JP6348355B2 - Magnetic resonance signal processing apparatus, magnetic resonance apparatus, and program - Google Patents

Description

  The present invention relates to a technique for processing a magnetic resonance signal.

  As one of imaging methods using a magnetic resonance apparatus, an imaging method for performing echo planner imaging (EPI) using a parallel imaging method is known (see Patent Document 1, Abstract, etc.). ). EPI is susceptible to static magnetic field inhomogeneity, and image distortion is likely to occur. For this reason, in this imaging method, the phase correction process is performed on the received signal of each channel coil of the surface coil (surface coil) arranged near the imaging region of the subject, and the corrected signal is used as a basis. Reconstruct the image.

  As a general idea of this phase correction processing, for each channel coil, a phase correction coefficient is obtained based on the signal obtained by the channel coil, and the phase correction coefficient is applied to the signal obtained by the channel coil. To correct the phase.

Japanese Patent No. 3734086

  However, in reality, if there is a channel coil that does not have a sufficient signal among a plurality of channel coils, the accuracy of the phase correction coefficient for that channel coil deteriorates, and eventually a ghost ( Creates a ghost-like artifact.

  As a countermeasure against this, one channel coil having the highest signal-to-noise ratio (signal noise ratio) of the received signal is selected from among the plurality of channel coils, and each channel coil is selected based on the signal obtained by the selected channel coil. A method for obtaining a phase correction coefficient to be applied to each channel coil is conceivable.

  However, with a single channel coil, the sensitivity is spatially limited, and it cannot be said that the phase information of the received signal at a location with low sensitivity is not very correct. For this reason, the phase correction coefficient may be greatly different from that of other channel coils, phase correction is insufficient in those channel coils, and as a result, artifacts may be generated in the reconstructed image.

  Due to such circumstances, when performing magnetic resonance imaging by EPI using the parallel imaging method, it is possible to perform more robust phase correction on the received signal to suppress artifacts in the reconstructed image. Technology is desired.

The invention of the first aspect
An acquisition means for acquiring a magnetic resonance signal received by a surface coil and a magnetic resonance signal received by a body coil by an EPI scan using a parallel imaging method;
Calculating means for calculating a phase correction coefficient for the magnetic resonance signal by the surface coil based on the magnetic resonance signal by the body coil;
Correction means for correcting a magnetic resonance signal from the surface coil using the phase correction coefficient;
There is provided a magnetic resonance signal processing apparatus comprising reconstruction means for reconstructing an image based on the corrected magnetic resonance signal.

The invention of the second aspect is
The calculation means calculates a phase correction coefficient for each channel signal of the surface coil based on the magnetic resonance signal from the surface coil,
For each channel of the surface coil, when the difference between the phase correction coefficient for the channel signal and the phase correction coefficient by the body coil is equal to or greater than a predetermined threshold, the correction means The magnetic resonance signal according to the first aspect, wherein the magnetic resonance signal is corrected using a phase correction coefficient by a coil, and the signal of the channel is corrected by using a phase correction coefficient by the surface coil when the difference is less than the threshold value. A processing device is provided.

The invention of the third aspect is
The magnetic resonance signal processing apparatus according to the first aspect or the second aspect, wherein the threshold value is a value of 10% to 50% of a phase correction coefficient based on the signal of the channel.

The invention of the fourth aspect is
An acquisition means for receiving and acquiring a magnetic resonance signal by a surface coil and a body coil by an EPI scan using a parallel imaging method;
Calculating means for calculating a phase correction coefficient for the magnetic resonance signal by the surface coil based on the magnetic resonance signal by the body coil;
Correction means for correcting a magnetic resonance signal from the surface coil using the phase correction coefficient;
There is provided a magnetic resonance apparatus comprising reconstruction means for reconstructing an image based on the corrected magnetic resonance signal.

The invention of the fifth aspect is
The calculation means calculates a phase correction coefficient for each channel signal based on the magnetic resonance signal of each channel of the surface coil,
For each channel of the surface coil, when the difference between the phase correction coefficient for the channel signal and the phase correction coefficient by the body coil is equal to or greater than a predetermined threshold, the correction means The magnetic resonance apparatus according to the fourth aspect, wherein correction is performed using a phase correction coefficient by a coil, and when the difference is less than the threshold value, the signal of the channel is corrected by using a phase correction coefficient by the surface coil. I will provide a.

The invention of the sixth aspect is
The magnetic resonance apparatus according to the fourth or fifth aspect, wherein the threshold value is a value of 10% to 50% of a phase correction coefficient based on the signal of the channel.

The invention of the seventh aspect
The magnetic resonance apparatus according to any one of the fourth to sixth aspects, wherein the acquisition unit simultaneously receives a magnetic resonance signal from the surface coil and a magnetic resonance signal from the body coil.

The invention of the eighth aspect
A program for causing a computer to function as the magnetic resonance signal processing apparatus according to any one of the first to third aspects is provided.

  According to the above aspect of the invention, when magnetic resonance imaging by EPI scan using the parallel imaging method is performed, phase correction is performed using a signal from a body coil with more stable sensitivity, and thus more robust phase correction is performed. To suppress artifacts in the reconstructed image.

1 is a diagram schematically showing a configuration of a magnetic resonance imaging apparatus according to a first embodiment. It is a figure showing the magnitude information and phase information of a received signal by a single channel coil with a surface coil part and a body coil part. It is a functional block diagram functionally showing the magnetic resonance imaging apparatus according to the first embodiment. FIG. 3 is a flowchart of imaging processing in the magnetic resonance imaging apparatus according to the first embodiment. It is a flowchart of the imaging | photography process in the magnetic resonance imaging apparatus which concerns on 2nd embodiment. It is a figure which shows the example of a result of the phase correction by this proposal method.

(First embodiment)
The present embodiment is a magnetic resonance imaging system that performs an EPI scan using a parallel imaging method. The magnetic resonance imaging apparatus according to the present embodiment applies a phase correction coefficient based on the reception signal of the body coil when phase correction is performed on the reception signal of each channel coil of the surface coil arranged in the imaging region of the subject.

  FIG. 1 is a diagram schematically showing a configuration of a magnetic resonance imaging apparatus according to the present embodiment.

  As shown in FIG. 1, the magnetic resonance imaging apparatus 1 includes a static magnetic field coil unit 11, a gradient coil unit 12, a body coil unit 13, a surface coil unit 14, a static magnetic field drive unit 21, a gradient drive unit 22, An RF drive unit 23, a data collection unit 24, a subject transport unit 25, a control unit 30, a storage unit 31, an operation unit 32, an image reconstruction unit 33, and a display unit 34 are provided.

  The static magnetic field coil unit 11 is a superconducting coil, for example, and generates a static magnetic field by receiving a current supply to generate a static magnetic field space.

  The gradient coil unit 12 is supplied with current and independently generates gradient magnetic fields in three axial directions, ie, a slice axis direction, a phase encode direction, and a frequency encode direction. Here, the frequency encoding direction, the phase encoding direction, and the slice axis direction correspond to the x direction, the y direction, and the z direction shown in FIG. 1, respectively.

  The body coil unit 13 receives a current supply and generates a high-frequency magnetic field, that is, an RF pulse (Radio Frequency pulse) for exciting the nuclear spin of the subject 40 in the static magnetic field space. The body coil unit 13 receives a magnetic resonance signal from the subject 40.

  The surface coil unit 14 is installed on the surface of the imaging region of the subject 40 and receives a magnetic resonance signal from the imaging region. The surface coil portion 14 is composed of a plurality of channel coils. The number of channel coils, that is, the number of channels is, for example, about 2 to 10. The surface coil is also referred to as a phased-array coil, a surface coil, or the like, and the channel coil is also referred to as a coil element.

  The static magnetic field drive unit 21 drives the static magnetic field coil unit 11 based on a control signal from the control unit 30 to generate a static magnetic field.

  The gradient driving unit 22 drives the gradient coil unit 12 based on a control signal from the control unit 30 to generate a gradient magnetic field in the static magnetic field space.

  The RF drive unit 23 drives the body coil unit 13 based on a control signal from the control unit 30 to generate a high-frequency magnetic field in the static magnetic field space.

  The data collection unit 24 performs phase detection on the magnetic resonance signals received by the body coil unit 13 and the surface coil unit 14 and performs AD (Analog-Digital) conversion to generate data of the magnetic resonance signals. Data of the generated magnetic resonance signal is output to the storage unit 31.

  The subject transport unit 25 transports the subject 40 into and out of the static magnetic field space based on a control signal from the control unit 30.

  Based on the operation signal from the operation unit 32, the control unit 30 executes a predetermined pulse sequence (pulse sequence), a static magnetic field drive unit 21, a gradient drive unit 22, an RF drive unit 23, and a data collection unit 24. Then, a control signal is sent to each part of the subject transport unit 25 for control.

  The storage unit 31 stores MR signal data collected by the data collection unit 24, image data obtained by image reconstruction processing by the image reconstruction unit 33, and the like.

  Under the control of the control unit 30, the image reconstruction unit 33 reads out magnetic resonance signal data from the storage unit 31, performs image reconstruction processing on the data, and generates image data. The image data is output to the storage unit 31.

  The display unit 34 displays information necessary for operation of the operation unit 32, an image represented by the image data, and the like.

  The control unit 30, the storage unit 31, and the image reconstruction unit 33 are configured by a computer, for example.

  The body coil portion 13 and the surface coil portion 14 are provided with a hardware decoupling measure. For example, the impedance of the preamplifier (pre-AMP, not shown) connected to each coil in the data collection unit 24 is designed to be as low as possible.

  Here, a phase correction method for the reception signal of the surface coil unit 14 in the present embodiment will be described.

  Normally, when performing EPI scan using the parallel imaging method, it is necessary to perform phase correction on each magnetic resonance signal received by each channel coil of the surface coil unit 14. Further, the phase correction processing for the reception signal of each channel coil is performed using the phase correction coefficient obtained for the channel coil. If sufficient reception signals can be obtained with all channel coils, calculating and applying the phase correction coefficient based on the reception signals of the channel coils for each channel coil will produce the best image quality. It is thought to be.

  Here, FIG. 2 is a diagram showing magnitude information and phase information of a received signal by the single channel coil having the surface coil portion 14 and the body coil portion 13. In this figure, MC and PC represent the magnitude information and phase information of the received signal by the channel coil having the highest received signal-to-noise ratio, respectively, and the magnitude and phase of the received signal corresponding to each position in the FOV are shaded. It is expressed by the difference. MB and PB represent the magnitude information and phase information of the received signal by the body coil, respectively, and represent the magnitude and phase of the received signal corresponding to each position in the FOV by the difference in density.

  In general, the channel coil of the surface coil is generally highly sensitive, and the SN ratio of the received signal is often high. However, as indicated by MC and PC in FIG. 2, the high sensitivity region of the channel coil is limited in the FOV, and the phase information in the low sensitivity region is ambiguous. For this reason, the signal-to-noise ratio may be extremely low for the reception signal of a specific channel coil. When the SN ratio of the received signal is extremely low, the calculated phase correction coefficient includes many error components, and phase correction within an appropriate range may not be possible. If even one phase correction is not appropriate for the received signal of the channel coil, ghost-like artifacts appear in the reconstructed image.

  On the other hand, the body coil is generally not very sensitive and the signal-to-noise ratio of the received signal is often not as high as that of the channel coil. However, as shown by MB and PB in FIG. 2, the sensitivity of the body coil is spatially substantially uniform, and a stable S / N ratio magnetic resonance signal can be obtained in almost all regions in the FOV. Phase information is generally less ambiguous.

  Therefore, in the present embodiment, the phase correction coefficient for each channel coil is calculated based on the received signal of the body coil unit 13 instead of the received signal of the channel coil. Then, the calculated phase correction coefficient is applied to the received signal of each channel coil to correct the phase. According to the method of the present embodiment, it may be difficult to obtain an optimal phase correction coefficient for all channel coils, but the possibility of calculating a phase correction coefficient out of the appropriate range is reduced as much as possible. it can. As a result, it is possible to perform more robust phase correction and suppress the occurrence of ghost-like artifacts in the reconstructed image.

  In addition, in the method of this embodiment, it is necessary to perform reception by the body coil unit 13 separately from reception by the surface coil unit 14, but simultaneous reception by both the body coil unit 13 and the surface coil unit 14 is performed. For example, an increase in shooting time can be avoided.

  FIG. 3 is a functional block diagram functionally showing the magnetic resonance imaging apparatus 1 according to the present embodiment. The magnetic resonance imaging apparatus 1 includes a signal acquisition unit 51, a phase correction coefficient calculation unit 52, a phase correction processing unit 53, and an image reconstruction unit 54. Each of these units 51 to 54 is an example of an acquisition unit, a calculation unit, a correction unit, and a reconstruction unit in the invention.

  The signal acquisition unit 51 receives and acquires magnetic resonance signals from the surface coil unit 14 for a plurality of views spreading in the phase encoding direction in the k-space by EPI scan using a parallel imaging method. Moreover, the signal acquisition part 51 receives and acquires a magnetic resonance signal from the body coil part 13 about the said several view by the same imaging | photography method.

  The phase correction coefficient calculation unit 52 calculates a phase correction coefficient corresponding to the channel coil for each channel coil of the surface coil unit 14 based on the magnetic resonance signal acquired by the body coil unit 13. The phase correction coefficient is composed of one or a plurality of coefficients depending on the order to be obtained, such as a zeroth order coefficient, a first order coefficient,.

  The phase correction processing unit 53 applies the calculated phase correction coefficient for each channel coil to the received signal of the channel coil, and performs a phase correction process on these received signals.

  The image reconstruction unit 54 reconstructs an image by performing a Fourier transform (inverse Fourier transform) on the received signal of each channel coil that has undergone the phase correction processing.

  Hereinafter, the flow of imaging processing in the magnetic resonance imaging apparatus 1 according to the present embodiment will be described.

  FIG. 4 is a flowchart of imaging processing in the magnetic resonance imaging apparatus 1 according to the present embodiment. Here, for the sake of convenience, it is assumed that a predetermined slice area of the subject 40 is imaged and an image of the slice is reconstructed.

  In step S <b> 1, the operator installs the surface coil unit 14 on the subject 40. Then, in response to a command from the operator, the signal acquisition unit 51 performs an EPI scan using a parallel imaging method on the predetermined slice region SR, and the magnetic resonance signal is transmitted to the body coil unit 13 and the surface coil unit. 14 and receive. Scanning may be performed separately for reception by the body coil unit 13 and reception by the surface coil unit 14, but scanning may be made common to receive by the body coil unit 13 and reception by the surface coil unit 14. May be performed simultaneously. In this way, the photographing time can be shortened.

Thereby, the reception signal B of the body coil portion 13 and the reception signals S _E1 , S _E2 ,..., S _{En of the} channel coils _{E1 to En of} the surface coil portion 14 are obtained.

  In step S <b> 2, the phase correction coefficient calculation unit 52 calculates the phase correction coefficient for the reception signal of each channel coil of the surface coil unit 14 based on the reception signal B from the body coil unit 13.

In step S3, the phase correction processing unit 53 applies the phase correction coefficient calculated for the channel coil to the received signals S _E1 , S _{E2 ,.} Process.

In step S4, the image reconstruction unit 54, the received signal S _E1 of each channel coil phase correction processed ', S _E2', ..., to reconstruct an image based on the S _En '.

(Second embodiment)
Hereinafter, the second embodiment will be described.

  As described above, generally, in the case of the channel coil constituting the surface coil section 14, the highest sensitivity is high, and the SN ratio of the magnetic resonance signal received at the high sensitivity portion is high, but the sensitivity distribution is limited. Therefore, the stability of the SN ratio is low. On the other hand, in the case of the body coil unit 13, the sensitivity is generally low, and the SN ratio of the received magnetic resonance signal is low, but the sensitivity distribution is almost uniform, so the SN ratio is highly stable. For this reason, the phase correction coefficient based on the received signal of the channel coil is higher in accuracy than the phase correction coefficient based on the received signal of the body coil unit 13, but lacks stability, and a sufficient received signal cannot be obtained. In some cases, the accuracy is extremely deteriorated. On the other hand, the phase correction coefficient based on the reception signal of the body coil unit 13 is lower in the maximum accuracy than the phase correction coefficient based on the reception signal of the channel coil, but is stable and the accuracy is not extremely reduced. Has properties.

  In the present embodiment, the following processing is performed in order to make good use of the good properties of both coils. For each channel coil of the surface coil section 14, a phase correction coefficient is calculated based on the received signal of the body coil section 13, and a phase correction coefficient is calculated based on the received signal of the channel coil. For each channel coil, the difference between the phase correction coefficient based on the received signal of the channel coil and the phase correction coefficient based on the received signal of the body coil unit 13 is examined. When the difference is equal to or greater than a certain level, the phase correction coefficient based on the received signal of the channel coil is regarded as having a large error, and the received signal of the channel coil is changed to the received signal of the body coil unit 13. Adopt a phase correction factor based. On the other hand, when the difference is less than a certain level, it is considered that the error is small, and a phase correction coefficient based on the received signal of the channel coil is adopted for the received signal of the channel coil. Therefore, it can be said that the method of the present embodiment enables more robust phase correction.

  Hereinafter, the flow of imaging processing in the magnetic resonance imaging apparatus 1 ′ according to the present embodiment will be described.

  FIG. 5 is a flowchart of imaging processing in the magnetic resonance imaging apparatus 1 ′ according to the present embodiment.

  In step T1, the signal acquisition unit 51 performs an EPI scan using a parallel imaging method on a predetermined slice region SR in response to an instruction from the operator, and the magnetic resonance signal is transmitted to the body coil unit 13 and the surface. Received and acquired by the coil unit 14.

  In step T2, the phase correction processing unit 53 selects a channel coil to be processed.

  In step T <b> 3, the phase correction coefficient calculation unit 52 calculates a phase correction coefficient for the channel coil to be processed based on the reception signal B from the body coil unit 13.

  In step T4, the phase correction coefficient calculation unit 52 calculates the phase correction coefficient for the reception signal of the channel coil for the channel coil to be processed based on the reception signal of the channel coil.

  In step T5, the phase correction processing unit 53 determines whether or not the difference between the phase correction coefficient based on the reception signal of the body coil unit 13 and the phase correction coefficient based on the reception signal of the channel coil is equal to or greater than a predetermined threshold value. judge. In this determination, when it is determined that the difference is equal to or greater than the threshold value, the error component of the phase correction coefficient based on the received signal of the channel coil is considered to be outside the allowable range, and the process proceeds to step T6. On the other hand, when it is determined that the difference is less than the threshold value, the error component of the phase correction coefficient based on the received signal of the channel coil is considered to be within the allowable range, and the process proceeds to step T7. Note that, for example, a value of 10% to 50% of the phase correction coefficient based on the received signal of the channel coil can be considered as the threshold value, and a value of 20% to 40% is assumed as one guideline, for example. be able to. In this example, the threshold value is about 30%.

  In step T <b> 6, the phase correction processing unit 53 applies phase correction to the reception signal of the channel coil to be processed by applying a phase correction coefficient based on the reception signal of the body coil unit 13. Thereafter, the process proceeds to step T8.

  In step T7, the phase correction processing unit 53 performs phase correction processing on the received signal of the channel coil to be processed by applying a phase correction coefficient based on the received signal of the channel coil.

  In step T8, the phase correction processing unit 53 determines whether there is a next channel coil as a processing target. If it is determined that there is, the process returns to step T2 to select a new channel coil as a processing target. If it is determined that there is not, the process proceeds to step T9.

  In step T9, an image is reconstructed based on the reception signal of each channel coil that has undergone the phase correction processing.

  FIG. 6 shows an example of the result of phase correction by the proposed method. The upper diagram U in FIG. 6 is a reconstructed image when the phase correction processing is performed using the phase correction coefficient based on the received signal of the channel coil having the highest SN ratio in the surface coil unit 14. Further, a lower diagram D in FIG. 6 is a reconstructed image when the phase correction processing is performed using the phase correction coefficient based on the reception signal of the body coil unit 13.

  In the upper diagram U, a ghost-like artifact appears remarkably on the lower left side. On the other hand, in the lower figure D, it can be seen that the occurrence of the artifact is moderately suppressed.

  As described above, according to the present embodiment, when it is determined that the reliability of the phase correction coefficient based on the received signal of the channel coil is high, the phase correction coefficient that tends to have high accuracy is added to the received signal of the channel coil. When applied to the phase correction process and it is determined that the reliability of the phase correction coefficient based on the received signal of the channel coil is low, the safer phase correction coefficient based on the received signal of the body coil unit 13 is applied to the channel coil. Applies to phase correction processing for received signals. Thereby, more robust phase correction can be performed, and the occurrence of ghost-like artifacts in the reconstructed image can be effectively suppressed.

  The invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

  Although the above embodiment is a magnetic resonance imaging apparatus, an MR signal processing apparatus that performs MR signal processing as described above, and a program for causing a computer to function as such an MR signal processing apparatus are also provided. 1 is an embodiment of the invention.

DESCRIPTION OF SYMBOLS 1,1 'Magnetic resonance imaging apparatus 11 Static magnetic field coil part 12 Gradient coil part 13 Body coil part 14 Surface coil part 21 Static magnetic field drive part 22 Gradient drive part 23 RF drive part 24 Data acquisition part 25 Subject conveyance part 30 Control part Reference Signs List 31 storage unit 32 operation unit 33 image reconstruction unit 34 display unit 40 subject 51 signal acquisition unit 52 phase correction coefficient calculation unit 53 phase correction processing unit 54 image reconstruction unit

Claims (8)

Acquisition means for acquiring the magnetic resonance signal received by the surface coil and the magnetic resonance signal received by the body coil by EPI scan using a parallel imaging method;
Calculating means for calculating a phase correction coefficient for the magnetic resonance signal by the surface coil based on the magnetic resonance signal by the body coil;
Correction means for correcting a magnetic resonance signal from the surface coil using the phase correction coefficient;
A magnetic resonance signal processing apparatus comprising: reconstruction means for reconstructing an image based on the corrected magnetic resonance signal. The calculation means calculates a phase correction coefficient for each channel signal of the surface coil based on the magnetic resonance signal from the surface coil,
For each channel of the surface coil, when the difference between the phase correction coefficient for the channel signal and the phase correction coefficient by the body coil is greater than or equal to a predetermined threshold value, the correction means The magnetic resonance signal according to claim 1, wherein the magnetic resonance signal is corrected using a phase correction coefficient by a coil, and when the difference is less than the threshold value, the signal of the channel is corrected by using a phase correction coefficient by the surface coil. Processing equipment. The magnetic resonance signal processing apparatus according to claim 2 , wherein the threshold value is a value of 10% to 50% of a phase correction coefficient based on a signal of the channel. An acquisition means for receiving and acquiring a magnetic resonance signal by a surface coil and a body coil by an EPI scan using a parallel imaging method;
Calculating means for calculating a phase correction coefficient for the magnetic resonance signal by the surface coil based on the magnetic resonance signal by the body coil;
Correction means for correcting a magnetic resonance signal from the surface coil using the phase correction coefficient;
A magnetic resonance apparatus comprising: reconstruction means for reconstructing an image based on the corrected magnetic resonance signal. The calculation means calculates a phase correction coefficient for each channel signal based on the magnetic resonance signal of each channel of the surface coil,
For each channel of the surface coil, when the difference between the phase correction coefficient for the channel signal and the phase correction coefficient by the body coil is greater than or equal to a predetermined threshold value, the correction means The magnetic resonance apparatus according to claim 4, wherein correction is performed using a phase correction coefficient by a coil, and when the difference is less than the threshold value, the signal of the channel is corrected by using a phase correction coefficient by the surface coil. . 6. The magnetic resonance apparatus according to claim 5 , wherein the threshold value is a value of 10% to 50% of a phase correction coefficient based on the signal of the channel.   The magnetic resonance apparatus according to any one of claims 4 to 6, wherein the acquisition unit simultaneously receives a magnetic resonance signal from the surface coil and a magnetic resonance signal from the body coil. The program for making a computer function as a magnetic resonance signal processing apparatus as described in any one of Claims 1-3.