JP5718148B2 - Magnetic resonance imaging apparatus and dual slice measurement method - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to a technique for reducing a luminance difference between slice images obtained by spin echo dual slice measurement.

  MRI equipment measures the NMR (echo) signal generated by the nuclear spins that make up the body of the subject, especially the human body, and the shape and function of the head, abdomen, limbs, etc. in two or three dimensions. A device for imaging. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, 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.

  Among the imaging methods in the MRI apparatus is an imaging method called Dual Slice measurement. This dual slice measurement is a function of imaging twice the number of slices in one measurement, and excites two slices simultaneously to detect one signal from the two slices. When two slices are excited simultaneously, two different carriers (f1, θ1) and (f2, θ2) corresponding to each slice are mixed (combined) and amplitude-modulated with a Sinc waveform to form a combined excitation pulse. An excitation pulse is emitted from the RF coil. Here, (f1, θ1) represents the excitation frequency and irradiation phase of slice 1, and (f2, θ2) represents the excitation frequency and irradiation phase of slice 2. At this time, a value obtained by inverting the value of θ2 by 180 degrees in the first time and the second time is set. The image can be separated into echo signals of two slices by adding and subtracting the obtained NMR signal by one-dimensional Fourier transform, and further, each echo signal of each slice is obtained by two-dimensional Fourier transform. An image is obtained.

  In the spin echo dual slice measurement, the NMR signal excited by the 90 degree excitation pulse is refocused by the 180 degree refocusing pulse. At that time, for the 90-degree excitation pulse, two slices are excited separately, but for the 180-degree refocusing pulse, the entire region including the two slices is excited. For this reason, the gradient magnetic field strength for slice selection differs between the 90-degree excitation pulse and the 180-degree refocusing pulse, and the gradient magnetic field strength of the 180-degree slice selection gradient magnetic field is lower than that of 90 degrees.

  On the other hand, it is known that an eddy current is generated by application of a gradient magnetic field pulse, and the magnitude of the generated eddy current varies depending on the applied gradient magnetic field intensity. As a method of reducing this eddy current, a correction current is applied in real time to shim coils, localized coils, or gradient magnetic field pulse generation systems based on eddy current characteristic data measured at the time of installation in advance. (Patent Document 1 and Patent Document 2) have been proposed.

JP 2004-261591 A JP 2008-22877

  Incorporating shim coils and localized coils into the system is a major factor in increasing costs, so it is particularly necessary not to install a B0 coil to apply a magnetic field pulse (B0 pulse) with a uniform spatial distribution in the imaging space. It is desired for an inexpensive MRI system.

  However, when the B0 coil is not installed, the B0 component of the incorrect magnetic field caused by the eddy current generated when the gradient magnetic field is applied cannot be corrected, and the B0 component of the incorrect magnetic field caused by the eddy current is relatively large. In, the static magnetic field intensity in the imaging space deviates from the original resonance frequency. In addition, in spin echo dual slice measurement, the slice selection gradient magnetic field strength when applying a 90-degree excitation pulse and a 180-degree refocusing pulse is different, so the B0 component of the incorrect magnetic field accompanying the generated eddy current is different and the 90-degree excitation is excited. The amount of offset between the excitation frequency of the pulse and the excitation frequency of the 180 ° refocusing pulse is different. As a result, the region re-converged with the 180-degree refocusing pulse shifts from the position excited with the 90-degree excitation pulse, and the brightness of one slice decreases relative to the brightness of the other slice. There remains an unsolved problem that a signal difference (luminance difference) occurs between images.

  Accordingly, an object of the present invention has been made in view of the above problems, and MRI capable of suppressing a luminance difference between slice images generated in spin echo system dual slice measurement without depending on the B0 coil. It is to provide a device and a Dual Slice measurement method.

  In order to solve the above problems, the present invention provides a first excitation pulse and a second slice for exciting the first slice according to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field. Calculate the frequency offset amount of at least one of the frequency of the second excitation pulse to be excited or the frequency of the refocusing pulse, and the frequency of the first excitation pulse and the second excitation pulse or the refocusing pulse. Dual Slice measurement is performed by adding a frequency offset amount to at least one of the frequencies.

  Specifically, the MRI apparatus of the present invention includes excitation high-frequency magnetic field application means for applying a high-frequency magnetic field to a subject placed in a static magnetic field, and gradient magnetic field application means for applying a gradient magnetic field to the subject. Applying a gradient magnetic field to the subject and combining a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice; a first excitation pulse; Dual Slice measurement control means for controlling Dual Slice measurement that applies a refocusing pulse that inverts both the slice and the second slice, and for the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field Accordingly, a Dual Slice measurement control unit is provided that includes a frequency offset amount calculation unit that calculates a frequency offset amount of at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. Is Dual Slice measurement is performed by adding a frequency offset amount to at least one of the frequency of the first excitation pulse and the second excitation pulse or the frequency of the refocusing pulse.

  In addition, the Dual Slice measurement method of the present invention, the frequency of the first excitation pulse and the second excitation pulse, or the refocusing pulse according to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field A frequency offset calculation step for calculating a frequency offset amount of at least one of the frequencies, and a frequency offset amount at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. And a step of performing Dual Slice measurement.

  According to the MRI apparatus and the Dual Slice measurement method of the present invention, it is possible to suppress the luminance difference between slice images generated in the spin echo system Dual Slice measurement regardless of the B0 coil. As a result, the B0 coil need not be installed, and the cost of the MRI apparatus can be reduced.

The block diagram which shows the whole structure of one Example of the MRI apparatus which concerns on this invention Timing chart of an example of pulse sequence for spin echo system dual slice measurement Diagram showing the state of the B0 component of the eddy current that contributes to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the slice selective gradient magnetic field Diagram for explaining the reason why a brightness difference occurs between slice images in spin echo dual slice measurement in an MRI apparatus that does not have a B0 coil or does not perform B0 component correction Functional block diagram showing each function of the CPU 8 according to the first embodiment The flowchart which shows the outline | summary of the processing flow of 1st Embodiment 6 is a flowchart showing a detailed processing flow of step 603 in FIG. 6 in the first embodiment. Functional block diagram showing each function of the CPU 8 according to the second embodiment A flowchart showing a detailed processing flow of step 603 of FIG. 6 in the second embodiment.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, A reception system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8 are provided.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generating system 3 includes a gradient magnetic field coil 9 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil. The gradient magnetic fields Gx, Gy, Gz are applied in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 described later. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the CPU 8 to collect tomographic image data of the subject 1. Various commands necessary for the transmission are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with RF pulses in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, and a high frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying to the high frequency coil 14a, the subject 1 is irradiated with the RF pulse.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b on the receiving side and a signal amplifier 15 And a quadrature phase detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The signal is divided into two orthogonal signals by the quadrature detector 16 at the timing according to the command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 (the echo signal converted into the digital quantity is echoed). Data)) and sent to the signal processing system 7.

  The signal processing system 7 performs various data processing and display and storage of processing results, and has an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 composed of a CRT, etc. When the echo data is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20, and the magnetic disk of the external storage device Record at 18 mag.

  The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed in the signal processing system 7, and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In FIG. 1, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side face the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted, in the case of the vertical magnetic field method. If the horizontal magnetic field method is used, the subject 1 is installed so as to surround it. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

(Dual Slice measurement sequence of spin echo system)
Next, a spin echo system dual slice measurement sequence according to the present invention will be described with reference to FIG. FIG. 2 is a timing chart of an example of a pulse sequence of spin echo system dual slice measurement. From the top, RF pulse is 201, which is a high-frequency magnetic field pulse, slice selection gradient magnetic field pulse Gs for slice selection: 202, phase encoding. The shapes of the phase encoding gradient magnetic field pulse Gp: 203 for frequency encoding and the frequency encoding gradient magnetic field pulse Gf: 204 for frequency encoding are respectively shown.

  In Dual Slice measurement, as described above, the 90 degree (synthetic) excitation pulse: 205 excites two adjacent slices separately, while the 180 refocusing pulse: 206 excites a region including two adjacent slices. For this reason, the gradient magnetic field strength is weaker in the slice selective gradient magnetic field strength: 208 of the 180-degree refocusing pulse than in the slice selective gradient magnetic field strength: 207 of the 90-degree excitation pulse.

  FIG. 3 shows the state of the B0 component of the eddy current that contributes to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the slice selective gradient magnetic field. For the sake of simplification, FIG. 3 shows the state of the B0 component 301 of the eddy current when a gradient magnetic field is applied only in the slice direction. Since the magnitude of the generated eddy current varies depending on the intensity of the slice selective gradient magnetic field, the magnitude of the B0 component 301 also varies. Since the intensity of the slice selection gradient magnetic field when applying the 90 degree excitation pulse is larger than the intensity of the slice selection gradient magnetic field when applying the 180 degree refocusing pulse, the B0 component 302 of the eddy current when applying the 90 degree excitation pulse is 180 It is understood that the eddy current is larger than the B0 component 303 when the refocusing pulse is applied. Therefore, in an MRI apparatus that is not equipped with a B0 coil or that does not correct the B0 component of eddy current, the influence of the B0 component of eddy current cannot be suppressed, and depending on the intensity of the slice selection gradient magnetic field, The B0 component of the incorrect magnetic field due to the eddy current remains, and the static magnetic field strength is shifted by the B0 component of the incorrect magnetic field.

  Based on the above, FIG. 4 shows the reason why a luminance difference occurs between slice images in a dual echo measurement of a spin echo system in an MRI apparatus that does not have a B0 coil or does not perform B0 component correction. It explains using. Correction of B0 component of incorrect magnetic field generated by eddy current by B0 coil: When 401, imaging slice position set by operator: 403, 90 degree excitation pulse excitation position: 404, 180 degree refocusing The pulse excitation position 405 does not shift. However, when the B0 component is not corrected by the B0 coil of the incorrect magnetic field generated by the eddy current: 402, the excitation position is shifted because the B0 component cannot be corrected with respect to the imaging slice position: 403. Furthermore, since the magnitude of the B0 component of the incorrect magnetic field generated by the eddy current differs between when applying a 90-degree excitation pulse and when applying a 180-degree refocusing pulse, when applying a 90-degree excitation pulse and when applying a 180-degree refocusing pulse Therefore, the displacement of the excitation position is different. For this reason, one of the imaging slice positions is completely refocused by the 180-degree refocusing pulse, but the other is incompletely refocused, resulting in signal degradation. As a result, a luminance difference occurs between slice images.

(Outline of the present invention)
The MRI apparatus and the Dual Slice measurement method of the present invention provide a first excitation pulse and a second slice for exciting the first slice according to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field. Calculate the frequency offset amount of at least one of the frequency of the second excitation pulse to be excited or the frequency of the refocusing pulse, and the frequency of the first excitation pulse and the second excitation pulse or the refocusing pulse. Dual Slice measurement is performed by adding a frequency offset amount to at least one of the frequencies.

  The frequency offset amount is determined so that the excitation region of the refocusing pulse includes the first slice and the second slice. Alternatively, the signal difference between the signal from the first slice and the signal from the second slice is determined to be zero or minimum. Alternatively, the frequency offset amount is determined in accordance with the difference between the intensity of the slice selection gradient magnetic field when the synthetic excitation pulse is applied and the intensity of the slice selection gradient magnetic field when the refocusing pulse is applied.

Hereinafter, each embodiment of the present invention will be described in detail.
(First embodiment)
First, a first embodiment of the MRI apparatus and dual slice measurement method of the present invention will be described. In the present embodiment, prior to the main measurement of Dual Slice measurement, pre-measurement (pre-scan) is performed, and using the echo data obtained by this pre-measurement, the signal difference between slices based on the displacement of the excitation position is zero or The minimum frequency offset amount is obtained, and the actual measurement of Dual Slice measurement is performed using the obtained frequency offset amount. Hereinafter, the present embodiment will be described in detail with reference to FIG.

  First, each function of the CPU 8 related to Dual Slice measurement of the present embodiment will be described based on the functional block diagram shown in FIG. The CPU 8 according to the present embodiment includes an imaging parameter setting unit 501, a frequency offset amount calculation unit 502, and a dual slice measurement setting unit 508.

  The imaging parameter setting unit 501 displays on the display 20 a GUI for accepting the selection of spin echo dual slice measurement and the setting of imaging parameters necessary for the dual slice measurement. A setting or change input is accepted, and the value of the input imaging parameter is notified to the frequency offset amount calculation unit 502 and the dual slice measurement setting unit 508.

The frequency offset amount calculation unit 502 obtains an offset amount of the excitation frequency for making the excitation position deviation between the two slices zero or minimum in the dual slice measurement. Specifically, the frequency of the 90-degree excitation pulse for slice 1 (f1), the frequency offset amount added to the frequency of the 90-degree excitation pulse for slice 2 (f2) (f ⁹⁰ _offset ), and 180-degree reconvergence frequency offset to be added to the frequency (f ₁₈₀₎ of the pulse and (f ¹⁸⁰ _offset), at least one of the, preferably, the frequency offset to be added to the frequency (f ₁₈₀₎ of 180 ° refocusing pulse (f ¹⁸⁰ _offset ) Only, and notifies the Dual Slice measurement setting unit 508. A detailed function for obtaining the frequency offset amount will be described later.

Dual Slice measurement setting section 508 adds the frequency offset amount obtained by frequency offset amount calculation section 502 to the frequency of the RF pulse, and executes Dual Slice measurement. Specifically, when the frequency offset amount (f ⁹⁰ _offset ) to be added to the 90-degree excitation pulse is obtained, the frequency of the 90-degree excitation pulse in slice 1 is set to (f1 + f ⁹⁰ _offset ) and 90 degrees in slice 2 the frequency of the excitation pulse and (f2 + f ⁹⁰ _offset). Further, when the frequency offset amount (f ¹⁸⁰ _offset ) to be added to the frequency (f ₁₈₀ ) of the 180-degree refocus pulse is obtained, the frequency of the 180-degree re-focus pulse is set to (f ₁₈₀ + f ¹⁸⁰ _offset ). Then, using the excitation frequency set in this way and the value of the imaging parameter notified from the imaging parameter setting unit 501, specific control data for performing Dual Slice measurement is generated and notified to the sequencer 4. Then, the sequencer 4 is caused to execute Dual Slice measurement.

  Next, detailed functions of the frequency offset amount calculation unit 502 will be described with reference to a functional block diagram shown in FIG. The frequency offset amount calculation unit 502 includes a search frequency offset amount setting unit 503, a pre-measurement execution unit 504, a Fourier transform unit 505, an inter-slice signal difference calculation unit 506, and an optimum frequency offset amount calculation unit 507. Have.

The search frequency offset amount setting unit 503 notifies the pre-measurement execution unit 504 of the search frequency offset amount (Δf) for the pre-measurement for examining the displacement amount of the excitation position in the dual slice measurement. Frequency offset for the search (Delta] f) can be set frequency offset of 90 ° excitation pulse and (Delta] f _90), either one or both of the frequency offset of 180 ° refocusing pulse (Delta] f ₁₈₀₎ . Preferably, only the frequency offset amount (Δf ₁₈₀ ) of the 180-degree refocusing pulse is set.

The pre-measurement execution unit 504 performs pre-measurement for adjusting the excitation frequency in the dual slice measurement for slice 1 and slice 2 using the pulse sequence in which the phase encoding is zero in the dual slice measurement sequence shown in FIG. To do. At that time, the frequency of the 90-degree excitation pulse for slice 1 is (f1 + Δf ₉₀ ), the frequency of the 90-degree excitation pulse for slice 2 is (f2 + Δf ₉₀ ), and these two 90-degree excitation pulses are synthesized, This is the combined excitation pulse in Dual Slice measurement. Further, the frequency of the 180-degree refocusing pulse is set to (f ₁₈₀ + Δf ₁₈₀ ). Then, using the value of the imaging parameter set by the imaging parameter setting unit 501, the pulse sequence control data for the pre-measurement is specifically generated, notified to the sequencer 4, and the pre-measurement is executed to the sequencer 4. Let Then, the sequencer 4 performs two measurements with the irradiation phase (θ2) of the 90-degree excitation pulse for slice 2 differing by 180 degrees, measures the echo signal, and notifies the pre-measurement execution unit 504 of the acquired echo data To do. The pre-measurement execution unit 504 notifies the Fourier transform unit 505 of the acquired echo data of the two measurements.

  The Fourier transform unit 505 performs one-dimensional Fourier transform on the acquired two-time echo data in the slice direction for addition or subtraction, or directly adds and subtracts two echo data to perform Fourier transform, thereby obtaining slice 1 and slice. Separated into 2 echo data. Then, the echo data for each slice is further subjected to a one-dimensional Fourier transform in the frequency encoding direction, and the absolute values of the real part and imaginary part are added for each slice to obtain the maximum signal intensity of slice 1 and slice 2, respectively. Then, the inter-slice signal difference calculation unit 506 is notified.

  The inter-slice signal difference calculation unit 506 calculates the difference between the maximum signal strengths of the slice 1 and the slice 2 for each excitation position, calculates the inter-slice signal difference for each excitation position, and the inter-slice signal at all excitation positions. Calculate the average of the differences.

The optimum frequency offset amount calculation unit 507 performs linear approximation using the average value of the signal difference between slices for each frequency offset amount (Δf ₉₀ and Δf ₁₈₀ ), and calculates the frequency offset amount at which the signal difference between slices becomes 0 or the minimum. Set the optimal frequency offset amount. By using this optimum frequency offset amount, the excitation region of the refocusing pulse includes the first slice and the second slice.

  Next, the processing flow of the present embodiment, which is performed in cooperation with the functional units of the CPU 8, will be described with reference to the flowcharts shown in FIGS. This flowchart is stored in advance on the magnetic disk 18 as a Dual Slice measurement program, and is executed by being loaded into the memory of the CPU 8 and executed as necessary. Hereinafter, the processing of each step will be described.

  In step 601, reception gain adjustment is performed so that the received signal has an optimal signal strength.

  In step 602, the imaging parameter setting unit 501 displays on the display 20 a GUI for accepting selection of spin echo dual slice measurement and setting of imaging parameters necessary for the dual slice measurement. And selection or change input of imaging parameter values are accepted. Then, it is determined whether or not the operator's selection is spin echo system dual slice measurement. If dual slice measurement is selected, the process proceeds to step 603, and if not dual slice measurement, the process ends.

  In step 603, the frequency offset amount calculation unit 502 obtains a frequency offset amount at which the signal difference between slices based on the displacement of the excitation position in Dual Slice measurement is zero or minimum. Details will be described later.

In Step 604, the Dual Slice measurement setting unit 503 adds the frequency offset amount at which the signal difference between slices obtained in Step 603 is zero or minimum to the RF pulse, and performs the main measurement of Dual Slice measurement. When the frequency offset amount (f ⁹⁰ _offset ) of the 90-degree excitation pulse is obtained, the frequency of the 90-degree excitation pulse in slice 1 is (f1 + f ⁹⁰ _offset ), and the frequency of the 90-degree excitation pulse in slice 2 is (f 2 + f ⁹⁰ _offset ), these two 90-degree excitation pulses are combined to form a combined excitation pulse in Dual Slice measurement. Further, when the frequency offset amount (f ¹⁸⁰ _offset ) of the 180-degree refocusing pulse is obtained, the frequency of the 180-degree refocusing pulse is set to (f ₁₈₀ + f ¹⁸⁰ _offset ). In this way, the frequency of the RF pulse is set, the main measurement of Dual Slice measurement is performed, and images of slice 1 and slice 2 are acquired. By using this frequency offset amount (f ⁹⁰ _offset , f ¹⁸⁰ _offset ), the excitation frequency shift due to the B0 component of the incorrect magnetic field due to the eddy current is corrected, and in the slice slice measurement of the spin echo system, two slices having no luminance difference An image can be obtained.
The above is the outline of the processing flow of the present embodiment.

  Next, details of the processing for obtaining the frequency offset amount at which the signal difference between slices is zero or minimized performed in step 603 will be described.

In step 611, the search frequency offset amount setting unit 503 sets the search frequency offset amount (Δf) in order to examine the displacement amount of the excitation position in the spin echo dual slice measurement. Specifically, to set the frequency offset of 90 ° excitation pulse and (Delta] f _90), either one or both of the frequency offset of 180 ° refocusing pulse (Delta] f _180). Preferably, only the frequency offset amount (Δf ₁₈₀ ) of the 180-degree refocusing pulse is used.

In step 612, the pre-measurement execution unit 504 performs pre-measurement for dual slice excitation frequency adjustment for slice 1 and slice 2 using the frequency offset amounts (Δf ₉₀ , Δf ₁₈₀ ) set in step 611. . Details are as described above. By this step, the pre-measurement execution unit 504 acquires echo data by two measurements that are different in the irradiation phase (θ2) of the 90-degree excitation pulse for slice 2 by 180 degrees.

  In step 613, the Fourier transform unit 505 performs one-dimensional Fourier transform on the two echo data measured in step 612 in the slice direction to separate the echo data of slice 1 and slice 2. Details are as described above.

  In step 614, the Fourier transform unit 505 further performs one-dimensional Fourier transform on the echo data for each slice obtained in step 613 in the frequency encoding direction to obtain the signal intensity of slice 1 and slice 2. Details are as described above.

  In step 615, the inter-slice signal difference calculation unit 506 calculates the inter-slice signal difference by obtaining the difference in signal intensity between slice 1 and slice 2 obtained in step 614.

  In step 616, in the case of measurement of a plurality of Dual Slices with different excitation positions, the frequency offset amount calculation unit 502 checks whether or not the signal difference between slices at all excitation positions has been calculated, and slices at all excitation positions. If the inter-signal difference is not calculated, the process returns to step 613 to repeat the calculation process of the inter-slice signal difference at the next excitation position. If already calculated, the process proceeds to step 617. In the case of Dual Slice measurement at one excitation position, this step, the next step, and the next step 617 are omitted, and the process proceeds to step 618.

  In step 617, the inter-slice signal difference calculation unit 506 calculates an average value of the inter-slice signal differences at all excitation positions obtained in steps 612 to 616.

In step 618, the frequency offset amount calculation unit 502 checks whether or not the calculation of the average value of the signal difference between slices in all the frequency offset amounts for search (Δf ₉₀ , Δf ₁₈₀ ) has been completed, and must be completed. For example, returning to step 611, the frequency offset amount (Δf ₉₀ , Δf ₁₈₀ ) is changed, and the calculation of the average value of the signal difference between the slices of the frequency offset amount is repeated.

In step 619, the optimum frequency offset amount calculation unit 507 performs linear approximation using the average value of the signal difference between slices for each frequency offset amount obtained in step 611 to step 618, and the signal difference between slices is 0 or minimum. An optimal frequency offset amount (f ⁹⁰ _offset , f ¹⁸⁰ _offset ) is calculated.

These optimal frequency offset amounts (f ⁹⁰ _offset , f ¹⁸⁰ _offset ) correspond to the difference between the strength of the slice selection gradient magnetic field when the synthetic excitation pulse is applied and the strength of the slice selection gradient magnetic field when the refocusing pulse is applied. Thus, by using the frequency offset amount (f ⁹⁰ _offset , f ¹⁸⁰ _offset ), the excitation region of the refocusing pulse includes the first slice and the second slice.
The details of the processing in step 603 have been described above.

  In the description of the processing in step 603 above, in step 612, the measurement is performed twice with the irradiation phase (θ2) of the 90-degree excitation pulse for slice 2 being 180 degrees apart, and the echo data of slice 1 and slice 2 are separated. However, the signals of slice 1 and slice 2 may be processed as a single unit without separation. An outline of the processing in step 603 in this case will be described below. In this description, a dash (') is added to each step number to clarify the correspondence.

  In step 611 ′, as in step 611, the search frequency offset amount setting unit 503 sets the search frequency offset amount (Δf) in order to investigate the displacement amount of the excitation position in the spin echo dual slice measurement. To do.

In step 612 ′, the pre-measurement execution unit 504 uses the frequency offset amount (Δf ₉₀ , Δf ₁₈₀ ) set in step 611 ′ to perform pre-measurement for dual slice excitation frequency adjustment for slice 1 and slice 2 I do. However, only one measurement is performed without performing two measurements in which the irradiation phase (θ2) of the 90-degree excitation pulse for slice 2 differs by 180 degrees.

  Step 613 is omitted.

  In step 614 ′, the Fourier transform unit 505 performs one-dimensional Fourier transform on the echo data obtained in step 612 ′ in the frequency encoding direction, and regards data that considers slice 1 and slice 2 as one body (hereinafter referred to as integral data). )

  Step 615 is omitted.

  In step 616 ′, in the case of a plurality of Dual Slice measurements with different excitation positions, the frequency offset amount calculation unit 502 confirms whether or not the integrated data of all excitation positions has been calculated, and the integrated data of all excitation positions. If not, the process returns to step 614 ′ to repeat the calculation process of the integrated data of the next excitation position. If already calculated, the process proceeds to step 617 ′. In the case of Dual Slice measurement at one excitation position, this step and the next step 617 'are omitted and the process proceeds to step 618'.

  In step 617 ′, the signal difference calculation unit 506 between slices calculates an average value of the integrated data at all the excitation positions obtained in steps 612 ′ to 616 ′.

In step 618 ′, the frequency offset amount calculation unit 502 checks whether or not the calculation of the average value of the integrated data for all the search frequency offset amounts (Δf ₉₀ , Δf ₁₈₀ ) has been completed. Returning to step 611 ′, the frequency offset amount (Δf ₉₀ , Δf ₁₈₀ ) is changed, and the calculation of the average value of the integrated data of the frequency offset amount is repeated.

In step 619 ′, the optimum frequency offset amount calculation unit 507 performs linear approximation using the average value of the integrated data for each frequency offset amount obtained in steps 611 ′ to 618 ′, and the optimal integrated data is maximized. The frequency offset amount (f ⁹⁰ _offset , f ¹⁸⁰ _offset ) is calculated.

  The above is the description of the processing in the case of processing the signals of slice 1 and slice 2 together without separating the echo data of slice 1 and slice 2.

  As described above, the MRI apparatus and the Dual Slice measurement method of the present embodiment obtain the frequency offset amount at which the signal difference between slices based on the displacement of the excitation position is zero or minimum in the previous measurement, and the obtained frequency offset amount Is added to the frequency of the 90-degree excitation pulse or 180-degree refocusing pulse for each slice, and the dual-slice measurement is performed. As a result, it is possible to correct the excitation frequency shift due to the B0 component of the incorrect magnetic field generated by the eddy current, and obtain two slice images having no brightness difference in the spin echo system dual slice measurement.

(Second embodiment)
Next, a second embodiment of the MRI apparatus and dual slice measurement method of the present invention will be described. This embodiment does not measure the excitation frequency deviation amount in the previous measurement described in the first embodiment, but changes the B0 component of the improper magnetic field generated by the eddy current accompanying the application of the gradient magnetic field over time. Is obtained in advance, and the frequency offset amount at which the signal difference between slices is zero or minimum is calculated using the pre-acquired eddy current characteristic data. Hereinafter, this embodiment will be described in detail with reference to FIGS.

In the present embodiment, the eddy current characteristic data is measured in advance when the MRI apparatus is installed, for example, by the method described in Patent Document 1, and the measured eddy current characteristic data is stored in the magnetic disk 18. And This eddy current characteristic data is a data representing the temporal change of the incorrect magnetic field (including the B0 component) due to the eddy current generated with the application of the unit gradient magnetic field pulse (G ₀ ), and is characteristic of the MRI apparatus. It is data. This eddy current characteristic data is expressed as a sum of exponential functions determined by a plurality of amplitudes and a set of time constants. The MRI apparatus and the Dual Slice measurement method of the present invention do not assume the B0 coil, but it is possible to virtually determine the correction current that flows through the B0 coil.

  First, each function of the CPU 8 related to Dual Slice measurement of the present embodiment will be described based on the functional block diagram shown in FIG. The CPU 8 according to this example includes an imaging parameter setting unit 501, a frequency offset amount calculation unit 502, and a dual slice measurement setting unit 508, as in the first embodiment described above. Only the specific function of the quantity calculation unit 502 is different. Therefore, only specific functions of the frequency offset amount calculation unit 502 will be described. The frequency offset amount calculation unit 502 includes a correction current calculation unit 801 and an excitation frequency deviation amount calculation unit 802.

The correction current calculation unit 801 includes a slice selection gradient magnetic field strength (G ₉₀ ) when a 90-degree excitation pulse is applied in an imaging condition, a slice selection gradient magnetic field strength (G ₁₈₀ ) when a 180-degree refocus pulse is applied, and a magnetic disk 18 in advance. Using the B0 component data in the eddy current characteristic data stored in the, current flows through the virtual B0 coil to correct the B0 component of the incorrect magnetic field due to the eddy current accompanying the application of each slice selective gradient magnetic field Calculate the correction current. The calculated virtual correction current is notified to the excitation frequency deviation amount calculation unit 802.

Here, the eddy current characteristic data is expressed as a sum of exponential functions determined by multiple amplitudes and a set of time constants. Each time constant is a slice selection when applying a 90-degree excitation pulse and a 180-degree refocusing pulse. It is sufficiently longer than the gradient magnetic field strength application time. For this reason, the correction current when applying a 90-degree excitation pulse is I ₉₀ [A], the correction current when applying a 180-degree refocusing pulse is I ₁₈₀ [A], and the current conversion factor for the gradient magnetic field strength to be applied is K ₁ , K Assuming ₂ , K ₃ ,..., The following equation can be approximated.

I ₉₀ = K ₁ × G ₉₀ + K ₂ × G ₉₀ + K ₃ × G ₉₀ + ・ ・ ・ ・ ----- (1)
I ₁₈₀ = K ₁ × G ₁₈₀ + K ₂ × G ₁₈₀ + K ₃ × G ₁₈₀ + ... ----- (2)
The excitation frequency deviation amount calculation unit 802 calculates the deviation amounts δf ₉₀ [Hz] and δf ₁₈₀ [Hz] of each excitation frequency from the correction currents I ₉₀ [A] and I ₁₈₀ [A] obtained by the correction current calculation unit 801. Is calculated. At this time, if the static magnetic field strength that changes when the current value I _B0 [A] is passed through the B0 coil is B _B0 [T], the correction currents I ₉₀ [A] and I ₁₈₀ [A] are passed through Expressions representing the changes in static magnetic field strength ΔB ₉₀ [T] and ΔB ₁₈₀ [T] are as follows.

式(3)、(4)とラーモア方程式より、求める励起周波数のずれ量δf₉₀[Hz]、δf₁₈₀[Hz]は以下となる。

From the equations (3) and (4) and the Larmor equation, the displacement amounts Δf ₉₀ [Hz] and Δf ₁₈₀ [Hz] of the excitation frequency to be obtained are as follows.

この励起周波数のずれ量を最適な周波数オフセット量(f⁹⁰ _offset、f¹⁸⁰ _offset)とする。

The amount of deviation of the excitation frequency is set as an optimum frequency offset amount (f ⁹⁰ _offset , f ¹⁸⁰ _offset ).

  Next, a processing flow of the present embodiment, which is performed in cooperation with each functional unit of the CPU 8, will be described based on a flowchart shown in FIG. This flowchart is stored in advance on the magnetic disk 18 as a Dual Slice measurement program, and is executed by being loaded into the memory of the CPU 8 and executed as necessary. Hereinafter, the processing of each step will be described.

  The processing flow of this embodiment is different only in the processing content of step 603 in the processing flow of the first embodiment shown in FIG. The details of the processing flow of step 603 in the present embodiment will be described below.

In step 901, the correction current calculation unit 801 reads B0 component data in the eddy current characteristic data stored in the magnetic disk 18. Then, using this B0 component data and the slice selection gradient magnetic field strength (G ₉₀ , G ₁₈₀ ) under the imaging condition determined by the value of the imaging parameter set in step 601, accompanying the application of each slice selection gradient magnetic field Correction currents (I ₉₀ [A], I ₁₈₀ [A]) that flow through the virtual B0 coil for correcting the B0 component of the incorrect magnetic field due to the eddy current are calculated. Details are as described above.

In step 902, the excitation frequency deviation amount calculation unit 802 uses the correction current (I ₉₀ [A], I ₁₈₀ [A]) that flows in the virtual B0 coil obtained in step 901 to generate the 90-degree excitation pulse. An excitation frequency deviation amount δf ₉₀ [Hz] and a 180-degree refocusing pulse excitation frequency deviation amount δf ₁₈₀ [Hz] are calculated. Details are as described above. These excitation frequency shift amounts Δf ₉₀ [Hz] and Δf ₁₈₀ [Hz] are optimum frequency offset amounts (f ⁹⁰ _offset , f ¹⁸⁰ _offset ).

The frequency of each RF pulse in Dual Slice measurement is synthesized with the frequency of the 90-degree excitation pulse in slice 1 as (f1 + δf ₉₀ ) and the frequency of the 90-degree excitation pulse in slice 2 as (f2 + δf ₉₀ ). Thus, the 90-degree excitation pulse in Dual Slice measurement is used. In addition, the excitation frequency of the 180-degree refocusing pulse is set to (f ₁₈₀ + Δf ₁₈₀ ). In this way, Dual Slice measurement is performed with the excitation frequency offset. As a result, it is possible to correct the excitation frequency shift due to the B0 component of the incorrect magnetic field generated by the eddy current and obtain two slice images having no luminance difference.

The above is the outline of the processing flow of the present embodiment. In the description of the above embodiment, the frequency offset of 90 ° excitation pulse and (f ⁹⁰ _offset), the frequency offset of 180 ° refocusing pulses has been described (f ¹⁸⁰ _offset) to seek, either Only the frequency offset amount of the excitation pulse may be obtained to correct only the frequency of the excitation pulse.

  As described above, the MRI apparatus and the Dual Slice measurement method of the present embodiment acquire in advance eddy current characteristic data including temporal change of the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field. The frequency offset amount at which the signal difference between slices is zero or minimum is calculated using the eddy current characteristic data acquired in advance. As a result, the frequency offset amount for correcting the excitation frequency shift due to the B0 component of the incorrect magnetic field generated by the eddy current can be obtained in a short time without performing the previous measurement, and in the dual slice measurement of the spin echo system, It is possible to obtain two slice images with no luminance difference.

  In each of the above-described embodiments, an example in which a 90-degree excitation pulse is used as the excitation pulse and a 180-degree refocusing pulse is used as the refocusing pulse has been described. However, the present invention is not limited to these angles. The excitation pulse may be an excitation pulse of any angle instead of 90 degrees, and the refocusing pulse may be a refocusing pulse of any angle instead of 180 degrees.

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 CPU, 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 high frequency oscillator, 12 Modulator, 13 High-frequency amplifier, 14a High-frequency coil (transmitting coil), 14b High-frequency coil (receiving coil), 15 signal amplifier, 16 quadrature detector, 17 A / D converter, 18 magnetic disk, 19 optical disk, 20 display, 21 ROM, 22 RAM, 23 Trackball or mouse, 24 keyboard, 25 operation unit

Claims (9)

A high-frequency magnetic field applying means for applying a high-frequency magnetic field to a subject placed in a static magnetic field;
A gradient magnetic field applying means for applying a gradient magnetic field to the subject;
With
A synthetic excitation pulse in which the gradient magnetic field is applied to the object and a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice are combined; Dual Slice measurement control means for controlling Dual Slice measurement for applying a refocusing pulse for exciting both one slice and the second slice;
A magnetic resonance imaging apparatus comprising:
According to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field, at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. A frequency offset amount calculation unit for calculating the frequency offset amount of the frequency is provided,
The frequency offset amount calculation unit obtains the frequency offset amount so that a signal difference between the signal from the first slice and the signal from the second slice is zero or minimum,
The Dual Slice measurement control means adds the frequency offset amount to at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse, and performs the Dual Slice measurement. Performing a magnetic resonance imaging apparatus. A high-frequency magnetic field applying means for applying a high-frequency magnetic field to a subject placed in a static magnetic field;
A gradient magnetic field applying means for applying a gradient magnetic field to the subject;
With
A synthetic excitation pulse in which the gradient magnetic field is applied to the object and a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice are combined; Dual Slice measurement control means for controlling Dual Slice measurement for applying a refocusing pulse for exciting both one slice and the second slice;
A magnetic resonance imaging apparatus comprising:
According to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field, at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. A frequency offset amount calculation unit for calculating the frequency offset amount of the frequency is provided,
The frequency offset amount calculation unit calculates the frequency offset amount corresponding to the difference between the strength of the slice selection gradient magnetic field when the synthetic excitation pulse is applied and the strength of the slice selection gradient magnetic field when the refocusing pulse is applied. Seeking
The Dual Slice measurement control means adds the frequency offset amount to at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse, and performs the Dual Slice measurement. Performing a magnetic resonance imaging apparatus. A high-frequency magnetic field applying means for applying a high-frequency magnetic field to a subject placed in a static magnetic field;
A gradient magnetic field applying means for applying a gradient magnetic field to the subject;
With
A synthetic excitation pulse in which the gradient magnetic field is applied to the object and a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice are combined; Dual Slice measurement control means for controlling Dual Slice measurement for applying a refocusing pulse for exciting both one slice and the second slice;
A magnetic resonance imaging apparatus comprising:
According to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field, at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. A frequency offset amount calculation unit for calculating the frequency offset amount of the frequency is provided,
The frequency offset amount calculation unit sets the first phase encoding to zero in the Dual Slice measurement, changes the frequency offset amount for search, and performs a plurality of pre-measurements to obtain a plurality of echo data. Determining a frequency offset amount at which a signal difference between the signal of the slice and the signal of the second slice is zero or minimum;
The Dual Slice measurement control means adds the frequency offset amount to at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse, and performs the Dual Slice measurement. Performing a magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to any one of claims 1 to 3 ,
The magnetic resonance imaging apparatus, wherein the frequency offset amount calculation unit obtains the frequency offset amount so that an excitation region of the refocusing pulse includes the first slice and the second slice . A high-frequency magnetic field applying means for applying a high-frequency magnetic field to a subject placed in a static magnetic field;
A gradient magnetic field applying means for applying a gradient magnetic field to the subject;
A synthetic excitation pulse in which the gradient magnetic field is applied to the object and a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice are combined; Dual Slice measurement control means for controlling Dual Slice measurement for applying a refocusing pulse for exciting both one slice and the second slice;
According to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field, at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. A frequency offset amount calculation unit for calculating the frequency offset amount of the frequency;
With
The Dual Slice measurement control means adds the frequency offset amount to at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse, and performs the Dual Slice measurement. A magnetic resonance imaging apparatus for performing
The frequency offset amount calculation unit
A search frequency offset amount setting unit for setting a search frequency offset amount;
The search frequency offset amount is added to at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse, and the phase encoding is set to zero. A pre-measurement execution unit that controls the pre-measurement to acquire two echo data by two measurements in which the phase of the first excitation pulse or the second excitation pulse is different by 180 degrees;
A Fourier transform unit that separates the echo data for each slice by performing addition / subtraction after the one-dimensional Fourier transform in the slice direction after the two echo data obtained in the previous measurement, and further Fourier transforms each echo data in the frequency encode direction When,
An inter-slice signal difference calculation unit for obtaining a signal difference between slices from data for each slice that is Fourier-transformed in the slice direction and the frequency encoding direction;
Based on the plurality of signal differences between slices obtained by changing the search frequency offset amount, an optimal frequency offset amount calculation unit that calculates a frequency offset amount at which the signal difference between slices is zero or minimum;
A magnetic resonance imaging apparatus comprising: A high-frequency magnetic field applying means for applying a high-frequency magnetic field to a subject placed in a static magnetic field;
A gradient magnetic field applying means for applying a gradient magnetic field to the subject;
A synthetic excitation pulse in which the gradient magnetic field is applied to the object and a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice are combined; Dual Slice measurement control means for controlling Dual Slice measurement for applying a refocusing pulse for exciting both one slice and the second slice;
According to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field, at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. A frequency offset amount calculation unit for calculating the frequency offset amount of the frequency;
With
The Dual Slice measurement control means adds the frequency offset amount to at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse, and performs the Dual Slice measurement. A magnetic resonance imaging apparatus for performing
The frequency offset amount calculation unit
A search frequency offset amount setting unit for setting a search frequency offset amount;
The search frequency offset amount is added to at least one of the frequency of the first excitation pulse and the second excitation pulse or the frequency of the refocusing pulse, the phase encoding is set to zero, and echo A pre-measurement execution unit for controlling pre-measurement before acquiring data;
A Fourier transform unit for Fourier transforming the echo data obtained in the previous measurement in the frequency encoding direction;
Based on a plurality of echo data Fourier-transformed in the frequency encoding direction obtained by changing the search frequency offset amount, an optimal frequency offset amount that maximizes the echo data Fourier-transformed in the frequency encoding direction is calculated. A frequency offset amount calculation unit;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1 .
The frequency offset amount calculation unit has a signal difference between slices of zero or zero based on eddy current characteristic data obtained in advance and including temporal changes in the B0 component of the incorrect magnetic field generated by the eddy current accompanying application of the gradient magnetic field. A magnetic resonance imaging apparatus that calculates a minimum frequency offset amount . The magnetic resonance imaging apparatus according to claim 7 , wherein the frequency offset amount calculation unit includes:
A correction current calculation unit for obtaining a correction current to be passed through a virtual B0 coil according to the B0 component of the incorrect magnetic field;
Based on the correction current, a frequency shift amount of the excitation pulse or the refocusing pulse is calculated, and an excitation frequency shift amount calculation unit using the frequency shift amount as the frequency offset amount;
Magnetic resonance imaging apparatus comprising: a. Synthetic excitation by applying a gradient magnetic field to a subject and synthesizing a first excitation pulse for exciting the first slice and a second excitation pulse for exciting the second slice using magnetic resonance imaging. A Dual Slice measurement method for performing Dual Slice measurement that applies a pulse and a refocusing pulse for exciting both the first slice and the second slice,
According to the B0 component of the incorrect magnetic field generated by the eddy current accompanying the application of the gradient magnetic field, at least one of the frequency of the first excitation pulse and the second excitation pulse, or the frequency of the refocusing pulse. A frequency offset calculating step for calculating a frequency offset amount of the frequency,
The frequency offset calculating step includes:
Obtaining the frequency offset amount so that the signal difference between the signal from the first slice and the signal from the second slice is zero or minimum, or
Corresponding to the difference between the intensity of the slice selective gradient magnetic field at the time of applying the synthetic excitation pulse and the intensity of the slice selective gradient magnetic field at the time of applying the refocusing pulse, the frequency offset amount is obtained, or
In the dual slice measurement, the phase encoding is set to zero, the frequency offset amount for search is changed, and a plurality of pre-measurements are performed, based on a plurality of echo data and the second slice signal and the second slice Find the frequency offset amount that makes the signal difference from the slice signal zero or minimum, or
Do one of the
The first excitation pulse and the second frequency of the excitation pulse, or by adding the frequency offset frequency, at least one of the refocusing pulses, and performing the Dual Slice measurement,
A Dual Slice measurement method characterized by comprising:

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