JP2012029999A - Magnetic resonance imaging apparatus and luminance nonuniformity correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct luminance nonuniformity of images based on sensitivity distribution nonuniformity of an RF coil for part imaging without unnaturally raising the pixel value of a background noise area.SOLUTION: When correcting the luminance nonuniformity based on the nonuniform sensitivity distribution of the RF coil for part imaging in images obtained using the RF coil for part imaging, the sensitivity distribution of the RF coil for part imaging is acquired, a correction value for correcting the luminance nonuniformity is determined on the basis of the sensitivity distribution, and the processing of preventing the rise of the background noise of the images is performed when correcting the luminance uniformity of the images using the correction value.

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density and relaxation time distributions (hereinafter referred to as nuclear magnetic resonance imaging). In particular, the present invention relates to a technique for correcting luminance non-uniformity of an image obtained by using a region imaging RF coil having a spatially non-uniform sensitivity distribution.

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In the MRI apparatus, in order to obtain a good signal-to-noise ratio (SNR), imaging is performed using a region imaging RF coil formed by combining at least one element coil. However, since the sensitivity distribution of the region imaging RF coil is spatially nonuniform, the echo data measured using such a region imaging RF coil is affected by the sensitivity distribution nonuniformity, and the reconstructed image The image quality is degraded due to non-uniform brightness. In order to correct image quality degradation based on such sensitivity distribution non-uniformity, the brightness non-uniformity of the image generated from the echo data is corrected using the correction value of non-sensitivity distribution obtained by performing auxiliary measurement. . In Patent Document 1, measurement using a whole-body RF coil having a spatially uniform sensitivity distribution over the entire examination region and measurement using a region imaging RF coil are performed as auxiliary measurements. Based on the obtained image data, a correction value for nonuniform luminance of the image obtained by the part imaging RF coil is obtained.

JP-A-8-56928 JP-A-10-248822 JP 2009-268569 A

  However, according to the method described in Patent Document 1, the sensitivity of the background noise region with low sensitivity is also corrected and emphasized, so that the pixel value of the background noise region rises unnaturally as a way to see the corrected image. The possibility of being left unresolved.

  Therefore, the present invention has been made in view of the above-mentioned unsolved problems, and the luminance of the image based on the non-uniform sensitivity distribution of the region imaging RF coil without unnaturally raising the pixel value of the background noise region. An MRI apparatus capable of correcting non-uniformity and a luminance non-uniformity correction method are provided.

  In order to achieve the above object, the present invention corrects a non-uniform brightness based on a non-uniform sensitivity distribution of an RF coil for site imaging in an image obtained using the RF coil for site imaging. Obtaining the sensitivity distribution of the imaging RF coil, obtaining a correction value for correcting the luminance nonuniformity based on the sensitivity distribution, and correcting the luminance nonuniformity of the image using the correction value, Performs processing to prevent background noise from rising.

  Specifically, the MRI apparatus of the present invention includes a sensitivity distribution calculation unit that acquires the sensitivity distribution of the region imaging RF coil, and a correction value calculation that calculates a correction value for correcting luminance nonuniformity based on the sensitivity distribution. And a background noise lifting prevention unit for preventing the background noise of the image from being raised when correcting the luminance unevenness of the image using the correction value.

  The brightness non-uniformity correction method of the present invention includes a step of obtaining a sensitivity distribution of the part imaging RF coil, and a correction value calculating step of obtaining a correction value for correcting the brightness non-uniformity based on the sensitivity distribution. And a step of preventing the background noise of the image from being lifted when correcting the luminance unevenness of the image using the correction value.

  According to the MRI apparatus and the luminance non-uniformity correction method of the present invention, the non-uniform luminance of the image based on the non-uniform sensitivity distribution of the region imaging RF coil is corrected without unnaturally raising the pixel value of the background noise region. It becomes possible.

The block diagram which shows an example of a structure of the MRI apparatus which concerns on this invention. 2 is a functional block diagram of an arithmetic processing unit 114 in Embodiment 1. FIG. 3 is a flowchart illustrating a processing flow of the first embodiment. FIG. 4 is a diagram showing an image and its center profile in each step of the flowchart shown in FIG. FIG. 3 is a diagram for explaining a problem when the background noise lifting prevention method of the first embodiment is simply applied. 10 is a flowchart showing a processing flow of Embodiment 2. FIG. 4 is a diagram showing an image and its center profile in each step of the flowchart shown in FIG. 10 is a flowchart showing a processing flow of Embodiment 3. An example of an image having uniform brightness that can be obtained by performing the first auxiliary measurement using a whole-body RF coil. An example of an image with non-uniform luminance that can be acquired by performing the second auxiliary measurement using the part imaging RF coil. Example of sensitivity distribution of RF coil for part imaging Example of correction values calculated from sensitivity distribution data Example of correction correction value

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiments of the invention, those having the same function are given the same reference numerals, and their repeated explanation is omitted.

  First, an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention.

  This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject 101. As shown in FIG. 1, a static magnetic field generating magnet 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 109, and an RF transmission coil 104, an RF transmitter 110, an RF receiver coil 105, a signal detector 106, a signal processor 107, a measurement controller 111, an overall controller 108, a display / operation unit 113, and a subject 101 are mounted. And a bed 112 for taking the top plate into and out of the static magnetic field generating magnet 102.

  The static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method and in the body axis direction in the horizontal magnetic field method. A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.

The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it. A current is supplied to the power source 109. Specifically, the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z.
When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and orthogonal to each other. Phase encoding gradient magnetic field pulse (Gp) and frequency encoding (leadout) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and position information in each direction is encoded in the NMR signal (echo signal). .

  The RF transmission coil 104 is a coil that irradiates the subject 101 with an RF pulse, and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, an NMR phenomenon is induced in the spins of atoms constituting the living tissue of the subject 101. Specifically, the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111 described later, and the RF transmission coil 104 is disposed in the vicinity of the subject 101 after the high frequency pulse is amplitude-modulated and amplified. , The subject 101 is irradiated with an RF pulse.

  The RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of spin that constitutes the living tissue of the subject 101. The received echo signal is connected to the signal detecting unit 106 and is received by the signal detecting unit 106. Sent. The RF receiving coil 105 includes a part imaging RF coil formed by combining one or more element coils and a whole body RF coil having spatially substantially uniform sensitivity over a wide area.

  The signal detection unit 106 performs processing for detecting an echo signal received by the RF receiving coil 105. Specifically, the echo signal of the response of the subject 101 induced by the RF pulse irradiated from the RF transmission coil 104 is received by the RF receiving coil 105 disposed in the vicinity of the subject 101, and measurement control described later is performed. In accordance with a command from the unit 111, the signal detection unit 106 amplifies the received echo signal, divides it into two orthogonal signals by quadrature detection, and samples each by a predetermined number (for example, 128, 256, 512, etc.) Each sampling signal is A / D converted into a digital quantity and sent to a signal processing unit 107 described later. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data.

  The signal processing unit 107 performs various processes on the echo data, and sends the processed echo data to the measurement control unit 111.

  The measurement control unit 111 mainly transmits various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106. And a control unit for controlling them. Specifically, the measurement control unit 111 operates under the control of the overall control unit 108 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 based on a predetermined pulse sequence. The echo necessary for reconstructing the image of the imaging region of the subject 101 is repeatedly executed by applying an RF pulse and applying a gradient magnetic field pulse to the subject 101 and detecting an echo signal from the subject 101. Control data collection. In the repetition, the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging. Values such as 128, 256, and 512 are usually selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings. With these controls, echo data from the signal processing unit 107 is output to the overall control unit 108.

  The overall control unit 108 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit 114 having a CPU and a memory, an optical disc, And a storage unit 115 such as a magnetic disk. Specifically, the measurement control unit 111 is controlled to execute the collection of echo data, and when the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the K space in the memory. A group of echo data stored in an area corresponding to the K space in the memory is also referred to as K space data. Then, the arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the K space data, and displays the resulting image of the subject 101 on the display / operation unit 113 described later. And is recorded in the storage unit 115.

  The display / operation unit 113 includes a display unit for displaying the reconstructed image of the subject 101, a trackball or a mouse and a keyboard for inputting various control information of the MRI apparatus and control information for processing performed by the overall control unit 108. Etc., and an operation unit. The operation unit is disposed in the vicinity of the display unit, and an operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display unit.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging the information on the spatial distribution of proton density and the spatial distribution of the relaxation time of the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged in two or three dimensions.

(Summary of Invention)
The present invention prevents an increase in background noise of an image when correcting a luminance non-uniformity based on a non-uniform sensitivity distribution of the region imaging RF coil in an image obtained using the region imaging RF coil. Processing is performed to prevent the pixel value in the background noise area from being unnaturally raised. That is, based on the sensitivity distribution of the part imaging RF coil, a correction value for correcting the luminance unevenness of the image is obtained, and the background noise of the image is corrected when correcting the luminance unevenness of the image using the correction value. Prevent lifting. Embodiments of the present invention that prevent the background noise of an image from rising will be described in detail below.

  Next, a first embodiment of the MRI apparatus and brightness nonuniformity correction method of the present invention will be described. In this example, after subtracting the average value of the pixel values of the background noise region from the image data acquired using the part imaging RF coil, the luminance of the image based on the nonuniform sensitivity of the part imaging RF coil is reduced. Correction processing is performed, and the average value is added to the corrected image data to restore the original. This prevents the background noise from lifting. Hereinafter, the present embodiment will be described in detail with reference to FIGS. The method of this embodiment is hereinafter referred to as a background noise lifting prevention method.

  First, each function of the arithmetic processing according to the present embodiment will be described based on a functional block diagram of the arithmetic processing unit 114 shown in FIG. The arithmetic processing unit 114 according to the present embodiment includes a measurement unit 201, an image reconstruction unit 202, a sensitivity distribution calculation unit 203, a correction value calculation unit 204, a correction unit 205, a background noise calculation unit 206, a background A noise subtracting unit 207 and a background noise adding unit 208 are provided.

  Measurement unit 201, based on the pulse sequence time chart specified by the operator and the imaging conditions set by the operator, the application timing and application intensity of the RF pulse and the gradient magnetic field pulse constituting the pulse sequence, Alternatively, the pulse sequence is generated by obtaining data that specifically defines the sampling timing and the like. Then, the measurement control unit 111 is notified of the obtained data, and the measurement control unit 111 is caused to execute a pulse sequence. In the present embodiment, the measurement unit 201 includes a pulse sequence for main measurement that measures echo data for an image using the part imaging RF coil, and sensitivity correction data for the part imaging RF coil prior to the main measurement. Each of the pulse sequences for auxiliary measurement for acquiring the above is generated, and the measurement control unit 111 is caused to execute the two pulse sequences. In addition, as auxiliary measurement, first auxiliary measurement using the whole body RF coil and second auxiliary measurement using the part imaging RF coil are performed. Either the first auxiliary measurement or the second auxiliary measurement may be performed first. As the pulse sequence for auxiliary measurement, for example, the pulse sequence described in Patent Document 3 can be used. In addition, the pulse sequence for this measurement may be any pulse sequence.

The image reconstruction unit 202 reconstructs an image (hereinafter referred to as a first sensitivity image) using the echo data acquired by the first auxiliary measurement. Similarly, an image (hereinafter referred to as a second sensitivity image) is reconstructed using the echo data acquired by the second auxiliary measurement. In addition, an image (hereinafter referred to as a main measurement image) is reconstructed using the echo data acquired in the main measurement.
The sensitivity distribution calculation unit 203 calculates a ratio of pixel values for each pixel between the first sensitivity image acquired by the first auxiliary measurement and the second sensitivity image acquired by the second auxiliary measurement. Then, obtain the sensitivity distribution of the part imaging RF coil. Specifically, it is obtained as in the following equation (1).

Sensitivity distribution (r) = normalization processing [first sensitivity image (r) / second sensitivity image (r)] (1)
Here, r represents coordinates, r = (x, y) for a two-dimensional image and r = (x, y, z) for a three-dimensional image. Also, the normalization processing means normalizing the whole so that the value of the sensitivity distribution at the pixel position where the pixel value of the second sensitivity image becomes the maximum value becomes a predetermined reference value (for example, 1).

  The correction value calculation unit 204 calculates a correction value for correcting the luminance non-uniformity of the main measurement image, using the sensitivity distribution of the part imaging RF coil obtained by the equation (1). Specifically, the correction value is calculated by the equation (2).

Correction value (r) = 1 / Sensitivity distribution (r) (2)
The correction unit 205 corrects the luminance non-uniformity of the main measurement image (pre-correction image) with the correction value calculated by the correction value calculation unit 204 to obtain a corrected image. Specifically, the correction is made using equation (3).

Corrected image (r) = Image before correction (r) * Correction value (r) (3)
The background noise calculation unit 206 calculates a threshold value for determining background noise in an image, considers pixel values less than the calculated threshold value as background noise, and calculates an average value of background noise. Specifically, 10% of the maximum pixel value of the image is set as a threshold value, and a pixel value less than the threshold value is determined as background noise. Then, an average value of pixel values, that is, an average value of background noise is obtained for all pixels having pixel values less than the threshold value.

  The background noise subtracting unit 207 subtracts, for each pixel, the average value of the background noise area calculated by the background noise calculating unit 206 from each pixel value of the main measurement image.

  The background noise adding unit 208 adds an average value of background noise to each pixel value of the main measurement image after the luminance nonuniformity correction for each pixel.

  Each of the above functions is realized by loading a program stored in advance in the storage unit 115 into a memory (not shown) and executing it.

  Next, the processing flow of the present embodiment, which is performed in cooperation with the functional units of the arithmetic processing unit 114, will be described with reference to FIGS. FIG. 3 is a flowchart showing the processing flow of the present embodiment, and FIG. 4 shows an image and its center profile in each step of the flowchart shown in FIG.

  In step 301, the sensitivity distribution data of the part imaging RF coil is acquired. For this purpose, the measurement unit 201 causes the measurement control unit 111 to execute the first auxiliary measurement and the second auxiliary measurement, and acquires echo data of each auxiliary measurement. Next, the image reconstruction unit 202 reconstructs the first sensitivity image and the second sensitivity image using echo data acquired for each auxiliary measurement. Next, the sensitivity distribution calculation unit 203 calculates the sensitivity distribution of the part imaging RF coil based on the first sensitivity image and the second sensitivity image. Details are as described above.

  In step 302, the main measurement is executed and an image of the subject is acquired. For this purpose, the measurement unit 201 causes the measurement control unit to perform main measurement and acquires echo data of the main measurement. Next, the image reconstruction unit 202 reconstructs the main measurement image using the echo data acquired in the main measurement. FIG. 4 shows an image 401 and a pixel value profile 405 on a straight line passing through the center of the image 401 as an example of the main measurement image. Due to the non-uniform sensitivity of the part imaging RF coil, non-uniform luminance occurs in the subject region of the main measurement image 401. An image 401 and a profile 405 show examples of non-uniform luminance in which the luminance decreases substantially linearly from the left end to the right end of the image.

  In step 303, the average value of background noise is subtracted for each pixel from each pixel value of the main measurement image. For this purpose, the background noise calculation unit 206 determines background noise in the main measurement image, and calculates an average value of pixel values determined as background noise. The background noise subtraction unit 207 subtracts the background noise average value from the pixel values of the main measurement image for each pixel, thereby removing the background noise from the main measurement image. FIG. 4 shows an image 402 and a pixel value profile 406 on a straight line passing through the center of the image 402 as an example of the result of subtracting the average value of background noise from the pixel values of the main measurement image for each pixel. By subtraction of the average value α of background noise, the pixel value (luminance) and profile value of the image are lowered by α as a whole.

  In step 304, a correction value for correcting luminance nonuniformity in the main measurement image is calculated. For this purpose, the correction value calculation unit 204 calculates a correction value using the sensitivity distribution of the region imaging RF coil acquired in step 301. As an example of the correction value, FIG. 4 shows a correction value distribution 410 and a profile 411 of values on a dotted line at the center thereof. The correction value distribution 410 and the profile 411 have opposite changes compared to the luminance distortion shown in the image 401 and the profile 405, and the luminance unevenness of the image 402 is corrected by the correction value distribution 410.

  In step 305, the luminance nonuniformity in the main measurement image is corrected using the correction value. For this purpose, the correction value distribution 410 calculated by the correction unit 205 is multiplied by each pixel value of the main measurement image to correct luminance nonuniformity of the main measurement image. FIG. 4 shows an image 403 and a pixel value profile 407 on a straight line passing through the center of the image 403 as an example of the result of the luminance nonuniformity correction. As a result of the luminance nonuniformity correction, the image 403 and the profile 407 both have a substantially flat luminance change.

  In step 306, the average value of background noise is added for each pixel to each pixel value of the main measurement image that has been corrected for nonuniform luminance. For this purpose, the background noise adding unit 207 adds the average value α of the background noise acquired in step 303 to each pixel value of the main measurement image corrected in luminance nonuniformity in step 305 for each pixel, Restore the noise. FIG. 4 shows an image 404 and a pixel value profile 408 on a straight line passing through the center of the image 404 as an example of the result of adding the average value α of background noise to each pixel value of the main measurement image. By adding the average value α of the background noise, the pixel value (luminance) and profile value of the image are increased by α as a whole.

  The above is the description of the processing flow of the present embodiment.

  As described above, the MRI apparatus and the luminance nonuniformity correction method according to the present embodiment perform the luminance nonuniformity correction process after subtracting the average value of the background noise from each pixel value of the image, and finally the background noise. Add the average value back. As a result, it is possible to obtain an image with uniform brightness by correcting the brightness unevenness of the image based on the sensitivity distribution unevenness of the region imaging RF coil without unnaturally raising the pixel value of the background noise. The image quality can be improved.

  Next, a second embodiment of the MRI apparatus and luminance nonuniformity correction method of the present invention will be described. In this embodiment, the correction value obtained for preventing background noise lifting in the first embodiment is corrected using image data having a high SNR output in the same measurement, and the SNR is calculated using the correction correction value. Correct low images. Hereinafter, this embodiment will be described in detail with reference to FIGS.

  When the background noise lifting prevention method described in Example 1 is applied to an image with a low SNR, the difference in pixel value between the subject area and the background noise area is small, so the background noise is determined from the pixel value of the image. If the average value is subtracted, not only the background noise region but also a part of the subject region becomes close to zero, and there is a possibility that luminance unevenness cannot be corrected correctly. A problem in the case of simply applying the background noise lifting prevention method of the first embodiment described above to an image with a low SNR will be described with reference to FIG.

  As shown in the image 501 having a low SNR and the pixel value profile 505 on a straight line passing through the center of the image 501, in the image having a low SNR, the average value α of the pixel values in the background noise region and the pixel value (D ) Does not change much. Therefore, as shown in the no-noise image 502 obtained by subtracting the average value α from the image 501 and the pixel value profile 506 on a straight line passing through the center of the no-noise image 502, the object region after subtracting the average value α is displayed. The magnitude (D−α) of the pixel value becomes extremely small. As a result, even if the correction value 410 is multiplied by the noiseless image 502, the luminance non-uniformity cannot be corrected sufficiently, and the image remains with the luminance non-uniformity remaining. An image 503 and a pixel value profile 507 on a straight line passing through the center of the image 503 are shown as an example of an image whose luminance nonuniformity is not corrected even when the correction value 410 is multiplied. As a natural result, even if the average value α of the background noise is added to the corrected image 503 at the end, the luminance non-uniformity remains. An example of this is shown in a pixel value profile 508 on a straight line passing through the center of the image 504 and the image 504.

  Therefore, in this embodiment, in the measurement in which an image with a high SNR and an image with a low SNR are output, the luminance enhancement of the subject region is reduced while reducing the luminance enhancement of the unnecessary background noise region even for an image with a low SNR. An image with uniform brightness is acquired by correcting the uniformity.

First, an outline of the technique of this embodiment for solving the problem that the background noise lifting prevention technique cannot be simply applied to an image with a low SNR will be described below.
First, the background noise lifting prevention method is expressed by equation (4).

Signalは原画像の画素値(絶対値)を、Signal_corrは、補正画像の画素値(絶対値)を表し、aは補正値を、Noise_aveは背景ノイズの平均値を表す。また、x、y、zはそれぞれ、周波数エンコード方向、位相エンコード方向、スライス方向の座標を表す。
原画像の画素値に対する補正画像の画素値の比は(5)式の様に求まる。

Signal represents the pixel value (absolute value) of the original image, Signal _corr represents the pixel value (absolute value) of the corrected image, a represents the correction value, and Noise _ave represents the average value of the background noise. X, y, and z represent coordinates in the frequency encoding direction, phase encoding direction, and slice direction, respectively.
The ratio of the pixel value of the corrected image to the pixel value of the original image can be obtained as shown in Equation (5).

また、(5)式に於いて、原画像の画素値が背景ノイズの平均値に対して、非常に大きい場合、(5)式は、(6)式の様になり、通常の補正処理となる。

In addition, in the equation (5), when the pixel value of the original image is very large with respect to the average value of the background noise, the equation (5) becomes the equation (6), which is a normal correction process. Become.

一方、原画像の画素値が背景ノイズの平均値にほぼ等しい場合には、(5)式は、(7)式の様になり、補正処理の効果が一切なくなる。

On the other hand, when the pixel value of the original image is substantially equal to the average value of the background noise, the expression (5) becomes the expression (7), and the effect of the correction process is completely lost.

ここで、(5)式は画像における、背景ノイズ持ち上がり防止手法を考慮した補正値といえる。また、補正値は、部位撮像用RFコイルの感度不均一に起因するが画像の輝度不均一を補正するための処理であり、SNRには影響されない。さらに、背景ノイズ領域の位置も、SNRの違いには影響を受けない。そこで、同じ計測で得られるSNRが高い画像を用いて得た補正値を、背景ノイズ持ち上がり防止処理を考慮して、(5)式の様に修正して修正補正値を得る。この修正補正値を、同じ計測で得られた全ての画像に共通な補正値とすることで、背景ノイズ持ち上がり防止処理を考慮した補正値を作成することができる。

Here, equation (5) can be said to be a correction value in consideration of the background noise lifting prevention method in the image. The correction value is a process for correcting the luminance non-uniformity of the image although it is caused by the non-uniform sensitivity of the region imaging RF coil, and is not affected by the SNR. Furthermore, the position of the background noise region is not affected by the difference in SNR. Therefore, the correction value obtained by using the image having a high SNR obtained by the same measurement is corrected as shown in the equation (5) in consideration of the background noise lifting prevention process to obtain a corrected correction value. By making this correction correction value a correction value common to all images obtained by the same measurement, it is possible to create a correction value considering the background noise lifting prevention process.

  Therefore, in this embodiment, the correction value is corrected using the equation (5), and the luminance unevenness of the image having a low SNR is corrected using the correction correction value, thereby preventing the background noise from being raised for the image having a low SNR. Perform brightness non-uniformity correction in consideration. The processing flow of this embodiment will be described with reference to FIGS. FIG. 6 is a flowchart showing the processing flow of the present embodiment, and FIG. 7 shows an image and its center profile in each step of the flowchart shown in FIG.

  In step 601, the background noise calculation unit 206 obtains the noise average value 602 of the image 701 using the image 701 having a high SNR.

  In step 602, the correction value calculation unit 203 calculates the correction value 703 by the process described in the first embodiment.

  In step 603, the correction value calculation unit 203 calculates a correction correction value 704 based on the equation (5) using the image 701 having a high SNR, the noise average value 702, and the correction value 703. The correction correction value 704 is the same correction value as that of the above-described first embodiment for a pixel (mainly the subject area at the center) in which the pixel value of the image 701 having a high SNR is very large with respect to the average value of the noise area. The correction value is close to 1 for a pixel (mainly the surrounding background area) in which the pixel value of an image with a high SNR is substantially equal to the noise average value.

  In step 604, the correction unit 205 multiplies the image 705 having a high SNR by multiplying the correction correction value 704 obtained in step 603 by the image data 706 having a high SNR in which only the subject region is corrected without correcting the noise region. Get. FIG. 7 shows an example of obtaining an image 706 having a high SNR after luminance nonuniformity correction by multiplying an image 705 having high luminance nonuniformity by a correction correction value 704. The non-uniformity of the pixel value profile 715 on the straight line passing through the center of the image 705 is corrected to make the pixel value profile 716 on the straight line passing through the center of the image 706 uniform.

  In step 605, the correction unit 205 multiplies the correction correction value 704 obtained in step 603 by the image 707 having a low SNR, so that the background noise region is not corrected in consideration of prevention of background noise lifting. Only the corrected image data 708 with a low SNR is obtained. FIG. 7 shows an example of obtaining an image 708 having a low SNR after luminance non-uniformity correction by multiplying an image 707 having non-uniform luminance by a low correction SNR 704 on a straight line passing through the center of the image 707. The non-uniformity of the pixel value profile 717 is corrected to be uniform like a pixel value profile 718 on a straight line passing through the center of the low image 708.

  The above is the description of the processing flow of the present embodiment.

  As described above, the MRI apparatus and the brightness non-uniformity correction method of the present embodiment correct the correction value obtained for preventing background noise lifting using image data having a high SNR output in the same measurement, The corrected luminance value is used to correct the luminance unevenness of the image. As a result, regardless of the SNR level, the pixel value of the background noise area is not raised unnaturally, and the brightness uniformity of the image is corrected by correcting the brightness non-uniformity of the image based on the sensitivity distribution non-uniformity of the part imaging RF coil. Image can be acquired, and the image quality of the image can be improved. For example, in measurement in which an image having a high SNR and an image having a low SNR are output, the correction value for correcting the luminance non-uniformity is corrected with reference to the image data having a high SNR, and the image is obtained using the corrected correction value. By correcting the luminance non-uniformity, it is possible to obtain an image with uniform luminance in which unnecessary luminance enhancement of the background noise of the image is reduced.

  Next, a third embodiment of the MRI apparatus and the luminance non-uniformity correction method according to the present embodiment will be described. In this example, luminance nonuniformity correction based on nonuniform sensitivity of the region imaging RF coil is applied to diffusion weighted measurement (hereinafter referred to as DWI measurement) as a measurement that can obtain an image with a high SNR and a low image with the same measurement. To do. Hereinafter, this embodiment will be described in detail with reference to FIGS.

  In the DWI measurement, a B0 image to which MPG (Motion Proving Gradient) is not applied and an MPG image to which MPG is applied are obtained using the same part imaging RF coil. The B0 image is an image having a high SNR, and the MPG image is an image having a low SNR. Since details of DWI measurement are described in Patent Document 2, detailed description thereof is omitted.

  In such DWI measurement, the processing flow of the present embodiment for performing luminance nonuniformity correction using the correction correction value described in the second embodiment will be described based on the flowchart shown in FIG.

  In step 801, auxiliary measurement for acquiring sensitivity distribution data of the region imaging RF coil is performed. Specifically, the measurement unit 201 performs the first auxiliary measurement using the whole-body RF coil, acquires an image with uniform brightness as shown in FIG. 9, for example, and uses the part imaging RF coil to obtain the second For example, an image with nonuniform luminance as shown in FIG. 10 is acquired.

  In step 802, sensitivity distribution data of the region imaging RF coil is created. Specifically, the sensitivity distribution calculation unit 203 creates sensitivity distribution data of the region imaging RF coil based on the equation (1). At that time, a mask process is performed on a region other than the subject region such as a background noise region, and then a reference value (for example, 1) is substituted into the background noise region. An example of the sensitivity distribution is shown in FIG.

  In step 803, a correction value is created. Specifically, the correction value calculation unit 204 calculates a correction value from the sensitivity distribution data created in step 802 based on equation (2). An example of the correction value data is shown in FIG.

  In step 804, a B0 image to which no MPG is applied is acquired using the region imaging RF coil. This B0 image is an image having a high SNR. Specifically, the measurement unit 201 generates data that specifically defines a DWI sequence that does not apply MPG for B0 image acquisition, notifies the measurement control unit 111 of the data, and sends the MPG to the measurement control unit 111. Echo data is acquired by executing a DWI sequence without applying. Then, the image reconstruction unit 202 reconstructs the B0 image using the acquired echo data.

  In step 805, the correction value calculated in step 803 is corrected to calculate a correction correction value. Specifically, the correction value calculation unit 204 corrects the correction value calculated in step 803 using the B0 image that is an image having a high SNR acquired in step 804, and calculates a correction correction value. FIG. 13 shows an example of the correction correction value data.

  In step 806, luminance nonuniformity correction processing of the B0 image is performed. Specifically, the correction unit 205 corrects the luminance non-uniformity of the B0 image acquired in step 804 using the correction correction value calculated in step 805.

  In step 807, the B0 image in which the luminance unevenness is corrected in step 806 is displayed on the display unit.

  In step 808, an MPG is obtained by applying MPG using the region imaging RF coil. This MPG image is an image having a low SNR. Specifically, the measurement unit 201 generates data that specifically defines the DWI sequence for applying the MPG for MPG image acquisition, notifies the measurement control unit 111 of the data, and notifies the measurement control unit 111 of the MPG. Echo data is acquired by executing a DWI sequence that applies. Then, the image reconstruction unit 202 reconstructs the MPG image using the acquired echo data.

  In step 809, a nonuniform luminance correction process for the MPG image is performed. Specifically, the correction unit 205 corrects the luminance non-uniformity of the MPG image acquired in step 808 using the correction correction value calculated in step 805.

  In step 810, the MPG image in which the luminance unevenness is corrected in step 809 is displayed on the display unit.

  The above is the description of the processing flow of the present embodiment.

  As described above, the MRI apparatus and the luminance non-uniformity correction method of this embodiment obtain a B0 image with a high SNR and a low MPG image by DWI measurement, and correct the correction value based on the B0 image. The value is obtained, and the brightness non-uniformity correction is performed on both the B0 image and the MPG image. As a result, regardless of the SNR level, both the B0 image and the MPG image do not raise the pixel values in the background noise region unnaturally, and the brightness of both images is not uniform due to the non-uniform sensitivity distribution of the part imaging RF coil. It is possible to correct the uniformity and acquire an image with uniform brightness, and to improve the image quality of both images.

  As mentioned above, although the Example of this invention was described, this invention is not limited to these. For example, in each of the above-described embodiments, the example in which the average value of the background noise is added to or subtracted from each pixel value of the image has been described. However, the background noise region and the subject region in the image are determined, and the background noise region pixels are determined. The brightness non-uniformity in the subject area may be corrected after being replaced with zero, and the background noise area replaced with zero after the correction may be restored.

  101 Subject, 102 Static magnetic field generating magnet, 103 Gradient magnetic field coil, 104 Transmitting RF coil, 105 RF receiving coil, 106 Signal detection unit 106, 107 Signal processing unit, 108 Overall control unit, 109 Gradient magnetic field power source, 110 RF transmission unit , 111 Measurement control unit, 112 beds, 113 Display / operation unit, 114 Arithmetic processing unit, 115 Storage unit

Claims (7)

A magnetic resonance imaging apparatus including an arithmetic processing unit that corrects luminance non-uniformity based on a non-uniform sensitivity distribution of a region imaging RF coil in an image obtained using the region imaging RF coil,
The arithmetic processing unit includes:
A sensitivity distribution calculation unit for obtaining a sensitivity distribution of the part imaging RF coil;
A correction value calculation unit for obtaining a correction value for correcting the luminance non-uniformity based on the sensitivity distribution;
A background noise lifting prevention unit for preventing the background noise of the image from being raised when correcting the brightness non-uniformity of the image using the correction value;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1.
The background noise lifting prevention unit is
A background noise calculator for calculating an average value of background noise of the image;
A background noise subtraction unit that subtracts an average value of background noise from each pixel value of the image;
A correction unit that corrects luminance non-uniformity of the image in which the average value of the background noise is reduced using the correction value;
A background noise adding unit that adds an average value of the background noise to each pixel value of the corrected image;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1.
The correction value calculation unit corrects the correction value using an image having a high SNR to obtain a correction correction value;
The background noise lifting prevention unit corrects luminance non-uniformity of an image having a low SNR using the correction correction value. The magnetic resonance imaging apparatus according to claim 1.
The high SNR image is a B0 image obtained without applying MPG in DWI measurement,
The magnetic resonance imaging apparatus according to claim 1, wherein the image having a low SNR is an MPG image obtained by applying MPG by DWI measurement. This is a luminance non-uniformity correction method for correcting luminance non-uniformity based on the non-uniform sensitivity distribution of the region imaging RF coil in an image obtained by operating the magnetic resonance imaging apparatus and using the region imaging RF coil. And
Obtaining a sensitivity distribution of the part imaging RF coil;
A correction value calculating step for obtaining a correction value for correcting the brightness non-uniformity based on the sensitivity distribution;
Preventing the background noise of the image from being raised during correction of non-uniform luminance of the image using the correction value;
A brightness non-uniformity correction method comprising: The brightness non-uniformity correction method according to claim 5,
The step of preventing the background noise from lifting is as follows:
Calculating an average value of background noise of the image;
Subtracting the average background noise from each pixel value of the image;
Correcting the brightness non-uniformity of the image in which the average value of the background noise is reduced using the correction value;
Adding an average value of the background noise to each pixel value of the corrected image;
A luminance non-uniformity correction method comprising: The brightness non-uniformity correction method according to claim 5,
The step of calculating the correction value acquires the correction correction value by correcting the correction value using an image having a high SNR,
The step of preventing the background noise from rising includes correcting the luminance non-uniformity of an image having a low SNR using the correction correction value.

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