WO2015115187A1 - 磁気共鳴イメージング装置及び脂肪抑制水画像算出方法 - Google Patents
磁気共鳴イメージング装置及び脂肪抑制水画像算出方法 Download PDFInfo
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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
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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
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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
- A61B5/4875—Hydration status, fluid retention of the body
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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
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/5607—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reducing the NMR signal of a particular spin species, e.g. of a chemical species for fat suppression, or of a moving spin species for black-blood imaging
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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
- 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
- G01R33/56545—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by finite or discrete sampling, e.g. Gibbs ringing, truncation artefacts, phase aliasing artefacts
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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
- G01R33/546—Interface between the MR system and the user, e.g. for controlling the operation of the MR system or for the design of pulse sequences
Definitions
- 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.
- MRI nuclear magnetic resonance imaging
- NMR nuclear magnetic resonance
- the present invention relates to a technique for acquiring a water image in which fat signals are suppressed at a desired ratio.
- 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.
- 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.
- an image in which a signal from a fat tissue (a fat signal) is suppressed may be required.
- a method for obtaining an image with suppressed fat signals multiple images with different TEs are acquired and reconstructed from images (water images) reconstructed from water signals (water signals) and fat signals.
- a method of separating the image (fat image) A typical method is called Dixon method.
- an image obtained in the opposite phase and an image obtained in the same phase are added to obtain a water image.
- In is an image obtained in the same phase
- Out is an image obtained in the opposite phase
- W is an image in which each pixel is constituted by a water signal (water image)
- F is constituted by a fat signal in each pixel. Represents an image (fat image).
- the fat signal After separating the water signal (water image) and the fat signal (fat signal) according to Equation (1), the fat signal can be left in the water image by adding the fat signal to the water signal at a desired ratio.
- the calculation time is extended. Further, since the number of images to be created increases, the amount of memory used by the computing unit increases.
- Equation (2) a negative fat signal remains in the water image obtained by adding the same phase and the opposite phase.
- Equation (2) assumes that the influence of T2 and T2 * attenuation of the water signal is the same as that of the fat signal.
- the present invention has been made in view of the above circumstances, and does not calculate a separated image obtained by separating a water signal and a fat signal, and the fat signal remains at a desired ratio without losing contrast by a simple method.
- An object is to provide a technique for obtaining a water image with high accuracy.
- the present invention obtains a water image in which fat signals are suppressed at a desired ratio by weighted addition of a plurality of images obtained by reconstructing echo signals acquired at a plurality of different echo times.
- the plurality of different echo times are set such that the phase difference between the water signal and the fat signal included in each image is different in at least two images.
- the weighting coefficient used for the weighted addition is determined so as to cancel the fat signal intensity difference due to the echo time difference and suppress the fat signal at a desired ratio in the water image.
- a water image in which a fat signal remains at a desired ratio is obtained with high accuracy by a simple method without calculating a separated image obtained by separating a water signal and a fat signal and without losing contrast. be able to.
- FIG. 1 is an overall configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention.
- Explanatory diagram for explaining the gradient echo (GE) sequence of the two-point Dixon method The block diagram of the signal processing part of embodiment of this invention (a) and (b) are explanatory diagrams for explaining the fat suppression coefficient input area of the embodiment of the present invention, and (c) is an explanation for explaining the fat ratio table of the embodiment of the present invention.
- GE gradient echo
- FIG. 1 is a functional block diagram of the MRI apparatus 100.
- the MRI apparatus 100 of this embodiment includes a static magnetic field generating magnet 102, a gradient magnetic field coil 103, an irradiation coil 104, a reception coil 105, a bed 106 on which the subject 101 lies, a gradient magnetic field power source 107, and an RF transmission Unit 108, signal detection unit 109, signal processing unit 110, display unit 111, control unit 112, and input unit 113.
- the static magnetic field generating magnet 102 generates a uniform static magnetic field around the subject 101 in a direction parallel or perpendicular to the body axis.
- the static magnetic field generating magnet 102 is composed of any one of a permanent magnet, a superconducting magnet, and a normal conducting magnet disposed in a space having a predetermined spread around the subject 101.
- 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.
- the irradiation coil 104 generates a high frequency magnetic field pulse (RF pulse) in response to a signal from the RF transmission unit 108.
- This RF pulse excites the atomic nuclei constituting the living tissue in the imaging cross section to induce an NMR phenomenon.
- An echo signal which is an NMR signal generated by the NMR phenomenon, is detected by the signal detection unit 109 through the reception coil 105 disposed close to the subject 101, and is signal-processed by the signal processing unit 110 and converted into an image.
- the display unit 111 displays the image converted by the signal processing unit 110. Further, an input interface screen of the input unit 113 is displayed as necessary.
- the input unit 113 accepts input of parameters from the operator.
- the input parameters are the repetition time (TR) and echo time (TE) necessary for imaging.
- TR repetition time
- TE echo time
- the input parameters are sent to the display unit 111 and displayed. Similarly, these parameters are sent to the control unit 112.
- the control unit 112 controls the gradient magnetic field power source 107, the RF transmission unit 108, and the signal processing unit 110 according to the parameters received from the input unit 113. Each of these units repeatedly generates slice encoding, phase encoding, and frequency encoding gradient magnetic fields and RF pulses according to a predetermined pulse sequence in accordance with control.
- the control unit 112 and the signal processing unit 110 include a CPU, a memory, and a storage device. These functions are realized by the CPU loading a program stored in the storage device into the memory and executing it.
- the water signal and the fat signal are TE (TE1; first echo time) having opposite phases and TE (TE2; second echo time) having opposite phases. Then, an echo signal is acquired for each. Then, an image is reconstructed from each echo signal to obtain an image.
- TE1 first echo time
- TE2 second echo time
- an image is reconstructed from each echo signal to obtain an image.
- an image reconstructed from the echo signal acquired at TE1 is referred to as an anti-phase image
- an image reconstructed from the echo signal acquired at TE2 is referred to as an in-phase image.
- the water image which suppressed the fat signal by the desired ratio is obtained, without isolate
- the pulse sequence and the processing of the signal processing unit for realizing this will be described.
- This pulse sequence 200 is a gradient echo (GE) sequence method sequence, and obtains two types of images having different TEs.
- images are obtained with TE1 and TE2.
- ⁇ Slice encode gradient magnetic field 202 is applied simultaneously with irradiation of RF pulse 201. Thereby, only the target tomographic plane is excited. Then, a phase encode gradient magnetic field 203 for encoding position information is applied, and a negative frequency encode gradient magnetic field (pre-pulse) 204 is simultaneously applied, and then a positive frequency encode gradient magnetic field 205 is applied and an RF pulse is applied. After elapse of TE1 from 201, the first echo signal 211 is generated.
- a frequency encode gradient magnetic field (rewind pulse) 206 is applied to generate the second echo signal 212 after TE2 has elapsed from the RF pulse.
- This sequence is repeatedly executed as many times as the number of phase encodings while changing the area of the gradient magnetic field 203 for phase encoding, and echo signals for the number of phase encodings are acquired and filled in the k space.
- the above-described pulse sequence is realized by the control unit 112 controlling the operations of the gradient magnetic field power source 107, the RF transmission unit 108, and the signal detection unit 109.
- the signal processing unit 110 performs two-dimensional Fourier transform on k-space data to obtain two types of images having different TEs. That is, the first image (anti-phase image) is obtained from the k-space data filled with the first echo signal, and the second image (in-phase image) is obtained from the k-space data filled with the second echo signal. Image). Then, a desired image is obtained from these two images.
- FIG. 3 is a functional block diagram of the signal processing unit 110.
- the signal processing unit 110 of the present embodiment includes a signal receiving unit 301, a k-space database 302, an image converting unit 303, an image database 304, an image processing unit 305, and an image transmitting unit 306. And a parameter holding unit 307.
- the signal reception unit 301 stores the echo signal detected by the signal detection unit 109 in the k space database 302 based on the arrangement information in the k space held in the parameter holding unit 307.
- the arrangement in the k space is specified by slice encoding, frequency encoding, and phase encoding. In this embodiment, a different k space is prepared for each TE, and an echo signal is stored in each k space.
- the image conversion unit 303 performs Fourier transform on the data for each k-space stored in the k-space database 302, reconstructs the images, and stores them in the image database 304. In this embodiment, an antiphase image and an inphase image are obtained.
- the image processing unit 305 performs image processing on each image stored in the image database 304 and passes it to the image transmission unit 306.
- the image processing includes, for example, processing for correcting unevenness in sensitivity of the receiving coil 105.
- a fat-suppressed image generation process is also performed in which a water image in which fat signals are suppressed at a desired ratio is calculated by weighted addition of the opposite-phase image and the same-phase image. Details of the fat suppression image generation processing will be described later.
- the image transmission unit 306 transmits the image processed image to the display unit 111.
- the parameter holding unit 307 is information on arrangement in the k space required by the signal receiving unit 301, that is, information on slice encoding, frequency encoding, and phase encoding of a pulse sequence, an image converting unit 303, an image processing unit 305, an image transmission
- the unit 306 holds parameters such as an image matrix and filtering, and control information. These parameters are acquired from the control unit 112.
- the parameter holding unit 307 further specifies an attenuation correction coefficient for correcting a difference in fat signal intensity due to T2 and T2 * attenuation, and a ratio for suppressing the fat signal, which are used during the fat suppression image generation processing. Parameter (fat suppression coefficient) to be retained.
- the attenuation correction coefficient and the fat suppression coefficient will be described later.
- the image processing unit 305 in the fat suppression image generation process, the image processing unit 305 generates a water image in which a fat signal is suppressed at a desired ratio without separating the reverse phase image and the same phase image into a water image and a fat image. obtain. At this time, the difference in fat signal intensity due to T2 and T2 * attenuation between signals due to the time difference of TE is also corrected.
- the attenuation correction coefficient multiplied to correct the difference in signal strength of fat, or the inverse of the attenuation correction coefficient, the attenuation coefficient of the signal intensity indicating the difference in signal intensity, and the fat signal is determined. That is, the difference between the T2 and T2 * attenuation of fat due to the difference in acquisition timing is corrected, and the anti-phase image and the same image are controlled so that the fat signal is suppressed according to the specified fat suppression ratio.
- a weighting factor for multiplying each of the phase images is determined.
- the fat suppression coefficient that is, the rate of suppressing the fat signal be (1 ⁇ ) (0 ⁇ ⁇ ⁇ 1).
- ⁇ indicates the fat remaining ratio. That is, a fat signal is left in the water image by a ratio of ⁇ .
- an attenuation correction coefficient for correcting the influence of T2 and T2 * attenuation of the fat signal depending on the acquisition timing is ⁇ F.
- weighting factors A and B applied to the anti-phase image Out and the in-phase image In are calculated as the following equation (3).
- W supF A ⁇ Out + B ⁇ In (3)
- W supF is a water image in which fat signals are suppressed at a desired rate.
- Equation (4) ⁇ W is the ratio of the signal attenuation of the water signal due to the effects of antiphase T2 and T2 * attenuation on the same phase
- ⁇ F is the fat due to the effects of antiphase T2 and T2 * attenuation on the same phase. It is the rate of signal attenuation of the signal.
- ⁇ W and ⁇ F are referred to as attenuation coefficients.
- the influence of the phase due to the static magnetic field inhomogeneity has been removed by correction.
- the weighting factor is not limited to A and B in the above formula (3).
- the following formulas (5) and (6) may be used, where one of the coefficients of the opposite phase image and the same phase image is 1.
- the weighting factor is determined so that the ratio of the signal intensity of the same phase / the signal intensity of the opposite phase increases as the ratio of leaving the fat signal in the water image increases.
- the fat suppression coefficient (1 ⁇ ) is input from the user via the input unit 113 and held in the parameter holding unit 307.
- the control unit 112 of the present embodiment includes an interface that accepts designation of the fat suppression coefficient from the user.
- the interface for example, the fat suppression coefficient input area 810 is displayed on the display unit 11, and the user inputs to the area via the input unit 113.
- the fat suppression coefficient can be input by inputting a fat suppression ratio in percentage via the fat suppression coefficient input area 810 displayed on the display unit 111. Also good. Further, as shown in FIG. 4B, a configuration may be adopted in which selection is made from preset values displayed in the fat suppression coefficient input area 810.
- the image processing unit 305 performs control so that only 5% of fat remains.
- items such as “strong”, “medium”, and “weak” are prepared as the degree of fat suppression, and can be selected from these.
- Each item is associated with the value of the suppression coefficient and held. For example, 100% is associated with “strong”, 90% is associated with “medium”, and 80% is associated with “weak”. That is, when the user selects “strong”, the image processing unit 305 performs control so that only 0% fat remains. When “medium” is selected, the image processing unit 305 performs control so that only 10% fat remains. Select to control so that only 20% of fat remains.
- what the user designates may not be the ratio of fat suppression but, conversely, the ratio at which fat is desired to remain. In this case, if you want to leave only 5% fat, enter 5%. In response to this, the image processing unit 305 performs control so that only 5% of fat remains.
- the ratio of leaving fat has an appropriate ratio depending on the imaging region and the imaging type. Therefore, the optimal ratio is previously stored in the parameter holding unit 307 as a fat ratio table according to the imaging region and imaging type, and is automatically adopted according to the selection of the imaging region and imaging type. May be.
- control unit 112 includes a fat ratio table as a database that holds the ratio of suppressing the fat signal in association with at least one of the imaging region and the imaging type, and the image processing unit 305 includes the imaging set by the user.
- the said ratio is acquired from a database according to a site
- the control unit 112 information parameters regarding the imaging region and the imaging type are transmitted to the control unit 112, and the fat ratio is calculated based on the relationship between the imaging region and the fat ratio of the image type stored in advance in the control unit 112.
- the parameter ⁇ is selected, transmitted to the signal processing unit 110, and used by the signal processing unit 110.
- FIG. 4 (c) is an example of a fat ratio table 800 that holds an appropriate fat suppression coefficient for each imaging region 801 and each imaging type 802.
- the image processing unit 305 calculates a fat suppression coefficient (1- ⁇ ) from the fat remaining ratio ⁇ and uses it to calculate a weighting coefficient.
- the fat residual ratio ⁇ is set to a low 5% for both the T1-weighted image and the T2-weighted image.
- the T1-weighted image and proton density image have a relatively large water signal, so the fat residual ratio ⁇ is set to a low 5%, but the T2-weighted image has a small water signal. Therefore, the fat remaining ratio ⁇ is set to 20% so that the tissue and the like can be easily grasped.
- the fat residual ratio ⁇ is 0% and is not left at all.
- the T1-weighted image of the knee and the proton density image have a relatively large water signal, but it is necessary to leave a bone signal, so the fat residual ratio ⁇ is 10%, and the T2-weighted image has a small water signal and a bone signal. Therefore, it is appropriate that the fat residual ratio ⁇ is 20%.
- the suppression coefficient input via the input unit 113 is sent to the signal processing unit 110 via the control unit 112, and is used when the signal processing unit 110 creates a water image in which fat signals are suppressed at a desired ratio. It is done.
- the fat ratio table 800 may be stored in the signal processing unit 110 in advance.
- the attenuation correction coefficient ⁇ F is a coefficient applied to either the in-phase image or the anti-phase image in order to correct the influence of T2 and T2 * attenuation. It determines so that the fat signal of an antiphase does not remain in the water image finally obtained. In the present embodiment, the signal strength of fat in the opposite phase is determined to be equal to the signal strength of fat in the same phase.
- the attenuation correction coefficient is set to 0.9.
- This attenuation correction coefficient ⁇ F is determined by the type of sequence and the TE of the same phase and the opposite phase. Therefore, the attenuation correction coefficient ⁇ F is determined in advance in various sequences based on the actual measurement values obtained by changing the TE, and is stored in the parameter storage unit 307 as a correction coefficient database or the like in association with the sequence type, TE, for example. Keep it. When the influence of T2 and T2 * attenuation is small or when high accuracy is not required, ⁇ F may be set to 1 and ignored.
- FIG. 5 is a processing flow of fat suppression image generation processing of the present embodiment by the image processing unit 305. As described above, this processing is stored as a program in the storage device, and the image processing unit 305 executes processing of each step.
- Step S1101 An image reconstructed from the echo signal acquired at the first echo time TE1 (antiphase image) and an image reconstructed from the echo signal acquired at the second echo time TE2 (in-phase image)
- Step S1102 Using the attenuation correction coefficient ⁇ F and the fat signal suppression ratio (1 ⁇ ), weight coefficients to be applied to the anti-phase image and the in-phase image are calculated.
- ⁇ F the attenuation correction coefficient
- ⁇ F the fat signal suppression ratio
- weight coefficients to be applied to the anti-phase image and the in-phase image are calculated.
- one of A and B in the above equation (3), A 1 in the above equation (5), and B 1 in the above equation (6) is calculated. These coefficients are calculated only once for imaging to obtain one image.
- Step S1103 The pixel values of the in-phase image and the anti-phase image are corrected using the weighting coefficient calculated in Step S1102. This calculation is performed for the number of image pixels and the number of slices taken.
- Step S1104 The corrected in-phase image and anti-phase image are added to obtain a water image in which the fat signal is suppressed at a desired ratio (1- ⁇ ).
- FIGS. 6 (a) to 6 (c) are water images (T2-weighted images) obtained using the anti-phase image and the in-phase image without performing signal intensity correction due to the difference in TE.
- the image 601 in FIG. 6 (a) is an image in which the fat signal ratio ⁇ to be left in the water image is 0, and the image 602 in FIG. 6 (b) is 10% in the fat signal ratio ⁇ to be left in the water image.
- An image 603 in FIG. 6C is an image in which the fat signal ratio ⁇ to be left in the water image is 20%.
- the fat signal increases moderately from the back of the head to the back by increasing the fat signal ratio ⁇ .
- FIGS. 7 (a) and 7 (b) are diagrams for explaining the effect of the method of the present embodiment, in which signal intensity correction is performed based on a difference in TE using an anti-phase image and an in-phase image.
- An image 701 in FIG. 7 (a) is an image in which the attenuation correction coefficient ⁇ is set so that the signal intensity of the in-phase image is larger than the signal intensity of the anti-phase image, and 20% of fat is left
- FIG. 702) is an image in which the attenuation correction coefficient ⁇ is set so that the signal strength of the opposite phase is larger than the signal strength of the same phase, and 20% of fat is left.
- the fat signal remaining in the water image can be aligned with the water phase by setting the fat signal in the opposite phase to be equal to or less than the same phase signal. .
- a water image in which a fat signal remains can be created at high speed without losing contrast.
- TE1 in which the fat signal and the water signal are in opposite phases and TE2 in the same phase
- TE2 in the same phase
- the phase difference between the water signal and the fat signal of the first image reconstructed from the echo signal acquired at the first echo time is reconstructed from the echo signal acquired at the second echo time. It is only necessary that the phase difference between the water signal and the fat signal in the image is different.
- the fat signal of the first image reconstructed from the echo signal acquired at the first echo time and the second echo time acquired may be different, and the phase of the water signal and the fat signal may be different in at least one of the first image and the second image.
- first echo time TE1 and a second echo time TE2 (TE1 ⁇ TE2)
- an image reconstructed from the echo signals acquired at the first echo time An image reconstructed from the first image and the echo signal acquired at the second echo time is referred to as a second image.
- the signal S1 of the first image and the signal S2 of the second image are each expressed by the following equation (7).
- equation (7) it is assumed that the influence of the phase due to the static magnetic field inhomogeneity has been removed by correction.
- ⁇ 1 is the phase due to the chemical shift of fat during TE1
- ⁇ 2 is the phase due to the chemical shift of fat during TE2
- ⁇ W is the first image relative to the first image due to T2 and T2 * attenuation of the water signal.
- the attenuation coefficient ⁇ F of the signal intensity of the second image is the same attenuation coefficient of the fat signal.
- a water image and a fat image can be obtained by solving the simultaneous equations of Equation (7).
- Expression (7) is represented by a matrix
- Expression (8) is obtained.
- Expression (10) is obtained by adding positive and negative ⁇ F terms on the right side to S 1 of Expression (7) and adding ⁇ W ⁇ F terms to S 2 . Since the positive and negative ⁇ F terms and ⁇ W ⁇ F terms cancel each other, they are equivalent to Equation (7).
- W + ⁇ is obtained as shown in the following equation (12). Note that a fat image F is also obtained at the same time in the calculation.
- the coefficients related to S1 and S2 at this time are the weighting coefficients A and B.
- a water image in which fat signals are suppressed at a desired ratio may be obtained by weighted addition of images acquired at three or more different echo times.
- each echo time may be set so that the phase difference between the water signal and the fat signal included in each image is different in at least two images.
- W + ⁇ F is obtained by solving the simultaneous equations with respect to W + ⁇ F and F as described above.
- n an image reconstructed from the nth echo signal.
- ⁇ nW is the attenuation coefficient due to the effects of T2 and T2 * attenuation of the water signal in the nth image relative to the water signal in the first image
- ⁇ nF is the nth image for the fat signal in the first image Is the attenuation coefficient due to the influence of T2 and T2 * attenuation of fat signal in.
- Equation (17) is solved for the vector P as shown in the following equation (18) to obtain a water image in which each element of P, that is, the fat signal is left by ⁇ .
- H represents an adjoint matrix
- the coefficient according to each S n becomes the above weighting factor.
- nW and ⁇ nF may be set to 1.
- Step S2101 The attenuation coefficient of T2 and T2 * attenuation between the echo signals is acquired from the parameter holding unit 307. Acquired, the attenuation coefficient for the n-th aqueous signals gamma nW, the attenuation coefficient for the n-th fat signal to gamma nF. Also in this embodiment, the fat signal suppression ratio (1- ⁇ ) designated by the user is acquired.
- Step S2102 Using the attenuation coefficients ⁇ nW and ⁇ nF and the fat signal suppression ratio (1 ⁇ ), a matrix shown in the following equation (19) is created. This matrix is created only once for imaging to obtain one image.
- Step S2103 The inverse matrix C ′ of the matrix C created in step S2102 is calculated.
- the matrix formula is expressed by the following formula (20).
- the matrix C is a square matrix. Therefore, in this case, the inverse matrix C ′ may be obtained by the following equation (21). This inverse matrix calculation is performed only once for one image pickup.
- Step S2104 According to the following equation (22), a water image in which only a fat signal is left is obtained.
- the element W + ⁇ F of the calculated vector P is a water image in which a fat signal is left by ⁇ .
- the calculation in this step is repeated for the number of image pixels and the number of slices taken.
- the magnetic resonance imaging apparatus 100 weights and adds a plurality of images obtained by reconstructing echo signals acquired with different lengths of echo times at a desired ratio.
- the different echo times are two echo times, a first echo time and a second echo time, the fat signal of the first image reconstructed from the echo signals acquired at the first echo time,
- the phase of the fat signal of the second image reconstructed from the echo signal acquired at the second echo time is different, and in at least one of the first image and the second image, a water signal
- the phase of the fat signal may be different.
- the water signal and the phase signal of the first image may be in opposite phases, and the water signal and the fat signal of the second image may be in phase.
- the weighting coefficient used for the weighted addition is determined so as to correct a difference in signal intensity due to a difference in each echo time and to suppress the fat signal at the desired ratio in the water image. Also good.
- the image processing unit 305 is an attenuation correction coefficient that is multiplied to correct the difference in signal intensity, or an inverse number of the attenuation correction coefficient, and indicates the difference in signal intensity, indicating the difference in signal intensity.
- a fat suppression coefficient that specifies a ratio of suppressing the fat signal, and the weighting coefficient to be multiplied to each of the plurality of images may be determined.
- the coefficient for multiplying each of the image data acquired by the TE in which the water signal and the fat signal are in phase and the TE in the opposite phase is determined, and the calculation is performed at a desired ratio.
- a water image with a suppressed fat signal is obtained. Therefore, it is possible to obtain a desired image without once creating a water image and a fat image in which the water signal and the fat signal are separated. Therefore, a desired image can be obtained at high speed without increasing the memory usage.
- the influence of T2 and T2 * attenuation due to the time difference between the TE having the same phase and the TE having the opposite phase is taken into consideration, and the water image finally obtained is reversed. Ensure that no phase fat signal remains. That is, after setting the fat signal having the opposite phase to the water signal to be equal to or less than the fat signal having the same phase, the weight coefficient is determined so that the fat signal remains in the water image at a desired ratio, The weight calculation is performed.
- the phase of the fat signal remaining in the water image is aligned with the phase of the water signal, and a water image in which the fat signal remains at a desired ratio can be obtained without impairing the contrast. That is, the contrast and fat suppression ratio of the finally obtained water image can be kept high, and a desired image can be obtained with high accuracy.
- a small amount of fat signal remaining in the water image is useful for grasping the positional relationship of the tissue, and can provide an image in which fat that is easy to interpret is suppressed.
- the operator can freely adjust the fat ratio according to the imaging region and imaging type, and a desired high-quality contrast for each imaging. Images can be obtained.
- fat ratio table 800 that holds a ratio for suppressing fat signals in association with at least one of the imaging region and imaging type, the operator can be aware of the fat ratio. It is possible to obtain a high-quality contrast image with an appropriate fat ratio.
- 100 MRI apparatus 100 magnetic resonance imaging apparatus, 101 subject, 102 static magnetic field generating 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, 113 input unit, 200 pulse sequence, 201 RF pulse, 202 slice encode gradient magnetic field, 203 phase encode gradient magnetic field, 204 frequency encode gradient magnetic field, 205 frequency encode gradient magnetic field 206, frequency encoding gradient magnetic field, 207, frequency encoding gradient magnetic field, 211 echo signal, 212 echo signal, 301 signal receiving unit, 302 k-space database, 303 image conversion unit, 304 image database, 305 image processing unit, 306 image transmission unit, 307 Parameter holding unit, 601 image, 602 image, 603 image, 701 image, 702 image , 800 fat percentage table, 801 an imaging part, 802 imaging species, 810 fat suppression coefficient input region
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Abstract
Description
ここで、In=W+F
Out=W-F
近年では、水画像と脂肪画像とから脂肪の含有率を示す画像を作成したのもが臨床に用いられている。このような画像は、TEの異なる画像を複数枚取得し、演算によって水画像と脂肪画像とに分離し、これらを数学的に結合することにより得る(例えば、特許文献1参照)。
=1.9W-0.1F ・・・(2)
意図的に残す信号とは異なり、消え残った脂肪信号の位相は、水画像の水信号の位相と異なることがあり、正しいコントラストが損なわれてしまう。この場合、再結合した画像のコントラストも損なわれてしまう。
本実施形態のMRI装置の一例の構成を説明する。図1は、MRI装置100の機能ブロック図である。本実施形態のMRI装置100は、静磁場発生用磁石102と、傾斜磁場コイル103と、照射コイル104と、受信コイル105と、被検体101が横たわるベッド106と、傾斜磁場電源107と、RF送信部108と、信号検出部109と、信号処理部110と、表示部111と、制御部112と、入力部113と、を備える。
2点Dixon法および本実施形態で用いるパルスシーケンスの一例を、図2に示すシーケンスチャートに基づいて説明する。このパルスシーケンス200は、グラジエントエコー(GE)シーケンス法のシーケンスであり、TEの異なる2つの種類の画像を得る。
本実施形態では、上述のように、TE1とTE2とで画像を得る。
次に、得られたエコー信号を処理する信号処理部110の詳細について説明する。本実施形態では、信号処理部110は、k空間のデータを2次元フーリエ変換し、TEの異なる2種類の画像を得る。すなわち、第一のエコー信号が充填されたk空間のデータから、第一の画像(逆位相画像)を得、第二のエコー信号が充填されたk空間データから、第二の画像(同位相画像)を得る。そして、これらの2画像から、所望の画像を得る。
k空間への配置は、スライスエンコード、周波数エンコード、位相エンコードにより特定される。本実施形態では、TE毎に、異なるk空間を用意し、それぞれにエコー信号を格納する。
本実施形態では、画像処理部305は、脂肪抑制画像生成処理において、逆位相画像および同位相画像から、水画像および脂肪画像に分離することなく、所望の割合で脂肪信号を抑えた水画像を得る。このとき、TEの時間差による信号間のT2およびT2*減衰による脂肪の信号強度の差も併せて補正する。
ここで、 A=(1-α)×βF
B=(1+α)
ここで、WsupFは、脂肪信号が所望の割合で抑制された水画像である。
=(1-α)×βF(γWW-γFF)+(1+α)×(W+F)
=(1+βFγW)W+(1-βFγW)αW+2αF ・・・(4)
なお、上記式(3)、式(4)では、静磁場不均一による位相の影響は補正によって除去済みとする。
A1=(1-α)/(1+α)×βF ・・・(5)
WsupF=Out+B1×In
B1=(1+α)/((1-α)×βF) ・・・(6)
なお、重み係数は、水画像に脂肪信号を残す割合が大きいほど、同位相の信号強度/逆位相の信号強度の比が大きくなるよう決定される。
なお、脂肪抑制係数(1-α)は、ユーザから入力部113を介して入力され、パラメータ保持部307に保持される。この場合、本実施形態の制御部112は、脂肪抑制係数の指定を、ユーザから受け付けるインタフェースを備える。インタフェースは、例えば、表示部11に、脂肪抑制係数入力領域810を表示し、当該領域に入力部113を介してユーザが入力するものとする。
減衰補正係数βFは、T2およびT2*減衰の影響を補正するために、同位相画像および逆位相画像のいずれかの画像にかける係数である。最終的に得る水画像に、逆位相の脂肪信号が残らないよう決定する。本実施形態では、逆位相の脂肪の信号強度が、同位相の脂肪の信号強度と等しくなるよう決定される。
図5は、画像処理部305による本実施形態の脂肪抑制画像生成処理の処理フローである。上述のように、本処理は、プログラムとして記憶装置に記憶され、画像処理部305が、各ステップの処理を実行する。
なお、上記実施形態では、通常の2点Dixon法同様、異なる2つのTEを、脂肪信号と水信号とが逆位相となるTE1および同位相となるTE2としているが、2つのTEは、これに限定されない。第一のエコー時間で取得したエコー信号から再構成される第一の画像の水信号と脂肪信号との間の位相差が、第二のエコー時間で取得したエコー信号から再構成される第二の画像の水信号と脂肪信号との間の位相差とが異なればよい。
さらに、本実施形態では、3以上の異なるエコー時間で取得した画像を重み付け加算し、所望の割合で脂肪信号が抑制された水画像を得てもよい。この場合、各エコー時間は、各画像に含まれる水信号と脂肪信号との位相差が、少なくとも2つの画像において異なるよう設定されればよい。
Claims (10)
- 異なる長さのエコー時間で取得したエコー信号をそれぞれ再構成して得た複数の画像を重み付け加算することにより、所望の割合で脂肪信号を抑制した水画像を得る画像処理部を備え、
各前記エコー時間は、前記画像に含まれる水信号と脂肪信号との位相差が、少なくとも2つの画像において異なるよう設定されること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
前記重み付け加算する際に用いられる重み係数は、各前記エコー時間の差による脂肪の信号強度の差を補正し、かつ、前記水画像において前記所望の割合で前記脂肪信号が抑制されるよう決定されること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記重み付け加算する際に用いられる重み係数は、前記水画像に前記脂肪信号を残存させる割合(α)と、各前記エコー時間の差による脂肪信号のT2及びT2*減衰の影響を補正する減衰補正係数(βF)とで表され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記載の磁気共鳴イメージング装置であって、
前記異なる長さのエコー時間は、第一のエコー時間と第二のエコー時間との2つのエコー時間であること
を特徴とする磁気共鳴イメージング装置。 - 請求項4記載の磁気共鳴イメージング装置であって、
前記第一のエコー時間で取得したエコー信号から再構成される第一の画像の脂肪信号と、前記第二のエコー時間で取得したエコー信号から再構成される第二の画像の脂肪信号とは位相が異なり、かつ、前記第一の画像および前記第二の画像の少なくとも一方において、水信号と脂肪信号との位相が異なること
を特徴とする磁気共鳴イメージング装置。 - 請求項5記載の磁気共鳴イメージング装置であって、
前記第一の画像の水信号と脂肪信号とは逆位相であり、
前記第二の画像の水信号と脂肪信号とは同位相であること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記画像処理部は、前記信号強度の差を補正するために乗算する減衰補正係数もしくは前記信号強度の差を示す減衰係数と、前記脂肪信号を抑制する割合を特定する脂肪抑制係数とを用い、各複数の画像それぞれに乗算する前記重み係数を決定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
前記脂肪信号を抑制する割合の指定を、ユーザから受け付けるインタフェースを備えること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
撮像部位および撮像種の少なくとも一方に対応づけて、前記脂肪信号を抑制する割合を保持するデータベースを備え、
前記画像処理部は、ユーザにより設定された撮像部位または撮像種に応じて、前記データベースから前記割合を取得すること
を特徴とする磁気共鳴イメージング装置。 - 複数の異なるエコー時間で取得したエコー信号間の、前記エコー時間の差による信号強度の差を補正する減衰補正係数もしくは前記エコー時間の差による信号強度の差を示す減衰係数と、脂肪信号を抑制する割合を特定する脂肪抑制係数とを取得する係数取得ステップと、
前記減衰補正係数もしくは前記減衰係数と前記脂肪抑制係数とから、前記複数の異なるエコー時間で取得した各エコー信号からそれぞれ再構成される複数の画像に乗算する重み係数を算出する重み係数算出ステップと、
前記算出した重み係数を用いて、前記複数の画像を重み付け加算し、前記脂肪信号を前記割合で抑制した水画像を得る脂肪抑制水画像算出ステップと、を含み、
前記各エコー時間は、前記画像に含まれる水信号と脂肪信号との位相差が、少なくとも2つの画像において異なるよう設定されること
を特徴とする磁気共鳴イメージング装置における脂肪抑制水画像算出方法。
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