Classifications

• • 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

Definitions

• the present invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus, and more particularly to a technique suitable for measuring a temperature distribution image in a living body by an MRI apparatus.
• MRI magnetic resonance imaging
• IV-MRI interventional MRI: hereafter referred to as IV-MRI.
• therapies performed with IV-MRI include laser therapy, injection of drugs such as ethanol, RF ablation, and cryotherapy.
• MRI plays a role in guiding real-time imaging to reach a puncture needle or tubule to the affected area, visualizing tissue changes during treatment, and monitoring local temperature during heating / cooling treatment.
• a typical application of IV-MRI is to image the temperature distribution in the body during laser treatment.
• Temperature distribution imaging methods include the method of obtaining from signal intensity, the method of obtaining from diffusion coefficient, and the method of obtaining from proton phase shift (PPS method: Proton Phase Shift method). Of these, the PPS method is the most measured. Excellent accuracy. This function can be used to monitor the temperature in the living body, monitor laser irradiation therapy, and use RF (Radio
• a temperature distribution is obtained from phase information of an echo signal obtained by reversing a gradient magnetic field.
• FIG. 1 shows an example of a method for measuring such phase information.
• the sample spin is excited by a 90 ° high-frequency pulse 101 (hereinafter referred to as a 90 ° pulse), and the gradient magnetic field Gpl03 is read out to change the spin phase.
• a gradient magnetic field Grl04 is sequentially applied to generate a gradient echo 105.
• the Fourier transform of the echo signal Phase distribution is obtained from the real part and the imaginary part of the complex image obtained by the above. For example, the phase distribution is obtained from equation (1).
• the temperature T is obtained from the frequency f and the temperature coefficient of water. For example, the temperature T is obtained from the equation (2).
• the temperature of the magnet and the pole piece may increase due to heating of the GC coil, and the static magnetic field and the gradient magnetic field offset may fluctuate. Fluctuations in the static and gradient magnetic field offsets are equivalent to fluctuations in the spatial distribution of the resonance frequency f. Therefore, unless the resonance frequency f at each position is measured for each imaging, the measured temperature change distribution will be affected by static magnetic field fluctuations, and accurate temperature changes cannot be captured.
• IV-MRI it is necessary to obtain a temperature distribution image in real time, so measuring the distribution of the resonance frequency f every time the temperature distribution image is measured leads to an increase in the measurement time and impairs the real-time performance .
• an object of the present invention is to solve these problems and provide a highly accurate temperature change distribution image. Disclosure of the invention
• the present invention provides a static magnetic field generating means for applying a static magnetic field to a test object, a slice-direction gradient magnetic field, a frequency encoding gradient magnetic field to the test object.
• Gradient magnetic field generating means for applying a phase encoding gradient magnetic field
• high-frequency pulse generating means for generating a high-frequency pulse for causing magnetic resonance in nuclei of atoms constituting the inspection object, and a nuclear magnetic resonance signal from the inspection object.
• a magnetic resonance system comprising: a detection unit for detecting; a reconstruction unit for reconstructing an image from the nuclear magnetic resonance signal; a display unit for displaying the reconstructed image; and a control unit for controlling each of the units.
• the control unit measures a spatial phase distribution at different times including temperature information, performs a complex difference calculation of the spatial phase distribution at different times, and obtains a reference phase from the spatial phase distribution. Calculating a difference between the reference phase and the spatial phase distribution on which the difference calculation is performed, and calculating a temperature change from the spatial phase distribution on which the difference calculation is performed. The distribution is determined.
• the image processing apparatus may further include a selection unit that selects an arbitrary position in the image displayed on the display unit, wherein the control unit obtains a reference phase from the position selected by the selection unit.
• the selection means may specify an arbitrary area in the image displayed on the display means and select a plurality of positions on the line of the area where the reference phase is to be obtained.
• control means may obtain the reference phase from at least one point of the region where the temperature does not change or from a plurality of points where the temperature does not change and are not arranged on a straight line.
• the display means superimposes the temperature image including the temperature information and the reconstructed morphological image in parallel, or superimposes the image of the temperature change region in the temperature image including the temperature information on the reconstructed morphological image. It may be displayed.
• the selection unit may specify an arbitrary region in the image displayed on the display unit, and may select a plurality of positions for obtaining a reference phase on a line of the region, A region including a temperature change region may be designated. Further, a fitting process for considering a temporal variation in the reference phase may be performed.
• the spatial phase distribution due to the static magnetic field variation is based on the phase variation of a plurality of points in a region where the temperature does not change, for example, any three points or a plurality of points on a region line in an arbitrary region.
• a change in the temperature distribution can be obtained.
• FIG. 1 is a diagram showing a photographing sequence used in the present invention
• FIG. 2 is a block diagram of an embodiment showing an apparatus configuration of the present invention
• FIG. 3 is a flowchart showing an embodiment of the present invention
• FIG. FIG. 5 is a flowchart illustrating an example of selection of a reference point in the embodiment of the present invention
• FIG. 6 is a diagram illustrating an example of another screen configuration of the present invention
• FIG. FIG. 8 is a flowchart showing an example of another reference point selection in the embodiment
• FIG. 8 is a diagram showing an example of another screen configuration of the present invention
• FIG. 9 is a diagram showing an example of another screen configuration of the present invention
• 10 is a diagram showing another embodiment of the present invention
• FIG. 11 is a flowchart showing another embodiment of the present invention.
• the MRI apparatus includes a static magnetic field generating magnetic circuit 202 composed of an electromagnet or a permanent magnet for generating a uniform static magnetic field H0 in a portion of the subject 201, and in three directions x, y, and z orthogonal to the subject 201.
• a static magnetic field generating magnetic circuit 202 composed of an electromagnet or a permanent magnet for generating a uniform static magnetic field H0 in a portion of the subject 201, and in three directions x, y, and z orthogonal to the subject 201.
• gradient Gx to-strength varies linearly, Gy, gradient coils 209 for generating the G Z, the transmission coil 214a for generating a high-frequency magnetic field to the subject 201, for detecting the nuclear magnetic resonance signal generated from the subject 201
• a detection coil 214b a sequencer 207 for generating a gradient magnetic field and a high-frequency pulse at a predetermined timing
• a computer 208 for performing various processes such as sequencer control and image processing
• a signal processing system 206 for displaying and storing images
• It has an operation unit 221 for performing operations such as setting various parameters.
• the high frequency generated by the synthesizer 211 is modulated by the modulator 212, amplified by the power amplifier 213, and supplied to the transmission coil 214a to generate a high frequency magnetic field inside the subject 201 to excite nuclear spins.
• Other nuclei with nuclear spins, such as forces 31 P and 13 C, usually for 1 H, may also be targeted.
• the nuclear magnetic resonance signal emitted from the subject 201 is received by the detection coil 214b, After passing through the amplifier 215, the signal is subjected to quadrature phase detection by the detector 216 and input to the computer 208 via the A / D converter 217.
• the transmission coil 214a and the detection coil 214b are provided separately, but may be used for both transmission and reception.
• an image corresponding to the nuclear spin density distribution, relaxation time distribution, spectrum distribution, etc. is displayed on a display 228 such as a CRT.
• the gradient magnetic field generation system 203 and the transmission system 204 are controlled by a sequencer 207, and the sequencer 207 and the detection system 205 are controlled by a computer 208.
• the computer 208 is controlled by commands from the operation unit 221.
• a complex signal from the subject is measured by a gradient echo (hereinafter referred to as GrE) method using a sequence as shown in Fig. 1, and the acquired complex signal is obtained by two-dimensional Fourier transform.
• the pulse sequence may be another sequence as long as it is a GrE-based sequence.
• the phase component of the echo signal contains (resonance frequency) X (static magnetic field) components, and the PPS method uses the temperature dependence of the resonance frequency.
• known pulse sequences such as a high-speed GrE sequence such as SARGE, TRSARGE, and RFSARGE, a sequence of SSFP (Steady State Free Precession), and a GrE-type EPI (Echo Planar Imaging) sequence can be used.
• the spatial phase distribution calculated by this difference reflects the phase fluctuation due to temperature change and static magnetic field fluctuation.
• the main cause of the static magnetic field fluctuation is that the temperature of the main magnetic field pole piece rises due to the heating of the GC coil during dynamic imaging.
• the variation of the spatial phase distribution due to the variation of the static magnetic field is corrected (step 321), and the temperature change distribution between time and is calculated (step 322).
• this correction means that the entire spatial phase distribution is uniformly affected by the static magnetic field fluctuation.
• a point may be selected and its phase variation may be subtracted from the entire spatial phase distribution. For example, if the phase of the selected reference point is ⁇ f> base , and the spatial phase distribution before and after correction is ⁇ before, y) and 0 after (x, y), then it is expressed by equation (3).
• the static magnetic field fluctuates linearly
• three or more points in the area where the temperature does not change on the image are selected as reference points, and from the phase fluctuation, the fluctuation of the spatial phase distribution due to the fluctuation of the static magnetic field is calculated. Subtract this from the spatial phase distribution.
• the three points must not be aligned on a straight line as shown in Fig. 4. This is because when the three points are on a straight line, the coefficient of the calculation formula representing the spatial phase distribution becomes ⁇ .
• the coordinates of the selected reference point are (x 15 y), (x 2 , y 2 ), 3 , y 3 ), their phases are ⁇ basel 'base2, and the spatial phase distribution changes! ]
• A, B, and C are constants and satisfy the following equations.
• a * + ⁇ * y n + ⁇ . C
• a series of procedures at this time will be described with reference to FIG.
• Fig. 5 first, time and are photographed respectively (step 500), the complex difference of the spatial phase distribution between the time is taken (step 501), and the temperature change between the time and is calculated. (Step 502), a temperature distribution image between the time and is displayed (Step 503). With the temperature distribution image displayed, the operator selects three reference points from an area where the temperature does not change (step 504).
• step 505 the variation of the spatial phase distribution due to the static magnetic field between the reference point phase force and the time is calculated (step 505), and the variation obtained in step 505 from the spatial phase distribution obtained in step 501 is calculated. Correction is performed by subtracting the spatial phase distribution of the data (step 506). Then, the temperature change distribution is recalculated from the corrected spatial phase distribution (step 507), and the corrected temperature distribution image is displayed (step 508).
• Fig. 5 (b) shows another procedure.
• a morphological image of the same part as the temperature distribution image is displayed in parallel, and if an area where the temperature does not change is known (predicted) in advance, three reference points are selected from the morphological image before photographing ( Step 510). If the coordinates of the morphological image are the same as the coordinates of the temperature distribution image, as in step 510 Even if the reference point is selected on the morphological image, the information of the reference point can be reflected on the temperature distribution image. Then, the photographing of the time and is performed respectively (step 511), and the complex difference of the spatial phase distribution between the time and is calculated (step 512).
• the variation of the spatial phase distribution due to the static magnetic field between time and is calculated from the phase of the reference point (step 513), and is calculated in step 513 from the spatial phase distribution obtained in step 512. Is corrected by subtracting the spatial phase distribution of the
• the corrected temperature distribution image is displayed (step 516).
• a method of selecting a reference point by specifying R0I will be described.
• the operator designates R0I so as to surround the temperature change area on the temperature distribution screen.
• R0I multiple reference points are automatically selected on the R0I line.
• eight reference points are selected on the line R0I.
• a series of procedures at this time will be described with reference to FIG.
• Fig. 7 (a) first, the images of time and are taken respectively (step 700), the complex difference of the spatial phase distribution between the time is obtained (step 701), and the temperature change between the time and is calculated. (Step 702), a temperature distribution image between the time and is displayed (Step 703).
• the operator specifies R0I, but specifies that the R0I line does not cross the temperature change area.
• a plurality of (8) reference points are selected on the R0I line (step 704).
• the reference point is selected, the variation of the spatial phase distribution due to the static magnetic field between time and is calculated from the phase of the reference point (step 705), and the variation of the spatial phase distribution determined in step 705 from the spatial phase distribution determined in step 701 is calculated. Correction is performed by subtracting the spatial phase distribution (step 706).
• the temperature change distribution is recalculated from the corrected spatial phase distribution (step 707), and the corrected temperature distribution image is displayed (step 708). If the reference point is selected by R0I in this manner, the operator can specify the R0I and save the trouble of inputting the reference point, and the temperature can be corrected by a simpler operation.
• Fig. 7 (b) shows another procedure. If the position and size of the temperature change area are known (predicted) in advance, specify R0I on the morphological image before photographing. By specifying this 0I, multiple reference points (8 points) are selected (step 710). If the coordinates of the morphological image are the same as the coordinates of the temperature distribution image as in the case of selecting the three points described above, even if the reference point is selected on the morphological image as in step 710, Information of the reference point can be reflected on the distribution image.
• imaging is performed at time t 1 (step 711), and the complex difference of the spatial phase distribution between time and is calculated (step 712) .
• the time is calculated from the phase of the reference point.
• the variation of the spatial phase distribution due to the static magnetic field is calculated (step 713), and correction is performed by subtracting the spatial phase distribution of the variation determined at step 713 from the spatial phase distribution determined at step 712 (step 714).
• the temperature change distribution is calculated from the corrected spatial phase distribution (step 715), and the corrected temperature distribution image is displayed (step 716).
• R0I When specifying R0I, it is desirable to specify so as to surround the temperature change region. This is because a reference point is selected on the R0I line, and if the temperature change region overlaps the R0I line, the temperature change region may be used as the reference point. Also, when the R0I line extends outside the subject, there is a possibility that the outside of the subject will be used as the reference point. If the temperature change region or the outside of the subject is used as the reference point, accurate correction may not be performed.
• a method for selecting a reference point when the temperature change region overlaps the R0I line or when the reference point extends outside the subject will be described.
• the temperature change area and the portion outside the subject are not referred to as reference points. In this way, only the point at which the temperature does not change can be selected as the reference point.
• the operator checks the image while performing the operation, selects the reference point at which the temperature change area or the outside of the subject overlaps, and determines the reference point. May be excluded.
• the operator can reset the R0I by himself, but it is necessary to re-select the R0I many times, or if there are too many criteria and the operator must select a reference point that is not the target Can automatically select the reference point by monitoring the ROI line.
• the temporal temperature fluctuation at the reference point is monitored, and if the temperature change exceeds a preset threshold value at each reference point, it is determined. It can be determined that the reference point overlaps with the temperature change area. It can be determined that it is located.
• the display of the corrected temperature distribution image has been described.
• the change in form is not considered. Therefore, by displaying the temperature distribution image and the morphological image in parallel as shown in FIG. 8, it is possible to observe changes in temperature and morphology simultaneously.
• the processing target may be limited to R0I.
• the corrected temperature distribution image in R0I can be displayed by being superimposed on the morphological image as shown in FIG. 9 (a).
• Fig. 9 (b) temporal changes in temperature within R0I are monitored, and only the part that has changed beyond a preset threshold is judged as a temperature change area, and only this area is extracted. And superimposed on the morphological image. Thereby, it is possible to easily confirm which part in the morphological image has changed in temperature.
• the reference phase may be the phase of one pixel, or the average of the phases of several pixels. Furthermore, since the static magnetic field generally fluctuates continuously, Fig.
• 10 (1001 to 1007) shows the actual phase of the reference point, where 1010 is a straight line with a first-order fitting.
• fitting may be performed in time.
• the fitting may be a higher-order fitting method (eg, a second-order fitting). In this case, it is possible to express more accurate magnetic field time variation than the first-order fitting.
• the temperature change Tl, i (x, y) between the times tl and ti is calculated by the equation (2). As a result, the temperature change can be measured accurately.
• the phase may turn more than ⁇ between the times and.
• shooting is performed at a time and a shorter time interval (during the time) (steps 1100 and 1110), and the spatial position between the time t ⁇ and After performing the complex difference calculation of the phase distribution (Step 1120), the correction of the spatial phase distribution (Step 1121), and the calculation of the temperature image distribution during the time (Step 1122), the temperature change at the time is calculated (Step 1123) ) You can.
• the temperature change in this case is expressed, for example, by equation (5).
• the present invention since the complex difference calculation of the spatial phase distribution at different times is performed, and the spatial phase distribution calculated by the difference is corrected by the static magnetic field fluctuation, there is no need to set the resonance frequency for each imaging. This makes it possible to measure a highly accurate temperature change distribution image without extending the measurement time.

Landscapes

• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• High Energy & Nuclear Physics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

Priority Applications (1)

Applications Claiming Priority (4)

Publications (1)

Family

ID=26542441

Family Applications (1)

Country Status (3)

Cited By (1)

Families Citing this family (38)

Citations (2)

Family Cites Families (7)

• 2000
  • 2000-03-28 JP JP2000089698A patent/JP4526648B2/ja not_active Expired - Fee Related
  • 2000-09-08 US US10/070,615 patent/US6566878B1/en not_active Expired - Fee Related
  • 2000-09-08 WO PCT/JP2000/006154 patent/WO2001017428A1/ja not_active Ceased

Patent Citations (2)

Also Published As

Similar Documents

Legal Events