JP7076224B2 - Magnetic resonance imaging device - Google Patents

Description

The present invention relates to a magnetic resonance imaging (hereinafter referred to as "MRI") apparatus, and more particularly to an adjustment of irradiation gain at the time of prescan in an MRI apparatus.

Conventionally, a magnetic resonance (NMR) signal generated by applying a radio frequency (RF) pulse to a subject while the subject (particularly the human body) is placed in a uniform static magnetic field is measured to measure the subject. There is known an MRI apparatus that images the morphology and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. In the MRI device, the NMR signal is frequency-encoded with different phase encodings depending on the gradient magnetic field and measured as time-series data, and the measured NMR signal is re-converted as an image by two-dimensional or three-dimensional Fourier transform. It is composed.

In the MRI apparatus, the irradiation intensity of the RF pulse for generating the NMR signal is determined by the irradiation gain. This irradiation gain is adjusted to the optimum value for each subject by performing a prescan before the main imaging. More specifically, before the main imaging, imaging is performed a plurality of times while changing the irradiation gain, and the irradiation gain at which the obtained NMR signal is maximized is determined as the optimum irradiation gain for the subject.

Regarding the technique for adjusting the irradiation gain and the RF pulse in such an MRI apparatus, for example, in Patent Document 1, the NMR signal is measured and measured under a lead-out gradient magnetic field excited by the RF pulse. It is disclosed that the NMR signal is subjected to Fourier transform and the intensity of the RF pulse is adjusted so that the evaluation value P using the sensitivity profile of the RF coil is maximized.

Japanese Unexamined Patent Publication No. 2002-325742

However, in the irradiation gain adjusting method disclosed in Patent Document 1, the operation of measuring the NMR signal after exciting with the RF pulse transmitted from the RF coil and weighting with the sensitivity profile of the RF coil to obtain the evaluation value P is repeatedly performed. Since the optimum value is searched for from the obtained plurality of evaluation values P, it takes a long time to adjust the intensity of the RF pulse. In addition, the measured sensitivity profile may have an error, and the effect of the sensitivity of the RF coil may not be sufficiently removed.

The present invention has been made in view of the above circumstances, and an object of the present invention is to adjust the irradiation gain at high speed without being affected by the reception sensitivity.

In order to solve the above problems, the present invention provides the following means.
One aspect of the present invention includes a transmitting unit that irradiates a subject with a high-frequency magnetic field pulse, an imaging unit having a receiving unit that detects a nuclear magnetic resonance signal from the subject, and a nuclear magnetic resonance signal acquired by the receiving unit. On the other hand, a calculation unit that performs predetermined calculation processing including imaging processing and a control unit that controls imaging by the imaging unit are provided, and the control unit has a high-frequency magnetic field having a predetermined intensity prior to the main imaging. It has an imaging sequence that prescans according to a pulse, and is generated by a high-frequency magnetic field pulse of a predetermined intensity by the transmitter based on the nuclear magnetic resonance signal detected by the receiver in the prescan. A high-frequency magnetic field distribution acquisition unit that acquires a high-frequency magnetic field distribution, an average value calculation unit that calculates the average value of the high-frequency magnetic field distribution, and an intensity that adjusts the intensity of the high-frequency magnetic field pulse emitted by the transmission unit based on the average value. Provided is a magnetic resonance imaging apparatus including an adjusting unit.

According to the present invention, the irradiation gain can be adjusted at high speed without being affected by the reception sensitivity.

It is a block diagram which shows the schematic structure of the MRI apparatus which concerns on 1st Embodiment of this invention. It is a graph about adjustment of irradiation gain in the MRI apparatus which concerns on 1st Embodiment of this invention. It is a timing chart which shows the image pickup sequence in the MRI apparatus which concerns on 1st Embodiment of this invention. It is a flowchart which shows the flow at the time of performing the irradiation gain adjustment in the MRI apparatus which concerns on 1st Embodiment of this invention. It is a flowchart which shows the flow of another example at the time of performing irradiation gain adjustment in the MRI apparatus which concerns on 1st Embodiment of this invention. It is a flowchart which shows the flow of the other example at the time of performing the irradiation gain adjustment in the MRI apparatus which concerns on the modification of 1st Embodiment of this invention. It is a flowchart which shows the flow at the time of performing the irradiation gain adjustment in the MRI apparatus which concerns on the 2nd Embodiment of this invention. It is a graph about the adjustment of the irradiation gain in the MRI apparatus which concerns on the 2nd Embodiment of this invention. It is a flowchart which shows the flow at the time of performing the irradiation gain adjustment in the MRI apparatus which concerns on 3rd Embodiment of this invention. It is a graph about the adjustment of the irradiation gain in the MRI apparatus which concerns on the 3rd Embodiment of this invention. It is a graph about the adjustment of the irradiation gain in the MRI apparatus which concerns on 4th Embodiment of this invention. It is a timing chart which shows the image pickup sequence in the MRI apparatus which concerns on 4th Embodiment of this invention.

The MRI apparatus according to the embodiment of the present invention has an imaging unit having a transmitting unit that irradiates a subject with a high-frequency magnetic field pulse and a receiving unit that detects a nuclear magnetic resonance signal from the subject, and a nuclear magnetic resonance acquired by the receiving unit. It is provided with a calculation unit that performs predetermined calculation processing including imaging processing on the signal and a control unit that controls imaging by the imaging unit, and the control unit has a high frequency of irradiation intensity predetermined prior to the main imaging. It has an imaging sequence that performs prescan according to the magnetic field pulse, and the high-frequency magnetic field distribution generated by the high-frequency magnetic field pulse of predetermined intensity by the transmitter based on the nuclear magnetic resonance signal detected by the receiver in the prescan. It is equipped with a high-frequency magnetic field distribution acquisition unit that acquires a high-frequency magnetic field distribution, an average value calculation unit that calculates the average value of the high-frequency magnetic field distribution, and an intensity adjustment unit that adjusts the intensity of the high-frequency magnetic field pulse emitted by the transmission unit based on the average value. ing.

According to the present embodiment, since the irradiation intensity is adjusted from the average value of the high frequency magnetic field distribution in the prescan performed according to the high frequency magnetic field pulse of the irradiation intensity determined in advance, it is necessary to perform multiple imagings for adjusting the irradiation intensity. It is possible to adjust the irradiation intensity at high speed. Further, since the average value of the high frequency magnetic field distribution is used for adjusting the irradiation intensity, stable intensity adjustment can be performed without depending on the reception sensitivity of the RF coil. In particular, by using the high-frequency magnetic field distribution calculation method described in International Publication WO2012 / 060192, the high-frequency magnetic field distribution can be calculated at high speed, so that the irradiation intensity of the high-frequency magnetic field pulse can be adjusted at higher speed. be able to. Further, since the high-frequency magnetic field distribution is used, the irradiation intensity can be adjusted only in a desired region, so that the irradiation gain can be optimized specifically for the region to be imaged.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<First Embodiment>
As shown in FIG. 1, the MRI apparatus according to the first embodiment of the present invention includes a static magnetic field generating unit 2, a gradient magnetic field generating unit 3, a sequencer 4, a transmitting unit 5, a receiving unit 6, a signal processing unit 7, and a signal processing unit 7. It is equipped with a central processing unit (CPU) 8.

The static magnetic field generation unit 2 generates a uniform static magnetic field in the space around the subject 1 in the direction orthogonal to the body axis in the case of the vertical magnetic field method, and in the body axis direction in the case of the horizontal magnetic field method. The static magnetic field generation unit 2 is realized by arranging a static magnetic field generation source of a permanent magnet system, a normal conduction system, or a superconducting system around the subject 1.

The gradient magnetic field generating unit 3 includes a gradient magnetic field coil 9 that applies a gradient magnetic field in the three axes of X, Y, and Z, which is a coordinate system (static coordinate system) of the MRI apparatus, and a gradient magnetic field that drives each gradient magnetic field coil 9. It is equipped with a power supply 10. By driving the gradient magnetic field power supply 10 of each coil according to the command from the sequencer 4 described later, the gradient magnetic fields Gx, Gy, and Gz are applied in the three axial directions of X, Y, and Z.

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 with respect to the subject 1, and the remaining 2 are 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 to encode the position information in each direction in the NMR signal.

The sequencer 4 operates under the control of the CPU 8 described later, transmits various commands necessary for collecting data of the tomographic image of the subject 1 to the transmitting unit 5, the gradient magnetic field generating unit 3, and the receiving unit 6, and transmits high-frequency magnetic field pulses. (Hereinafter referred to as "RF pulse") and the gradient magnetic field pulse are controlled to be repeatedly applied in a predetermined pulse sequence.

The transmission unit 5 irradiates the subject 1 with an RF pulse in order to cause nuclear magnetic resonance in the nuclear spins of the atoms constituting the biological tissue of the subject 1, and the high frequency oscillator 11, the modulator 12, and the high frequency It includes an amplifier 13 and a high frequency coil (transmission coil) 14a on the transmission side. The RF pulse output from the high frequency oscillator 11 is amplitude-modulated by the modulator 12 at the timing instructed by 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 the high frequency coil 14a, an RF pulse is applied to the subject 1.

The receiving unit 6 detects an NMR signal (nuclear magnetic resonance signal) emitted by the nuclear magnetic resonance of the nuclear spins constituting the biological tissue of the subject 1, and the high frequency coil (reception coil) 14b on the receiving side, It includes a signal amplifier 15, an orthogonal phase detector 16, and an A / D converter 17. The NMR signal of the response of the subject 1 induced by the RF pulse emitted from the high frequency coil 14a on the transmitting side was detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15. After that, it is divided into two orthogonal systems of signals by the orthogonal phase detector 16 at the timing according to the command from the sequencer 4, each of which is converted into a digital quantity by the A / D converter 17, and is sent to the signal processing unit 7 as measurement data. Sent.

The signal processing unit 7 processes various data and displays and saves processing results, and includes a CPU 8, a storage device 18 such as RAM and ROM, an external storage device 19 such as a magnetic disk and an optical disk, a CRT, and the like. It is equipped with a display 20 of the above.

The CPU 8 functions as a control unit that controls the entire MRI apparatus, and when data from the reception unit 6 is input to the CPU 8, it functions as a calculation unit that executes processing such as signal processing and image reconstruction in the CPU 8. The tomographic image of the subject 1 as a result of various calculations is displayed on the display 20 and recorded on a storage device 18 such as a magnetic disk or an external storage device 19.

Further, the CPU 8 has an imaging sequence in which a prescan is performed according to an RF pulse having an irradiation gain (specified value) indicating a predetermined intensity prior to the main imaging, and this imaging sequence is performed when the prescan is performed in the MRI apparatus. The sequencer 4 is operated according to the above.

Further, the CPU 8 realizes the functions of the high frequency magnetic field distribution acquisition unit 30, the average value calculation unit 31, and the irradiation gain adjustment unit (intensity adjustment unit) 32. These functions can be realized as software by reading and executing the program in the storage device 18 by the CPU, or can be realized by hardware such as ASIC.

The high-frequency magnetic field distribution acquisition unit 30 acquires a high-frequency magnetic field distribution (hereinafter referred to as “B1 distribution”) generated by an irradiation gain of a predetermined intensity based on the NMR signal detected by the reception unit 6 in the prescan. Various methods can be applied to the method for acquiring the B1 distribution, and in particular, the B1 distribution can be calculated at high speed by using the method for calculating the B1 distribution described in International Publication WO2012 / 060192. Can be done.

The average value calculation unit 31 calculates the average value of the B1 distribution (hereinafter referred to as “B1 average value”) acquired by the high frequency magnetic field distribution acquisition unit 30.
The irradiation gain adjusting unit 32 corrects the irradiation gain so as to have a B1 average value at which an optimum FA (FlipAngle) can be obtained by using the slope of the B1 average value with respect to the irradiation gain stored in the storage device 18. .. It is also possible to specify a region of interest (ROI) based on the B1 distribution and adjust the irradiation intensity of the RF pulse, that is, the irradiation gain from the B1 average value in the specified ROI.

The storage device 18 stores a table in which the slope of the B1 average value with respect to the irradiation gain is recorded. Here, the stored table is a record of the data obtained as follows. That is, a plurality of subjects are irradiated with an RF pulse while changing the irradiation gain in advance to acquire a B1 distribution, and the average value of the B1 distribution with respect to the irradiation gain is calculated for each irradiation gain. Since the graph plotting the B1 average value with respect to the irradiation gain changes linearly, the average value of the slope of the straight line indicating the relationship between the B1 average value and the irradiation gain (the slope of the B1 average value with respect to the irradiation gain) is calculated for each irradiation gain. And record it to get the table. In the present specification, for convenience of explanation, the imaging target for which the B1 distribution is acquired when the table to be stored in the storage device 18 is generated is defined as the “subject”, and the subject to be actually photographed by applying this table is defined as the subject. Distinguish as "subject".

Further, since the B1 average value at which FA = 90 ° is the ideal B1 average value in the storage device 18, this is set as the target B1 average value. This is because FA = 90 ° is the intensity of the RF pulse that maximizes the NMR signal to be measured. A plurality of such inclinations and target B1 average values are obtained for each part, weight, age, and gender, and are stored in the storage device 18 in advance.

Therefore, the irradiation gain adjusting unit 32 corrects the irradiation intensity, that is, the irradiation gain, based on this table. That is, a slope that matches the imaging conditions such as the subject and its site is extracted from the table stored in the storage unit 18, and plotted on the graph shown in FIG. 2, for example, with an RF pulse having a specified value for this slope. By fitting the obtained B1 average value, an irradiation gain value corresponding to the target B1 average value can be obtained. Since the plot position of the average value of the B1 distribution with respect to the irradiation gain differs depending on the subject, adjustment is required for each subject.

The operation unit 25 inputs various control information of the MRI apparatus and control information of the processing performed by the signal processing unit 7, and includes an input unit 23. As the input unit 23, one or a plurality of input devices such as a mouse, a keyboard, and a trackball can be applied. Further, the input unit 23 receives the input of the imaging condition from the user and transmits the input imaging condition to the CPU 8. Here, the imaging conditions include a portion to be imaged, a bandwidth of a high-frequency magnetic field, a number of slice encodings, a number of phase encodings, a sequence, a GAP, an imaging range, and the like.
By arranging the operation unit 25 close to the display 20, the operator can interactively control various processes of the MRI apparatus via the operation unit 25 while looking at the display 20.

Next, the B1 distribution imaging sequence in the MRI apparatus configured as described above will be described with reference to FIG.
As shown in FIG. 3, the transmission unit 5 irradiates the subject with a test pulse (201). Low-resolution images (202, 203, 204) are acquired at the timing of three points before and after irradiation. Each image is acquired by hitting the image acquisition RF (205) of a minute FA. In FIG. 2, Gs (206) is the axis of the gradient gradient magnetic field, Gp (207) is the axis of the phase gradient gradient magnetic field, and Gr (208) is the axis of the readout direction gradient magnetic field. Based on the signal values of the three images, the B1 distribution, which is a value representing the ratio of the FA in which the nuclear magnetization in the subject actually collapses to the FA of the test pulse, is calculated.

Subsequently, the irradiation gain adjustment using such a B1 distribution imaging sequence will be described with reference to the flowchart of FIG.
As shown in FIG. 4, in step S11, the high-frequency magnetic field distribution acquisition unit 30 acquires the B1 distribution by the RF pulse of the irradiation gain of the specified value in the prescan, and in step S12, the mean value calculation unit 31 acquires the high-frequency magnetic field. The B1 average value is calculated from the B1 distribution acquired by the distribution acquisition unit 30. In step S13, the irradiation gain adjusting unit 32 calculates an irradiation gain value such that the B1 average value calculated in step S12 becomes the target B1 average value, and adjusts to the obtained irradiation gain value.

Here, in particular, for example, when the MRI device is a device having a static magnetic field intensity of 3T or more, the RF pulse irradiation distribution often becomes non-uniform, and the RF pulse irradiation distribution is adjusted by performing RF shimming. ..
In such a case, the irradiation gain can be adjusted according to the flowchart shown in FIG.

As shown in FIG. 5, in step S21, the high frequency magnetic field distribution acquisition unit 30 acquires the B1 distribution by the RF pulse of the irradiation gain of the specified value in the prescan, and in step S22, determines the necessity of RF shimming. If it is determined that RF shimming is unnecessary, the process proceeds to step S24. If it is determined that RF shimming is necessary, the process proceeds to step S23, and the RF irradiation pulse is adjusted in step S23.

In step S24, an input from the user is accepted as to whether or not to set the region of interest (ROI), and when setting the ROI, the ROI set in step S25 is cut out and the process proceeds to step S26. If the ROI is not set in step S24, the process proceeds to step S26 as it is.

In step S26, the average value calculation unit 31 calculates the B1 average value from the B1 distribution acquired by the high frequency magnetic field distribution acquisition unit 30 when the ROI is not set. When the ROI is set, the B1 average value in the ROI of the B1 distribution acquired by the high frequency magnetic field distribution acquisition unit 30 is calculated. In the next step S27, the irradiation gain adjusting unit 32 calculates an irradiation gain value such that the B1 average value calculated in step S26 becomes the target B1 average value, and adjusts to the obtained irradiation gain value.

As described above, according to the present embodiment, the irradiation intensity (irradiation gain) is adjusted from the average value of the B1 distribution in the prescan performed according to the RF pulse of the specified value. That is, the irradiation after adjustment is performed by correcting the irradiation gain of the specified value to the irradiation gain specified value corresponding to the target B1 average value so that the B1 average value obtained by the irradiation gain of the specified value becomes the target B1 average value. Get a gain. In other words, the value obtained by multiplying the irradiation gain of the specified value by the B1 average value obtained by the irradiation gain of the specified value from the target B1 average value multiplied by the slope of the B1 average value with respect to the irradiation gain ((target). The adjusted irradiation gain is obtained by adding B1 average value-B1 average value) / slope at the specified irradiation gain value. Therefore, it is not necessary to perform a plurality of imagings for adjusting the irradiation intensity, and the irradiation intensity can be adjusted at high speed. Further, since the average value of the B1 distribution is used for adjusting the irradiation intensity, stable intensity adjustment can be performed without depending on the reception sensitivity of the RF coil.

(Modification example)
In addition, when the irradiation gain in the first embodiment described above is adjusted, it is conceivable that the B1 average value acquired by the RF pulse with the irradiation gain of the specified value deviates from the predetermined range for each part. Be done. In this case, the B1 average value is calculated using the irradiation gain corrected in step S27, and the irradiation gain is adjusted again. Specifically, the process is performed according to the flowchart shown in FIG. Since steps S21 to S27 in the flowchart of FIG. 6 are the same as the processes of steps S21 to S27 in the flowchart of FIG. 5, the same reference numerals are given and the description thereof will be omitted.

In the processing of the flowchart shown in FIG. 6 according to this modification, in step S28, it is determined whether or not the B1 average value acquired by the RF pulse with the irradiation gain of the specified value deviates from the predetermined range for each part. If it is determined that the device is out of alignment, the process proceeds to step S29. In step S29, the irradiation gain adjusted by the process of step S27 is used as the irradiation gain of the specified value, and the process returns to step S21. In step S21, the B1 distribution is acquired again with the irradiation gain value of a new specified value, and the irradiation gain is adjusted by repeating the subsequent processing. The width of the threshold value of the B1 average value as a reference can be determined for each site and according to the uniformity of the B1 distribution.

<Second embodiment>
Hereinafter, a second embodiment of the present invention will be described. In the MRI apparatus according to the first embodiment described above, irradiation is performed based on the B1 average value obtained from the B1 distribution generated by the RF pulse of the specified value and the slope of the B1 average value previously stored in the storage unit 18. Although the gain was adjusted, in the present embodiment, the irradiation gain is adjusted when the table is not stored in the storage unit 18. In the following description, the same components as those in the first embodiment described above are designated by the same reference numerals, and the description thereof will be omitted.

In the present embodiment, the high-frequency magnetic field distribution acquisition unit 30 acquires the B1 distribution with two different irradiation gains, and adjusts the irradiation gain by obtaining the slope from the two obtained values.
Specifically, as shown in the flowchart of FIG. 7, in step S31, the high-frequency magnetic field distribution acquisition unit 30 acquires the B1 distribution by the RF pulse of the irradiation gain of the two specified values in the prescan. In step S32, the necessity of RF shimming is determined. If it is determined that RF shimming is unnecessary, the process proceeds to step S34. If it is determined that RF shimming is necessary, the process proceeds to step S33, and the RF irradiation pulse is adjusted in step S33.

In step S34, an input from the user is accepted as to whether or not to set the region of interest (ROI), and when setting the ROI, the ROI set in step S35 is cut out and the process proceeds to step S36. If the ROI is not set in step S34, the process proceeds to step S36 as it is.

In step S36, the average value calculation unit 31 calculates the B1 average value from the B1 distribution acquired by the high frequency magnetic field distribution acquisition unit 30 when the ROI is not set. When the ROI is set, the B1 average value in the ROI of the B1 distribution acquired by the high frequency magnetic field distribution acquisition unit 30 is calculated.
In the next step S37, the slope of the irradiation gain −B1 average value is calculated for each subject from the B1 average values of the two points (see FIG. 8). In step S38, the irradiation gain value calculated by the irradiation gain adjusting unit 32 so that the B1 average value calculated in step S36 becomes the target B1 average value is calculated based on the slope calculated in step S37, and the obtained irradiation gain value is calculated. The adjusted irradiation gain value.

As described above, according to the present embodiment, since the slope of the irradiation gain −B1 average value suitable for each subject is used, it is possible to adjust the irradiation gain with higher accuracy.

<Third embodiment>
Subsequently, a third embodiment of the present invention will be described. In the first and second embodiments described above, the irradiation gain was adjusted using the slope obtained based on the B1 average value, but in the present embodiment, parameters other than the B1 average value (temporarily X and ) Is used to calculate the slope of the irradiation gain-B1 average value for each subject.
Hereinafter, only the points different from each of the above-described embodiments will be described.

The inclination obtained as follows is stored in advance in the MRI apparatus.
A high-frequency magnetic field pulse is applied to a plurality of subjects in advance while changing the irradiation gain to calculate the B1 average value, and the slope of the B1 average value with respect to the irradiation gain and the value of the parameter X are calculated. An equation for calculating the slope of the irradiation gain-B1 mean value of each subject is determined based on the value of X. That is, the slope of the straight line indicating the relationship between the B1 average value and the irradiation gain, which is optimal for each parameter X indicating the characteristics of the subject , is calculated. Here, as parameters indicating the characteristics of the subject , the size of the ROI set for the B1 distribution, the height, weight, age, T1 value, etc. of the subject can be mentioned.

in short,
Pseudo slope = A × (value of X) + B (A and B are unknowns)
Objective function = Σ _{All subjects} (pseudo slope-actual slope for each subject) ²
As a result, the coefficients A and B that minimize the objective function are determined.
At the time of adjusting the irradiation gain, the pseudo slope is calculated for each subject using the above formula, and the irradiation gain value is set so that the B1 average value becomes the target B1 average value according to the method of the first embodiment described above. To adjust.

In the MRI apparatus that stores the inclination obtained in this way, the gain adjustment is performed according to the flowchart of FIG. That is, in step S41, the high-frequency magnetic field distribution acquisition unit 30 acquires the B1 distribution by the RF pulse of the irradiation gain of the specified value in the prescan. In step S42, the necessity of RF shimming is determined. If it is determined that RF shimming is unnecessary, the process proceeds to step S44. If it is determined that RF shimming is necessary, the process proceeds to step S43, and the RF irradiation pulse is adjusted in step S43.

In step S44, an input from the user is accepted as to whether or not to set the region of interest (ROI), and when setting the ROI, the ROI set in step S35 is cut out and the process proceeds to step S46. If the ROI is not set in step S44, the process proceeds to step S46 as it is.

In step S46, the average value calculation unit 31 calculates the B1 average value from the B1 distribution acquired by the high frequency magnetic field distribution acquisition unit 30 when the ROI is not set. When the ROI is set, the B1 average value in the ROI is calculated from the B1 distribution acquired by the high frequency magnetic field distribution acquisition unit 30. At the same time, the value of the parameter X is calculated.

In the next step S47, the slope of the irradiation gain −B1 average value is calculated from the value of X for each subject (see FIG. 10). In step S48, the irradiation gain value calculated by the irradiation gain adjusting unit 32 so that the B1 average value calculated in step S46 becomes the target B1 average value is calculated based on the slope calculated in step S47, and the obtained irradiation gain value is calculated. The adjusted irradiation gain value.
As described above, according to the present embodiment, since the slope of the irradiation gain −B1 average value is calculated for each subject, the irradiation gain can be adjusted with higher accuracy.

In this embodiment as well, as in the modification of the first embodiment described above, when the B1 average value acquired by the RF pulse with the irradiation gain of the specified value deviates from the predetermined range for each part. The B1 average value can be calculated using the corrected irradiation gain, and the irradiation gain can be adjusted again by repeating the same process.

<Fourth Embodiment>
Subsequently, a fourth embodiment of the present invention will be described. In each of the above-described embodiments, the irradiation gain is adjusted using the slope obtained based on the B1 average value, but in the present embodiment, the irradiation is performed without using the slope obtained based on the B1 average value. The slope is calculated on the assumption that the gain-B1 average value passes through the origin, that is, the B1 average value is also zero when the irradiation gain value is zero.
Hereinafter, only the points different from each of the above-described embodiments will be described.

The flow of the irradiation gain adjustment processing in the present embodiment is the same as the flowchart of FIG. 5 described above, and the contents of the calculation of the B1 average value in step S26 of FIG. 5 and the processing of the correction of the irradiation gain value in step S27 are as follows. different. That is, the high-frequency magnetic field distribution acquisition unit 30 acquires the B1 average value with one irradiation gain. Then, the irradiation gain adjusting unit 32 has a ratio of a predetermined target average value to the acquired B1 average value and a ratio of the corrected irradiation gain value to the irradiation gain value of the specified value when the B1 average value is acquired. Adjust the irradiation gain so that the ratio is the same. Specifically, as shown in FIG. 11, the irradiation gain adjusting unit 32 makes the B1 average value proportional to the target B1 average value according to the graph connecting the B1 average value obtained by the high-frequency magnetic field distribution acquisition unit 30 and the origin. While adjusting the irradiation gain.

Also in this embodiment, when the B1 average value acquired by the RF pulse with the irradiation gain of the specified value deviates from the width of the threshold value predetermined for each site, the B1 average value is used by using the corrected irradiation gain. The value can be calculated and the irradiation gain can be adjusted again.

In order to connect the B1 average value and the origin as in this embodiment and adjust the irradiation gain in proportion to this, it is necessary to perform measurement more accurately than in each of the above-described embodiments and obtain the value of the B1 distribution. be. This is because, although the irradiation gain and the B1 distribution are theoretically proportional to each other, the value of the B1 distribution obtained as an actual measurement result is not strictly proportional to the irradiation gain due to a measurement error or the like.

Therefore, the irradiation gain can be adjusted accurately by eliminating the measurement error and the like as much as possible, performing the measurement accurately, acquiring the B1 distribution, and applying the present embodiment. As a method for performing accurate measurement, it is necessary to relax the longitudinal magnetization of the image acquisition RF, and for example, it is conceivable to allow a time between the acquisition of the reference image and the test pulse as shown in FIG.
As described above, according to the present embodiment, it is not necessary to perform a plurality of imagings for adjusting the irradiation intensity, and the irradiation intensity can be adjusted at high speed by a simpler process. Further, since the average value of the B1 distribution is used for adjusting the irradiation intensity, stable intensity adjustment can be performed without depending on the reception sensitivity of the RF coil.

<Fifth Embodiment>
Subsequently, a fifth embodiment of the present invention will be described. In each of the above-described embodiments, the irradiation gain is adjusted each time, but in the present embodiment, the value is determined in advance without adjustment and adopted. Hereinafter, only the points different from each of the above-described embodiments will be described.

In the present embodiment, a large amount of B1 average value data for each irradiation gain is collected in advance, and the irradiation gain value that gives the optimum B1 average is stored in the storage device 18 separately for each part, height, weight, and the like. At the time of this measurement, an appropriate value is selected and applied from the irradiation gain values stored in the storage device 18 based on the site, height, body weight, etc. corresponding to the subject. In this embodiment, the plot position of the average value of the B1 distribution with respect to the irradiation gain differs depending on the subject, but by classifying to some extent according to the site, height, body weight, etc., the irradiation gain that becomes the optimum B1 average becomes approximately close. It is based on the idea.

2 ... Static magnetic field generator, 3 ... Diagonal magnetic field generator, 4 ... Sequencer, 5 ... Transmitter, 6 ... Receiver, 7 ... Signal processing unit, 8 ... Central processing device (CPU), 9 ... gradient magnetic field coil, 10 ... gradient magnetic field power supply, 11 ... high frequency oscillator, 12 ... modulator, 13 ... high frequency amplifier, 14a, 14b ... High frequency coil, 15 ... signal amplifier, 16 ... orthogonal phase detector, 17 ... A / D converter, 18 ... storage device, 19 ... external storage device, 20 ... display, 23 ... Input unit, 25 ... Operation unit, 30 ... High frequency magnetic field distribution acquisition unit, 31 ... Average value calculation unit, 32 ... Irradiation gain adjustment unit

Claims (7)

An imaging unit having a transmitting unit that irradiates a subject with a high-frequency magnetic field pulse and a receiving unit that detects a nuclear magnetic resonance signal from the subject.
An arithmetic unit that performs predetermined arithmetic processing including imaging processing on the nuclear magnetic resonance signal acquired by the receiving unit, and an arithmetic unit.
A control unit that controls imaging by the imaging unit,
A high-frequency magnetic field pulse is applied to a plurality of subjects in advance while changing the irradiation gain to obtain a high-frequency magnetic field distribution, the gradient of the average value of the high-frequency magnetic field distribution with respect to the irradiation gain is calculated, and the obtained gradient is calculated for each subject. Equipped with a recorded storage unit
The control unit has an imaging sequence in which a prescan including irradiation of a high-frequency magnetic field pulse of a predetermined intensity is performed on the subject prior to the main imaging.
The high-frequency magnetic field distribution acquisition unit that the arithmetic unit acquires the high-frequency magnetic field distribution generated by the high-frequency magnetic field pulse of the predetermined intensity based on the nuclear magnetic resonance signal detected by the reception unit in the prescan for the subject. And the average value calculation unit that calculates the average value of the high frequency magnetic field distribution acquired by the high frequency magnetic field distribution acquisition unit, and the high frequency magnetic field pulse that the transmission unit irradiates based on the average value calculated by the average value calculation unit. Equipped with a strength adjustment unit that adjusts the strength of
The mean value calculation unit uses the parameter X indicating the characteristics of the subject and the inclination of each of the plurality of subjects recorded in the storage unit to determine the subject according to the following equation (1). The pseudo-slope of the average value of the high-frequency magnetic field distribution with respect to the irradiation gain of is calculated by determining the coefficients A and B that minimize the objective function obtained by the following equation (2).
Pseudo slope = A × (value of X) + B (however, A and B are unknown)
... (1)
Objective function = Σ _{All subjects} (pseudo slope-slope for each subject) ² ... (2)
The intensity adjusting unit sets an irradiation gain value such that the average value of the high-frequency magnetic field distribution of the subject calculated by the average value calculation unit becomes the average value of the predetermined high-frequency magnetic field distribution. The intensity of the high-frequency magnetic field pulse irradiated by the transmitter is adjusted by using the obtained irradiation gain value as the adjusted irradiation gain value, which is calculated based on the pseudo-inclination.
A magnetic resonance imaging device characterized by this. In the magnetic resonance imaging apparatus according to claim 1,
The parameter X indicating the characteristics of the subject is characterized by being one of the size of the region of interest set for the high-frequency magnetic field distribution, the height, weight, age, and T1 value of the subject. Magnetic resonance imaging device . In the magnetic resonance imaging apparatus according to claim 1,
The intensity adjusting unit calculates the ratio of the average value of the high-frequency magnetic field distribution to the predetermined target high-frequency magnetic field distribution to the average value of the high-frequency magnetic field distribution of the subject, which is calculated by the average value calculation unit, and the subject. A magnetic resonance imaging device that adjusts the intensity so that the ratio of the adjusted irradiation gain value to the irradiation gain value of the specified value when the average value of the high-frequency magnetic field distribution is obtained is the same ratio. In the magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging apparatus in which the mean value calculation unit sets a region of interest in the high-frequency magnetic field distribution and calculates the mean value within the set region of interest. In the magnetic resonance imaging apparatus according to claim 1,
When the average value of the high-frequency magnetic field distribution calculated by the average value calculation unit deviates from a predetermined range,
The high-frequency magnetic field distribution acquisition unit acquires the high-frequency magnetic field distribution by the prescan again with the intensity of the high-frequency magnetic field pulse after being adjusted by the intensity adjustment unit.
A magnetic resonance imaging device in which the intensity adjusting unit adjusts the intensity of a high-frequency magnetic field pulse based on the average value of the high-frequency magnetic field distribution obtained again. In the magnetic resonance imaging apparatus according to claim 5 ,
A magnetic resonance imaging device in which the predetermined range is determined according to the site and body weight of the subject and the uniformity of a high-frequency magnetic field distribution. In the magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging device that determines the value of the irradiation gain for acquiring the high-frequency magnetic field distribution in the prescan and the interval of irradiating the high-frequency magnetic field pulse according to the site and weight of the subject and the uniformity of the high-frequency magnetic field distribution.

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