WO2014125876A1 - 磁気共鳴イメージング装置及びその処理方法 - Google Patents
磁気共鳴イメージング装置及びその処理方法 Download PDFInfo
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/48—Other medical applications
- A61B5/4869—Determining body composition
- A61B5/4872—Body fat
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/74—Details of notification to user or communication with user or patient ; user input means
- A61B5/742—Details of notification to user or communication with user or patient ; user input means using visual displays
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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
- G01R33/4828—Resolving the MR signals of different chemical species, e.g. water-fat imaging
-
- 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
- G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
- G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/48—Other medical applications
- A61B5/4869—Determining body composition
-
- 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
Definitions
- the present invention relates to a magnetic resonance imaging apparatus, and more particularly to a tissue contrast image processing technique in the magnetic resonance imaging apparatus.
- Nuclear Magnetic Resonance Imaging Magnetic Resonance Magnetic Imaging, hereinafter referred to as MRI
- MRI Nuclear Magnetic Resonance Magnetic Imaging
- NMR nuclear magnetic resonance
- TE Echo Time
- TR repetition time
- Images with tissue contrast can be obtained.
- one method for obtaining an image in which water and fat are separated is a method called the Dixon method.
- phase unwrapping is performed in order to prevent the water and fat from being switched.
- This phase unwrapping process is a discontinuous jump that occurs because a phase that exceeds the range of - ⁇ to ⁇ (this state is said to cause the main value) is again expressed within the range of - ⁇ to ⁇ .
- the spatial phase change is made continuous and expressed by a phase value exceeding the range of - ⁇ to ⁇ .
- the signal-to-noise ratio (Signal Noise Ratio, hereinafter referred to as SNR) is large when the static magnetic field inhomogeneity is large and the phase difference between adjacent pixels is large. It was found that there are two factors when is low.
- the above unwrapping process is performed on two-dimensional or three-dimensional data, and a technique called area expansion method is used.
- the region expansion method is a method of first determining a pixel to start processing (hereinafter referred to as a start pixel) and spatially expanding the processing from the start pixel to the adjacent pixels. Pixel values that have already been processed are used.
- Patent Document 1 proposes a method of reducing the vicinity of the main value by processing in order from the pixel with the smallest phase difference among the adjacent pixels. Specifically, a phase difference gradient image is created and processed in order from the pixel with the smallest phase difference.
- Patent Document 1 proposes a method for reducing the rotation around the main value by creating a phase difference tilt image and processing sequentially from the pixel having the smallest phase difference.
- the SNR is not taken into account at all, the accuracy of the image representing the finally composed tissue is reduced as a result of the fact that the SNR is low and cannot be reduced around the main value caused by noise. There is a problem to do.
- An object of the present invention is to provide a magnetic resonance imaging apparatus capable of displaying an image representing a state of a tissue more accurately or a processing method thereof.
- a magnetic resonance imaging apparatus for solving the above-described problems includes a magnetic field generation unit that generates a static magnetic field and a gradient magnetic field on a subject, a high-frequency pulse irradiation unit that irradiates a high-frequency pulse, and the subject
- a receiving unit that receives the NMR signal from the display unit, a display unit that displays the created diagnostic image, a first image based on the NMR signal, and a phase unwrapping process for each pixel constituting the first image
- unprocessed pixels having strong signal strength are selected from among a plurality of unprocessed pixels of the phase unwrap process adjacent to the processed pixels of the phase unwrap process among the pixels of the first image.
- a signal processing unit that determines by selecting and performs phase unwrapping processing of the unprocessed pixels in accordance with the determined order to generate the second image. It is characterized by.
- a magnetic resonance imaging apparatus that can display an image representing a tissue state more accurately.
- FIG. 1 is a configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention.
- Explanatory drawing showing the gradient echo sequence of the 2-point Dixon method Configuration diagram showing the processing functions of the signal processor Processing flow diagram using the two-point Dixon method Configuration diagram showing the processing functions of the image processing unit based on the 2-point Dixon method
- processing can be performed in a relatively short time.
- Patent Document 1 since a phase difference tilt image is created and adjacent pixels are processed, the processing time becomes long.
- FIG. 1 is a schematic diagram of the overall configuration of an MRI apparatus 1 according to an embodiment of the invention.
- the MRI apparatus 1 includes a static magnetic field magnet 102 that generates a static magnetic field, a gradient magnetic field coil 103 that generates a gradient magnetic field, and an irradiation coil 104 that irradiates a subject 101 with a high-frequency magnetic field pulse (hereinafter referred to as an RF pulse) and a subject 101
- RF pulse high-frequency magnetic field pulse
- a receiving coil 105 for detecting the NMR signal is provided around the subject 101, and a bed 106 on which the subject 101 is placed is further provided.
- the static magnetic field magnet 102 is disposed in a wide space where the subject 101 is placed, and is composed of a permanent magnet, a superconducting magnet, or a normal conducting magnet, and is parallel or perpendicular to the body axis of the subject 101.
- a uniform static magnetic field is generated in any direction.
- the gradient magnetic field coil 103 applies a gradient magnetic field in the three-axis directions of the X axis, the Y axis, and the Z axis to the subject 101 based on a signal from the gradient magnetic field power source 107.
- the imaging cross section of the subject 101 is set depending on how the gradient magnetic field is applied.
- the irradiation coil 104 generates an RF pulse according to a signal from the RF transmission unit 108.
- 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, is 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, and this echo signal approaches the subject 101. It is detected by the signal detection unit 109 through the receiving coil 105 arranged in this manner. The detected echo signal is signal-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 controls the gradient magnetic field power supply 107 and the RF transmission unit 108 in order to repeatedly generate each gradient magnetic field and RF pulse of slice encoding, phase encoding, and frequency encoding in a predetermined pulse sequence, and further, a signal processing unit 110 is controlled.
- the static magnetic field generated in the space where the subject 101 is placed by the static magnetic field magnet 102 of the MRI apparatus 1 is placed in the static magnetic field space and the spatial nonuniformity of the static magnetic field itself due to the magnet structure.
- a static magnetic field spatial non-uniformity is generated due to a difference in magnetic sensitivity for each part of the subject 101 (hereinafter, these are collectively referred to as a static magnetic field non-uniformity).
- the two-point Dixon method with static magnetic field correction and the three-point Dixon method are methods in which a function for correcting the influence of static magnetic field inhomogeneity is added to the Dixon method.
- the TE is changed to perform three shootings, and in the two-point Dixon method, the TE is changed to perform two shootings.
- the three-point Dixon method takes three times by changing the TE, so it takes more time than the two-point Dixon method, but a more accurate image can be obtained.
- the case where the two-point Dixon method is used will be described.
- FIG. 2 is an example of a pulse sequence used in the two-point Dixon method used in the embodiment of the present invention.
- This pulse sequence is a gradient echo (Gradient Echo) sequence, and is a sequence for obtaining two types of image data with different TEs.
- the control unit 112 transmits the pulse sequence via the RF transmission unit 108 by performing the following control. That is, the control unit 112 controls the excitation of only the target tomographic plane by applying the slice encode gradient magnetic field 202 simultaneously with the irradiation of the RF pulse 201.
- GTE Echo Gradient Echo
- a phase encoding gradient magnetic field 203 for encoding position information is applied by the gradient coil 103, and after applying a negative frequency encoding gradient magnetic field (dephase pulse) 204, a positive frequency encoding gradient magnetic field 205 is applied.
- the first echo signal is generated after TE1 has elapsed from the RF pulse 201.
- the frequency encode gradient magnetic field (rewind pulse) 206 in the negative direction and the frequency encode gradient magnetic field 207 in the positive direction are applied again to generate a second echo signal after TE2 has elapsed from the RF pulse 201.
- TE1 is the time when the echo signals obtained from water and fat are in opposite phases
- TE2 is the phase where the echo signals obtained from water and fat are in phase with each other It's time.
- Such a sequence is repeatedly executed for the number of times of phase encoding while changing the area of the phase encoding gradient magnetic field 203, and echo signals for the number of phase encodings are received by the signal detection unit 109 via the reception coil 105 in the k space.
- Data is acquired and processed by the signal processing unit 110.
- the signal processing unit 110 performs two-dimensional Fourier transform on k-space data to obtain two types of image data having different TEs. These images are displayed on the display unit 111.
- two types of pulses are set, a TE in which water and fat are in the same phase and a TE in which water and fat are in opposite phases.
- the pulse sequence method shown in FIG. 2 is an example, and there are various other gradient echo pulse sequences used in the two-point Dixon method.
- the pulse sequence method shown in FIG. 13 is another example, and the pulse sequence method shown in FIG. 14 is still another example.
- the second echo signal is generated with a frequency encoding gradient magnetic field in the negative direction after TE2 has elapsed.
- the control unit 112 controls the excitation of only the target tomographic plane by applying the slice encode gradient magnetic field 1302 simultaneously with the irradiation of the RF pulse 1301. Apply a phase encoding gradient magnetic field 1303 to encode position information, apply a negative frequency encoding gradient magnetic field (dephase pulse) 1304, then apply a positive frequency encoding gradient magnetic field 1305, and then apply an RF pulse.
- a first echo signal is generated after TE1 elapses from 1301.
- a frequency encode gradient magnetic field 1306 in the negative direction is applied to generate a second echo signal after TE2 has elapsed from the RF pulse 1301.
- the RF pulse 1301, the slice encode gradient magnetic field 1302, the phase encode gradient magnetic field 1303, the frequency encode gradient magnetic field 1304, and the frequency encode gradient magnetic field 1305 are respectively the RF pulse 201, the slice encode gradient magnetic field 202, and the phase encode for FIG.
- the actions and effects are basically the same.
- FIG. 14 shows a method of generating the first echo after TE1 and the second echo after TE2 with separate RF pulses.
- the control unit 112 controls the excitation of only the target tomographic plane by applying the slice encode gradient magnetic field 1402 simultaneously with the irradiation of the RF pulse 1401.
- a phase encoding gradient magnetic field 1403 for encoding position information is applied, a negative frequency encode gradient magnetic field (dephase pulse) 1404 is applied, and then a positive frequency encode gradient magnetic field 1405 is applied to generate an RF pulse.
- a first echo signal is generated after elapse of TE1 from 1401.
- the slice encode gradient magnetic field 1407 is applied again simultaneously with the irradiation of the RF pulse 1406 to control only the target tomographic plane to be excited.
- a phase encoding gradient magnetic field 1408 for encoding position information is applied, a negative frequency encode gradient magnetic field (dephase pulse) 1409 is applied, and then a positive frequency encode gradient magnetic field 1410 is applied to generate an RF pulse 1406.
- a second echo signal is generated after TE2 has elapsed.
- the method shown in FIG. 2 is a method for generating an echo signal after elapse of TE1 based on the RF pulse 201 and further generating an echo signal after elapse of TE2 based on the same RF pulse 201.
- the method shown in FIG. 14 is a method in which an echo signal is generated after the lapse of TE1 based on the RF pulse 1401, and an echo signal generated after the lapse of TE2 is generated based on the RF pulse 1406.
- the RF pulse 1401 and the RF pulse 1406 have the same basic effects as the RF pulse 201 of FIG.
- the slice encode gradient magnetic field 1402 and the slice encode gradient magnetic field 1407 have the same basic effects as the slice encode gradient magnetic field 202.
- the phase encoding gradient magnetic field 1403 and the phase encoding gradient magnetic field 1408 have the same basic effects as the phase encoding gradient magnetic field 203 of FIG. Fig. 2 shows negative frequency encoding gradient magnetic field (dephase pulse) 1404 and positive frequency encoding gradient magnetic field 1405, or negative frequency encoding gradient magnetic field (dephase pulse) 1409 and positive frequency encoding gradient magnetic field 1410.
- the basic effects are the same as those of the dephase pulse 204 and the frequency encoding gradient magnetic field 205.
- FIG. 3 is a functional block diagram for explaining the processing function of the signal processing unit 110.
- the signal reception unit 301 stores the echo signal from the signal detection unit 109 in the k-space database 302 based on the arrangement information in the k-space including the slice encoding, the frequency encoding, and the phase encoding of the parameter 307.
- the image conversion unit 303 performs Fourier transform on the k-space data stored in the k-space database 302 to convert it into an image, and stores the image in the image database 304.
- the image processing unit 305 performs image processing on the image stored in the image database 304 and passes it to the image transmission unit 306.
- Image processing includes, for example, processing for creating a water image and a fat image, processing for correcting unevenness in sensitivity of the receiving coil 105, and the like.
- the image transmission unit 306 transmits the image processed image to the display unit 111.
- the parameter 307 is information on slice encoding, frequency encoding, and phase encoding of the pulse sequence required by the signal receiving unit 301, image matrix and filtering required by the image converting unit 303, the image processing unit 305, and the image transmitting unit 306. And obtained from the control unit 112.
- FIG. 4 is a processing flowchart for explaining the present embodiment.
- a program for executing this processing flow is stored in the image processing unit 305, and the image processing unit 305 executes processing of each step described with reference to FIG.
- FIG. 5 is a functional configuration diagram illustrating processing functions of the image processing unit 305 that executes the processing flow of the present embodiment.
- the phase unwrapping process by area expansion in the two-point Dixon method will be described.
- the complex data image obtained by setting TE with water and fat in opposite phases is designated as the first echo image 501, and the complex data image obtained by setting TE with water and fat in the same phase is designated as the first echo image 501.
- a 2-echo image 502 is assumed.
- a mask image creation unit 503 creates a mask image 504 indicating a region to be subjected to phase unwrapping from the first echo image 501 and the second echo image 502 stored in the image database 304.
- the mask image 504 is created in order to exclude the noise-only area where there is clearly no object from the phase unwrapping area.
- Method 1 for creating mask image As a method 1, there is a method of creating the first echo image 501 by performing binarization processing.
- S1 (x, y, z) is the first echo image 501
- M (x, y, z) is the mask image 504.
- x represents the abscissa of the image
- y represents the ordinate of the image
- z represents the slice number of the image.
- Threshold is a threshold value that divides a region with an object and a region without an object. Since the first echo image 501 is an image in which water and fat are in opposite phases, a low SNR region where water and fat cancel each other can be excluded from the phase unwrapped region. A method for obtaining the threshold will be described later.
- Method 2 for creating a mask image As a method 2, there is a method of creating the second echo image 502 by performing binarization processing.
- S2 (x, y, z) is the second echo image 502. Since the second echo image 502 is an image in which water and fat are in phase, water and fat do not cancel each other, and an area where an object is present can be accurately extracted. A method for obtaining the threshold will be described later.
- Method 3 for creating a mask image there is also a method of further identifying a region using a ratio between the first echo image 501 and the second echo image 502. Since the first echo image 501 is an image in which water and fat are in opposite phases, and the second echo image 502 is an image in which water and fat are in phase, the first echo image 501 is divided by the second echo image 502. By performing the threshold processing, it is possible to accurately remove a region where water and fat are mixed. Since an area where water and fat are mixed is likely to cause a phase error, excluding the mixed area of water and fat from the area expansion is effective as a method of reducing the phase unwrapping around the main value.
- the mask image of (Equation 3) is used in combination with the mask image created in (Equation 1) or (Equation 2), and (Equation 1) and (Equation 3), or (Equation 2) and (Equation 3).
- the threshold Threshold2 is set to an appropriate value in the range of 0 to 1, taking into account the mixing ratio of water and fat. A specific method will be described later.
- [Method 3] is used as a method for creating a mask image, but Methods 1 and 2 may be used.
- a method for obtaining the threshold will be described. For example, there are the following two methods for obtaining the threshold value.
- a method 1 there is a method of determining as 1/10 of the maximum pixel value of the first echo image 501.
- method 2 there is also a method of obtaining a noise level and determining a threshold value from the noise level (for example, three times the noise level).
- the noise level uses the fact that the ratio of the noise component is large in the high-frequency component of the image, creates a high-pass filter (High Pass Filer) on the first echo image 501 or the second echo image 502, creates an absolute value image, The average value of the absolute value image can be set as the noise level.
- High Pass Filer High Pass Filer
- the noise absolute value image can be realized by using the feature of the Rayleigh distribution, creating a histogram of the absolute value image, and then obtaining the mode value from the cumulative density distribution. Even when there is a high signal component other than noise on the absolute value image, the influence on the noise level can be reduced by creating a histogram and obtaining the mode value.
- method 2 is used as a method for obtaining the threshold value, but method 1 may be used.
- Step S402 A phase difference image creating unit 505 creates a phase difference image 506 from the first echo image 501 and the second echo image 502. First, the phase of the first echo image 501 is subtracted from the second echo image 502.
- the signal intensity of the phase difference image 506 obtained from (Equation 4) and (Equation 5) is the signal intensity of the second echo image 502 in which water and fat have the same phase.
- the signal intensity of the phase difference image is used for the phase unwrapping of the area expansion described later. Therefore, when the region is expanded, it is possible to perform phase unwrapping while avoiding the low SNR region of the image in which water and fat have the same phase.
- Method 1 is a case where the signal intensity of the phase difference image 506 is the signal intensity of the first echo image 501 in which water and fat are in opposite phases, and instead of (Equation 4), the following equation (Equation 6) is used. be able to.
- Method 1 can perform phase unwrapping avoiding a low SNR region where water and fat cancel each other.
- Method 2 is a case in which both the first echo image 501 and the second echo image 502 are taken into consideration for the signal intensity of the phase difference image 506, and (Equation 7) or (Equation 8) is used instead of (Equation 4). be able to.
- phase difference image 506 is created by using the method 2, but any of the above methods may be used.
- phase difference image 506 can reduce the influence of noise by applying a low pass filter (Low Pass Filer) or smoothing at the end, and reduce the phase unwrap around the main value.
- Low Pass Filer Low Pass Filer
- Step S403 Using the list 508, the region expansion phase unwrapping unit 507 performs phase unwrap processing on the phase difference image 506 by region expansion to create a static magnetic field inhomogeneity map U (x, y, z) 509.
- a list 508 is used to perform region expansion processing while repeating storage and extraction for each pixel, with coordinates for region expansion, phase unwrapped phases, and weights indicating the order of region expansion. Details of the phase unwrapping process 403 by area expansion will be described later.
- Step S404 The phase correction unit 510 corrects the phase of the second echo image S2 (x, y, z) 502 using the static magnetic field inhomogeneity map U (x, y, z) 509.
- the static magnetic field inhomogeneity map U (x, y, z) 509 does not contain a value for the pixel whose mask image M (x, y, z) 504 is 0, the value is obtained by extrapolation. .
- the half phase of the static magnetic field inhomogeneity map U (x, y, z) 509 is set to the second echo image S2 (x, y, z ) 502 is subtracted from the second phase to obtain a second echo image S2 ′ (x, y, z) 511 after phase correction.
- Step S405 The water / fat image separation unit 512 creates a water image by complex addition of the first echo image 501 and the second echo image 511 after phase correction, and creates a fat image by complex subtraction, respectively. Send.
- a program for executing each procedure of the processing flow is stored in the image processing unit 305, and the image processing unit 305 executes processing of each step.
- FIG. 6 shows the specific processing contents of step S403 regarding the phase unwrapping process by the region extension of the region extension phase unwrapping unit 507, and the phase unwrapping process by the region extension will be described based on the processing flow of FIG.
- Step S602 The phase is obtained using the phase difference image 506 corresponding to the coordinates of the start pixel obtained in step 601 as the target pixel. If the coordinates of the start pixel are (x 0 , y 0 , z 0 ), the phase can be obtained by the following equation.
- Step S603 The phase of the pixel of interest is set in the static magnetic field inhomogeneity map 509.
- the static magnetic field inhomogeneity map 509 is expressed by the following equation.
- Step S604 It is determined whether there is an unprocessed pixel in a pixel adjacent to the pixel of interest (denoted as an adjacent pixel).
- the adjacent pixel is a pixel listed below when the coordinates of the pixel of interest are (x 0 , y 0 , z 0 ).
- Step S605 when the adjacent pixel of the target pixel is not processed, that is, in the case of “YES” in the figure, the unwrapped phase of the adjacent pixel is obtained.
- the unwrapped phase ⁇ unwrap of the adjacent pixel is represented by the following expression, for example, where the coordinates of the adjacent pixel are (x 0 -1, y 0 , z 0 ).
- the above method is a method of obtaining the phase difference (range: ⁇ to ⁇ ) between the adjacent pixel and the target pixel, and adding the obtained phase difference to the phase of the target pixel to obtain the phase of the adjacent pixel.
- the unwrapped phase is obtained by this method, but may be obtained by another method.
- the unwrapped phase ⁇ unwrap of adjacent pixels may be obtained by the following equation.
- n in (Expression 14) uses a value in which ⁇ unwrap is in the range of U (x 0 , y 0 , z 0 ) ⁇ ⁇ .
- Step S606 the weighting of the target pixel and the unprocessed adjacent pixel is obtained.
- This weighting is an element that determines the order of region expansion processing, which is a feature of the present embodiment, and there are the following four methods.
- the first method is a method in which the inner product of the target pixel and the adjacent pixel is calculated, and weighting is performed based on the calculation result.For example, if the coordinates of the adjacent pixel are (x 0 -1, y 0 , z 0 ), The following equation can be used.
- the value that makes the inner product negative is excluded. That is, a value for which the inner product is negative is not stored in the list 508 in the next step 607.
- the square root of the inner product may be used.
- the inner product indicates a scalar of a vector of adjacent pixels, and increases as the phase difference decreases and increases as the signal strength increases. Therefore, by expanding the region in order from the pixel with the largest inner product, it is possible to expand the region while avoiding the region having a large phase difference and low SNR.
- the second method is weighting that excludes the signal intensity of the pixel of interest from the inner product.
- the third method is a method in which the difference between the target pixel and adjacent pixels is weighted.
- the range of ⁇ is set to ⁇ / 2 to ⁇ / 2, and if it falls outside the range, processing such as skipping step 607 and not storing in the list 508 is performed.
- the weighting of (Expression 20) and (Expression 21) decreases as the phase difference between the target pixel and the adjacent pixel decreases, and decreases as the difference in signal intensity decreases. Therefore, in the third method, by expanding the region in order from the pixel with the smallest weight, it is possible to expand the region while avoiding the region where the phase change is large and the signal intensity is largely changed.
- the fourth method is a method in which the product of the signal intensity of the target pixel and the adjacent pixel or the signal intensity of the adjacent pixel is weighted.
- step 607 is skipped, and the list 508
- step 607 is skipped, and the list 508
- the order of region expansion processing is determined using Method 1 in which the inner product of the pixel of interest and adjacent pixels is weighted, but other methods may be used.
- Step S607 the unwrapped phase obtained in step 605 is stored in the list 508 together with the coordinates according to the weight obtained in step 606.
- the internal structure of the list 508 is shown in FIG.
- the list 508 stores, for each pixel, the weighted and unwrapped phases and coordinates as one set, arranged in the order of weighting.
- the larger the value the more the value is stored at the exit of the list.
- the largest weighting is “weighting 701”
- the next largest weighting is “weighting 702”
- the next largest weighting is “weighting 703”
- the weighting 701 from the side closer to the exit of the list of FIG. Weight 702, and weight 703.
- the list at this time uses a first-in first-out method (First-in-First-Out, hereinafter referred to as FIFO).
- Step S608 For example, the list of FIG. 7 or the list of FIG. 8 is checked to determine whether it is empty. If there is a non-empty list, the process moves to step 609, and coordinates and phase are extracted from the list exit. If all lists are empty, the process is terminated. When all the lists are empty, the static magnetic field inhomogeneity map U (x, y, z) 509 is completed. That is, using the list of FIG. 7 and FIG. 8, it is checked in step S608 whether all the phase unwrapping processes for the pixels that require the phase unwrapping process by area expansion are completed in the pixels of the phase difference image 506, and all the processes are finished. In this case, the phase unwrapping process shown in FIG. 6 is terminated, and then step S404 in FIG. 4 is executed. If the phase unwrapping process has not been completed, the process moves to step S609, and the target pixel to be processed next is determined using the list in FIG. 7 or FIG.
- Step S609 The coordinates and phase stored in the list 508 are extracted and set as the coordinate and phase of the pixel of interest.
- the list of FIG. 7 When the list of FIG. 7 is used, coordinates and phases are extracted from the exit of the list.
- a plurality of list diagrams 8 prepared for speeding up are used, for example, in the case of an inner product, coordinates and phases are extracted in order from a list with a large number that is not empty.
- step 603. the phase unwrap process 403 is performed.
- the unwrapping phase of the adjacent pixel with respect to the target pixel is obtained in step S605 of FIG. 6, and the weighting of the adjacent pixel is calculated in step S606.
- the adjacent pixel may be a pixel that is in direct contact with the pixel of interest, or may have a little width as well as a pixel that is in direct contact.
- the order of performing the unwrap processing has been described as if, for example, it is a method of processing one pixel at a time.
- the processing order may be determined in units of two or several pixels, and may be processed in order by giving a width according to the processing order. Even if it does in this way, there can exist the effect of this invention.
- the above-described processing can be performed using a plurality of pixels as a unit for determining the order.
- the processing width may be changed based on the weighted calculation result. For example, based on the calculation result of weighting, in the part where pixels with high weighting are gathered, multiple pixels are processed together, and conversely the part with low weighting result, that is, the part with low SNR or the phase difference from the target pixel In a large portion, the number of pixels to be processed at one time may be reduced, and for example, the processing order may be determined based on weighting for each pixel.
- FIG. 9 shows the phase difference image described in step 402 of FIG.
- FIG. 9 is a photograph showing the phase difference image. Since the data of the phase image is a complex number, it is represented by an absolute value and a phase.
- FIG. 9 is a photograph showing an image showing one slice of the phase difference image 506 in the two-point Dixon method
- FIG. 901 is a photograph showing an image based on the absolute value of the signal of each pixel of the phase difference image.
- FIG. 902 is a photograph showing an image based on the phase of the signal of each pixel of the phase difference image.
- FIG. 10 is a photograph showing an image showing the result of phase unwrapping of the phase difference image of FIG. 9 by region expansion.
- Fig. 1001 shows a static magnetic field inhomogeneity map created by expanding a region in the order of weighting according to the phase difference of conventional adjacent pixels (hereinafter referred to as the conventional method), and is weighted by the inner product of adjacent pixels as an example of the present invention.
- FIG. 1002 shows a static magnetic field inhomogeneity map created by area expansion (hereinafter referred to as the method of the present invention) in this order.
- the conventional method the phase difference of conventional adjacent pixels
- FIG. 1002 shows a static magnetic field inhomogeneity map created by area expansion (hereinafter referred to as the method of the present invention) in this order.
- the background area is excluded from the area expansion, and simulation is performed with data of 11 slices, and only one slice is shown. It can be seen that there is a difference in results between point A and point B in the static magnetic field inhomogeneity map 1001 of the conventional method and the static magnetic field inhomogeneity map 1002 of the method of the present invention.
- FIG. 11 shows a conventional water image and fat image, and water subjected to the phase correction processing by the two-point Dixon method described in step S404 and the water / fat image separation processing described in step S405 as an example to which the present invention is applied. It is a photograph showing the image explaining an image and a fat image.
- water image 1101 and the fat image 1102 of the conventional method water has moved to the fat image at the point P, but the water image 1103 and the fat image 1104 to which the present invention is applied are correctly separated into water and fat at the point P. I understand that. This indicates that the static magnetic field inhomogeneity map 1002 to which the present invention is applied is more accurate.
- FIG. 12 is a diagram for explaining a profile of absolute values of a static magnetic field inhomogeneity map and a phase difference image.
- the phase from point A to point B of the conventional static magnetic field inhomogeneity map 1001 in FIG. 10 is shown in profile 1201, and the phase from point A to point B in the static magnetic field inhomogeneity map 1002 in FIG. Shown in Further, the signal intensity from the point A to the point B of the absolute value 901 (FIG. 9) of the phase difference image is shown in the profile 1203.
- the phase change between the point C and the point D is different between the profile 1201 of the conventional method and the profile 1202 to which the present invention is applied.
- the D point is continuous, and the phase increases by 2 ⁇ at the C point. Since the water in this region has moved to the fat image, the main value around the C point and the D point has occurred.
- phase D was prioritized over the point D over the point C, so the main value around the point D (actually it must be increased by 2 ⁇ , but it was accidentally made continuous) was generated. Oops. At point C, the main value was generated without phase unwrapping.
- phase unwrapping by region expansion was performed with priority given to C point over D point, so that the phase was continuous at point C, and the phase was increased by 2 ⁇ without phase unwrapping at D point.
- the price did not occur.
- This is an effect obtained by adding the signal strength to the priority of region expansion. Since the signal intensity at the point D is low in the profile 1203 of the absolute value of the phase difference image, in the application example of the present invention, the signal intensity is low, that is, the SNR is low. A non-uniform map 1002 could be obtained.
- the embodiments of the present invention have been described.
- the main value in the phase unwrap processing by area expansion can be reduced, and a more stable and accurate static magnetic field inhomogeneity map can be created.
- By creating an accurate static magnetic field inhomogeneous map it is possible to obtain a water image and a fat image with reduced replacement of water and fat.
- the present invention it is possible to create a stable and accurate static magnetic field inhomogeneous map, so that the static magnetic field inhomogeneous map is particularly effective in measuring a part where the static magnetic field inhomogeneous map tends to be inaccurate, for example, the cervical spine.
- MRI apparatus 101 subject, 102 static magnetic field magnet, 103 gradient magnetic field coil, 104 irradiation coil, 105 reception coil, 106 bed, 107 gradient magnetic field power supply, 108 RF transmission unit, 109 signal detection unit, 110 signal processing unit, 111 Display unit, 112 control unit, 201 RF pulse, 202 slice encode gradient magnetic field, 203 phase encode gradient magnetic field, 204, 206 negative frequency encode gradient magnetic field, 205, 207 positive frequency encode gradient magnetic field, 301 signal receiver , 302 k-space database, 303 image conversion unit, 304 image database, 305 image processing unit, 306 image transmission unit, 307 parameters, 501 first echo image, 502 second echo image, 503 mask image creation unit, 504 mask image, 505 phase difference image creation unit, 506 phase difference image, 507 region expansion phase unwrapping unit, 508 list, 509 static magnetic field inhomogeneity map, 510 position complementary 511, second echo image after phase correction, 512 water / fat image separation unit, 701 to
Abstract
Description
マスク画像作成部503は画像データベース304に格納されている第1エコー画像501と第2エコー画像502から位相アンラップ処理を行う領域を示すマスク画像504を作成する。マスク画像504は明らかに物体が無いノイズだけの領域を位相アンラップ処理の領域から除外するために作成する。マスク画像を作成する方法は例えば以下の3つの方法がある。
方法1として、第1エコー画像501を2値化処理することによって作成する方法がある。
方法2として、第2エコー画像502を2値化処理することによって作成する方法がある。
方法3として、第1エコー画像501と第2エコー画像502の比を用いて領域をさらに識別する方法もある。第1エコー画像501は水と脂肪が逆位相の画像であり、第2エコー画像502は水と脂肪が同位相の画像であるため、第1エコー画像501を第2エコー画像502で除算して、閾値処理をすることによって、正確に水と脂肪が混在する領域を除去することができる。水と脂肪が混在する領域は位相のエラーが発生し易いため、水と脂肪の混在領域を領域拡張から除外することは、位相アンラップの主値回りを低減する方法として有効である。
方法1として、第1エコー画像501の最大となるピクセル値の1/10として決める方法がある。方法2として、ノイズレベルを求め、ノイズレベルから閾値を決める方法(例えばノイズレベルの3倍)もある。ノイズレベルは、画像の高周波成分にはノイズ成分の割合が大きいことを利用し、第1エコー画像501あるいは第2エコー画像502にハイパスフィルタ(High Pass Filer)を施して絶対値画像を作成し、絶対値画像の平均値をノイズレベルとすることができる。より正確にノイズレベルを求めるには、ノイズの絶対値画像はレイリー分布となる特徴を利用し、絶対値画像のヒストグラムを作成した後、累積密度分布から最頻値を求めることで実現できる。前記絶対値画像上にノイズ以外の高信号の成分がある場合でも、ヒストグラムを作成し最頻値を求めることでノイズレベルに与える影響を低減できる。本発明の実施例では閾値を求める方法として方法2を用いるが、方法1を用いてもよい。
位相差画像作成部505が、前記第1エコー画像501および第2エコー画像502から位相差画像506を作成する。最初に、第2エコー画像502から第1エコー画像501の位相を減算する。
領域拡張位相アンラップ部507は、リスト508を用いて、位相差画像506を領域拡張による位相アンラップ処理し、静磁場不均一マップU(x,y,z)509を作成する。リスト508は領域拡張する座標、位相アンラップした位相、領域拡張の順番を示す重み付けをピクセル毎に格納および取り出しを繰り返しながら領域拡張処理していくために利用する。領域拡張による位相アンラップ処理403の詳細については後述する。
位相補正部510は、静磁場不均一マップU(x,y,z)509を用いて、第2エコー画像S2(x,y,z)502を位相補正する。このとき、静磁場不均一マップU(x,y,z)509にはマスク画像M(x,y,z)504が0となるピクセルには値が入っていないため、外挿によって値を求める。また、ステップ402の位相差画像作成処理で位相を2倍にしているため、静磁場不均一マップU(x,y,z)509の半分の位相を第2エコー画像S2(x,y,z)502から減算し、位相補正後の第2エコー画像S2’(x,y,z)511を求める。
水・脂肪画像分離部512は、第1エコー画像501と位相補正後の第2エコー画像511を複素加算して水画像を、複素減算して脂肪画像を、それぞれ作成し、画像送信部306に送信する。
最初に処理を開始するピクセル(上述したとおり開始ピクセルと以下記載する)の座標を求める。開始ピクセルの座標の決め方は幾つかあり、以下に列挙する。
1)マスク画像M(x,y,z)=1となる領域で、第1エコー画像501の最大信号強度のピクセルを開始ピクセルとする方法
2)マスク画像M(x,y,z)=1となる領域で、第2エコー画像502の最大信号強度のピクセルを開始ピクセルとする方法
3)マスク画像M(x,y,z)=1となる領域で、位相差画像506の最大信号強度のピクセルを開始ピクセルとする方法
上記、3つの方法の決め方により、結果に大差はないが、本実施例では1)の決め方を用いる。しかし上記2)や3)に記載の方法で開始ピクセルの座標を決めてもよい。
ステップ601で求めた開始ピクセルの座標に対応する位相差画像506を注目ピクセルとし位相を求める。開始ピクセルの座標を(x0,y0,z0)とすると、位相は以下の式で求めることができる。
注目ピクセルの位相を静磁場不均一マップ509に設定する。注目ピクセルの座標を(x0,y0,z0)とすると、静磁場不均一マップ509は下式となる。この処理により注目ピクセルに対してアンラップ処理された位相が静磁場不均一マップ509の対応ピクセルに対して設定される。
注目ピクセルに隣接するピクセル(隣接ピクセルと記す)に未処理のピクセルがあるかどうかが判断される。隣接ピクセルとは注目ピクセルの座標を(x0,y0,z0)とすると、以下に列挙したピクセルである。
1)マスク画像M(x0-1,y0,z0)=1となるピクセル
2)マスク画像M(x0+1,y0,z0)=1となるピクセル
3)マスク画像M(x0,y0-1,z0)=1となるピクセル
4)マスク画像M(x0,y0+1,z0)=1となるピクセル
5)マスク画像M(x0,y0,z0-1)=1となるピクセル
6)マスク画像M(x0,y0,z0+1)=1となるピクセル
また、未処理とは、該当の座標の静磁場不均一マップに値がまだ設定されていない、かつ、この注目ピクセルの隣接ピクセルとして、次に説明するステップ605とステップ606の処理をしていないことを示す。
ステップ604で注目ピクセルの隣接ピクセルが未処理である場合すなわち図示の「YES」の場合には、隣接ピクセルのアンラップした位相を求める。隣接ピクセルのアンラップした位相θunwrapは、例えば隣接ピクセルの座標を(x0-1,y0,z0)とすると、次の式になる。
別の方法として、隣接ピクセルのアンラップした位相θunwrapを次の式で求めてもよい。
次に注目ピクセルと未処理の隣接ピクセルとの重み付けを求める。この重み付けは本実施例の特徴である領域拡張の処理の順番を決定する要素であり、以下に示す4つの方法がある。
最初の方法は、注目ピクセルと隣接ピクセルとの内積が演算され、演算結果に基づいて重み付けを行う方法であり、たとえば隣接ピクセルの座標を(x0-1,y0,z0)とすると、次の式を用いることができる。
2つ目の方法は、内積に対して、注目ピクセルの信号強度を除外した重み付けである。
3つ目の方法は、注目ピクセルと隣接ピクセルとの差分を重み付けとする方法である。
4つ目の方法は、注目ピクセルと隣接ピクセルの信号強度の積あるいは隣接ピクセルの信号強度を重み付けとする方法である。
次に、ステップ606で求めた重み付けにしたがって、ステップ605で求めたアンラップした位相を座標と一緒にリスト508に格納する。リスト508の内部構造を図7に示す。リスト508は、ピクセル毎に、重み付けとアンラップした位相と座標を1組として、重み付けの順番で並ぶように格納する。
例えば、図7のリスト又は図8のリストを確認し、空であるかどうかを判断する。空でないリストがある場合には、ステップ609に移り、リストの出口から座標と位相を取り出す。また、全てのリストが空の場合は、処理を終了する。全てのリストが空になったとき、静磁場不均一マップU(x,y,z)509が完成する。すなわち図7や図8のリストを使用し、位相差画像506のピクセルの内、領域拡張による位相アンラップ処理が必要なピクセルに対する位相アンラップ処理が全て終了したかを、ステップS608で調べ、全て終了した場合に、図6に示す位相アンラップ処理を終了し、次に図4のステップS404を実行する。また、位相アンラップ処理が終了していない場合に、ステップS609に実行が移り、図7や図8のリストを使用して次に処理する注目ピクセルを決める。
リスト508に格納した座標と位相を取り出し、注目ピクセルの座標および位相とする。図7のリストを使用した場合には、リストの出口から座標と位相を取り出す。また、高速化用に複数用意したリスト図8を使用した場合には、例えば内積の場合には空でない番号の大きいリストから順に座標と位相を取り出す。
Claims (15)
- 被検体に対して静磁場および傾斜磁場を発生する磁場発生部と、
高周波パルスを照射する高周波パルス照射部と、
前記被検体からのNMR信号を受信するNMR信号受信部と、
作成された診断用画像を表示する表示部と、
前記NMR信号に基づいて第1画像を作成し、前記第1画像を構成する各ピクセルの位相アンラップ処理を行う順番を、前記第1画像のピクセルの内の前記位相アンラップ処理の処理済みピクセルに隣接する複数の前記位相アンラップ処理の未処理ピクセルの中から、信号強度の強い未処理ピクセルを優先して選択することにより決定し、前記決定された順番に従って前記未処理ピクセルの位相アンラップ処理を行って第2画像を作成する信号処理部と、
を備えることを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、前記信号処理部は、前記信号強度の強さに加え、前記処理済みピクセルの信号に対する位相差の小さい未処理ピクセルを優先して選択することにより、前記位相アンラップ処理を行う前記順番を決定する、ことを特徴とする磁気共鳴イメージング装置。
- 請求項1に記載の磁気共鳴イメージング装置において、
前記第1画像は位相差画像であり、前記第2画像は補正用マップであって、
前記信号処理部は、前記位相差画像の信号をピクセル毎に前記順番に従って位相アンラップ処理を行って前記補正用マップを作成し、
前記信号処理部は、前記位相差画像の前記処理済みピクセルに対して隣接する複数の前記未処理ピクセルの中から、信号強度が強くさらに前記処理済みピクセルの信号に対する位相差が小さい前記未処理ピクセルを優先して選択することにより、前記位相差画像の前記未処理ピクセルの前記位相アンラップ処理を行う前記順番を決定し、さらに前記補正用マップを基に水画像あるいは脂肪画像を作成し、
前記表示部が前記水画像と前記脂肪画像の少なくとも一方を表示する、ことを特徴とする磁気共鳴イメージング装置。 - 請求項3に記載の磁気共鳴イメージング装置において、
前記信号処理部は、前記受信部からの前記NMR信号に基づいて少なくとも第1と第2エコー画像を作成し、さらに前記第1と第2エコー画像を基に各ピクセル毎に複素数で表された複素信号を有する前記位相差画像を作成し、前記位相差画像の内の前記未処理ピクセルの前記複素信号を演算して重み付けを行い、前記未処理ピクセルの位相アンラップ処理を行う前記順番を前記重み付けに基づいて決定する、ことを特徴とする磁気共鳴イメージング装置。 - 請求項4に記載の磁気共鳴イメージング装置において、前記信号処理部は、前記処理済みピクセルの複素信号と前記未処理ピクセルの複素信号との積の演算に基づいて前記重み付けを演算する、ことを特徴とする磁気共鳴イメージング装置。
- 請求項4に記載の磁気共鳴イメージング装置において、
前記信号処理部は、前記処理済みピクセルの複素信号と前記未処理ピクセルの複素信号との差分を演算することにより前記重み付けを演算する、ことを特徴とする磁気共鳴イメージング装置。 - 請求項3に記載の磁気共鳴イメージング装置において、
前記信号処理部は前記受信部からの前記NMR信号に基づいて少なくとも第1エコー画像と第2エコー画像を作成し、
前記信号処理部は、前記第1エコー画像あるいは第2エコー画像に基づいてマスク画像を作成し、
前記信号処理部は、前記第1エコー画像あるいは第2エコー画像に基づいて前記位相差画像を作成し、
前記信号処理部は、前記位相差画像の前記ピクセルの内、前記マスク画像でマスクされていないピクセルに対して、前記ピクセル毎に前記順番に従って位相アンラップ処理を行い、静磁場不均一マップを作成する、ことを特徴とする磁気共鳴イメージング装置。 - 請求項3に記載の磁気共鳴イメージング装置において、
前記信号処理部は、前記受信部からの前記NMR信号に基づいて少なくとも第1と第2エコー画像を作成し、さらに前記第1と第2エコー画像を基に前記位相差画像を作成し、前記位相差画像の内の前記未処理ピクセルに対して前記順番に従って位相アンラップ処理を行って静磁場不均一マップを作成し、前記静磁場不均一マップを基に少なくとも水画像と脂肪画像の少なくとも一方を作成し、
前記表示部が前記水画像と前記脂肪画像の少なくとも一方を前記診断用画像として表示する、
ことを特徴とする磁気共鳴イメージング装置。 - 請求項8に記載の磁気共鳴イメージング装置において、
前記高周波パルス照射部から前記高周波パルスが照射された後、水と脂肪からの前記NMR信号が互いに逆相となる第1時間経過後に第1NMR信号を受信し、また前記高周波パルスが照射された後、水と脂肪からの前記NMR信号が互いに同相となる第2時間経過後に第2NMR信号を受信し、前記第1NMR信号により前記第1エコー画像を作成し、前記第2NMR信号により前記第2エコー画像を作成し、
前記位相差画像を位相アンラップ処理して静磁場不均一マップを作成し、前記静磁場不均一マップと前記第1エコー画像と前記第2エコー画像を基に、少なくとも前記水画像あるいは前記脂肪画像を作成する、ことを特徴とする磁気共鳴イメージング装置。 - 請求項9に記載の磁気共鳴イメージング装置において、
前記磁場発生部による傾斜磁場の発生状態で、前記高周波パルス照射部から高周波パルスを照射し、その後さらに前記磁場発生部が傾斜磁場を発生すると共に負方向のエンコード傾斜磁場を発生し、
前記高周波パルスの照射から前記第1時間が経過した時点で第1極性のエンコード傾斜磁場を発生して前記第1NMR信号を受信し、
又は/且つ、
前記高周波パルスの照射から前記第2時間が経過した時点で第2極性のエンコード傾斜磁場を発生して前記第2NMR信号を受信し、
前記エンコード傾斜磁場を変更して前記第1NMR信号と前記第2NMR信号の受信を繰り返し行い、
前記信号処理部は、受信した前記第1NMR信号と前記第2NMR信号とをそれぞれ画像変換して、前記第1エコー画像と前記第2エコー画像を作成する、ことを特徴とする磁気共鳴イメージング装置。 - 請求項3に記載の磁気共鳴イメージング装置において、前記信号処理部は、前記処理済みピクセルの内の最後に位相アンラップ処理を行った前記処理済みピクセルを注目ピクセルとし、前記注目ピクセルに隣接する前記未処理ピクセルに対して、前記未処理ピクセルの信号強度および前記注目ピクセルの信号に対する前記未処理ピクセルの信号の位相差に基づいて、重み付けの演算を行い、前記各未処理ピクセルの前記重み付けの演算結果に従って前記各未処理ピクセルに関するデータを各未処理ピクセルで順に配置したリストを作成し、前記リストに従って新たな注目ピクセルを決定すると共にそのアンラップ位相を静磁場不均一マップの対応するピクセルの位相として設定し、前記新たな注目ピクセルに対して、再び上記処理を繰り返す、ことを特徴とする磁気共鳴イメージング装置。
- 請求項2に記載の磁気共鳴イメージング装置において、前記信号処理部は、前記処理済みピクセルの信号と前記処理済みピクセルに隣接する複数の前記未処理ピクセルのそれぞれの信号との内積を演算し、前記内積の演算結果に従って、前記未処理ピクセルの内から位相アンラップ処理を行うべき未処理ピクセルを決定する、ことを特徴とする磁気共鳴イメージング装置。
- 請求項2に記載の磁気共鳴イメージング装置において、
前記信号処理部が行う前記位相アンラップ処理は、前記第1画像である位相差画像を構成する各ピクセルに対してそのアンラップ処理した位相を前記第2画像の対応するピクセルの位相として設定することであり、
前記高周波パルス照射部からの前記高周波パルスの照射後、水と脂肪からの前記NMR信号が互いに逆相となる第1時間経過時点で第1NMR信号を受信し、また前記高周波パルスの照射後、水と脂肪からの前記NMR信号が互いに同相となる第2時間経過時点で第2NMR信号を受信し、
前記信号処理部は、前記第1NMR信号を基に画像変換して前記第1エコー画像を作成し、また前記第2NMR信号を基に画像変換して前記第2エコー画像を作成し、
前記信号処理部は、前記第1エコー画像と前記第2エコー画像に基づき前記第1画像である前記位相差画像を作成し、さらに前記位相差画像を構成するピクセルの内前記未処理ピクセルの内から前記処理を開始する注目ピクセルを決定し、前記注目ピクセルに対してアンラップ処理した位相を前記第2画像の対応するピクセルの位相として設定し、
前記信号処理部は、さらに前記注目ピクセルに隣接する未処理ピクセルの有無を判断し、未処理ピクセルが存在する場合には前記未処理ピクセルに対する優先度やアンラップ処理した位相を前記各未処理ピクセルを単位とするリストとして記憶し、
前記信号処理部は、記憶した前記リストに基づき前記優先度に従って処理すべき未処理ピクセルを選択し、選択した未処理ピクセルの前記アンラップ処理した位相を前記第2画像の対応するピクセルの位相として設定し、さらに前記選択した未処理ピクセルを新たな注目ピクセルとして前記動作を繰り返し、
前記信号処理部が、前記優先度に従って、前記位相差画像を構成する各ピクセルのアンラップ処理した位相を対応する前記第2画像のピクセルにそれぞれ設定して、前記第2画像を作成する、ことを特徴とする磁気共鳴イメージング装置。 - 被検体に対して静磁場および傾斜磁場を発生する磁場発生部と、高周波パルスを照射する高周波パルス照射部と、前記被検体からのNMR信号を受信する受信部と、信号処理を行う信号処理部と、診断用画像を表示する表示部と、を備える磁気共鳴イメージング装置の処理方法であって、
前記信号処理部は、前記NMR信号に基づいて第1画像を作成し、
前記信号処理部は、前記第1画像の既に位相アンラップ処理を行った処理済みピクセルに対して隣接する複数の前記位相アンラップ処理が行われていない未処理ピクセルの中から、信号強度の大きさおよび前記処理済みピクセルの信号に対する位相差の小ささに基づいて重み付けを行い、
前記信号処理部は、前記重み付けに基づいて、前記未処理ピクセルの内から次に位相アンラップ処理を行うピクセルを選択して、前記未処理ピクセルを順番に処理して第2画像を作成し、
前記信号処理部はさらに、前記第2画像を使用して前記診断用画像を構成する、ことを特徴とする磁気共鳴イメージング装置の処理方法。 - 請求項14に記載の磁気共鳴イメージング装置の処理方法において、前記信号処理部は、前記第1画像の前記処理済みピクセルの信号と前記処理済みピクセルに隣接する複数の前記未処理ピクセルのそれぞれの信号との内積を演算し、演算した前記内積の結果に従って、前記重み付けを行い、前記重み付けに従って前記未処理ピクセルの内から次に位相アンラップ処理を行うべき未処理ピクセルを選択する、ことを特徴とする磁気共鳴イメージング装置の処理方法。
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