Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
  • A61B5/7235—Details of waveform analysis
  • A61B5/7253—Details of waveform analysis characterised by using transforms
  • A61B5/7257—Details of waveform analysis characterised by using transforms using Fourier transforms
• • 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
• • 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
  • G01R33/4804—Spatially selective measurement of temperature or pH

Definitions

• the present invention relates to a magnetic resonance diagnostic apparatus (MRI), and particularly to a medical MRI apparatus suitable for measuring a temperature distribution.
• MRI magnetic resonance diagnostic apparatus
• MRI Magnetic resonance Imaging
• IVMR minimally invasive treatment
• One of its applications is the detection of tissue temperature distribution.
• Laser ablation which burns and treats the affected tissue such as a tumor or hernia, and real-time treatment status of the affected area during focused ultrasound ablation. It is attracting attention as a means for monitoring in the field.
• the parameters that show temperature dependence include spin density P, longitudinal relaxation time Tl, transverse relaxation time ⁇ 2, water diffusion coefficient, and water proton chemical shift ⁇ ( JC Hindman, J. Chem. Phys. Vol. 44. p. 4582, 1966). Among them, it is said that the reliability of the chemical shift of water protons is high because there is little dependence on factors other than temperature.
• This method uses a chemical shift sensitive sequence such as the Gradient Eco (GrE) method to detect the change in chemical shift before and after a temperature change as the phase difference of the MR signal.
• the frequency shift of the water proton with temperature is 0.01 ppm / ° C.
• the phase difference ⁇ 0 is expressed by the following equation (1).
• ⁇ 0 is the phase difference at the pixel of interest
• ⁇ is the change in the chemical shift of the water protons at that pixel
• ⁇ is the nuclear magnetic rotation ratio
• Bo is the static magnetic field strength
• ⁇ is the echo time. the same.
• ⁇ is the temperature dependence of the chemical shift of the water proton [0,01 ppm / ° C]. The same applies hereinafter.
• the accuracy of temperature measurement by this method depends on the S / N ratio of the signal and the stability of the hardware, but is about ⁇ 1 ° C.
• phase map creation by the conventional GrE method sequence it is necessary to repeat the phase encoder droop in one direction of space in two-dimensional measurement, and it is necessary to repeat double phase encoder droop in two directions of space in three-dimensional measurement. Therefore, it is difficult to create a phase map in a short time.
• the echo time TE 20ms
• the repetition time TR 30ms
• the number of phase encoding steps 64 in the high-speed GrE method it takes about 2 seconds to create an image. Furthermore, it takes 32 seconds to perform 16 steps of slice code.
• the temperature measurement in IVMR it is necessary to monitor the temperature change of the affected part due to convergent ultrasound treatment etc. in real time, it is desirable to take several images per second, and also to display the three-dimensional temperature distribution It is desirable. However, as described above, these are difficult with the conventional GrE method.
• an object of the present invention is to provide an MRI apparatus capable of creating and displaying a temperature distribution image in a very short time.
• the present invention uses a series of high-frequency pulses (hereinafter referred to as burst waves and laser waves) composed of a plurality of sub-pulses as a high-frequency magnetic field for exciting a water proton. ) And by generating a gradient magnetic field echo that is more phase sensitive than a spin echo, it was possible to create a phase map and display the temperature distribution at extremely high speed.
• a high-speed imaging sequence using a burst wave is known as a burst method (Japanese Patent Publication No. 6-34784).
• a sequence modified from this burst method sequence is executed to perform chemical shift. It is characterized by generating a gradient magnetic field echo to which a phase rotation proportional to is applied, so that a phase map and a temperature map can be obtained.
• the MRI apparatus of the present invention comprises: a magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a radio frequency (RF) magnetic field in a space where a subject is placed; and a detecting means for detecting an MR signal generated from the subject.
• Image reconstruction means for reconstructing an image based on the detected MR signal; image display means; and control means for controlling these.
• the MR signal While applying a gradient magnetic field in the same direction as the gradient magnetic field as a readout gradient magnetic field, the MR signal is detected as a gradient magnetic field echo,
• phase distribution or the temperature distribution obtained from the phase distribution is displayed on the display unit as an image.
• the burst wave is a series of RF pulses composed of a plurality of sub-pulses p as shown in Fig. 3A.
• the burst wave on the time axis is Fourier-transformed, on the frequency axis, as shown in Fig. 3B
• a series of pulse trains of the same number are obtained.
• the interval between sub-pulses constituting the RF burst on the time axis is U (second) and the length of the entire pulse train is W (second)
• the interval between square waves constituting the pulse train on the frequency axis is 1 / u (Hz)
• width is l / W (Hz).
• phase-sensitive data can be obtained by measuring the MR signal as a gradient magnetic field echo generated by spin dephasing and rephasing.
• a high frequency magnetic field pulse for magnetization reversal is applied together with a gradient magnetic field for slice selection.
• the gradient magnetic field echo is generated at a different time from the spin echo generated by applying a high frequency magnetic field pulse for magnetization reversal.
• a phase rotation proportional to the time difference t0 and the chemical shift is applied to the signal.
• the effective TE can be set longer, and the phase sensitivity can be improved.
• slice selection using a high-frequency magnetic field pulse for magnetization reversal cannot be performed, it is suitable for three-dimensional measurement in which phase encoding is performed in the slice direction.
• a gradient magnetic field for a phase code is also applied in the slice direction, data is collected by repeating the encode step in the slice direction, and a three-dimensional phase distribution is performed.
• a high frequency magnetic field pulse for magnetization reversal may be applied after applying the ballast wave, or a high frequency magnetic field pulse for magnetization reversal may not be used.
• the frequency of the burst wave is changed in each cycle of the encoding step to excite different portions parallel to the read direction and to generate the burst wave for excitation without waiting for the longitudinal magnetization recovery time. It may be applied.
• the temperature distribution is obtained by measuring the phase distribution twice or more at different times, calculating the difference between these phase distributions, and converting this phase difference into a temperature change.
• the temperature distribution image can display a temperature difference based on a normal temperature (for example, the body temperature of a subject) in hue, gradation, or a combination thereof.
• the temperature distribution image is preferably displayed so as to be superimposed on the tissue image. This allows the IVMR procedure (treatment) to proceed while confirming the tissue that is undergoing temperature changes by color or the like.
• an MRI monitor can display a three-dimensional temperature distribution at a high speed by employing an imaging sequence having a high phase sensitivity using a burst wave as a high-frequency magnetic field for excitation. It can improve the safety of IVMR surgery below.
• FIG. 1 is a diagram showing a procedure of measuring a temperature distribution in the MRI apparatus of the present invention.
• FIG. 2 is a diagram showing an embodiment of an imaging sequence employed in the MRI apparatus of the present invention.
• FIGS. 3A and 3B are diagrams showing a burst wave employed in the sequence of FIG. 2, and FIG. 3C is a diagram showing a region excited by a burst wave.
• FIG. 4 is a diagram showing the overall configuration of an MRI apparatus to which the present invention is applied.
• FIG. 5 is a diagram schematically showing a phase map obtained by the MRI apparatus of the present invention.
• FIG. 6 is a diagram showing another embodiment of the imaging sequence employed in the MRI apparatus of the present invention.
• FIG. 7 is a diagram showing another embodiment of the imaging sequence employed in the MRI apparatus of the present invention.
• FIG. 8 is a diagram showing another embodiment of the imaging sequence employed in the MRI apparatus of the present invention.
• FIG. 4 is a schematic configuration diagram of a magnetic resonance diagnostic apparatus to which the present invention is applied.
• the MRI apparatus includes a static magnetic field generating magnetic circuit 402 having an electromagnet or a permanent magnet for generating a uniform static magnetic field Bo inside a subject 401 as a magnetic field generating means, and a high frequency magnetic field. And a gradient magnetic field coil 409 that generates gradient magnetic fields Gx, Gy, and Gz whose intensities linearly change in three orthogonal directions x, y, and z.
• the gradient coil 409 is connected to a power supply 410 for supplying a current to the gradient coil.
• a detection coil 414b for detecting an MR signal generated from the subject 401 as a detection means, a computer 408 for performing various calculations for image reconstruction on the MR signal, and a storage device for storing the calculation results (424 to 427)
• a signal processing system 406 having a display 428 for displaying, and an operation unit 421 including a keyboard 422 and a mouse 423 for inputting to a computer 408 and the like.
• the coils 414a and 414b may be separate for transmission and reception as shown in the figure, but may be coils for both purposes.
• the computer 408 also functions as a control unit that controls each magnetic field generating unit and the detecting unit, and controls the gradient magnetic field generating system 403 (the gradient magnetic field coil 409 and its power supply 410), the transmitting system 404, and the detecting system 405 via the sequencer 407. Control each operation.
• the high frequency generated by the synthesizer 411 is modulated by the modulator 412 at the timing controlled by the sequencer 407, amplified by the power amplifier 413, and supplied to the coil 414a.
• a high-frequency magnetic field is generated inside the subject 401 to excite nuclear spins.
• a series of high-frequency pulses composed of a plurality of sub-pulses, ie, burst waves, are generated as the high-frequency magnetic field.
• Nucleus shall be the subject E 1 ⁇ other, is a 3 1 P, 1, etc., in a temperature measurement of the present invention is directed to water protons.
• the gradient magnetic field coil 410 is driven via the gradient magnetic field power supply 410 to apply the respective gradient magnetic fields in the slice direction, the phase encode direction, and the frequency encode direction, and select a region (slice) for exciting nuclear spins.
• the generated nuclear magnetic resonance signal is phase encoded and Z or frequency encoded.
• the MR signal emitted from the subject 401 is received by a coil 414b, passes through an amplifier 415, is subjected to quadrature phase detection by a detector 416, and is input to a computer 408 via an A / D converter 417.
• the computer 408 displays on the CRT display 428 an image corresponding to the density distribution of the nuclear spins, the density distribution to which contrast is applied by relaxation, the spectrum distribution, and the like.
• the tissue-image image is a density distribution of the nuclear spins, information, for example, color display showing the temperature distribution of the phase image and the z or tissue 5 View by. Note that the data being calculated or the final data is stored in the memories 424 and 425 '.
• the above-described gradient magnetic field generation system 403, transmission system 404, and detection system 405 are controlled by a sequencer 407 in accordance with a pulse sequence determined according to the purpose of measurement, and the sequencer 407 is controlled by a computer 408.
• the computer 408 executes a sequence using a burst wave and obtains a phase distribution by calculating a detected MR signal. Further, measurement is performed to determine the phase distribution at two different times, the temperature distribution of the tissue is determined from the difference between the obtained phase distributions, and this is displayed on the display 428.
• Figure 1 shows the temperature measurement procedure.Temperature measurement is performed at least before laser ablation, that is, before temperature change and during or after laser ablation processing. Take two measurements. In each measurement, measurement of data by executing an imaging sequence (17, 11), Fourier transformation of measurement data (18, 12), creation of phase map (19, 13), and phase unwrap (unwrap) processing (101 , 14).
• phase difference map is created from the phase maps obtained by these two measurements (15), and a temperature difference map is created and displayed from this (16).
• FIG. 2 shows an embodiment of the imaging sequence executed in steps (17, 11).
• RF represents a high-frequency magnetic field pulse
• Gz, Gy, and Gx represent gradient magnetic field pulses in three orthogonal directions
• Sig represents an echo signal
• Gz is a slice direction
• Gy is a phase encode direction
• Gx is a gradient magnetic field in the direction of the door.
• the RF burst 91 is a series of RF pulses composed of a plurality of sub-pulses p as shown in FIG. 3A, where the amplitude of the sub-pulses is set so that the flip angle of the entire burst is 90 degrees. . Due to such RF burst 91 and gradient magnetic field Gx95, the x-direction scan is performed as shown in Fig. 3C. Only the magnetization of the trip-like region 301 is excited.
• the width of the RF burst sub-pulse is 200 ⁇ s
• the intensity of Gx applied simultaneously with RF is 23.3 mT / m
• ⁇ X (5 hidden) xGx 5 kHz
• magnetization is excited at 5 ° intervals in the X direction.
• an inversion (180.) RF pulse 92 is applied.
• a gradient magnetic field Gz93 is applied simultaneously with the application of the inversion RF pulse 92, and a slice is selected.
• the signal is measured while applying the gradient magnetic field Gx96 in the readout direction having the same sign as the gradient magnetic field Gx95 applied during the transverse magnetization excitation.
• the magnetization in the strip region 301 (Fig.
• the application of the inversion RF pulse 92 causes the magnetization to be inverted, and a spin echo occurs at a time 99 after a lapse of TE / 2 from the application, but a time at which an echo at the center of the gradient magnetic field echo 97 occurs.
• the application timing of the readout gradient magnetic field Gx96 is set so that 98 differs from the spin echo generation time 99 by time t0.
• the gradient magnetic field Gx95 and the gradient magnetic field Gx96 have the same sign, the order of generation of the echoes corresponding to the sub-pulses of the burst wave is switched with the order of application of the sub-pulses.
• the subpulses A, B, and C in Fig. 2 correspond to the echoes a, b, and c because they converge again with the gradient magnetic field Gx96 having the same effect as the gradient magnetic field Gx95.
• each echo and deviation also occur at the time difference between the time of the spin echo and tO.
• each echo is given a phase rotation proportional to the static magnetic field inhomogeneity Es (x, y) and tO including the chemical shift. Therefore, by performing a Fourier transform on these echo signals, a phase image can be obtained for the selected slice plane (steps (18, 12)).
• each echo is expressed by the following equation (3) 5 It is represented by The phase component is represented by the exp term of the imaginary factor.
• equation (3) can be approximated by equation (4).
• the phase map is obtained by performing a Fourier transform on the measurement signal (complex number) subjected to quadrature phase detection and calculating a signed arctan (imaginary part / real part) from the real part and imaginary part of the pixel value (Fig. 1 Steps (19, 13)).
• the phase map (phase image) thus obtained is schematically shown in FIG. In FIG. 5, a region 27 is a portion where the temperature has increased due to laser ablation or the like, and the phase is largely changed due to a temperature change as compared with the peripheral region.
• phase unwrap process is performed (steps (101, 14) in Fig. 1).
• a reference point (xo, yo) of the phase map is determined, processing is started from the phase reference point, and 2 ⁇ is added or subtracted when there is a change of 2 ⁇ or more between adjacent pixels. To reduce change.
• a temperature map is created from the phase map.
• the imaging The steps from sequence execution, Fourier transform of measurement data, creation of phase map, and phase unwrapping process are performed at least twice before and after temperature change (or during temperature change). Then, a phase difference map ⁇ 0 (x, y) is created (step 15 in FIG. 1). A temperature difference map AT (x, y) is created by calculation from this phase difference map (step 16).
• the calculation for creating the temperature difference from the phase difference is as follows.
• Equation (1) yt o E s (x, y) -7t Q B 0 A 6 (6)
• the temperature change ⁇ can be calculated from Equations (1) and (2) as follows: Is the temperature dependence of the chemical shift [0.01 ppm / ° C]), the temperature change ⁇ is obtained from the phase change ⁇ in equation (6) by equation (7).
• a 7t o B o Display the temperature difference map thus obtained on the display (step 16).
• the display of the temperature difference is preferably displayed so as to be superimposed on a tissue image acquired in advance.
• a method of displaying the temperature difference map for example, a color bar to which an arbitrary color is assigned for each predetermined temperature width is prepared, and the temperature of a certain pixel is displayed in a color corresponding to the temperature of the color bar.
• a temperature of 30 ° C to 200 ° C at 10 ° C increments is assigned to the rubber that changes its hue sequentially, for example, blue-green-yellow-orange-red, and the temperature of a certain pixel is 100 ° C (or If the difference from the temperature before the temperature change is 74 ° C), the color of the color bar corresponding to that temperature (temperature difference), for example, orange, is superimposed on the pixel brightness and displayed.
• the temperature display method in addition to the above-described color display, a method of changing the gradation with the same hue, using an isotherm, or the like can be adopted.In each case, the temperature is superimposed on the tissue image. It is desirable to display. Thereby, the site where the temperature change occurs can be confirmed.
• a high-speed sequence using a burst wave is employed as an imaging sequence, a gradient magnetic field echo is generated at a time different from that of the spin echo, and this is measured and calculated.
• a two-dimensional phase map can be created in less than 0.1 second. Therefore, a two-dimensional temperature map can be measured and displayed almost in real time during IVMR operation.
• the sequence in which the phase encoding gradient magnetic field Gy94 is applied during echo measurement is shown as the imaging sequence.However, as shown in FIG. 6, the phase encoding Gy61 can be performed even when the transverse magnetization is excited. Also in this case, the same effect as that of the embodiment of FIG. 2 can be obtained.
• the sequence in FIG. 6 is the same as the sequence in FIG. 2 except that the timing for applying the phase-encoding gradient magnetic field Gy61 is different.
• a three-dimensional measurement can be performed by adding a phase encoder droop in the slice direction.
• FIG. 7 shows a sequence of three-dimensional measurement as a second embodiment of the present invention.
• This sequence is basically the same as the sequence shown in FIG. 2, and the same pulse is indicated by the same number.
• the slice direction phase encoding gradient magnetic field Gz71 is applied between the burst wave 91 and the inversion RF pulse 92.
• the gradient magnetic field Gz93 applied together with the inversion RF pulse 92 is a gradient magnetic field for selecting a region (slab) including the entire slice to be measured here.
• This sequence is repeated at the repetition time TR while changing the intensity of the phase encode gradient magnetic field Gz71 in the slice direction to obtain three-dimensional measurement data.
• This measurement data is data having a phase sensitivity as in the case of the two-dimensional measurement, and a three-dimensional phase map can be obtained by performing a three-dimensional Fourier transform on the data.
• phase difference map and a temperature difference map are created from the obtained three-dimensional phase map and the temperature is displayed in the same way as in two-dimensional measurement.
• the magnetization excited by applying the readout direction gradient magnetic field Gx95 simultaneously with the RF burst 91 is the magnetization of the strip-like region 301 in the X direction as shown in FIG. 3C. Therefore, the magnetization existing in the region 302 between the regions 301 is not excited, and can be excited by shifting the burst frequency.
• the frequency is shifted by, for example, 1 / (the number of pixels in the ux readout direction) (Hz) for each excitation.
• three-dimensional measurement assuming that the echo time TE is 40 ms, the repetition time TR is 60 ms, and the number of phase steps in the slice direction (the number of pixels in the slice direction) is 16, three-dimensional measurement can be performed in about 1 second.
• the method of shifting the carrier frequency of the burst 91 for each excitation can be applied to not only three-dimensional measurement but also two-dimensional measurement. However, in the case of such a measurement involving repetition of the sequence, the unexcited longitudinal magnetization is reduced by It is important to note that the phase of the signal is inverted because the direction changes to + z and -z directions for each vehicle.
• the readout gradient magnetic field Gx96 has a different sign from the gradient magnetic field Gx95 applied simultaneously with the burst wave 91. Since no spin echo is generated in principle, TE can be set arbitrarily. Since the phase of the spin changes in proportion to TE, the phase sensitivity can be increased by setting TE longer. However, looking at the individual echoes that make up the echo train 97, the time from the excitation RF pulse 91 to the generation of the echo is different, and TE is different in the ky axis direction on k space (phase space). However, if the total length of the echo train is shorter than the effective TE (the TE of the center echo a), that is, if TE >> td, the difference in TE between individual echoes can be ignored approximately.
• the phase encoding in the Gy direction may be performed at the time of exciting the longitudinal magnetization or at the time of measuring the echo. Also, by shifting the carrier frequency of the excitation burst for each excitation to excite the unexcited longitudinal magnetization, the repetition time TR can be made shorter than the normal longitudinal relaxation time T1.
• burst wave used in the present invention various modifications other than the pulse train having a constant amplitude as shown in FIGS. 3A and 3B can be used.
• a technique for improving the signal strength by performing appropriate frequency modulation or amplitude modulation on the excitation burst and widening the pulse width of the excitation spectrum may be used in combination.
• phase distribution can be used.
• phase wrapping occurs, but if the time difference before and after the temperature change is small, the phase change will be less than 2 ⁇ , and no phase wrapping will occur. In such a case, the phase distribution before conversion to the temperature distribution can also be used for the temperature monitor.
• ⁇ in Equation 1 represents a change in chemical shift due to a temperature change from the ambient temperature where the subject is placed.
• a phase map can be obtained by the sequence steps 17, 18, 19, 101 or 11, 12, 13, and 14 in FIG. 1, and an image representing a temperature change can be created and displayed from the phase map. .

Landscapes

• Health & Medical Sciences (AREA)
• Physics & Mathematics (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Animal Behavior & Ethology (AREA)
• Veterinary Medicine (AREA)
• Biophysics (AREA)
• Pathology (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Biomedical Technology (AREA)
• Heart & Thoracic Surgery (AREA)
• Medical Informatics (AREA)
• Molecular Biology (AREA)
• Surgery (AREA)
• High Energy & Nuclear Physics (AREA)
• General Health & Medical Sciences (AREA)
• Public Health (AREA)
• Radiology & Medical Imaging (AREA)
• Mathematical Physics (AREA)
• Artificial Intelligence (AREA)
• Computer Vision & Pattern Recognition (AREA)
• Physiology (AREA)
• Psychiatry (AREA)
• Signal Processing (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)
• Image Processing (AREA)
• Image Analysis (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=18368976

Family Applications (1)

Country Status (3)

Cited By (1)

Families Citing this family (26)

Citations (5)

Family Cites Families (1)

• 1998
  • 1998-12-03 JP JP34440198A patent/JP4318774B2/ja not_active Expired - Fee Related
• 1999
  • 1999-12-03 WO PCT/JP1999/006797 patent/WO2000032107A1/ja not_active Ceased
  • 1999-12-03 US US09/857,304 patent/US6445183B1/en not_active Expired - Fee Related

Patent Citations (5)

Also Published As

Similar Documents

Legal Events