JP4698231B2 - Magnetic resonance diagnostic equipment - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) that uses nuclear magnetic resonance to visualize nuclear density distribution, relaxation time distribution, and the like, and in particular, uses a plurality of RF receiving coils and phase encoding. The present invention relates to an MRI apparatus to which an imaging method (parallel imaging method) in which a signal obtained by thinning out is developed by matrix calculation using sensitivity distribution of each RF receiving coil.

  In recent years, as a receiving coil for detecting NMR signals generated from a subject in an MRI apparatus, a highly sensitive multiple coil or phased array coil in which a plurality of small coils are arranged is frequently used. In addition, an imaging method using such multiple coils and thinning out data in the phase encoding direction has been proposed (for example, Non-Patent Document 1, Non-Patent Document 2, etc.).

This method is called parallel imaging, and the aliasing that occurs when phase-encoded data is thinned out by using the fact that the sensitivity distribution of the small coils constituting the multiple coil is spatially different from each other is removed by calculation. The RF coil sensitivity distribution used for the parallel imaging calculation can be obtained from each RF coil. Specifically, from the phantom image having a uniform density, the projected spatial shading is used as the sensitivity distribution, There is a method in which a sensitivity distribution is obtained by applying a low frequency filter to image data obtained by photographing for sensitivity. Further, Patent Document 1 proposes a method of using a part of main measurement data for acquiring a cross-sectional image for sensitivity distribution.
Daniel Sodickson, Warren J. Manning, "Simultaneous acquisition of spatial harmonics (SMASH): fast imaging with radio-frequency coilarrays" Magnetic Resonance in Medicine 38, 591-603 (1997) J. Wang, A. Reykowski, "A SMASH / SENSE related method usingratios of array coil profiles" ISMRM Japanese Patent Laid-Open No. 2001-161657

  By the way, in general, the spatial sensitivity distribution of the RF coil has a high sensitivity at a portion close to the coil, and a low sensitivity as the distance from the coil increases. In an image obtained from an RF coil having such characteristics, the portion near the RF coil has high luminance, and the portion far from the RF coil has low luminance, resulting in uneven sensitivity. Conventionally, in order to eliminate such sensitivity unevenness, the image data is weighted in accordance with the spatial sensitivity distribution to perform reception coil sensitivity correction. In the sensitivity correction calculation, normal image data is subjected to inverse Fourier transform, then subjected to low-pass filter processing, and further subjected to Fourier transform to obtain a spatial sensitivity distribution.

The parallel imaging described above is an imaging with phase encoding thinned out and is effective for shortening the imaging time. However, when sensitivity correction is applied to this parallel imaging, sensitivity distribution correction calculation is performed in addition to aliasing removal calculation accompanying parallel imaging. Therefore, the image reconstruction time is extended, and the overall effect of shortening the photographing time including the image reconstruction time is hindered. Particularly, when real-time continuous shooting is performed, the real-time property is impaired.
Therefore, the present invention has an object to provide an MRI apparatus capable of eliminating luminance unevenness due to non-uniform spatial sensitivity distribution of RF coils and obtaining a good image without impairing the effect of shortening the imaging time of parallel imaging. And

  An MRI apparatus of the present invention that solves the above-described problems includes a transmission unit that applies a high-frequency magnetic field to a subject placed in a static magnetic field space, a gradient magnetic field generation unit that generates a gradient magnetic field in the static magnetic field space, and a plurality of small-sized magnetic fields. A receiving means configured to include a receiving coil configured to detect an echo signal generated from the subject; and applying a high-frequency magnetic field and a gradient magnetic field to the subject and an echo signal generated from the subject according to a predetermined pulse sequence. A sequence control means for controlling the measurement, and an image reconstruction means for reconstructing the image of the subject using the measured echo signal, and the processing performed by the image reconstruction means includes sensitivity distribution calculation for each small coil and Including a reconstruction calculation of the subject image using the sensitivity distribution, and executing the sequence for obtaining the image data in the sequence control means using the sensitivity distribution calculation Characterized by comprising means for controlling the sequence control unit and image reconstruction means to perform the.

  In the MRI apparatus of the present invention, the processing performed by the image reconstruction means can further include sensitivity correction calculation of the reconstructed image, and the sensitivity correction calculation is performed by each sensitivity distribution obtained by the sensitivity distribution calculation of the small coil. Is used during the execution of a sequence for acquiring image data. The sensitivity distribution of the entire receiving coil is combined and the sensitivity distribution of the entire receiving coil is combined.

Further, in the MRI apparatus of the present invention, for example, the sequence control means is such that the phase encoding in the high frequency region is sparse and the phase encoding in the low frequency region is dense among the phase encoding necessary for image reconstruction. The measurement is performed by controlling the gradient magnetic field generation means, and the image reconstruction means includes, from the data obtained by the measurement, data in which the phase encoding of the entire area is sparse and data in the low frequency area in which the phase encoding is dense. Are used to calculate the sensitivity distribution of the small coil and the sensitivity distribution of the entire receiving coil using the data in the low frequency region where the phase encoding is dense.
Alternatively, the sequence control means thins out the first measurement in which the gradient magnetic field generating means is controlled so that the phase encoding in the low frequency region is dense and the phase encoding necessary for one image reconstruction at a predetermined rate. The image reconstructing means obtains the sensitivity distribution calculation of the small coil and the sensitivity distribution of the entire receiving coil using the data obtained by the first measurement. Perform calculations.

According to the MRI apparatus of the present invention, the operation of the sequence control unit and the image reconstruction unit is controlled, and the sensitivity distribution calculation is executed using the sensitivity data collected prior to the main measurement during the main measurement. Therefore, the aliasing removal calculation using the sensitivity distribution can be performed immediately after the main measurement, and the measurement time can be effectively shortened by parallel imaging.
Further, according to the MRI apparatus of the present invention, sensitivity correction can be performed using the sensitivity distribution used for parallel imaging, and the image quality can be further improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing an overall outline of an MRI apparatus to which the present invention is applied. This MRI apparatus includes a static magnetic field generation system 101 for generating a static magnetic field in a space around a subject 108, a gradient magnetic field generation system 102 for applying a gradient magnetic field to the space, and atomic nuclei constituting the tissue of the subject 108 Transmitting system 103 that generates high-frequency pulses to cause nuclear magnetic resonance by exciting the signal, receiving system 104 that receives echo signals generated by magnetic resonance generated from the subject 108, processing such as Fourier transform and correction calculation on the echo signals A signal processing system 105 that performs the above operation, a control unit 106 that controls operations of the gradient magnetic field generation system 102, the transmission system 103, and the reception system 104, and a central processing unit (CPU) 107 that sends commands to the signal processing system 105 and the control unit 106 It has.

  The static magnetic field generation system 101 includes magnetic field generation means such as a permanent magnet, a normal conducting magnet, or a superconducting magnet, and generates a uniform static magnetic field in a direction along the body axis of the subject 108 or in a direction perpendicular to the body axis. The gradient magnetic field generation system 102 is composed of gradient magnetic field coils in the X, Y, and Z axes, and an imaging cross section of the subject 108 is set by adding this gradient magnetic field, and an echo signal is encoded to obtain position information. Can be granted.

  The transmission system 103 includes a high-frequency oscillator 111, a modulator 112, a high-frequency amplifier 113, and a high-frequency irradiation coil 114. The high-frequency pulse output from the high-frequency oscillator 111 is amplified by the high-frequency amplifier 113 and then placed close to the subject 108. The high frequency irradiation coil 114 is supplied to irradiate the subject.

  The receiving system 104 includes a high-frequency receiving coil 115, a receiving circuit 116, and an A / D converter 117. The NMR signal received by the receiving coil 115 is detected by the receiving circuit 116 and then converted into a digital signal by the A / D converter 117. Then, it is sent to the signal processing system 105 as collected data sampled at a timing according to a command from the control unit 116. In the MRI apparatus of the present invention, a receiving coil comprising a plurality of small receiving coils (hereinafter simply referred to as small coils) is used as a receiving coil in order to perform parallel imaging. As such a receiving coil, a known receiving coil called a multiple coil or a phased array coil (for example, those described in Non-Patent Documents 1 to 3) can be used. The receiving circuit 116 and the A / D converter 117 are provided for each small coil.

  The configuration of the receiving system 104 is shown in FIG. In the illustrated embodiment, the receiving coil includes four small coils 2011 to 2014, which are connected to preamplifiers 2021 to 2024, respectively, to form one multiple coil 203. The receiving circuit 204 includes an A / D converter for each small coil and orthogonal detection circuits 2051 to 2054 arranged in parallel, and the outputs of the preamplifiers 2021 to 2024 are connected thereto. The NMR signals detected by the small coils 2011 to 2014 are detected by the receiving circuit 204, converted into digital complex signals, and sent to the signal processing system 206.

  A signal processing system 105 (206) performs processing such as Fourier transform, correction calculation, and image reconstruction on the collected data sent from the CPU 107 and reception system 104, and a signal processing device 118 that performs signal processing necessary for the calculation. A data storage unit for storing reconstructed image data, a temporary storage of data, a memory 119 for storing image analysis processing over time, a program for a specified pulse sequence, parameters used for the execution, and the like A storage device such as a magnetic disk 120 and an optical disk 121, a display 122 that displays image data as an image, and an operation unit (not shown) such as a trackball, a mouse, and a keyboard are provided.

  The control unit 106 operates under the control of the CPU 107 and controls the gradient magnetic field generation system 102, the transmission system 103, the reception system 104, and the signal processing system 105 according to a predetermined pulse sequence determined by the imaging method. The CPU 107 controls the operation of the control unit 106 that controls the sequence and the signal processing system (signal processing device 118) 105 that performs operations such as image reconstruction in accordance with the shooting mode selected via the operation unit. In the MRI apparatus of the present invention, two measurement modes for executing the parallel imaging method are set in advance. One is a measurement mode (SCM: self-calibration mode) that collects sensitivity distribution data during measurement (main measurement) for obtaining an image of the subject, and the other is sensitivity distribution data prior to the main measurement. Is a measurement mode (PCM: pre-scan calibration mode). These can be selected, for example, via the operation unit. In any case, when the collection of sensitivity distribution data is completed, the signal processing system 107 is instructed to perform sensitivity distribution calculation in parallel with the main measurement. send.

  Next, a first embodiment of an imaging method based on a parallel imaging method using the MRI apparatus having the above configuration will be described. In this embodiment, a case (SCM: self-calibration mode) in which data (main measurement data) and sensitivity distribution data are acquired simultaneously to obtain a tomographic image of a subject will be described.

  In general, the parallel imaging method does not measure the phase-encoded echoes necessary for reconstructing one image, and performs measurement with the phase encoding thinned out evenly. In principle, if the number of small coils constituting the receiving coil is N, measurement can be performed with a phase encode number 1 / N of the number of phase encodes necessary for reconstruction of one image. In this embodiment, phase encoding is thinned out by applying a parallel imaging method in measurement for obtaining a tomographic image of a subject (hereinafter referred to as main measurement), and data is measured without thinning out phase encoding in a low frequency region. The sensitivity distribution is calculated using the data in this region.

  An example of the pulse sequence employed in this embodiment is shown in FIG. This pulse sequence is a gradient echo sequence. An RF pulse 301 is applied together with a slice encoding gradient magnetic field 302 to excite a nuclear spin in a specific region of the subject to generate transverse magnetization, and then a phase encoding gradient magnetic field pulse. 303 is applied, then a readout gradient magnetic field pulse 304 is applied, and the echo signal 305 is measured. The time (echo time) TE from application of the RF pulse 301 to measurement of the echo signal 305 is a parameter that determines the image contrast, and is set in advance in consideration of the target tissue and the like. This sequence is repeated a plurality of times while changing the application amount of the phase encoding gradient magnetic field pulse (the value obtained by integrating the magnetic field intensity with respect to the application time) to obtain k-space data. At this time, the phase encoding change method is not uniform, and in a region where the phase encoding amount is large (high frequency region in k space), phase encoding is thinned out at a predetermined thinning rate N defined in parallel imaging (that is, the phase encoding amount). (N-1) steps are omitted), and measurement is performed without thinning out in a small region (low frequency region of k space).

  FIG. 4 shows the k-space data array thus measured. In the figure, measured data is shown in white, and data not measured (thinned out) is shown in gray. As the k-space data, a row of data in the horizontal direction (kx direction) is obtained from one echo signal, and the value of ky is determined by the phase encoding amount. In order to reconstruct one image, data of all phase encoding amounts are usually obtained. However, in the sequence of this embodiment, as shown in the figure, the measurement is performed in the high-frequency region b of the k-space data 401. There is data that is not performed, and all data is measured in the low frequency range a. There are several methods for measuring the k-space data (measurement method). For example, here, measurement starts from the highest frequency data, and then goes through the low frequency range to the high frequency range data with different polarity. The sequential order to measure is adopted.

  The signal processing system 105 divides the data 401 thus collected to create two k-space data. One is image reconstruction data 404 in which phase encoding is thinned out at equal intervals in the entire k-space region. The other is sensitivity data 405 composed of data in the low frequency region a. When the measurement order of the k space is sequential ordering, sensitivity data is collected before the measurement of all the areas in the k space is completed. Therefore, the sensitivity distribution is calculated immediately after the sensitivity data is collected.

  This is shown in FIG. In the figure, the horizontal axis is time, and the operations 501 to 503 of the control unit 106 that controls the sequence and the operations 504 to 506 of the signal processing system 105 are shown in blocks. As shown in the figure, the sequence first measures the region b (data measurement of the portion where the phase encoding is sparse) 501 shown in FIG. 4, and then measures the region a (data measurement of the portion where the phase encoding is dense) 502. Finally, measurement 503 of the remaining area b is performed. In the progress of this sequence, sensitivity distribution calculation 504 is performed using the sensitivity data 405 when the measurement 503 in the region a is completed, and the main measurement data obtained in the measurements 501 to 503 when the measurement 503 in the region b is completed. Using the sensitivity distribution of each small coil obtained by 404 and sensitivity distribution calculation 504, image data from which aliasing has been removed is obtained. Finally, coil sensitivity correction 506 of the image data is performed using the sensitivity distribution of the entire receiving coil obtained by combining the sensitivity distributions of the small coils.

Sensitivity distribution calculation, image reconstruction calculation, and sensitivity correction calculation performed by the signal processing system 105 can be performed using a known method. Specifically, the sensitivity distribution calculation 504 is obtained from image data obtained by performing two-dimensional Fourier transform processing after low-pass filter processing. That is, first, pixel data (sensitivity data) w _i (x, y) (subscript i indicates a coil number; the same applies hereinafter) after two-dimensional Fourier transform is performed on collected data from each small coil. To synthesize.

Next, the sensitivity data w _i (x, y) of each small coil is divided by Sc (x, y) to obtain a sensitivity image C _i (x, y) of each small coil.

これをＭ個の小型コイルについて行列で表すと、次式（数４）となり、

これら行列をＳ、Ｃ、Ｐで表すと、折り返しのない画像Ｐは以下のように求めることができる。

Further, the calculation 505 for obtaining the image P (x, y) from which the aliasing is removed uses the image reconstruction data 404 and the sensitivity image C _i (x, y) of each RF coil obtained by the above-described calculation as follows. Do as follows. That is, when the measurement data is thinned to 1 / N, the phase encode direction matrix of the image is ΔY = Y / N, and the folded image S _i (x, y) of the RF coil i is expressed by the following equation (Equation 3). The

When this is expressed in a matrix for M small coils, the following equation (Equation 4) is obtained.

When these matrices are represented by S, C, and P, an image P without aliasing can be obtained as follows.

これによりパラレルイメージング法の画像であって感度補正された画像データが得られる。 Sensitivity correction 506 is performed by the following equation (Equation 6) using Sc (x, y), which is a combination of sensitivity distributions of the small coils.

As a result, image data that has been subjected to sensitivity correction and is an image of the parallel imaging method can be obtained.

  FIG. 6 schematically shows the above processing performed by the signal processing system. According to the present embodiment, the time-consuming sensitivity distribution creation processing 600 is executed in parallel with the main measurement. Immediately after measurement data collection is completed, aliasing calculation (matrix calculation 606, inverse matrix calculation 607) using sensitivity distribution 6011-6014 of the small coil obtained in process 600 can be performed, and sensitivity distribution creation processing Coil sensitivity correction 608 can be performed using the composite sensitivity distribution data 603 created at 600. As a result, a result image 609 with excellent image quality after sensitivity correction is obtained. As described above, according to this embodiment, it is possible to prevent the image formation time from being extended as the amount of calculation increases, and it is possible to effectively increase the speed of parallel imaging.

  Next, measurement of a prescan calibration mode (PCM) will be described as a second embodiment of the present invention. In PCM, sensitivity data is acquired as a pre-scan for the main measurement.

  Also in the present embodiment, the same pulse sequence as in the first embodiment can be adopted as the pulse sequence. However, the measurement for obtaining the sensitivity data prior to the actual measurement measures only the data in the low frequency region of the k space. That is, for example, when the phase encoding gradient magnetic field 303 is changed in the repetition of the pulse sequence shown in FIG. 3, the phase encoding amount is not changed from the maximum value to the minimum value, but is changed in a narrow range centered on 0. .

As shown in FIG. 7A, the sensitivity data thus obtained is the same as the data (FIG. 4, 405) divided from the main measurement data in the first embodiment, and the high-frequency region data 703 (the portion shown in gray) ) (Not shown in white) only in the low frequency region 702. This sensitivity data is obtained for each small coil. As shown in FIG. 6, the sensitivity data is imaged by performing low-pass filter processing and two-dimensional Fourier change processing, as in the first embodiment, and sensitivity images of the respective RF coils are obtained by Equations 1 and 2. .
On the other hand, the sequence control unit performs the main measurement following the pre-scan for obtaining sensitivity data. In the main measurement, measurement is performed by thinning out the phase encode according to a predetermined thinning rate defined by the number of small coils constituting the receiving coil, and main measurement data 705 as shown in FIG. 7B is obtained.

  FIG. 8 shows the relationship between the operations (sequence control) 801 and 802 of the control unit and the operations (calculations) 803 to 805 of the signal processing system in this embodiment. As shown in the figure, the sensitivity distribution calculation 803 in the signal processing system described above is performed in parallel with the main measurement 802. Therefore, when the acquisition of the main measurement data is completed, the sensitivity distribution of each small coil and the entire receiving coil is required. Therefore, the signal processing system first uses the sensitivity distribution of each small coil and follows the equations 3 to 5. The measurement data is returned and the image reconstruction calculation 805 is performed, and then the sensitivity correction 806 of the image obtained according to Equation 6 is performed using the sensitivity distribution of the receiving coil.

  As described above, also in this embodiment, by performing sensitivity distribution calculation in parallel with the main measurement after the completion of the sensitivity data acquisition, the aliasing removal calculation and the sensitivity correction calculation can be performed immediately after the main measurement. A corrected good image can be obtained.

  The operation of the MRI apparatus of the present invention has been described using the first embodiment in which the operation is performed in the self-calibration mode and the second embodiment in which the operation is performed in the pre-scan calibration mode. It is possible to select and execute one mode as appropriate. Further, the pulse sequence executed in the sensitivity data measurement and the main measurement is not limited to the pulse sequence by the gradient echo method shown in FIG. 3, and a known imaging sequence, for example, a spin according to the tissue or region to be imaged. An echo sequence, a high-speed spin echo sequence, an EPI sequence, a spiral sequence, an SSFP sequence, or the like can be employed.

  According to the present invention, in an MRI apparatus that employs a parallel imaging method that has excellent real-time properties and can effectively suppress image degradation due to body movement, sensitivity data necessary for parallel imaging calculation is acquired in parallel with the main measurement. At the same time, by applying it to subsequent sensitivity correction, an image whose sensitivity is corrected can be obtained in a short time. In addition, by using the time reduction, various applications such as an improvement in spatial resolution and an increase in the number of slices can be applied depending on the application.

The figure which shows the whole outline | summary of the MRI apparatus with which this invention is applied. The block diagram which shows the structure of the receiving system of the MRI apparatus of FIG. The figure which shows an example of the pulse sequence which the MRI apparatus of this invention employ | adopts. The figure which shows the k space arrangement | sequence of the collection data in the 1st Embodiment of this invention. The figure which shows the relationship between the measurement and signal processing in the 1st Embodiment of this invention. The figure which shows typically the process which the signal processing system of the MRI apparatus of this invention performs. The figure which shows the k space arrangement | sequence of the collection data in the 2nd Embodiment of this invention. The figure which shows the relationship between the measurement and signal processing in the 2nd Embodiment of this invention.

Explanation of symbols

101 ... Static magnetic field generation system, 102 ... Gradient magnetic field generation system, 103 ... Transmission system, 104 ... Reception system, 105 ... Signal processing system, 115, 203 ... Reception coil, 2011 ~ 2014 ・ ・ ・ Small receiver coil

Claims (3)

A subject including a transmitting means for applying a high-frequency magnetic field to a subject placed in a static magnetic field space; a gradient magnetic field generating means for generating a gradient magnetic field in the static magnetic field space; and a receiving coil comprising a plurality of small coils. Receiving means for detecting an echo signal generated from the signal, sequence control means for controlling the application of a high-frequency magnetic field and a gradient magnetic field to the subject and the measurement of the echo signal generated from the subject according to a predetermined pulse sequence, and the measured echo In a magnetic resonance diagnostic apparatus comprising image reconstruction means for reconstructing an image of a subject using a signal,
The sequence control means generates a gradient magnetic field so that the phase encoding of the high frequency region of k space is sparse and the phase encoding of the low frequency region of k space is dense among the phase encoding necessary for image reconstruction of one sheet. Measure the controlled means,
The processing performed by the image reconstruction means includes sensitivity distribution calculation of each small coil, reconstruction calculation of the subject image using the sensitivity distribution, and sensitivity correction calculation of the reconstructed image,
The image reconstruction means performs the sensitivity distribution calculation during measurement of a high-frequency region of k space in which phase encoding is sparse among image data,
The sensitivity distribution of each small coil obtained by the sensitivity distribution calculation is synthesized to obtain the sensitivity distribution of the entire receiving coil, and the sensitivity correction calculation of the image reconstructed immediately before is performed using the sensitivity distribution of the entire receiving coil. A magnetic resonance diagnostic apparatus characterized by being performed. The magnetic resonance diagnostic apparatus according to claim 1.
The image reconstruction means creates, from the data obtained by the measurement, data of the entire k-space where the phase encoding is sparse and data of the low frequency region of the k-space where the phase encoding is dense. 2. The magnetic resonance diagnosis according to claim 1, wherein the sensitivity distribution calculation of the small coil and the calculation of obtaining the sensitivity distribution of the entire receiving coil are performed using data in a low-frequency region of k-space with dense encoding. apparatus. A subject including a transmitting means for applying a high-frequency magnetic field to a subject placed in a static magnetic field space; a gradient magnetic field generating means for generating a gradient magnetic field in the static magnetic field space; and a receiving coil comprising a plurality of small coils. Receiving means for detecting an echo signal generated from the signal, sequence control means for controlling the application of a high-frequency magnetic field and a gradient magnetic field to the subject and the measurement of the echo signal generated from the subject according to a predetermined pulse sequence, and the measured echo In a magnetic resonance diagnostic apparatus comprising image reconstruction means for reconstructing an image of a subject using a signal,
The sequence control means performs the first measurement in which the gradient magnetic field generating means is controlled so that the phase encoding in the low frequency region of the k space is dense and the phase encoding necessary for one image reconstruction at a predetermined ratio. Execute the second measurement in which the gradient magnetic field generating means is controlled so as to be thinned out;
The processing performed by the image reconstruction means includes sensitivity distribution calculation of each small coil, reconstruction calculation of the subject image using the sensitivity distribution, and sensitivity correction calculation of the reconstructed image,
The image reconstruction means performs, during the second measurement, the sensitivity distribution calculation of the small coil and the calculation for obtaining the sensitivity distribution of the entire receiving coil using the data obtained by the first measurement .
A magnetic resonance diagnostic apparatus characterized in that the sensitivity correction calculation of the image reconstructed immediately before is performed using the sensitivity distribution of the entire receiving coil .

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