WO2011108314A1 - 磁気共鳴撮影装置 - Google Patents
磁気共鳴撮影装置 Download PDFInfo
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- WO2011108314A1 WO2011108314A1 PCT/JP2011/051785 JP2011051785W WO2011108314A1 WO 2011108314 A1 WO2011108314 A1 WO 2011108314A1 JP 2011051785 W JP2011051785 W JP 2011051785W WO 2011108314 A1 WO2011108314 A1 WO 2011108314A1
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- water
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- fat
- phase difference
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- 238000002595 magnetic resonance imaging Methods 0.000 title claims abstract description 40
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 109
- 238000012937 correction Methods 0.000 claims abstract description 4
- 230000003068 static effect Effects 0.000 claims description 41
- 238000004364 calculation method Methods 0.000 claims description 16
- 238000001208 nuclear magnetic resonance pulse sequence Methods 0.000 claims description 13
- 238000005481 NMR spectroscopy Methods 0.000 claims description 11
- 238000003384 imaging method Methods 0.000 claims description 7
- 230000001678 irradiating effect Effects 0.000 claims description 6
- 238000005259 measurement Methods 0.000 abstract description 31
- 238000000926 separation method Methods 0.000 abstract description 13
- 238000000034 method Methods 0.000 description 29
- RGCLLPNLLBQHPF-HJWRWDBZSA-N phosphamidon Chemical compound CCN(CC)C(=O)C(\Cl)=C(/C)OP(=O)(OC)OC RGCLLPNLLBQHPF-HJWRWDBZSA-N 0.000 description 7
- 238000012545 processing Methods 0.000 description 6
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- 238000002474 experimental method Methods 0.000 description 3
- 230000014509 gene expression Effects 0.000 description 3
- 239000011159 matrix material Substances 0.000 description 3
- 229910021586 Nickel(II) chloride Inorganic materials 0.000 description 2
- 210000001015 abdomen Anatomy 0.000 description 2
- 239000007864 aqueous solution Substances 0.000 description 2
- 239000000696 magnetic material Substances 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- QMMRZOWCJAIUJA-UHFFFAOYSA-L nickel dichloride Chemical compound Cl[Ni]Cl QMMRZOWCJAIUJA-UHFFFAOYSA-L 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 238000000701 chemical imaging Methods 0.000 description 1
- 239000013626 chemical specie Substances 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000004069 differentiation Effects 0.000 description 1
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- 230000005284 excitation Effects 0.000 description 1
- 235000015220 hamburgers Nutrition 0.000 description 1
- 125000004435 hydrogen atom Chemical class [H]* 0.000 description 1
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- 238000004904 shortening Methods 0.000 description 1
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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
-
- 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
-
- 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
-
- 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
Definitions
- the present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and in particular in water / fat separation imaging, measurement is performed with an echo time such that the phase between water and fat is the same phase and opposite phase. Achieves good water / fat separation even without it. Furthermore, the present invention relates to an apparatus and a method capable of discriminating between the obtained two separated images as a water image or a fat image.
- an MRI apparatus magnetic resonance imaging apparatus
- the present invention relates to an apparatus and a method capable of discriminating between the obtained two separated images as a water image or a fat image.
- the radionuclide that is widely used in clinical practice is a hydrogen nucleus (proton) that is the main constituent of the subject.
- MRI images the form or function of the human head, abdomen, limbs, etc. in two or three dimensions by imaging the spatial distribution of relaxation information of proton density and nuclear magnetic resonance signals.
- the main protons that can be detected by MRI are water and fat. Fat present in the human body reduces image contrast in the abdomen, spine, limbs, etc. of the subject.
- Non-patent Document 1 a Dixon method that suppresses fat using a phase difference between water and fat.
- Water and fat have a “chemical shift” in which the resonance frequency differs by 3.5 ppm due to the difference in molecular structure.
- the frequency difference due to this chemical shift is proportional to the magnetic field strength, and is approximately 224 Hz when the magnetic field strength is 1.5 Tesla.
- the Dixon method uses a phase difference caused by a difference in frequency between water and fat.
- ⁇ Acquire a plurality of image data with different time (hereinafter referred to as echo time) from excitation of nuclear spins to acquisition of signals. Specifically, an image acquired at an echo time in which water and fat are in phase (in phase) and an image acquired at an echo time in which water and fat are out of phase (out of phase) are acquired.
- n is an integer.
- echo time is to get the two images I 2 of t out.
- I 1 and I 2 can be expressed by the following equations (1) and (2), respectively.
- I 1 W + F
- I 2 WF
- the water signal and the fat signal are separated as in the following equations (3) and (4).
- W (I 1 + I 2 ) / 2 (3)
- F (I 1 ⁇ I 2 ) / 2 (4)
- the water signal W and the fat signal F can be calculated, respectively, and an image in which water and fat are separated can be acquired.
- Non-Patent Document 2 and Non-Patent Document 3 are known as water / fat separation methods in consideration of static magnetic field inhomogeneity.
- Non-Patent Document 2 discloses that the least-square estimation process is repeatedly performed on three variables of a water signal W, a fat signal F, and a frequency difference f due to static magnetic field inhomogeneity from three images having different echo times. It is a method to determine the variable and separate water and fat.
- Non-Patent Document 3 describes that acquiring two images I 1 acquired with an echo time in which water and fat are in the same phase and an image I 2 acquired in an echo time in which water and fat are in opposite phases is the Dixon method. It is the same. However, the method of Non-Patent Document 3 separates water and fat by estimating the phase around due to the static magnetic field inhomogeneity using the region enlargement method. In Non-Patent Document 3, after measuring the image I 1 with an echo time t in when water and fat are in phase, the gradient magnetic field pulse is inverted, and the water and fat are in reverse phase at the echo time t out. to get the image I 2. That is, an image having the same phase and the opposite phase is acquired by one measurement.
- Non-Patent Document 2 described above has a problem that the measurement time is extended because it is necessary to perform three measurements.
- the gradient magnetic field pulse is inverted to acquire an image in which water and fat are in phase and an image in opposite phase in one measurement.
- the time interval dt between t in and t out is inversely proportional to the magnetic field strength.
- n is an integer, for example, when the magnetic field strength is 3.0 Tesla, dt is (1.12 + 2.24 ⁇ n) ms, and when the magnetic field strength is 1.5 Tesla, dt is (2.24 + 4.48).
- dt is (6.70 + 13.39 ⁇ n) ms.
- the magnetic field strength is 3.0 Tesla or 1.5 Tesla
- the measurement band is widened, the signal-to-noise ratio (hereinafter referred to as SNR) is lowered and the image is deteriorated.
- SNR signal-to-noise ratio
- the measurement time per slice is extended because dt is wide. That is, in the two-dimensional multi-slice measurement, since the number of slices per desired repetition time TR has to be reduced, the number of slices to be acquired is reduced. In the three-dimensional measurement, the measurement time is simply extended. When the measurement time is extended, the body motion artifact of the subject increases. When the artifact increases, an error in the arithmetic processing increases, and it becomes difficult to separate water and fat.
- two original images are measured at two echo times in which the phase difference between water and fat is not opposite, and is not an integral multiple of ⁇ .
- the phase difference was P 2
- Two original images are obtained by measuring at echo times t 1 and t 2 that satisfy the above condition.
- Two water-to-fat ratio maps are calculated algebraically or numerically from the two original images acquired under the above conditions.
- Two phase maps are calculated from the two calculated water-to-fat ratio maps.
- Two minimum phase difference maps that minimize the spatial phase difference are calculated by combining the two calculated phase maps. It is determined whether the minimum phase difference map is a correct minimum phase difference map from the variation of the differential map obtained by spatially differentiating the minimum phase difference map. Using the determined phase difference map, the phase of the original image is corrected. Water / fat separation calculation processing is performed from the corrected original image.
- a static magnetic field generating means for generating a static magnetic field in a space where the subject is placed, an irradiating means for irradiating the subject with a high frequency magnetic field pulse, and a nuclear magnetism generated from the subject by the irradiation of the high frequency magnetic field pulse
- Receiving means for receiving a resonance signal, gradient magnetic field applying means for applying a gradient magnetic field for adding position information to the nuclear magnetic resonance signal, the irradiating means, the gradient magnetic field generating means, and the receiving means are subjected to predetermined pulses.
- a magnetic resonance imaging apparatus comprising: a control unit that operates according to a sequence to perform imaging at two different echo times; and an image reconstruction unit that reconstructs an original image from nuclear magnetic resonance signals of the different echo times.
- a phase difference map calculating means for calculating a phase difference map due to inhomogeneous static magnetic field from the original image, and positioning one of the original images from the phase difference map.
- a phase-corrected image calculating unit that calculates a phase-corrected image by correcting, and a water-fat separated image calculating unit that calculates an image obtained by separating water and fat from two images of the original image that is not subjected to phase correction and the phase-corrected image.
- a magnetic resonance imaging apparatus is provided.
- the original image measuring means measures the original image with two echo times satisfying the formulas (5), (6), and (7), for example.
- the phase difference map calculating means includes, for example, a water-to-fat ratio map calculating means for calculating two water-to-fat ratio maps from the two original images in an algebraic or numerical analysis manner, and the two water-to-fat ratio map calculating means. From the fat ratio map, two minimum phase difference maps are calculated so as to minimize the spatial phase difference from the phase map calculating means for calculating the two phase maps and the combination of the points of the two phase maps. And a phase difference map determining means for determining whether the minimum phase difference map is a correct minimum phase difference map from variations in a differential map obtained by spatially differentiating the minimum phase difference map.
- the limitation on measurement is eased in the method of acquiring images at different echo times for water / fat separation, and good water / fat separation can be realized. Furthermore, it is possible to determine whether the obtained two separated images are water images or fat images.
- FIG. The external view of other MRI apparatuses of a perpendicular magnetic field system.
- (A) is a flowchart of the method of calculating the image which isolate
- (b) is the position applied to the magnetic resonance imaging apparatus of embodiment of this invention.
- (A) And (b) is a figure explaining two phase maps obtained on the measurement conditions applied to the magnetic resonance imaging apparatus of embodiment of this invention.
- (A) And (b) is a figure explaining two phase maps obtained on the conventional measurement conditions. The figure explaining two minimum phase difference maps applied to the magnetic resonance imaging apparatus of embodiment of this invention.
- (A) And (b) is a figure explaining the effect by this invention using a computer simulation result.
- (A)-(d) is a figure explaining the effect by this invention using a phantom experiment result.
- FIG. 1A to 1C illustrate an overall configuration and an external view of a magnetic resonance imaging apparatus to which the present invention is applied.
- FIG. 1A shows a horizontal magnetic field type MRI apparatus using a tunnel magnet that generates a static magnetic field with a solenoid coil.
- FIG. 1B shows a hamburger type (open type) vertical magnetic field type MRI apparatus in which magnets are separated into upper and lower sides in order to enhance the feeling of opening.
- FIG. 1C shows the same tunnel-type MRI apparatus as FIG. However, the feeling of opening is enhanced by shortening the depth of the magnet and tilting it diagonally.
- the present invention can be applied to an MRI apparatus having a known structure including the MRI apparatus of these forms, but is not limited to this, and can be applied regardless of the form and type of the MRI apparatus.
- This MRI apparatus has a static magnetic field coil 2 for generating a static magnetic field in a space where a subject 1 is placed, a gradient magnetic field coil 3 for applying a gradient magnetic field in three directions orthogonal to the static magnetic field, and a static magnetic field uniformity.
- a shim coil 4 that can be adjusted, a measurement high-frequency coil 5 that irradiates a measurement region of the subject 1 with a high-frequency magnetic field (hereinafter simply referred to as a transmission coil), and a reception high-frequency that receives a nuclear magnetic resonance signal generated from the subject 1
- a coil 6 hereinafter simply referred to as a receiving coil
- FIG. 2 Various types of static magnetic field coils 2 are employed depending on the structure of the apparatus shown in FIG.
- the gradient magnetic field coil 3 and the shim coil 4 are driven by a gradient magnetic field power supply unit 12 and a shim power supply unit 13, respectively.
- the transmission coil 5 and the reception coil 6 are shown separately, but there is a configuration in which only one high-frequency coil is used for both transmission and reception.
- the high frequency magnetic field irradiated by the transmission coil 5 is generated by the transmitter 7.
- the nuclear magnetic resonance signal detected by the receiving coil 6 is sent to the computer 9 through the receiver 8.
- the computer 9 performs various arithmetic processes on the nuclear magnetic resonance signal to generate image information and the like.
- the calculation process includes various calculation processes for calculating a separated image of water and fat.
- a display 10, a storage device 11, an input device 15 and the like are connected to the computer 9.
- the above-described image information and shooting area information are displayed on the display 10 or recorded in the storage device 11.
- the input device 15 is used to input measurement conditions, shooting area information settings, conditions necessary for arithmetic processing, and the like, and these measurement conditions are recorded in the storage device 11 as necessary.
- the sequence control device 14 controls the gradient magnetic field power supply unit 12 for driving the gradient magnetic field coil 3, the shim power supply unit 13 for driving the shim coil 4, the transmitter 7 and the receiver 8.
- the control time chart (pulse sequence) is set in advance by the imaging method and is stored in the storage device 11.
- FIG. 3 shows a time chart of a pulse sequence employed in the present embodiment.
- the RF pulse 301 is irradiated to excite the spin of the subject.
- a slice selection gradient magnetic field Gs is applied simultaneously with the RF pulse 301 in order to select a specific slice of the subject.
- a phase encoding gradient magnetic field Gp for phase encoding is applied to the nuclear magnetic resonance signal, then a read gradient magnetic field Gr302 is applied, and an echo signal (first echo) after time TE 1 from the first RF pulse 301 irradiation. 303 is measured.
- a read gradient magnetic field Gr304 having an inverted polarity is applied, and an echo signal (second echo) 305 is measured after time dt from the first echo 303 measurement.
- the echo time TE 1 and the echo time TE 2 are set so that the phase difference between water and fat does not become positive or negative as shown in Expression (7). Further, as shown in the equations (5) and (6), the echo times TE 1 and the echo times TE 2 are set so that the phase differences P 1 and P 2 between water and fat do not become integer multiples of ⁇ , respectively. Set.
- This sequence is repeated a predetermined number of times while changing the intensity of the phase encoding gradient magnetic field Gp.
- the number of echo signals necessary for image reconstruction is repeatedly acquired 128 times, 256 times, and the like.
- one image is reconstructed from the first echo 303 for the number of repetitions
- another image is reconstructed from the second echo 305 for the number of repetitions.
- FIG. 4 shows a spin echo type pulse sequence.
- an RF pulse 401 is irradiated to excite the spin of the subject.
- a slice selection gradient magnetic field Gs is applied simultaneously with the RF pulse 401 in order to select a specific slice of the subject.
- a phase encoding gradient magnetic field Gp for phase encoding is applied to the nuclear magnetic resonance signal, and further, an RF pulse 402 for inverting spin is irradiated together with the slice selection gradient magnetic field Gs.
- a readout gradient magnetic field Gr403 it measures the echo signal (first echo) 404 from the first RF pulse 401 after irradiation time TE 1.
- a read gradient magnetic field Gr405 having an inverted polarity is applied, and an echo signal (second echo) 406 is measured after time dt from the first echo 404 measurement.
- the echo time TE 1 when measuring the first echo 404 and the echo time TE 2 when measuring the second echo 406 are the same as in the gradient pulse sequence, as shown in Equation (5), Equation (6), Equation (7). ) Is selected so as to satisfy the above condition.
- two signals having different echo times may be measured within one repetition time, or two measurements may be performed. Thus, signals with different echo times may be measured.
- an image in which water and fat are separated is calculated. That is, by controlling the respective units by a sequence controller 14, executes a pulse sequence shown in FIG. 3 or FIG. 4, each at different times TE 1 and TE 2 to obtain an echo signal (captured; Step 511). Then, the computer 9 reconstructs the original image from the echo signals obtained at different times TE 1 and TE 2 (image reconstruction; step 512).
- the computer 9 calculates a phase difference map due to the static magnetic field inhomogeneity from the two original images obtained in step 512 (phase difference map calculation; step 513), and uses the obtained phase difference map to perform step.
- One of the two original images obtained in 512 is corrected (phase-corrected image calculation; step 514), and water and fat are separated from the corrected image and the original image not corrected in step 514.
- An image is calculated (water fat separation image calculation; step 515).
- step (501) two water to fat ratio maps are calculated.
- a calculation formula for calculating a water-to-fat ratio map is obtained.
- the phase of water at the echo time t 1 when measuring the first echo is ⁇ w1
- the phase of fat is ⁇ F1
- the phase around the phase due to the static magnetic field inhomogeneity is ⁇ 1
- the echo time t 2 when measuring the second echo The phase of water is ⁇ w2
- the phase of fat is ⁇ F2
- the phase around the phase due to the static magnetic field inhomogeneity is ⁇ 2
- the initial phase is ⁇
- the absolute value of the signal obtained as the first echo is r 1
- the declination angle is ⁇ 1
- the ratio of the water signal contained in the voxel is R W
- the ratio of the fat signal is R F
- the proportionality constant is k
- equation (9) is divided by equation (8) to obtain equation (10).
- ⁇ ⁇ 2 ⁇ 1
- r r 2 / r 1
- ⁇ ⁇ 2 ⁇ 1 .
- a and B are represented by the following formulas (15) and (16).
- R W + , R F + , R W ⁇ , and R F ⁇ may be obtained numerically from Equation (11) and Equation (12). That is, while maintaining the relationship of equation (12), (when varying the R F 1 to 0) when changing the left side of R W of the formula (11) from 0 to 1, the absolute value after complex division R W + , R F + , R W ⁇ , and R F ⁇ may be obtained by searching for a combination that minimizes the difference from r.
- step (502) two phase maps ⁇ + and ⁇ ⁇ are calculated from the two water-to-fat ratio maps calculated in step (501).
- the phase map calculated from R W + and R F + is defined as ⁇ + .
- a phase map calculated from R W ⁇ and R F ⁇ is defined as ⁇ ⁇ .
- ⁇ + represents the correct static magnetic field map in the region where the water signal is the main component
- ⁇ ⁇ is the correct static magnetic field in the region where the fat signal is the main component.
- ⁇ + represents the correct static magnetic field map in the region where the fat signal is the main component
- ⁇ ⁇ is the correct static magnetic field in the region where the water signal is the main component. Represents a map.
- step (503) two minimum phase difference maps that minimize the spatial phase change at each point of the two phase maps are calculated.
- the procedure will be described using a model.
- the conditions of the echo time t 1 of the first echo and the echo time t 2 of the second echo are compared in the following four patterns (A) to (D).
- t 1 is 4.5 ms (in phase)
- t 2 is 6.0 ms
- B t 1 is 6.7 ms (reverse phase)
- t 2 is 8.2 ms
- C t 1 is 4.5 ms (same phase)
- t 2 is 6.7 ms (reverse phase)
- D t 1 is 6.7 ms (reverse phase)
- t 2 is 8.9 ms (in-phase)
- Conditions (A) and (B) are the conditions of this embodiment, and conditions (C) and (D) are conditions conventionally used.
- the phase difference may not be 0 as long as the conditions of equations (5) to (7) are satisfied. Further, the water / fat phase difference when the echo time of the condition (B) is t 1 is ⁇ , but the phase difference may not be ⁇ if the conditions of the equations (5) to (7) are satisfied.
- phase map [delta] + under the condition (A), [delta] - in FIGS. 6 (a) and phase map [delta] + under the condition (B), [delta] - in Figure 6 (b) are shown respectively.
- phase map [delta] + under the condition (C), [delta] - in FIGS. 7 (a) and phase map [delta] + under the condition (D), [delta] - in Figure 7 (b) are shown respectively.
- 6 (606: black circle) and FIG. 7 (706: black circle) are graphs of ⁇ + under each condition
- FIG. 6 (607: white frame) and FIG. 7 (707: white frame) are ⁇ under each condition. - shows the graphs respectively.
- phase difference (608) between ⁇ + and ⁇ ⁇ changes depending on the ratio of water to fat.
- the sudden change in the phase difference (608) between ⁇ + and ⁇ ⁇ depending on the ratio of water and fat is called phase jump.
- phase around due to the static magnetic field inhomogeneity changes smoothly in space unless a magnetic material such as metal is in the vicinity. For this reason, the phase jump like the phase difference (608) shown in FIG. 6A does not occur in the region where protons are continuously present. Therefore, in the regions (601) and (602) where fat occupies the main component, ⁇ ⁇ represents the phase around due to the static magnetic field inhomogeneity, and water occupies the main component (604) and (605). In this region, ⁇ + represents the phase around due to the static magnetic field inhomogeneity. In the region (603), since the ratio of water and fat is the same, both ⁇ + and ⁇ ⁇ represent the phase around due to non-uniform static magnetic field.
- the phase difference (609) may, when the echo time t 1 of the first echo of antiphase, the phase difference shown in FIG. 6 (a) (608) and inversely related become. That is, in the regions (601) and (602) where fat occupies the main component, ⁇ + represents the phase around due to the static magnetic field inhomogeneity, and water occupies the main component (604) and (605). In this region, ⁇ ⁇ represents the phase around due to the static magnetic field inhomogeneity.
- the phase difference (708) between ⁇ + and ⁇ ⁇ is ⁇ or ⁇ regardless of the ratio of water to fat. Therefore, it cannot be determined which region of ⁇ + and ⁇ ⁇ reflects the correct phase.
- the phase difference (709) may, when the echo time t 1 of the first echo of antiphase, the phase difference shown in FIG. 6 (b) similar to (609), FIG. The relationship is reversed from the phase difference (708) shown in FIG.
- the ratio of water and fat as in the phase difference (608) shown in FIG. 6 (a) or the phase difference (609) shown in FIG. 6 (b). Utilizes the phase jump caused by the change of.
- the phase around due to inhomogeneous static magnetic field changes smoothly in space unless a magnetic material such as metal is in the vicinity. Therefore, an arbitrary point on the calculated phase map ⁇ + , ⁇ ⁇ is used as a reference point, and a point having the smallest phase difference between the reference point and the adjacent point is searched and calculated while enlarging the region, and finally Thus, the minimum phase difference maps ⁇ +1 and ⁇ ⁇ 1 are calculated.
- One of the minimum phase difference maps ⁇ +1 and ⁇ ⁇ 1 correctly represents the phase around due to the static magnetic field inhomogeneity.
- a reference point x 0 at ⁇ + and ⁇ ⁇ is determined.
- is set as the reference point x 0 .
- the point (610) in FIG. 6A is set as the reference point x0.
- Step 2 in order to calculate the minimum phase difference map [delta] +1 and [delta] -1, defines the value of [delta] +1 and [delta] -1 at the reference point x 0.
- ⁇ +1 (x 0 ) ⁇ + (x 0 )
- ⁇ ⁇ 1 (x 0 ) ⁇ ⁇ (x 0 ).
- FIG. 8 shows minimum phase difference maps ⁇ +1 and ⁇ ⁇ 1 , respectively.
- (801: white circle) represents ⁇ +1
- (802: American mark) represents ⁇ ⁇ 1 . It can be seen that the difference between each of the minimum phase difference maps ⁇ +1 and ⁇ ⁇ 1 from the adjacent point is minimum.
- top minimum phase difference map [delta] +1 it will vary with the position of the reference point x 0.
- the reference point x 0 is set to a point (610) of FIG. 6 (a), there is a region where the phase jump remains.
- One of the two minimum phase difference maps ⁇ +1 and ⁇ ⁇ 1 obtained here represents a phase difference map due to correct static magnetic field inhomogeneity.
- step (504) in FIG. 5 differential maps D + and D ⁇ of the two minimum phase difference maps calculated in step (503) are calculated. Instead of taking the differentiation, it may be obtained by complex division with adjacent points, as shown in equations (27) and (28).
- n is an integer.
- step (505) in FIG. 5 the variation of the calculated differential maps D + and D ⁇ is calculated, and the minimum phase difference map with the minimum variation is selected.
- This variation may be obtained, for example, from the standard deviation or the examiner may judge from the histogram of the differential values.
- phase difference map ⁇ due to the correct static magnetic field inhomogeneity is calculated.
- phase correction image calculation method in step 514 and the water image and fat image calculation method in step 515 will be described.
- step 514 when the image measured with the first echo is I 1 and the image measured with the second echo is I 2 , the phase of the image I 2 is corrected by equation (29) using the phase difference map ⁇ described above. I 2 'is calculated.
- I 1 ′ may be calculated by correcting the phase of the image I 1 using equation (30).
- step 515 using the image I 1 and I 2 'calculates the water and fat images.
- the water phase at the first echo time is ⁇ w1
- the fat phase is ⁇ F1
- the water phase at the second echo time is ⁇ w2
- the fat phase is ⁇ F2
- the water image is W
- the fat image the when the F, I 1 and I 2 'can be expressed by the following simultaneous equations.
- Equation (33) deformed by using a transposed matrix A t and the inverse matrix, using the obtained formula (35), to calculate the water and fat images.
- FIG. 9A is a water image
- FIG. 9B is a fat image.
- the model used here is a model in which the above-described regions where the ratio of water to fat is 0: 100, 25:75, 50:50, 75:25, and 100: 0 are continuously arranged.
- the first echo time was 4.5 ms and the second echo time was 6.0 ms.
- FIG. 10 (1001) is a nickel chloride aqueous solution phantom
- FIG. 10 (1002) is a fat mat.
- the measurement conditions were a repetition time TR of 80 ms, a flip angle of 15 degrees, an imaging field of view of 360 mm, a measurement matrix of 128 ⁇ 128, and an integration count of 2.
- FIG. 10A shows a first echo image measured at an echo time of 4.5 ms
- FIG. 10B shows a second echo image measured at an echo time of 6.0 ms
- FIGS. 10C and 10D show a water image and a fat image separated according to the present invention, respectively. As shown in FIGS. 10C and 10D, the water image and the fat image can be separated even if the images are not acquired with echo times in which water and fat have the same phase and opposite phases, respectively. Recognize. It can also be seen that the water image and the fat image are discriminated without making a mistake.
- the present invention in water / fat separation photography, it is possible to achieve good water / fat separation without measuring with echo times such that the phase between water and fat is the same phase and opposite phase. Is realized. Furthermore, it is possible to determine whether the obtained two separated images are water images or fat images.
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Abstract
Description
I1=W+F (1)
I2=W-F (2)
W=(I1+I2)/2 (3)
F=(I1-I2)/2 (4)
以上より、水信号Wと脂肪信号Fとがそれぞれ算出でき、水と脂肪が分離した画像を取得できる。
P1≠n1π(n1は整数) (5)
かつ
P2≠m1π(m1は整数) (6)
かつ
2n2πP1≠-2m2πP2(n2およびm2は整数) (7)
を満たすようなエコー時間t1およびt2で計測し2つの原画像を得る。
図1(a)~(c)に、本発明が適用される磁気共鳴撮影装置の全体構成と外観図を例示する。図1(a)はソレノイドコイルで静磁場を発生するトンネル型磁石を用いた水平磁場方式のMRI装置である。図1(b)は開放感を高めるために磁石を上下に分離したハンバーガー型(オープン型)の垂直磁場方式のMRI装置である。また、図1(c)は、図1(a)と同じトンネル型のMRI装置である。しかし、磁石の奥行を短くし且つ斜めに傾けることによって、開放感を高めている。本発明は、これらの形態のMRI装置を含む公知の構造のMRI装置に適用することができるが、これに限定されるものではなく、MRI装置の形態やタイプを問わず適用可能である。
P1≠n1π(n1は整数) (5)
かつ
P2≠m1π(m1は整数) (6)
かつ
2n2πP1≠-2m2πP2(n2およびm2は整数) (7)
を満たすエコー時間TE1およびエコー時間TE2が選択される。
RW+RF=1 (12)
δ+=θ-ω+ (25)
δ-=θ-ω- (26)
(B)t1を6.7ms(逆位相)、t2を8.2ms
(C)t1を4.5ms(同位相)、t2を6.7ms(逆位相)
(D)t1を6.7ms(逆位相)、t2を8.9ms(同位相)
Claims (6)
- 被検体が置かれる空間に静磁場を発生する静磁場発生手段と、
前記被検体に高周波磁場パルスを照射する照射手段と、
高周波磁場パルスの照射により前記被検体から生じる核磁気共鳴信号を受信する受信手段と、
前記核磁気共鳴信号に位置情報を付加するための傾斜磁場を印加する傾斜磁場印加手段と、
前記照射手段、前記傾斜磁場発生手段及び前記受信手段を所定のパルスシーケンスに従って動作させて異なる2つのエコー時間の撮影を実行する制御手段と、
前記異なるエコー時間の核磁気共鳴信号から原画像を再構成する画像再構成手段と、
を備える磁気共鳴撮影装置であって、
前記原画像から、静磁場不均一による位相差マップを算出する位相差マップ算出手段と、
前記位相差マップから前記原画像のうちの1画像を位相補正して位相補正画像を算出する位相補正画像算出手段と、
前記位相補正画像と位相補正を実施しない前記原画像の2画像から水と脂肪を分離した画像を算出する水脂肪分離画像算出手段と
を有することを特徴とする磁気共鳴撮影装置。 - 請求項1記載の磁気共鳴撮影装置であって、
前記制御手段は、水と脂肪との位相差が正負逆とならず、かつ、水と脂肪との位相差がπの整数倍とならないよう、前記異なる2つのエコー時間を設定するエコー時間設定手段を有すること
を特徴とする磁気共鳴撮影装置。 - 請求項2記載の磁気共鳴撮影装置であって、
前記位相差マップ算出手段は、
前記原画像から、水と脂肪の比を算出する水対脂肪比算出手段と、
前記水対脂肪比から、位相マップを算出する位相マップ算出手段と、
前記位相マップの基準点から隣り合う点の位相差が最も小さくなる点を、領域を拡大しながら探索し、最小位相差マップを算出する最小位相差マップ算出手段と、
前記最小位相差マップから、静磁場不均一による位相差マップを判定する位相差マップ判定手段と、を有すること
を特徴とする磁気共鳴撮影装置。 - 前記水対脂肪比算出手段は、水と脂肪との割合の和が一定となる条件を付加して代数的に算出する算出手段を有すること
を特徴とする請求項3記載の磁気共鳴撮影装置。 - 前記水対脂肪比算出手段は、水と脂肪の割合の和が一定となる条件を付加して数値解析的に算出する算出手段を有すること
を特徴とする請求項3記載の磁気共鳴撮影装置。 - 前記位相差マップ判定手段は、前記最小位相差マップの微分値から、標準偏差が最小となる最小位相差マップを選択する選択手段を有すること
を特徴とする請求項3記載の磁気共鳴撮影装置。
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