KR101725681B1 - Method for unwrapping phase in magnetic resonance image deive and magnetic resonance image device using the method - Google Patents

Method for unwrapping phase in magnetic resonance image deive and magnetic resonance image device using the method Download PDF

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KR101725681B1
KR101725681B1 KR1020150164302A KR20150164302A KR101725681B1 KR 101725681 B1 KR101725681 B1 KR 101725681B1 KR 1020150164302 A KR1020150164302 A KR 1020150164302A KR 20150164302 A KR20150164302 A KR 20150164302A KR 101725681 B1 KR101725681 B1 KR 101725681B1
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phase
magnetic resonance
angular velocity
size information
noise
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KR20170003355A (en
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황도식
어태준
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연세대학교 산학협력단
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/5601—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution involving use of a contrast agent for contrast manipulation, e.g. a paramagnetic, super-paramagnetic, ferromagnetic or hyperpolarised contrast agent
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/5608—Data processing and visualization specially adapted for MR, e.g. for feature analysis and pattern recognition on the basis of measured MR data, segmentation of measured MR data, edge contour detection on the basis of measured MR data, for enhancing measured MR data in terms of signal-to-noise ratio by means of noise filtering or apodization, for enhancing measured MR data in terms of resolution by means for deblurring, windowing, zero filling, or generation of gray-scaled images, colour-coded images or images displaying vectors instead of pixels

Abstract

A phase spreading method in a magnetic resonance imaging apparatus and a magnetic resonance imaging apparatus using the method are disclosed. The disclosed apparatus includes an image acquiring unit acquiring magnetic resonance images for a plurality of echo times; A size information extracting unit for extracting size information of each pixel from the acquired magnetic resonance image; A noise removing unit for removing noise from the extracted size information; And the expanded phase is linearly proportional to the echo time, a subtracted value obtained by subtracting the modeled extended phase from the magnetic resonance image acquired for each echo time is subtracted from the noise-removed magnitude, and the difference value To obtain an angular velocity and an initial phase of the modeled expanded phase that minimizes the sum of the angular velocity and the initial phase. According to the disclosed apparatus and method, it is possible to obtain an expanded phase with respect to the folded phase of a magnetic resonance image measured with high accuracy.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging apparatus,

Embodiments of the present invention relate to a method of spreading a phase in a magnetic resonance imaging apparatus and a magnetic resonance imaging apparatus using the method.

The image data obtained from the MRI apparatus can be divided into a magnitude component and a phase component due to its complex shape, but phase data is not normally used and size information is generally used.

However, in recent years, it has become known that phase information of magnetic resonance images can provide information such as magnetic susceptibility, elasticity and heat of tissue, and so on.

However, the phase information obtained through actual measurement is

Figure 112015114275774-pat00001
in
Figure 112015114275774-pat00002
Phase only. For example,
Figure 112015114275774-pat00003
The phase being measured is < RTI ID = 0.0 >
Figure 112015114275774-pat00004
It must be expressed as.

Therefore, phase spreading must be performed to obtain accurate phase information.

Conventionally, phase spreading has predominantly been performed in the spatial domain. However, there is a limit to accuracy in spreading the phase in the spatial domain, and a method of performing phase spreading in the echo time domain has been proposed. However, the phase spreading in the echo time domain is a method of deriving only a phase change between magnetic resonance images for each adjacent echo time, and this method also has a problem that the accuracy can not be relied upon.

An aspect of the present invention provides a method for acquiring an expanded phase for a collapsed phase of a magnetic resonance image measured with high accuracy.

Another aspect of the present invention provides a method for acquiring an expanded phase by comprehensively considering all magnetic resonance images obtained for each echo time.

According to an aspect of the present invention, there is provided an image processing apparatus including an image acquiring unit acquiring magnetic resonance images of a plurality of echo times; A size information extracting unit for extracting size information of each pixel from the acquired magnetic resonance image; A noise removing unit for removing noise from the extracted size information; And the expanded phase is linearly proportional to the echo time, a subtracted value obtained by subtracting the modeled extended phase from the magnetic resonance image acquired for each echo time is subtracted from the noise-removed magnitude, and the difference value And obtaining an angular velocity and an initial phase of the modeled expanded phase that minimizes a value obtained by summing the angular velocity and the initial phase.

The modeled open phase is set according to the following equation.

Figure 112015114275774-pat00005

In the above equation,

Figure 112015114275774-pat00006
Is an angular velocity,
Figure 112015114275774-pat00007
Is an initial phase,
Figure 112015114275774-pat00008
Is the pixel value.

A value obtained by applying the modeled open phase to the noise-removed size is set as the following equation.

Figure 112015114275774-pat00009

In the above equation,

Figure 112015114275774-pat00010
Is the noise-removed size information
Figure 112015114275774-pat00011
Is modeled and unfolded.

The expanded phase obtaining unit obtains an expanded phase through the following equation.

Figure 112015114275774-pat00012

Figure 112015114275774-pat00013

In the above equation,

Figure 112015114275774-pat00014
Is the acquired magnetic resonance image,
Figure 112015114275774-pat00015
Is the noise-removed size information,
Figure 112015114275774-pat00016
Is an angular velocity,
Figure 112015114275774-pat00017
Is an initial phase,
Figure 112015114275774-pat00018
Means a pixel.

The expanded phase acquiring unit obtains an angular velocity and an initial phase for each echo image by a least squares method.

The expanded phase acquiring unit acquires an angular velocity and an initial phase using a dictionary that stores in advance a possible angular velocity and an initial phase set.

According to another aspect of the present invention, there is provided a method comprising: (a) obtaining magnetic resonance images for a plurality of echo times; (B) separating and extracting size information of each pixel from the acquired magnetic resonance image; (C) removing noise from the extracted size information; And the expanded phase is linearly proportional to the echo time, a subtracted value obtained by subtracting the modeled extended phase from the magnetic resonance image acquired for each echo time is subtracted from the noise-removed magnitude, and the difference value (D) obtaining an angular velocity and an initial phase of the modeled expanded phase which minimizes a value obtained by summing up the sum of the angular velocity and the initial phase, thereby obtaining an expanded phase.

According to the present invention, there is an advantage that an expanded phase with respect to the folded phase of a magnetic resonance image measured with high accuracy can be obtained.

1 is a block diagram showing the structure of a magnetic resonance imaging apparatus according to an embodiment of the present invention;
2 is a graph showing a result of performing phase spread according to an embodiment of the present invention.
3 is a view showing a concept of acquiring an echo image in a magnetic resonance imaging apparatus;
4 is a flowchart showing a flow of a method of spreading a phase of a magnetic resonance image according to an embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Like reference numerals are used for like elements in describing each drawing.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram showing the structure of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

Referring to FIG. 1, a MRI apparatus according to an embodiment of the present invention includes an image acquisition unit 100, a size information extraction unit 110, a size information noise removal unit 120, and an expanded phase acquisition unit 130 ).

The image acquisition unit 100 acquires a magnetic resonance image from the subject. A magnetic resonance imaging apparatus includes a main coil set that generates a uniform and uniform magnetic field. The main coil generates a magnetic field so as to surround the tunnel-shaped inspection space. The subject is placed in the examination space of the main coil. Also, the magnetic resonance imaging apparatus includes a gradient coil and the gradient magnetic field superimposed on the uniform magnetic field exhibits a spatial gradient of the temporary gradient shape in the individual directions by the gradient coil.

The tilt coil is connected to a controllable power supply and is activated by applying a current by the power supply. The intensity, direction and duration of the gradient are controlled by control of the power supply. Further, the magnetic resonance imaging apparatus includes a transmitting coil and a receiving coil, and the transmitting coil serves as a transmitting antenna for transmitting an RF excitation pulse. The receiving coil 15 is a surface coil disposed on or near the body of the subject 30 to be examined. This surface coil 5 has high sensitivity to spatially inhomogeneous magnetic resonance signal reception. The receiving coil is connected to a demodulator 24 and the received magnetic resonance signal MS is demodulated by a demodulator 24. [ The demodulated magnetic resonance signal (DMS) is applied to the image acquisition unit 100 and the image acquisition unit 100 acquires a magnetic resonance image based on the demodulated magnetic resonance signal and the coil sensitivity profile of the reception coil. MRI images are acquired for each echo time.

3 is a diagram showing a concept of acquiring an echo image in a magnetic resonance imaging apparatus.

Referring to FIG. 3, a magnetic resonance image is acquired for each echo time, and a magnetic resonance image obtained has a complex-valued pixel value for each pixel.

The size information extracting unit 110 extracts and extracts Mangitude information from a magnetic resonance image obtained for each echo time. The image signal for each echo time of the magnetic resonance image can be divided into size information and phase information, and the size information extracting unit 110 extracts only size information from an image for each echo time. The size information extracting unit 110 extracts size information for each pixel constituting the image.

Any pixel

Figure 112015114275774-pat00019
Of the magnetic resonance image
Figure 112015114275774-pat00020
Can be expressed as the following equation.

Figure 112015114275774-pat00021

In the above equation, TE i means echo time.

Figure 112015114275774-pat00022
The
Figure 112015114275774-pat00023
≪ / RTI >< RTI ID = 0.0 >
Figure 112015114275774-pat00024
The
Figure 112015114275774-pat00025
≪ / RTI >

Each pixel value of the magnetic resonance image has a plurality of types including a real number component and an imaginary number component as shown in Equation 1. The size information extraction unit 110 extracts a pixel value of a magnetic resonance image And extracts size information from the size information.

Since the pixel value of the magnetic resonance image is a complex number, the size information extracting unit 110 can calculate the size information of each MRI image in the following manner.

Figure 112015114275774-pat00026

The size information noise removing unit 120 performs noise removal on the pixel-by-pixel size information output from the size information extracting unit 110. Since noise removal for size information in a magnetic resonance image is a well-known method, a detailed description thereof will be omitted. The size information noise removing unit 120 may perform noise removal by applying various known denoding methods.

The expanded phase acquiring unit 130 acquires the expanded phase information using the pixel-by-pixel size information output from the size information extracting unit 110 and the pixel-by-pixel size information output from the size information noise removing unit 120 .

According to a preferred embodiment of the present invention, complex signal modeling is used to obtain the expanded phase, and the expanded phase is linear with respect to echo time.

The phase measured in magnetic resonance imaging

Figure 112015114275774-pat00027
To
Figure 112015114275774-pat00028
And even if the actual phase value deviates from the above range, the expanded phase corresponding to the actual phase value can not be obtained through the measurement.

The per-pixel phase obtained through the image acquisition unit 100 may be obtained through measurement as shown in Equation 3 below,

Figure 112015114275774-pat00029
To
Figure 112015114275774-pat00030
The actual phase value can not be known through measurement.

Figure 112015114275774-pat00031

Magnetic resonance images have the characteristic that their phase decreases as the echo time increases. Because of this characteristic, the actual phase value

Figure 112015114275774-pat00032
And the measured phase value
Figure 112015114275774-pat00033
Has a relationship expressed by the following equation (4).

Figure 112015114275774-pat00034

In Equation (4) above,

Figure 112015114275774-pat00035
The
Figure 112015114275774-pat00036
this
Figure 112015114275774-pat00037
The total number of times the phase is folded.

In the present invention, an expanded phase value is calculated by using the fact that the actual phase is linear in echo time as shown in Equation (5) to obtain the expanded phase. The folded phase can not be linear with respect to the echo time, but the expanded phase can be represented linearly compared to the echo time, and the expanded phase can be expressed by the following equation (5)

Figure 112015114275774-pat00038
And initial phase
Figure 112015114275774-pat00039
. ≪ / RTI >

Figure 112015114275774-pat00040

The expanded phase obtaining unit 130 according to an embodiment of the present invention calculates the angular velocity

Figure 112015114275774-pat00041
And initial phase
Figure 112015114275774-pat00042
And obtains an expanded phase.

On the other hand, the expanded phase acquiring unit 130 acquires an expanded phase using all of the magnetic resonance images obtained for each echo time as a variable, and the obtained image per echo time is subjected to complex number modeling as shown in Equation (6) .

Figure 112015114275774-pat00043

In the above equation,

Figure 112015114275774-pat00044
And the initial phase
Figure 112015114275774-pat00045
Is usually within a certain range, and it is assumed that the angular velocity and the initial phase have a range as shown in the following Equation (7).

Figure 112015114275774-pat00046

Figure 112015114275774-pat00047

The expanded phase acquiring unit 130 acquires a magnetic resonance image for each echo time acquired through measurement

Figure 112015114275774-pat00048
An angular velocity and an initial phase for minimizing a value obtained by summing squares of values obtained by complex-number-modeling by applying an expanded phase to size information from which noises have been removed from the echo-time- Set to the expanded phase.

More specifically, the magnetic resonance image acquired at one echo time (TE 1 )

Figure 112015114275774-pat00049
The size of the image obtained in the first echo time,
Figure 112015114275774-pat00050
The phase in
Figure 112015114275774-pat00051
And the square of the subtracted value is performed.

Such an operation is performed on magnetic resonance images obtained in all echo times, and when the angular speed and the initial phase for minimizing the calculation result for each echo time are found, the value found is set to the expanded phase.

The above-described method can be expressed as the following Equation (7).

Figure 112015114275774-pat00052

Figure 112015114275774-pat00053

The angular velocity for minimizing Equation (7)

Figure 112015114275774-pat00054
Wow
Figure 112015114275774-pat00055
Is obtained for each echo time, and the expanded phase is
Figure 112015114275774-pat00056
.

According to an embodiment of the present invention, the echo time-specific angular velocity and initial phase for minimizing Equation (7) can be obtained by Least Squares Method.

According to another embodiment of the present invention, the angular velocity and the initial phase for each echo time may be determined by a dictionary method. For example, an angular velocity and an initial phase may be obtained for each echo time minimizing the above equation (7) by applying the angular velocity and initial phase set sets preliminarily stored and stored with the possible angular velocity and the initial phase set to Equation (7) .

Of course, those skilled in the art will appreciate that various methods other than those described above can be used to obtain the initial phase and angular velocity.

2 is a graph illustrating a result of phase spreading according to an embodiment of the present invention.

Referring to FIG. 2, it can be seen that the obtained original phase has a folded shape according to time, but a linearly expanded phase can be obtained according to the proposed method according to the echo time.

FIG. 4 is a flowchart illustrating a method of spreading a phase of a magnetic resonance image according to an exemplary embodiment of the present invention. Referring to FIG.

Referring to FIG. 4, a magnetic field is applied to an examinee and a response signal to the applied magnetic field signal is demodulated to obtain a magnetic resonance image (step 400). Magnetic resonance images are acquired for each echo time to acquire magnetic resonance images for a plurality of echo times.

When the magnetic resonance image is acquired, only the size information of each pixel of the magnetic resonance image is separated and extracted (step 402). Since each pixel value of the magnetic resonance image has a complex number form, the size information is extracted through Equation (2).

When size information is extracted, denoising of the size information is performed (step 404). As described above, various known grooming methods may be used.

When the degrowth is complete, the phase is modeled as a linear function of echo time to obtain an expanded phase (step 406).

As described above, since the expanded phase has a linear characteristic, in the present invention, the phase is modeled as a linear function for the echo time, and the modeled expanded phase includes the angular velocity and the initial phase as shown in Equation (5) . The modeled phase is applied to the above-mentioned goniometer size, and a subtraction operation is performed on the pixel value of the measured magnetic resonance image and the value obtained by applying the expanded phase modeled to the magnitude of the dino-pixel value, This is done for all echo images.

An expanded phase is obtained in order to minimize the sum of subtraction operations for all echo images. Eventually, the angular velocity and the initial phase are acquired for each pixel using all the echo images.

Conventionally, in order to obtain an expanded phase, a complex number division of neighboring echo images is used. However, this method has a problem that only a local change can be observed, and it is difficult to obtain an accurate expanded phase considering all echo images.

The expanded phase acquisition method of the present invention is advantageous in that it can acquire a phase that is expanded at one time in consideration of all echo images, and more accurately acquire an expanded phase.

As described above, the present invention has been described with reference to particular embodiments, such as specific elements, and limited embodiments and drawings. However, it should be understood that the present invention is not limited to the above- Various modifications and variations may be made thereto by those skilled in the art to which the present invention pertains. Accordingly, the spirit of the present invention should not be construed as being limited to the embodiments described, and all of the equivalents or equivalents of the claims, as well as the following claims, belong to the scope of the present invention .

Claims (12)

An image acquiring unit acquiring magnetic resonance images of a plurality of echo times;
A size information extracting unit for extracting size information of each pixel from the acquired magnetic resonance image;
A noise removing unit for removing noise from the extracted size information; And
And calculates a subtracted value obtained by subtracting the modeled open phase from a magnitude obtained by removing the noise from the magnetic resonance image obtained for each echo time and outputs the difference value to all echo images And obtaining an angular velocity and an initial phase of the modeled expanded phase that minimizes the summed value.
The method according to claim 1,
Wherein the modeled open phase is set according to the following equation.
Figure 112016124343086-pat00057

In the above equation,
Figure 112016124343086-pat00058
Is an angular velocity,
Figure 112016124343086-pat00059
Is an initial phase,
Figure 112016124343086-pat00060
Is the pixel value, and TE i is the echo time. The method according to claim 1,
Wherein a value obtained by applying the modeled open phase to the noise-removed size is set according to the following equation.
Figure 112016124343086-pat00061

In the above equation,
Figure 112016124343086-pat00062
Is the noise-removed size information
Figure 112016124343086-pat00063
Is the modeled open phase, and TE i is the echo time. The method according to claim 1,
Wherein the expanded phase obtaining unit obtains an expanded phase through the following equation.
Figure 112016124343086-pat00064

Figure 112016124343086-pat00065

In the above equation,
Figure 112016124343086-pat00066
Is the acquired magnetic resonance image,
Figure 112016124343086-pat00067
Is the noise-removed size information,
Figure 112016124343086-pat00068
Is an angular velocity,
Figure 112016124343086-pat00069
Is an initial phase,
Figure 112016124343086-pat00070
Denotes a pixel, and TE i denotes an echo time.
5. The method of claim 4,
Wherein the expanded phase acquiring unit acquires an angular velocity and an initial phase for each echo image by a least squares method.
5. The method of claim 4,
Wherein the expanded phase acquiring unit obtains an angular velocity and an initial phase using a dictionary in which a possible angular velocity and an initial phase set are stored in advance.
(A) obtaining magnetic resonance images for a plurality of echo times;
(B) separating and extracting size information of each pixel from the acquired magnetic resonance image;
(C) removing noise from the extracted size information; And
And calculates a subtracted value obtained by subtracting the modeled open phase from a magnitude obtained by removing the noise from the magnetic resonance image obtained for each echo time and outputs the difference value to all echo images (D) acquiring an angular velocity and an initial phase of the modeled expanded phase that minimizes a summed value to obtain an expanded phase. 8. The method of claim 7,
Wherein the modeled open phase is set according to the following equation: < EMI ID = 1.0 >
Figure 112016124343086-pat00071

In the above equation,
Figure 112016124343086-pat00072
Is an angular velocity,
Figure 112016124343086-pat00073
Is an initial phase,
Figure 112016124343086-pat00074
Is the pixel value, and TE i is the echo time. 8. The method of claim 7,
Wherein a value obtained by applying the modeled expanded phase to a size obtained by removing the noise is set according to the following equation: < EMI ID = 1.0 >
Figure 112016124343086-pat00075

In the above equation,
Figure 112016124343086-pat00076
Is the noise-removed size information
Figure 112016124343086-pat00077
Is the modeled open phase, and TE i is the echo time.
8. The method of claim 7,
Wherein the step (d) acquires an expanded phase through the following equation: < EMI ID = 17.0 >
Figure 112016124343086-pat00078

Figure 112016124343086-pat00079

In the above equation,
Figure 112016124343086-pat00080
Is the acquired magnetic resonance image,
Figure 112016124343086-pat00081
Is the noise-removed size information,
Figure 112016124343086-pat00082
Is an angular velocity,
Figure 112016124343086-pat00083
Is an initial phase,
Figure 112016124343086-pat00084
Denotes a pixel, and TE i denotes an echo time.
8. The method of claim 7,
Wherein the step (d) acquires the angular velocity and the initial phase for each echo image by the least squares method.
8. The method of claim 7,
Wherein the step (d) acquires an angular velocity and an initial phase using a dictionary in which a possible angular velocity and a set of initial phases are stored in advance.
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US20130002247A1 (en) 2011-05-02 2013-01-03 Magnetic Resonance Innovations, Inc. Catalytic multiecho phase unwrapping scheme
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EP2503350A1 (en) 2011-03-25 2012-09-26 Medizinische Universität Wien Unwrapping phase images in magnetic resonance imaging
US20130002247A1 (en) 2011-05-02 2013-01-03 Magnetic Resonance Innovations, Inc. Catalytic multiecho phase unwrapping scheme
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
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