at the calibration position z _C and the calibration position z _C ratio. That is, G _C =B(z _C )/z _C . The characteristic expressed by (Equation 4) corresponds to the straight line indicated by the dotted line in the graph of FIG. Here, the characteristic represented by (Formula 4) shall be called a 2nd ideal linear gradient magnetic field characteristic.

As described above, conventionally, the calibration process represented by (Equation 4) and the nonlinear correction process represented by (Equation 3) are performed. However, in these conventional processes, since the gradient _GC of the second ideal linear gradient magnetic field characteristic (equation 4) and the gradient _G0 of the first ideal linear gradient magnetic field characteristic (equation 3) are different, the distortion correction table is Even after performing the correction used, a residual error would occur. This residual error is caused by the fact that the slopes of the two straight lines of (Equation 3) and (Equation 4) are different. In the graph of FIG. 3, the hatched area corresponds to the residual error.

FIG. 4 is a diagram schematically showing how the magnitude of this residual error changes depending on the width of the linear region of the gradient magnetic field. In the two graphs shown in FIG. 4, the horizontal axis indicates the position in the z direction as in FIG. 3 (however, FIG. 4 shows both positive and negative regions with respect to the magnetic field center). On the other hand, the vertical axis indicates the gradient of the gradient magnetic field, not the magnitude of the gradient magnetic field.

As shown in the upper part of FIG. 4, when the magnitude of the gradient magnetic field is not so large and the slew rate of the gradient magnetic field pulse is not so large, the range of the linear region of the gradient magnetic field is wide, that is, the gradient of the gradient magnetic field has a wide flat area. In this case, the difference between the gradient _G0 of the gradient magnetic field at the magnetic field center and the gradient _GC at the calibration position is small. Therefore, the residual error after nonlinear distortion correction by the distortion correction table is relatively small. This residual error corresponds to the difference between the slope _G0 and the slope _Gc indicated by hatching in the graph of FIG.

On the other hand, in recent years, attempts have been made to increase the intensity of the gradient magnetic field and increase the slew rate of the gradient magnetic field pulse. Therefore, as shown in the lower part of FIG. 4, the linear region of the gradient magnetic field has been narrowed. In this case, the difference between the gradient G ₀ of the gradient magnetic field at the magnetic field center and the gradient G _C at the calibration position becomes large, resulting in a large residual error after nonlinear distortion correction by the distortion correction table.

The above is an explanation of the matters to be improved in the prior art regarding the correction of the nonlinearity of the gradient magnetic field and the reasons thereof. A more specific operation of this embodiment for solving these problems will be described below with reference to FIGS. 5 to 8. FIG. 5 and 7 are flowcharts showing an example of the operation of the magnetic resonance imaging apparatus 1 according to this embodiment, and FIGS. 6 and 8 are diagrams for explaining the concept of the operation.

At step ST100 in FIG. 5, the gradient magnetic field characteristics of the gradient magnetic field coil 11 are obtained by calculation or actual measurement. The gradient magnetic field characteristics calculated or actually measured here are, for example, the nonlinear characteristics shown in the graph shown in FIG. As described above, the gradient magnetic field characteristics generally change linearly in the vicinity of the magnetic field center in proportion to the distance from the magnetic field center, and exhibit nonlinear characteristics in which the gradient gradually decreases as the distance from the magnetic field center increases.

When obtaining the gradient magnetic field characteristics of the gradient magnetic field coil 11 by calculation, the gradient magnetic field characteristics are based on design parameters such as the diameter size of the gradient magnetic field coil 11, the axial length, the size of the coil conductor, and the arrangement pattern of the coil conductor. Calculated.

In step ST101, the nonlinear characteristics of the gradient magnetic field calculated or measured in step ST100 are inclined so as to become linear characteristics based on the gradient magnetic field at the correction reference position set at a position away from the magnetic field center of the gradient coil 11. Distortion correction data 412 is generated to correct for non-linear distortion of the magnetic field. The process of step ST101 is performed by the distortion correction data generation function 406. FIG.

FIG. 6 is a diagram showing the concept of generating the distortion correction data 412 according to the magnetic resonance imaging apparatus 1 of this embodiment. A straight line obliquely extending from the origin in the graph of FIG. 6 is the linear characteristic after correction by the distortion correction data 412 . This linear characteristic is based on the gradient magnetic field B(z _C ) at the correction reference position z _C away from the magnetic field center. Here, the “linear characteristic with reference to the gradient magnetic field B(z _C ) at the correction reference position z _C ” is the ratio of the gradient magnetic field B(z _C ) at the correction reference position z _C to the correction reference position z _C That is, it is a linear characteristic represented by a straight line passing through the origin with a gradient of G _C =B(z _C )/z _C , that is, an ideal linear gradient magnetic field characteristic. This ideal linear gradient magnetic field characteristic is represented, for example, by the following (Equation 5).
ω'(z)=γ· _GC ·z (Formula 5)
The distortion correction data 412 is generated, for example, as data that offsets the difference between the ideal linear gradient magnetic field characteristic represented by (Equation 5) and the actual nonlinear gradient magnetic field characteristic represented by (Equation 2).

The distortion correction data 412 may be, for example, a correction table in the form of a lookup table. Alternatively, the distortion correction data 412 may be parameters of a conversion function for converting nonlinear characteristics into linear characteristics.

So far, for convenience of explanation, one-dimensional correction in the Z-axis direction has been described. It can be generated as three-dimensional data. Also, by providing a plurality of two-dimensional distortion correction data (for example, two-dimensional distortion correction data on the XY plane) in another axial direction (for example, in the Z-axis direction), three-dimensional distortion correction data is generated. can do.

Also, the gradient magnetic field characteristics are usually symmetrical in the positive and negative directions of the X-, Y-, and Z-axes when viewed from the center of the magnetic field. For this reason, for example, in the case of two-dimensional distortion correction data, only the distortion correction data for the first quadrant is calculated and stored, and for the other three quadrants, the distortion correction data for the first quadrant is used. can be

In step ST102, before a diagnostic image is acquired, for example, when the magnetic resonance imaging apparatus 1 is installed in a facility such as a hospital, the phantom is imaged, and the calibration set at the same position as the correction reference position described above is performed. A position-based calibration is performed to obtain calibration data.

The calibration processing in step ST102 has already been described as a function performed by the calibration function 403, and is substantially the same as the conventional calibration processing, so description thereof will be omitted.

Note that in the description of the process of step ST102, it is stated that "a phantom is imaged and calibration is performed with reference to the calibration position set at the same position as the correction reference position described above". However, the calibration positions (for example, the position of the outer edge of the phantom, the position of the marker inside the phantom, etc.) in the calibration process using the phantom are determined for each phantom and cannot normally be changed. Therefore, actually, instead of setting the calibration position at the same position as the correction reference position, the correction reference position when generating the distortion correction data is set so as to match the calibration position determined for each phantom. will be set.

As for the calibration position and correction reference position, it is preferable to set them at a position at least 7 cm away from the center of the magnetic field from the viewpoint of nonlinear distortion correction accuracy and calibration accuracy.

Step ST103 and subsequent steps are processes for obtaining normal diagnostic images. In step ST103, a diagnostic image of the subject is generated by reconstructing the MR signals acquired by imaging the subject. In step ST104, the diagnostic image is calibrated using the calibration data acquired in step ST102. The reconstruction function 402 performs the process of step ST103, and the calibration function 403 performs the process of step ST104.

In step ST105, the distortion correction data generated in step ST101 is used to correct nonlinear distortion of the calibrated diagnostic image. The correction function 404 performs the processing of step ST105.

The flowchart shown in FIG. 5 corresponds to a processing example corresponding to the software calibration processing described above. In this case, the calibration data acquired in step ST102 is data that associates the actual size of the phantom with the size of the phantom drawn in the calibration image. It is the ratio of the sizes of the rendered phantoms.

On the other hand, as mentioned above, a hardware calibration process is also possible. FIG. 7 shows a flowchart based on an example process corresponding to the hardware calibration process. In the flowchart shown in FIG. 7, the processes from step ST100 to step ST102 are substantially the same as those in FIG. 5, so description thereof will be omitted.

However, the position calibration process performed in step ST102 is performed by calibrating the value of the gradient magnetic field by adjusting the gradient magnetic field current. Also, the calibration data acquired in step ST102 is data relating to the adjustment value of the gradient magnetic field current.

In step ST200, the subject is imaged using the calibration data to acquire MR signals. That is, a calibrated gradient magnetic field based on the calibration data acquired in step ST102 is applied to image the subject and acquire MR signals. The acquired MR signals are then reconstructed to produce a diagnostic image of the subject.

Since the diagnostic image generated in step ST200 is an image whose position has already been calibrated, the process of step ST104 in FIG. unnecessary. In step ST201, the distortion of the diagnostic image generated in step ST200 is corrected using the distortion correction data generated in step ST101. The correction function 404 performs the process of step ST200.

Although the calibration processing method differs between the processing shown in FIG. 5 and the processing shown in FIG. A similar technical effect is obtained.

FIG. 8 is a diagram for explaining the technical effect of the nonlinear distortion of the present embodiment by comparing the conventional nonlinear distortion correction processing and the nonlinear distortion correction processing of the present embodiment. The upper diagram in FIG. 8 is substantially the same diagram as the lower diagram in FIG.

In conventional nonlinear distortion correction processing, position calibration using a phantom is performed with reference to the calibration position _zC away from the magnetic field center, while distortion correction data for correcting the nonlinearity of the gradient magnetic field is obtained from the magnetic field center It was generated based on the gradient of the gradient magnetic field at . In other words, in conventional nonlinear distortion correction processing, the correction reference position used for distortion correction is different from the calibration position used as the reference for calibration processing. For this reason, the slope _GC of the ideal linear gradient magnetic field characteristic assumed in the calibration process and the slope _G0 of the ideal linear gradient magnetic field characteristic after the distortion correction process do not match. As a result, after the nonlinear distortion correction, a residual error occurs due to the difference between the slopes of the two.
This residual error cannot be ignored as the intensity of the gradient magnetic field increases or as the slew rate of the gradient magnetic field pulse increases.

On the other hand, in the nonlinear distortion correction processing of the present embodiment, as shown in the lower part of FIG. I am trying to generate data. Therefore, the gradient G _C of the ideal linear gradient magnetic field characteristic assumed in the calibration process and the gradient G _C of the ideal linear gradient magnetic field characteristic after the distortion correction process are matched. As a result, after the nonlinear distortion correction, the residual error caused by the difference between the slopes of the two can be made substantially zero.

As shown in the upper part of FIG. 8, conventional nonlinear distortion correction processing using distortion correction data increases the inclination of the gradient magnetic field over almost the entire region other than the magnetic field center. On the other hand, in the nonlinear distortion correction processing using the distortion correction data of the present embodiment, as shown in the lower part of FIG. 8 (or FIG. 6), in the first region from the magnetic field center to the correction reference position, the gradient magnetic field The diagnostic image is corrected so that the tilt of is equivalently reduced. On the other hand, the diagnostic image is corrected so that the gradient of the gradient magnetic field is equivalently increased in the second area beyond the correction reference position, which is further away from the magnetic field center. Therefore, the correction processing of this embodiment not only makes the residual error substantially zero, but also reduces the magnitude of the absolute value of the correction amount at a position away from the magnetic field center as compared with the conventional method. becomes possible.

As described above, according to the magnetic resonance imaging apparatus 1 and the magnetic resonance imaging method according to the above-described embodiments, the linear region of the gradient magnetic field is expanded by increasing the strength of the gradient magnetic field and increasing the slew rate of the gradient magnetic field pulse. Even if it is narrowed, it is possible to ensure linearity over a wider range than before by correcting the nonlinear distortion using the distortion correction data.

In the above description, the distortion correction data generation function 406 performs processing for generating and storing distortion correction data, and the correction function 404 performs processing for correcting a diagnostic image using the distortion correction data. However, these two functions may be combined so that correction function 404 generates distortion correction data and uses the generated distortion correction data to correct a diagnostic image.

Up to this point, the nonlinear distortion of the gradient magnetic field has been described as being corrected in the real space, but it is also possible to perform the distortion correction processing in the k-space. In this case, the distortion correction data generation function 406 or the correction function 404 generates distortion correction data for correcting the nonlinear characteristics of the gradient magnetic field to linear characteristics on the k-space. Then, the correction function 404 phase-corrects the MR signal before reconstruction processing using the generated distortion correction data on the k-space. Thereafter, the reconstructing function 402 reconstructs the corrected MR signals, thereby obtaining a diagnostic image in which the distortion due to the nonlinear characteristics of the gradient magnetic field is corrected.

FIG. 9 is a diagram showing an example of a calibration phantom used in the magnetic resonance imaging apparatus 1 of this embodiment. This calibration phantom has a configuration in which a plurality of (four in the example of FIG. 9) thin pin-shaped markers are provided inside a cubic case. Each pin-shaped marker is placed at a distance Pc, for example, from the center of the phantom.

Depending on the type of substance filled inside the pin-shaped marker and the substance filled outside the pin-shaped marker, for example, the MR signal is generated only from the pin-shaped marker, and the MR signal is not generated from the outside. can be Alternatively, conversely, it is also possible to generate MR signals from the entire cubic area excluding the area of the pin-shaped marker and not generate the MR signal from the pin-shaped marker. In either case, when observed from a direction parallel to the axis of the pin-shaped marker, the pin-shaped marker is literally depicted as a pinpoint point, so positional calibration can be performed with high accuracy.

When performing the calibration process, the center of this calibration phantom is positioned at the center of the magnetic field, and the axial direction of the pin-shaped marker is positioned perpendicular to the imaging cross section of the calibration image. With such an arrangement, pin-shaped markers are depicted as dots in the calibration image.

When performing position calibration in the directions of the X-axis, Y-axis and Z-axis, as illustrated in the right diagram of FIG. Orientation is set to acquire at least 3 cross-sectional calibration images. For example, calibration is performed by acquiring calibration images of three cross sections, an axial cross section (XY cross section), a coronal cross section (XZ cross section), and a sagittal cross section (YZ cross section). In this example, the above-described calibration position is the distance Pc from the center position of the phantom placed at the center of the magnetic field to the pin-shaped marker for any axis. Therefore, the correction reference position, which is the reference for generating the distortion correction data, is also the position away from the center of the magnetic field by the distance Pc.

(Second embodiment)
FIG. 10 is a diagram for explaining the operation concept of the magnetic resonance imaging apparatus 1 according to the second embodiment. There is no difference between the second embodiment and the first embodiment in terms of matching the correction reference position and the calibration position. However, in the first embodiment, the correction reference position is a predetermined fixed position away from the center of the magnetic field (for example, a position at least 7 cm away from the center of the magnetic field), whereas in the second embodiment , the imaging range, for example, is changed according to the FOV (Field of View) size condition.

The upper left diagram of FIG. 10 is a diagram illustrating a case where a large imaging range is set, and the ellipse in the diagram schematically shows an axial section of the body of the subject. When trying to image the entire torso, the imaging range is set to be relatively large. In such a case, as illustrated in the upper right diagram of FIG. 10, the correction reference position Xc1 is positioned relatively far away from the center of the magnetic field, for example, 15 cm or more away from the center of the magnetic field in the X direction. position.

On the other hand, the lower left diagram in FIG. 10 is a diagram illustrating a case where a small imaging range is set. In such a case, as illustrated in the lower right diagram of FIG. 10, the correction reference position Xc2 is positioned relatively close to the center of the magnetic field, for example, about 7 cm from the center of the magnetic field in the X direction. set.

When the correction reference position is close to the magnetic field center, the characteristics of the actual gradient magnetic field also approach linear characteristics. Therefore, as shown in the lower right diagram of FIG. 10, the difference between the linear characteristics for correction and the actual nonlinear characteristics of the gradient magnetic field is reduced, and the amount of correction is also reduced.

If the correction is performed ideally, the nonlinear characteristics of the actual gradient magnetic field will completely match the linear characteristics for correction, and the error (residual error) after the correction process will be zero as described above. becomes. In practice, however, the correction is rarely ideal, and some non-zero residual error will occur even after the correction process, even if the value is small. This residual error is considered to decrease as the correction amount decreases.

That is, when the imaging range (FOV) is small and, as a result, it is possible to use an area where the characteristics of the gradient magnetic field are nearly linear, the correction reference position is set to a position close to the magnetic field center (see the lower part of FIG. 10). case), the residual error can be small.

On the other hand, when the field of view (FOV) is large and as a result, the gradient magnetic field characteristics must be used up to a region where the characteristics are non-linear to some extent, the correction reference position should be set at a position further away from the magnetic field center. (the upper case in FIG. 10), and the residual error may become large.

In the second embodiment, since the correction reference position can be changed according to the size of the imaging range (FOV), even though the imaging range (FOV) is small, the correction reference position By setting the position farther from the center of the magnetic field, it is possible to avoid a situation in which the residual error is increased.

(Third Embodiment)
FIG. 11 is a diagram for explaining the operational concept of the magnetic resonance imaging apparatus 1 according to the third embodiment. Also in the third embodiment, as in the first and second embodiments, the corrected reference position and the calibrated position are matched. However, in the third embodiment, the correction reference position is changed according to the imaging region of the subject. Here, the imaging region of the subject means an organ of the subject to be imaged (eg, lungs, stomach, heart, etc.), or a body region of the subject to be imaged (eg, head, lower limbs, right shoulder, etc.). , left shoulder, right arm, left arm, etc.).

The top and bottom left diagram of FIG. 11 is a diagram schematically showing an axial section of the subject, like the top and bottom left diagram of FIG. 10 . The upper left diagram of FIG. 11 shows an example in which the right shoulder is set as the imaging region. When imaging the right shoulder, it is preferable to set the correction reference position so as to reduce the error in the area centered on the right shoulder. Therefore, as shown in the upper right diagram of FIG. 11, the correction reference position is set at a position separated to the right by Xc1 in the X direction from the center of the magnetic field. By setting the correction reference position in this way, the residual error around the right shoulder can be reduced.

On the other hand, the lower left diagram of FIG. 11 shows an example in which the heart is set as the imaging region. The heart, in the X direction, is usually contained within a region close to the magnetic field center. Therefore, in such a case, the correction reference position is set to a position (Xc2) near the magnetic field center in the X direction. By setting the correction reference position in this manner, the residual error during cardiac imaging can be reduced.

As described above, in the third embodiment, since the correction reference position can be changed according to the imaging region of the subject, the distortion caused by the nonlinear characteristics of the gradient magnetic field can be minimized according to the imaging region. can do.

(Fourth embodiment)
FIG. 12 is a diagram for explaining the operation concept of the magnetic resonance imaging apparatus 1 according to the fourth embodiment. In the fourth embodiment, similarly to the first to third embodiments, the correction reference position and the calibration position are matched. However, in the fourth embodiment, two or more correction reference positions are set, and a plurality of linear characteristics for correction are calculated for each of the plurality of correction reference positions. Then, the distortion correction data is generated for each of the plurality of correction reference positions so that the nonlinear characteristics of the gradient magnetic field match each of the plurality of linear characteristics. That is, distortion correction data (for example, lookup table for distortion correction) is generated for each correction reference position.

In the example shown in the right diagram of FIG. 12, the first distortion correction data corresponding to the correction reference position Xc1, the second distortion correction data corresponding to the correction reference position Xc2, and the third distortion correction data corresponding to the correction reference position Xc3. , three sets of distortion correction data are generated. Then, using these three sets of distortion correction data, the reconstructed diagnostic images are corrected respectively. As a result, three post-correction diagnostic images are obtained corresponding to each of the three sets of distortion correction data.

As can be inferred from the description of the second and third embodiments described above, the region R1 farthest from the magnetic field center in the left diagram of FIG. The corrected diagnostic image will be corrected with the least distortion.

On the other hand, in the region R3 closest to the magnetic field center, the diagnostic image corrected using the third distortion correction data corresponding to the correction reference position Xc3 is corrected with the least distortion. In the area R2 between the areas R1 and R3, the diagnostic image corrected using the second distortion correction data corresponding to the correction reference position Xc2 is corrected with the least distortion.

Thus, in the fourth embodiment, by providing a plurality of correction reference positions, it is possible to generate a plurality of distortion correction data and obtain a plurality of diagnostic images after distortion correction. As a result, the user can select the diagnostic image with the least distortion in the region of interest from among the multiple diagnostic images.

(Image processing device)
FIG. 13 is a block diagram showing functions of the image processing apparatus 700 of the embodiment. A difference from the magnetic resonance imaging apparatus 1 shown in FIG. 2 is that the image processing apparatus 700 does not have an imaging unit 600 for acquiring MR signals. The image processing apparatus 700 also does not have a reconstruction function 402 for reconstructing MR signals and an imaging condition setting function 401 for setting imaging conditions for the imaging unit 600 .

On the other hand, the processing circuit 701 of the image processing apparatus 700 has an acquisition function 703 that acquires a reconstructed reconstructed image before correction. Other functions of the processing circuit 701, that is, the functions of the calibration function 403, the correction function 404, the calibration data generation function 405, and the distortion correction data generation function 406 are the same as the functions of the magnetic resonance imaging apparatus 1 described above. Since they are the same, the description is omitted.
The image processing apparatus 700 also provides substantially the same technical effects as the embodiments of the magnetic resonance imaging apparatus 1 described above.

As described above, the magnetic resonance imaging apparatus and image processing apparatus of each embodiment can appropriately correct image distortion caused by the nonlinearity of the gradient magnetic field generated by the gradient magnetic field coil.
Note that the reconstruction function and the correction function in the description of each embodiment are examples of the reconstruction unit and the correction unit in the description of the claims, respectively.

While several embodiments have been described, these embodiments are provided 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, changes, and combinations of embodiments 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.

1 magnetic resonance imaging apparatus 11 gradient magnetic field coil 12 WB coil 20 RF coil 31 gradient magnetic field power supply 34 sequence controller 40 processing circuit 41 memory circuit 42 display 43 input device 100 magnet mount 300 control cabinet 400 console 402 reconstruction function 403 calibration function 404 correction Function 405 Calibration data generation function 406 Distortion correction data generation function 700 Image processing device 703 Acquisition function

Claims (16)

It comprises at least a high-frequency coil and a gradient magnetic field coil, and collects magnetic resonance signals emitted from a subject in response to application of a high-frequency pulse output from the high-frequency coil and a gradient magnetic field formed by the gradient magnetic field coil. an imaging unit;
a reconstruction unit that reconstructs the acquired magnetic resonance signals to generate a diagnostic image of the subject;
Distortion correction data for correcting the nonlinear characteristics of the gradient magnetic field to linear characteristics defined by the strength of the gradient magnetic field at a position away from the magnetic field center of the gradient coil and the distance from the magnetic field center to the position. a correction unit that corrects the reconstructed diagnostic image using the distortion correction data;
A magnetic resonance imaging apparatus comprising. wherein the correction unit changes the position for generating the distortion correction data according to the range of the imaging area;
The magnetic resonance imaging apparatus according to claim 1. The correction unit changes the position for generating the distortion correction data according to an imaging region of the subject.
The magnetic resonance imaging apparatus according to claim 1. The correction unit adjusts the nonlinear characteristics of the gradient magnetic field to the intensity of each of the gradient magnetic fields at the two or more positions away from the magnetic field center of the gradient coil and from the magnetic field center to the two or more positions. generating two or more distortion correction data for correcting two or more linear characteristics respectively defined by the respective distances of and correcting the reconstructed diagnostic image using the two or more distortion correction data, respectively do,
4. The magnetic resonance imaging apparatus according to claim 1. The correction unit uses the distortion correction data to
correcting the diagnostic image so that the gradient of the gradient magnetic field is reduced in a first region from the magnetic field center to the position;
correcting the diagnostic image such that the gradient of the gradient magnetic field is increased in a second region beyond the position, further away from the magnetic field center;
4. The magnetic resonance imaging apparatus according to claim 1 . The diagnostic image is an image calibrated with reference to a calibration position set at the same position as the position for generating the distortion correction data.
6. The magnetic resonance imaging apparatus according to claim 1. The diagnostic image is calibrated using a calibration image of a phantom.
7. A magnetic resonance imaging apparatus according to claim 6. The diagnostic image is an image whose dimensions have been calibrated using pre-acquired calibration data,
The calibration data are drawn on the calibration position, which is the position of a specific marker in the phantom, and the calibration image, based on the calibration image obtained by imaging the phantom prior to imaging the subject. Data that associates the position of the marked object with each other,
The magnetic resonance imaging apparatus according to claim 7. adjusting the strength of the gradient magnetic field using the calibration image obtained by imaging the phantom before imaging the subject;
The adjustment of the intensity of the gradient magnetic field is performed so that the calibration position, which is the position of a specific marker within the phantom, and the position of the marker depicted in the calibration image are associated with each other.
The magnetic resonance imaging apparatus according to claim 7 or 8. The marker in the phantom is a pin-shaped marker, and when the phantom is imaged, the phantom is arranged such that the axial direction of the pin-shaped marker is orthogonal to the cross section of the calibration image, In the calibration image, the marker is depicted as a point.
The magnetic resonance imaging apparatus according to claim 8 or 9. nonlinear characteristics of the gradient magnetic field are calculated based on design parameters of the gradient coil;
A magnetic resonance imaging apparatus according to any one of claims 1 to 10. the calibration position is set at a distance of at least 7 centimeters from the magnetic field center;
7. A magnetic resonance imaging apparatus according to claim 6. It comprises at least a high-frequency coil and a gradient magnetic field coil, and collects magnetic resonance signals emitted from a subject in response to application of a high-frequency pulse output from the high-frequency coil and a gradient magnetic field formed by the gradient magnetic field coil. an imaging unit;
a reconstruction unit that reconstructs the acquired magnetic resonance signals to generate a diagnostic image of the subject;
Correcting the nonlinear characteristics of the gradient magnetic field to a linear characteristic defined by the intensity of the gradient magnetic field at a position away from the magnetic field center of the gradient coil and the distance from the magnetic field center to the position. a correction unit that generates distortion correction data and phase-corrects the acquired magnetic resonance signals on the k-space using the distortion correction data on the k-space;
a reconstruction unit that reconstructs the magnetic resonance signals corrected using the distortion correction data on the k-space to generate a diagnostic image of the subject;
A magnetic resonance imaging apparatus comprising. It comprises at least a high-frequency coil and a gradient magnetic field coil, and collects magnetic resonance signals emitted from a subject in response to application of a high-frequency pulse output from the high-frequency coil and a gradient magnetic field formed by the gradient magnetic field coil. an imaging unit;
a reconstruction unit that reconstructs the acquired magnetic resonance signals to generate a diagnostic image of the subject;
Distortion correction data for correcting the nonlinear characteristics of the gradient magnetic field to linear characteristics defined by the strength of the gradient magnetic field at a position away from the magnetic field center of the gradient coil and the distance from the magnetic field center to the position. a correction unit that corrects the reconstructed diagnostic image using the distortion correction data;
with
wherein the correction unit changes the position for generating the distortion correction data according to the range of the imaging area;
Magnetic resonance imaging equipment. an acquisition unit that acquires a reconstructed diagnostic image of the subject;
The nonlinear characteristics of the gradient magnetic field formed by the gradient magnetic field coil are converted to linear characteristics defined by the strength of the gradient magnetic field at a position away from the magnetic field center of the gradient magnetic field coil and the distance from the magnetic field center to the position. a correction unit that generates distortion correction data to be corrected and corrects the reconstructed diagnostic image using the distortion correction data;
An image processing device comprising: The nonlinear characteristics of the gradient magnetic field of the gradient magnetic field coil are defined by the intensity of the gradient magnetic field at a correction reference position set away from the magnetic field center of the gradient magnetic field coil and the distance from the magnetic field center to the correction reference position. Generates distortion correction data that corrects to the linear characteristics that are
imaging a phantom to generate a calibration image, and a position at a calibration position of a specific landmark in the phantom depicted in the calibration image, the calibration position being set at the same position as the corrected reference position; calibrate and
After the position calibration, applying a high-frequency pulse and the gradient magnetic field to the subject to collect magnetic resonance signals emitted from the subject,
reconstructing the acquired magnetic resonance signals to produce a diagnostic image of the subject;
correcting the reconstructed diagnostic image using the distortion correction data;
Magnetic resonance imaging method.

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