JP2013183967A - Magnetic resonance imaging apparatus and phase correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a phase rotation amount due to non-uniform static magnetic field by phase unwrapping by accurately obtaining and correcting a phase gradient generated between different TEs in order to obtain an image with reduced exchange of water and fat. At this time, a static magnetic field inhomogeneity map in which the vicinity of the main value of the phase is reduced is created.
SOLUTION: The phase gradient generated in an image between different echoes is accurately calculated by obtaining an average of phase differential values (phase differences between adjacent pixels) or a weighted average value. By correcting the phase gradient generated in the image between echoes by this phase gradient, the phase principal value is obtained when the phase rotation amount due to the static magnetic field inhomogeneity (static magnetic field inhomogeneity map) is obtained by phase unwrapping Reduce the rotation. Thereby, the image which reduced the interchange of water and a fat image is obtained.
[Selection] Figure 4

Description

  The present invention relates to a nuclear magnetic resonance imaging (MRI) apparatus that measures nuclear magnetic resonance (NMR) signals from hydrogen, phosphorus, etc. in a subject and visualizes nuclear density distribution, relaxation time distribution, etc. And a phase correction technique for acquiring fat images.

  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 and is 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.

  When an image is obtained with an MRI apparatus, an image having various tissue contrasts can be obtained by changing parameters such as echo time (TE) and repetition time (TR) or performing image calculation. In clinical practice, an image in which signals from adipose tissue are suppressed is often required. As an example of a method for obtaining an image in which a signal from adipose tissue is suppressed, there is a method for obtaining a plurality of images having different TEs and obtaining a water / fat separated image by calculation. A typical method is called a Dixon method (see Non-Patent Document 1).

  Furthermore, in the MRI apparatus, the spatial non-uniformity of the static magnetic field itself due to the magnet structure and the spatial non-uniformity of the static magnetic field due to different magnetic sensitivities for each part of the subject placed in the static magnetic field space. (Hereinafter, collectively referred to as static magnetic field inhomogeneity). Thus, there are a two-point Dixon method with static magnetic field correction (see Non-Patent Document 2), a three-point Dixon method (see Patent Document 1), and the like, in which a function for correcting the influence of static magnetic field inhomogeneity is added to the Dixon method.

  In the three-point Dixon method and the two-point Dixon method with static magnetic field correction, when calculating the amount of phase rotation due to inhomogeneous static magnetic field, it is necessary to perform a calculation process called phase unwrapping to prevent the water and fat from being switched . This phase unwrapping process eliminates the fact that the phase exceeding the range of -π to π is again expressed within the range of -π to π, and makes the phase change continuous, thereby reducing the range of -π to π. This is a process of expressing with the phase value exceeding.

  The phase unwrap processing has a problem that the main value cannot be detected. In other words, if the inhomogeneity of the static magnetic field is large or the time interval between echoes of the same phase and opposite phase is wide, the phase becomes −π or less, or more than π (this state is caused around the main value). It is said that). When the main value occurs, the images of water and fat obtained by calculation are switched.

  Therefore, by correcting and removing in advance the phase gradient (that is, the primary component of the phase change) that occurs between the in-phase and anti-phase TEs, the main value in the phase unwrap process is reduced, and water and fat are reduced. There is known a method for reducing the replacement of images (see Non-Patent Document 3).

  For the phase gradient, it is effective to obtain the phase gradient in the image space by the least square method using a linear function. However, since the phase range is -π to π, the phase is unwrapped after the phase unwrapping process. It is necessary to calculate the slope of the phase by the least square method using a function.

JP 2002-301041 A

W. Thomas Dixon “Simple Proton Spectroscopic Imaging” RADIOLOGY, Vol.153, p.189-194, (1984) Bernard D. Cooms “Two-Point Dixon Technique for Water-Fat Signal Decomposition with B0 Inhomogeneity Correction” Magnetic Resonance in Medicine, vol.38, p.884-889, (1997) J.Ma “Linear phase error correction for improved water and fat separation in the dual-echo Dixon techniques” International Society for Magnetic Resonance in Medicine 16 (2008) p.655

  As described above, in the two-point Dixon method and the three-point Dixon method with static magnetic field correction, when the phase rotation amount due to the static magnetic field inhomogeneity is obtained by phase unwrapping, the image of water and fat is generated when the phase is around the main value. There is a problem that is replaced. A method of removing in advance the phase gradient generated between the in-phase and anti-phase TEs by correction is effective, but at this time, it is necessary to obtain the phase gradient with high accuracy.

  However, the phase unwrapping process for obtaining the phase gradient may not correctly represent the phase continuity due to the influence of noise in a region where the SNR (signal to noise ratio) is low, and the phase gradient may not be obtained correctly.

  The object of the present invention has been made in view of the above problems, and by accurately obtaining and correcting the phase gradient generated between different TEs, the amount of phase rotation due to non-uniform static magnetic fields is obtained by phase unwrapping. An object of the present invention is to provide an MRI apparatus and a phase correction method capable of creating a static magnetic field inhomogeneous map in which the area around the principal value of the phase is reduced when obtaining.

  In order to solve the above-mentioned problem, the present invention provides a phase gradient of the phase rotation amount between images of different TEs before the phase rotation amount (static magnetic field inhomogeneity map) due to the static magnetic field inhomogeneity is obtained by the phase unwrapping process. Obtaining with high accuracy, correcting this and removing it, reduces the phase around the main value due to the phase unwrapping process, and obtains an image with reduced exchange of water and fat images.

  To this end, the present invention is characterized in that a phase differential value (phase difference between adjacent pixels) in an image space is calculated, and a phase gradient is obtained by obtaining an average of the phase differential values.

  Specifically, the present invention calculates the phase gradient between image data with different echo times or the phase of the image data, removes the phase gradient from at least one image data, and removes the phase gradient image data. A water image and a fat image are acquired using at least two image data including different echo times. At this time, the phase gradient is calculated from the average value or the weighted average value of the differential value of the phase.

  Since the MRI apparatus and the phase correction method of the present invention are configured as described above, the inclination of the phase of the image is accurately removed. Thereby, when the amount of phase rotation due to non-uniformity of the static magnetic field is obtained by the phase unwrapping process, the phase around the main value can be reduced. As a result, it is possible to obtain an image with reduced replacement of water and fat.

Overall configuration of magnetic resonance imaging system Gradient echo (GE) sequence diagram for capturing two TE images Functional block diagram showing each function of phase correction processing of the present invention The flowchart showing the processing flow of the Example which correct | amends the phase inclination component between the image of water and fat in the same phase of this invention, and the image of antiphase. Flow chart showing processing flow explaining creation of water and fat image

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment of the invention, and the repetitive description thereof is omitted.

  First, the configuration of an example of the MRI apparatus of the present invention will be described. FIG. 1 is a schematic diagram of a configuration example of an MRI apparatus. This MRI apparatus includes a static magnetic field generating magnet 102 for generating a static magnetic field around the subject 101, a gradient coil 103 for generating a gradient magnetic field, an irradiation coil 104 for irradiating the subject with a high-frequency magnetic field pulse, A receiving coil 105 for detecting the NMR signal and a bed 106 on which the subject 101 lies are provided.

  The static magnetic field generating magnet 102 is a permanent magnet, a superconducting magnet, or a normal conducting magnet arranged in a wide space around the subject 101, and is parallel to the body axis of the subject 101 or A uniform static magnetic field is generated in the vertical direction.

  The gradient magnetic field coil 103 applies a gradient magnetic field in the X, Y, and Z axial directions to the subject 101 in accordance with a signal from the gradient magnetic field power source 107. The imaging cross section of the subject is set depending on how the gradient magnetic field is applied.

  Irradiation coil 104 generates a high-frequency magnetic field pulse (hereinafter abbreviated as “RF pulse”) in accordance with a signal from RF transmitter 108. By this RF pulse, atomic nuclei constituting the biological tissue of the imaging cross section of the subject 101 set by the gradient magnetic field coil 103 are excited and an NMR phenomenon is induced.

  An echo signal, which is an NMR signal generated by the NMR phenomenon of the atomic nucleus constituting the biological tissue of the subject 101 induced by the RF pulse emitted from the irradiation coil 104, is received close to the subject 101. The signal is detected by the signal detection unit 109 through the coil 105, processed by the signal processing unit 110, and converted into an image. The converted image is displayed on the display unit 111.

  The control unit 112 generates slice encoding, phase encoding, and frequency encoding gradient magnetic fields and RF pulses based on a predetermined pulse sequence, and repeats the process of measuring the echo signal in order to repeat the gradient magnetic field power supply 107, RF transmission The unit 108 and the signal processing unit 110 are controlled.

(Example of Pal sequence)
Next, an example of a pulse sequence used in water / fat separation measurement according to the present invention will be described based on the sequence chart shown in FIG. This pulse sequence is a pulse sequence for obtaining two types of image data having different TEs based on a gradient echo (GE) sequence method.

  The controller 112 performs the following control to execute this pulse sequence. That is, the slice encode gradient magnetic field 202 is applied simultaneously with the irradiation of the RF pulse 201 to excite only the target tomographic plane. Then, after applying a phase encoding gradient magnetic field 203 for encoding position information and simultaneously applying a negative frequency encoding gradient magnetic field (pre-pulse) 204, applying a positive frequency encoding gradient magnetic field 205 to RF The first echo signal 208 is generated after TE1 has elapsed from the pulse 201. Next, a negative frequency encode gradient magnetic field (rewind pulse) 206 and a frequency encode gradient magnetic field 207 are applied again to generate a second echo signal 209 after TE2 has elapsed from the RF pulse 201. Then, such a pulse sequence is repeatedly executed for the number of times of phase encoding while changing the area of the gradient magnetic field 203 for phase encoding, and echo signals for the number of phase encoding are acquired, and data is filled in the k space. . Two types of image data with different TEs can be obtained by performing two-dimensional Fourier transform on the filled k-space data.

  In the two-point Dixon method, the timing at which water and fat are in opposite phases is set to TE1, and the timing at which water and fat are in phase is set to TE2 (> TE1).

(Outline of processing of the present invention)
Next, the main part of the phase correction process of the present invention will be described.
First, each function of the phase correction processing of the present invention will be described based on the functional block diagram shown in FIG.

  The functions of the phase correction processing of the present invention are roughly divided into a phase gradient removing unit 310 that removes a phase gradient of an image, a static magnetic field inhomogeneous map creating unit 320 that creates a static magnetic field inhomogeneous map, and a static magnetic field non-uniformity. It includes a phase correction unit 331 that corrects the phase of an image with a uniform map, and a water fat separation unit 341 that obtains a water image and a fat image by performing complex addition and subtraction of two images.

  Then, the phase gradient removing unit 310 includes a phase doubling unit 311, a Fourier transform unit 312, a phase difference calculation unit 313, a phase differentiation calculation unit 314, a phase gradient calculation unit 315, a phase gradient subtraction unit 316, It has.

  The static magnetic field inhomogeneity map creation unit 320 includes a phase map creation unit 321 and a phase unwrap processing unit 322. Each of these functions is implemented by the signal processing unit 110 executing each process on various data stored in the memory 301 based on a predetermined program. The processing contents of these functions will be described in detail in the description of the processing flow described below.

  Next, the processing flow of the phase correction processing of the present invention performed by cooperation of these functions will be described based on the flowchart shown in FIG. This processing flow is stored in the signal processing unit 110 as a program, for example, and is executed by the signal processing unit 110 executing the processing of each step.

  In this processing flow, a method for correcting the phase gradient in the frequency encoding direction in the two-point Dixon method will be described. Therefore, an image of complex data obtained from the first echo signal 208 measured in TE1 where water and fat are in opposite phases is a first echo image 401, and the first obtained in TE2 where water and fat are in phase. An image of complex data obtained from the two echo signals is a second echo image 402. These image data are stored in the memory in the signal processing unit 110. When the phase inclination in the phase encoding direction is corrected, the same correction can be performed by replacing the following x coordinate and y coordinate.

Hereinafter, each processing step will be described in detail.
First, each processing step from Step 403 to Step 413 related to the phase gradient removal processing will be described.

  In step 403, the phase doubling unit 311 removes the phase difference (180 °) due to the chemical shift of water and fat in the first echo image S1 (x, y) 401 and the second echo image S2 (x, y) 402. Therefore, the phases of the first echo image 401 and the second echo image 402 are each doubled.

ここで、S1’(x,y)は第1エコー画像301の位相を2倍にした画像であり、S2’(x,y)は第2エコー画像302の位相を2倍にした画像である。また、xは周波数エンコード方向の座標とし、yは位相エンコード方向の座標とする。||は絶対値を示す。

Here, S1 ′ (x, y) is an image obtained by doubling the phase of the first echo image 301, and S2 ′ (x, y) is an image obtained by doubling the phase of the second echo image 302. . Further, x is a coordinate in the frequency encoding direction, and y is a coordinate in the phase encoding direction. || indicates an absolute value.

  In step 404, the Fourier transform unit 312 performs Fourier transform on the images S1 ′ (x, y) and S2 ′ (x, y) obtained in step 403 in the phase encoding direction, and converts the phase encoding direction into the real space (image space). ) To k-space. In other words, the phase encoding direction of the image S1 ′ (x, y) is Fourier-transformed and the data s1 (x, ky) 405 returned to the k space, and the phase encoding direction of the image S2 ′ (x, y) is Fourier-transformed. The data s2 (x, ky) 406 returned to the k space is created respectively.

ステップ407で、位相差分算出部313は、位相エンコード方向のk空間の中心データを取り出す。s1(x,ky)405の位相エンコード方向のk空間の中心データをs1(x,0)408とし、s2(x,ky)406の位相エンコード方向のk空間の中心データをs2(x,0)409とする。s1(x,0)408およびs2(x,0)409は、それぞれ画像S2’(x,y)および画像S1’(x,y)を位相エンコード方向に平均化したデータである。

In step 407, the phase difference calculation unit 313 extracts the center data of the k space in the phase encoding direction. The center data of k space in the phase encoding direction of s1 (x, ky) 405 is s1 (x, 0) 408, and the center data of k space in the phase encoding direction of s2 (x, ky) 406 is s2 (x, 0 ) 409. s1 (x, 0) 408 and s2 (x, 0) 409 are data obtained by averaging the image S2 ′ (x, y) and the image S1 ′ (x, y) in the phase encoding direction, respectively.

  By applying a low-pass filter to s1 (x, 0) 408 and s2 (x, 0) 409 in the frequency encoding direction to remove noise components, the calculation accuracy of the phase gradient can be improved.

  In step 410, the phase difference calculation unit 313 calculates the phase difference D (x) between s2 (x, 0) and s1 (x, 0) acquired in step 407. The phase difference D (x) is obtained by the following equation.

ここで、*は複素共役を示す。

Here, * indicates a complex conjugate.

  In step 411, the phase differential calculation unit 314 differentiates the phase difference D (x) obtained in step 410 in the frequency encoding direction to obtain a differential value θ (x) of the phase. The differential value θ (x) of the phase is obtained by the following equation. This differentiation is a phase difference between adjacent pixels.

argは複素データから位相を求めることを示す。

arg indicates that the phase is obtained from complex data.

  In step 412, the phase inclination calculation unit 315 calculates an average value a (that is, a primary coefficient of phase change) of the phase differential value θ (x) obtained in step 410. At this time, in order to remove the influence of low SNR, an average value is calculated using weighting of data in which water and fat are in opposite phases. By using data in which water and fat have opposite phases, the influence of the low SNR region where water and fat cancel each other can be eliminated.

重み付けは、例えば閾値処理によって行なっても良い。例えば、水と脂肪が逆位相となるデータが閾値以上のときは1、閾値未満のときは0とすることで、閾値以上の領域での平均値を求めることができる。このような重み付けによって、信号強度にムラがある場合でも、位相の傾斜を精度良くの算出することができる。

The weighting may be performed by threshold processing, for example. For example, when the data in which water and fat are in opposite phases is equal to or greater than the threshold value, the average value in the region equal to or greater than the threshold value can be obtained by setting 1 when the data is equal to or greater than the threshold value. By such weighting, even when the signal intensity is uneven, the phase gradient can be calculated with high accuracy.

  In step 413, the phase gradient subtraction unit 316 subtracts the phase of the second echo image S2 (x, y) 402 by the phase gradient half that of the average value a calculated in step 412 to obtain water and fat. The phase gradient generated between the in-phase image and the anti-phase image is removed. A corrected second echo image S2 ″ (x, y) 414 obtained by subtracting the phase gradient of the second echo image S2 (x, y) 402 is obtained by the following equation.

以上までが、本発明である位相の傾斜の除去方法に関して説明した。

Up to this point, the phase gradient removal method according to the present invention has been described.

  Next, a method of creating a water image and a fat image using the first echo image 401 and the second echo image 414 after removing the phase gradient will be described with reference to FIG.

  In step 503, the phase map creation unit 321 creates a phase map from the first echo image S1 (x, y) 401 and the corrected second echo image S2 ″ (x, y) 414 from which the phase gradient is removed. The phase map is created by subtracting the phase of S1 (x, y) from S2 ″ (x, y) and doubling the remaining phase.

φ(x,y)は位相マップを示す。

φ (x, y) indicates a phase map.

  In step 504, the phase unwrap processing unit 322 performs phase unwrap processing on the phase map φ (x, y) calculated in step 503 to generate a static magnetic field inhomogeneity map ψ (x, y) 505.

  Since the phase range is in the range of −π to π, a value exceeding the range of −π to π cannot be identified. The phase unwrap process is a process for identifying the phase exceeding the range of -π to π, and 2π is added to the phase of the pixel so that the phase difference between adjacent pixels on the image is within the range of -π to π. This is a process of adding or subtracting.

  This phase unwrapping process selects the correct phase if the true phase difference between adjacent pixels is within the range of -π to π, but the true phase difference between adjacent pixels is outside the range of -π to π. When it becomes, the wrong phase is selected (the main value rotation occurs).

  In response to this problem, the phase correction method of the present invention removes the phase gradient, which is a factor causing an error in phase selection, in advance, as described in step 413, so that adjacent pixels can be detected. The true phase difference falls within the range of −π to π, and the phase can be selected correctly by the phase unwrapping process.

  In step 506, the phase correction unit 331 corrects the phase of the corrected second echo image S2 ″ (x, y) 414 using the static magnetic field inhomogeneity map 405 calculated in step 504. Specifically, the phase The correction unit 331 subtracts half the phase of the static magnetic field inhomogeneity map ψ (x, y) 505 from the corrected second echo image S2 ″ (x, y) 414. When expressed by an expression, the following expression is obtained.

ここで

は位相補正した第2エコー画像である。

here

Is the second echo image after phase correction.

と、を複素加算および減算し、それぞれ水画像W(x,y)508と脂肪画像F(x,y)509を得る。即ち、

以上までが、水画像と脂肪画像の作成方法の説明である。 In step 507, the water fat rib 341 includes the first echo image S1 (x, y) 401 and the second echo image phase-corrected in step 506.

Are subjected to complex addition and subtraction to obtain a water image W (x, y) 508 and a fat image F (x, y) 509, respectively. That is,

The above is the description of the method for creating the water image and the fat image.

  As described above, according to the embodiment of the present invention, the phase gradient is obtained by the phase differentiation process and the weighted average process without using the phase unwrapping process. Therefore, it is possible to obtain a water image and a fat image with reduced replacement of water and fat.

  Although the two-point Dixon method has been described in the above-described embodiments, the present invention can be similarly applied to the three-point Dixon method. In the three-point Dixon method, for example, if three images are acquired at equal intervals in the order that the phases of water and fat are the same phase, opposite phase, and same phase, the phase between the first same phase and the third same phase The inclination may be calculated, the third in-phase image may be corrected with the calculated phase inclination, and the anti-phase image may be corrected with half the phase inclination. At this time, the weighted average of the differential value of the phase when obtaining the slope of the phase may be weighted average with the signal intensity of the same phase.

  Further, in the above embodiment, the phase gradient between images of different TEs is accurately removed, but the phase gradient of the image can be simply removed in the same manner.

  101 subject, 102 static magnetic field magnet, 103 gradient coil, 104 irradiation coil, 105 receiver coil, 106 top plate, 107 gradient magnetic field power supply, 108 RF transmitter, 109 signal detector, 110 signal processor, 111 display, 112 Control unit

Claims (5)

A phase inclination removal unit that calculates the inclination of the phase of image data between image data having different echo times or removes the inclination of the phase from at least one image data;
A water / fat separation unit for acquiring a water image and a fat image using at least two image data having different echo times, including image data from which the phase gradient has been removed,
A magnetic resonance imaging apparatus comprising:
The magnetic resonance imaging apparatus, wherein the phase gradient removal unit includes a phase gradient calculation unit that calculates the phase gradient from an average value or a weighted average value of phase differences between adjacent pixels of the image data. The magnetic resonance imaging apparatus according to claim 1.
The phase inclination calculation unit weights the phase difference using image data in which water and fat have opposite phases, and calculates the phase inclination. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the phase inclination calculation unit calculates the inclination of the phase by weighting the phase difference by thresholding the phase difference. The magnetic resonance imaging apparatus according to claim 1.
The phase inclination calculation unit calculates the inclination of the phase using data of one line at the center of k-space obtained by Fourier transforming the image data in a phase encoding direction or a frequency encoding direction. Magnetic resonance imaging device. A phase gradient removal step of calculating a phase gradient between image data having different echo times or a phase gradient of the image data, and removing the phase gradient from at least one image data;
A water fat separation step for obtaining a water image and a fat image, respectively, using at least two image data having different echo times, including image data from which the phase gradient has been removed;
A phase correction method in a magnetic resonance imaging apparatus comprising:
The phase inclination removing step includes a phase inclination calculating step of calculating an inclination of the phase from an average value or a weighted average value of phase differences between adjacent pixels of the image data.

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