JP6233965B2 - Magnetic resonance imaging apparatus and RF shimming method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus, and in particular, suitably masks an image obtained by imaging a subject and stably acquires a B1 map necessary for RF shimming. It relates to an MRI apparatus capable of

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. Different phase encodings are applied depending on the gradient magnetic field, and frequency encoding is performed, and measurement is performed as time-series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In recent years, high magnetic field MRI apparatuses of 3T or more have become widespread. In the high magnetic field MRI apparatus, since the subject is placed in a high magnetic field as compared with the MRI apparatus, an image having a high S / N can be obtained. However, there is a problem that unevenness occurs in the image due to abdominal imaging or the like. One of the causes of this unevenness is that the magnetic field distribution (B1 map) of the RF irradiation magnetic field used in the MRI apparatus is not spatially uniform, that is, the magnetic field distribution of the transmission RF irradiation magnetic field is spatially non-uniform. It is thought that it has an ingredient. In the high magnetic field MRI system, the frequency of the RF pulse used is high, so the RF irradiation magnetic field irradiated to the subject is easily absorbed, and the difference in the local distribution of the RF irradiation magnetic field is generated. Appears as uneven.

  There is RF shimming as a technique for solving the unevenness of the MRI image due to such non-uniform magnetic field distribution of the RF pulse. RF shimming is a technology that uses an RF coil for transmission with multiple channels and controls the intensity and phase of the RF pulse applied to each channel independently, so that the subject is evenly irradiated with RF pulses. It is. However, in order to perform RF shimming, it is necessary to measure the irradiation magnetic field distribution irradiated by each transmission RF coil as a B1 map in a state where the subject is actually arranged in the uniform magnetic field space.

As prior art relating B1 map measurements, there is a technology described in Patent Document 1.

International Publication No.2012 / 060192

  However, Patent Document 1 does not discuss creation of a mask necessary for obtaining a B1 map. In particular, for the measurement of the B1 map, if the area where the subject is placed (signal area) and the area where the background subject is not placed (mask area) are not properly distinguished by creating a mask, B1 The map could not be obtained accurately, and there was a risk of artifacts in the obtained MRI image.

  An object of the present invention is to provide an MRI apparatus capable of suitably masking an image obtained by imaging a subject and stably acquiring a B1 map necessary for RF shimming.

In order to solve the above problems, a magnetic resonance imaging apparatus having an RF irradiation coil that includes a plurality of RF coils and irradiates a subject with a high-frequency magnetic field for causing nuclear magnetic resonance is provided. Multiple echo signals are measured from the subject, and the images of each RF coil are generated by reconstructing the images of the multiple echo signals, and the image for creating the mask is generated by combining the images for each RF coil. Calculate the noise statistic based on the pixel value of the area below the bed in the image for creation, determine the threshold based on the noise statistic, generate a mask image using the image for mask creation and the threshold, Calculate the B1 map of the RF irradiation coil using the image and mask image for each coil, calculate the amplitude and phase of the RF pulse applied to each RF coil based on the B1 map, and calculate the RF calculated for each RF coil Duplicate using the amplitude and phase of the pulse. Carry out the measurement of the echo signal.

  According to the present invention, it is possible to provide an MRI apparatus capable of suitably masking an image obtained by imaging a subject and stably acquiring a B1 map necessary for RF shimming.

Block diagram showing an embodiment of an MRI apparatus Figure showing each function executed by the CPU in Figure 1 The figure which shows the internal structure of the mask preparation part 29 in Example 1. The figure explaining calculating the average value and standard deviation of the pixel value of the area below the bed The figure which shows the flowchart which shows the procedure which performs B1 distribution measurement in Example 1. The figure which shows the flowchart of the production | generation step 204 of the mask in Example 1. FIG. The figure which shows the relationship between the pre-pulse 301 and signal acquisition sequence 303,305,307 Diagram showing gradient echo (GrE) pulse sequence Diagram showing the so-called centric order pulse sequence starting from the echo in the center of k-space The figure which showed the internal structure of the mask creation part 29 in Example 2. The figure which shows the flowchart of the creation step 204 of the mask in Example 2. FIG.

  Embodiments of the present invention will be described below. First, an overall configuration of an MRI apparatus to which the present invention is applied will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an MRI apparatus to which the present invention is applied. This 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, a central processing unit ( arithmetic processing unit, CPU) 8 And is configured.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the space in which the subject 1 is placed, and includes a permanent magnet type, normal conduction type or superconductivity type static magnetic field generation source (not shown). The static magnetic field generation source is arranged so as to generate a uniform static magnetic field in the direction perpendicular to the body axis of the subject 1 in the vertical magnetic field method, and in the body axis direction in the horizontal magnetic field method. Yes.

The gradient magnetic field generation system 3 drives a gradient magnetic field coil 9 that applies a gradient magnetic field in the three orthogonal directions of X, Y, and Z, which are the coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil 9 It consists of a gradient magnetic field power supply 10. By driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 to be described later, desired gradient magnetic fields Gx, Gy, and Gz can be applied in the three axial directions of X, Y, and Z. Depending on how the gradient magnetic field is applied, the imaging slice of the subject can be selectively excited, and position information can be added to an echo signal (NMR signal) generated from the excitation region.

  The sequencer 4 is a control means that repeatedly applies an RF pulse and a gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 4 operates under the control of the CPU 8, and sends various commands necessary for collecting tomographic image data of the subject 1 to the transmission system 5. To 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. In this embodiment, the transmission coil 14a has a plurality of feeding points, and is configured to be able to adjust the intensity and phase of the supplied high frequency. A plurality of high-frequency oscillators 11, modulators 12, and high-frequency amplifiers 13 are provided for each channel. Although the figure shows a case where there are two feeding points, the number of feeding points is not limited to two.

  The RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at the timing according to the command from the sequencer 4, and the amplitude-modulated RF 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 timing from the sequencer 4 and the modulation by the modulator 12 are controlled by reflecting the measurement result of the B1 distribution described later.

  The receiving system 6 detects an echo 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, a signal amplifier 15, and quadrature detection. And an A / D converter 17. The NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the transmission coil 14a is detected by the reception coil 14b arranged close to the subject 1, amplified by the signal amplifier 15, and then from the sequencer 4. Are divided into two orthogonal signals by the quadrature detector 16, converted into digital quantities by the A / D converter 17, and sent to the signal processing system 7.

  Although FIG. 1 shows a configuration in which a high-frequency coil for transmission and a high-frequency coil for reception are separately provided, a configuration in which one high-frequency coil (including multiple coils) serves both for transmission and reception It is also possible.

  The signal processing system 7 performs CPU 8, various data processing, processing result display and storage, and the like, and includes an external storage device such as an optical disk 19 and a magnetic disk 18, and a display 20 including a CRT or the like. When data from the receiving system 6 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 an external storage device On the magnetic disk 18 or the like.

The CPU 8 has a function as a control unit that controls each element of the apparatus in addition to a function as a calculation unit of the signal processing system 7, and executes various pulse sequences via the sequencer 4. The pulse sequence is incorporated in advance as a program. In the present embodiment, a B1 distribution measurement sequence for measuring the irradiation magnetic field distribution (B1 distribution) by the transmission coil is provided. The signal processing system 7, using the measurement results of the B1 distribution measurement sequence, performs calculation of the phase and amplitude of a given radio frequency pulses to the calculation and transmission coil 14a of the B1 distribution, based on this calculation result, the transmitting coil 14a Controls the phase and amplitude of the high-frequency pulse applied to.

  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.

  Next, a block diagram of the MRI apparatus according to Embodiment 1 of the present invention will be described. FIG. 2 is a diagram illustrating each function executed by the CPU in FIG. 1, and includes a positioning unit 26, a B1 map measurement sequence execution unit 27, an image reconstruction unit 28, a mask creation unit 29, and a B1 map calculation. The unit 30 includes an RF pulse amplitude and phase calculation / setting unit 31, and an imaging control unit 32.

The positioning unit 26 controls to place the subject 1 at the center of the imaging space by capturing a scout image or the like.

  The B1 map measurement sequence execution unit 27 is connected to the positioning unit 26, and executes a sequence for measuring the B1 map as described later to obtain an echo signal for each channel.

  The image reconstruction unit 28 is connected to the B1 map measurement sequence execution unit 27, and reconstructs an echo signal for each channel obtained by the B1 map measurement sequence execution unit 27 to generate an image for each channel. Is.

The mask creation unit 29 is connected to the image reconstruction unit 28 and creates a mask image for making the B1 map creation only from the signal region where the subject 1 is arranged.

  The B1 map calculation unit 30 is connected to the image reconstruction unit 28 and the mask creation unit 29, and creates a B1 map using the image obtained by the image reconstruction unit 28 and the mask image created by the mask creation unit 29. Is for.

  The RF pulse amplitude and phase calculation / setting unit 31 is connected to the B1 map calculation unit 30 and uses the B1 map created by the B1 map calculation unit 30 to determine the amplitude and phase of the RF pulse applied to each RF coil channel. For calculating and setting.

The imaging control unit 32 is connected to the RF pulse amplitude and phase calculation / setting unit 31 for imaging the subject 1 with the RF pulse amplitude and phase and the RF pulse amplitude and phase set by the phase calculation / setting unit 31. Is.

  FIG. 3 is a diagram illustrating an internal configuration of the mask creating unit 29 according to the first embodiment. The mask creating unit 29 includes a mask creating image generating unit 29-1, an average value / standard deviation calculating unit 29-2, a threshold determining unit 29-3, and a threshold processing unit 29-4.

  The mask creation image generation unit 29-1 is connected to the image reconstruction unit 28, and adds the images for each channel obtained by the image reconstruction unit 28 to generate a single mask creation image. Is.

  The average value / standard deviation calculation unit 29-2 is connected to the mask creation image generation unit 29-1, and is an area below the bed from the mask creation image created by the mask creation image generation unit 29-1. This is for calculating the average value and standard deviation of the pixel values (35 portion in FIG. 4). In FIG. 4, 1 is a subject, 33 is a top plate, and 34 is an image frame.

  The threshold value determination unit 29-3 is connected to the average value / standard deviation calculation unit 29-2, and the threshold value (Av) / standard deviation (Sd) calculated by the average value / standard deviation calculation unit 29-2 For example, Th) is determined as Th = Av + 20 × Sd.

  The threshold processing unit 29-4 is connected to the mask creating image generating unit 29-1 and the threshold determining unit 29-3, and uses the threshold determined by the threshold determining unit 29-3 to use the mask generating image generating unit 29- This is for applying a threshold process to the mask creation image created in 1 to create a mask image. More specifically, for example, a 64 × 64 mask image is created.

  Next, FIG. 5 is a flowchart illustrating a procedure for performing B1 map measurement according to the first embodiment. Figure 5 shows the procedure.

(Step 201)
The operator arranges the subject 1 in the static magnetic field space, acquires a scout image using a program or the like provided in the positioning unit 26, and the target imaging region of the subject 1 is positioned almost at the center of the static magnetic field space. The subject 1 is moved so as to perform positioning.

(Step 202)
The B1 map measurement sequence execution unit 27 executes the B1 map measurement sequence for each channel of the transmission coil and acquires an echo signal for each channel of the transmission coil. The B1 map measurement sequence will be described later.

(Step 203)
The image reconstruction unit 28 performs an image reconstruction operation using the echo signal for each channel of the transmission coil acquired in step 202, and generates an image for each channel of the transmission coil.

(Step 204)
Based on the image for each channel of the transmission coil obtained in step 203, the mask creation unit 29 generates a mask for distinguishing between the signal region and the noise region. A specific mask generation method will be described later.

(Step 205)
The B1 map calculation unit 30 calculates a B1 map for each channel using the image acquired in step 203 and the mask generated in step 204.

(Step 206)
The RF pulse amplitude and phase calculation / setting unit 31 uses the B1 map calculated in step 205 to calculate and set the amplitude and phase of the high-frequency pulse applied to each channel.

(Step 207)
The imaging control unit 32 performs desired measurement (imaging) under the conditions set in step 206.
Details of step 202, step 204, and step 205 will be described below.

<< B1 map measurement (step 202) >>
The B1 map measurement sequence consists of a combination of a pre-pulse consisting of RF pulses with a relatively large flip angle and a signal acquisition sequence using RF pulses with a small flip angle. In the signal acquisition sequence, the elapsed time (TI ) To obtain multiple images. Different modes are possible depending on the combination of the prepulse and the signal acquisition sequence, and the B1 map calculation (step 205) differs depending on the mode.

  The present embodiment is characterized in that at least three signal acquisition sequences are executed following the application of the pre-pulse, and the B1 map is calculated from the images acquired in each signal acquisition sequence.

FIG. 7 is a diagram showing the relationship between the pre-pulse 301 and the signal acquisition sequences 303, 305, and 307, and the change in signal intensity depending on the elapsed time (TI) since the application of the pre-pulse 301 is shown in the graph on the upper side of the figure. . In the graph shown, the horizontal axis represents the elapsed time after applying the pre-pulse, and the vertical axis represents the signal intensity. The pre-pulse 301 is a non-selective RF pulse, for example, and has a large flip angle, for example, a 90-degree pulse. While the nuclear spin excited by the prepulse 301 is longitudinally relaxed, at least three signal acquisition sequences 303, 305, and 307 are executed to acquire three k-space data (or image data) having different TIs. . In the present invention, it is important to obtain data with different longitudinal relaxation from the prepulse, and the TI is set so that all signal acquisition sequences are completed while the influence of the prepulse remains sufficiently. As a result, the B1 map can be calculated with high accuracy.

Each signal acquisition sequence is not particularly limited as long as it is a pulse sequence that can collect k-space data in a short time. For example, a gradient echo (GrE) pulse sequence as shown in FIG. 8 can be adopted. In this GrE system sequence, a low-flip angle RF pulse 401 is applied together with a slice gradient magnetic field pulse 402, and then a phase encode gradient magnetic field 403 is applied. At the same time, the readout gradient magnetic field 404 is applied, and the echo signal 405 is measured during the application of the readout gradient magnetic field whose polarity is reversed. Finally, a rephase gradient magnetic field 406 is applied in the phase encoding direction. As the RF pulse 401, a low flip angle pulse of preferably 10 degrees or less, more preferably 5 degrees or less is used in order to reduce the influence on the longitudinal magnetization.

In this pulse sequence, a low flip angle pulse is used as the RF pulse 401 and a rephase gradient magnetic field 406 in the phase encoding direction is used. Therefore, the repetition time TR can be set to about several ms (milliseconds). The RF pulse 401 to the rephase gradient magnetic field 406 are repeated while changing the intensity of the phase encode gradient magnetic field pulse 403, and the data (k-space data) of the slice selected by the slice gradient magnetic field 402 is obtained.

  This k-space data is used for calculation of the B1 map described later, and the matrix size may be about 64 × 64. As a result, the entire k-space data can be acquired in a very short time, specifically in a measurement time of about 200 ms.

  The contrast of the image data formed from the k-space data is mainly governed by the data at the center of the k-space data, so the timing for measuring the echo at the center of the k-space data in each signal acquisition sequence is the elapsed time from the pre-pulse application. (TI). In FIG. 8, each signal acquisition sequence is a so-called centric order pulse sequence that starts measurement from an echo in the center of the k-space, and the start points of the signal acquisition sequence are TI1, TI2, and TI3, respectively. Further, in the present embodiment, the relationship between these elapsed times is set to the relationship of TI2 = 2 × TI1 and TI3 = 3 × TI1.

<< Mask Generation Step 204 >>
Next, the mask generation step 204 in Embodiment 1 of the present invention will be described using the flowchart of FIG. Hereinafter, each step of the flowchart of the figure will be described in order.

(Step 204-1)
The mask creation image generation unit 29-1 generates a mask creation image. Specifically, the images for each channel obtained by executing the B1 distribution measurement sequence in step 202 are combined to form a mask creation image.

(Step 204-2)
The average value / standard deviation calculation unit 29-2 calculates the average value and standard deviation of the pixel values below the bed from the mask creation image created in Step 204-1.

(Step 204-3)
Based on the average value (Av) and standard deviation (Sd) calculated in step 204-2, the threshold value determination unit 29-3 calculates the threshold value (Th) necessary for creating a mask necessary for creating the B1 map. decide. For example, Th = Av + 20 × Sd.

(Step 204-4)
The threshold processing unit 29-4 creates a mask using the mask creation image obtained in step 204-1 based on the threshold set in step 204-3. Specifically, in step 204-3, a pixel having a pixel value equal to or higher than the threshold is replaced with 1, and a pixel having a pixel value equal to or lower than the threshold is replaced with 0 to obtain a mask image.

<< B1 distribution calculation (step 205) >>
Next, in this embodiment, a method for calculating a B1 map from k-space data obtained by the B1 map measurement sequence shown in FIG. 7 will be described.

When the image data obtained by performing inverse Fourier transform on the k-space data obtained in the first signal acquisition sequence is multiplied by the mask image created in step 204-4 described above, the signal intensity S (B1, TI) is given by equation (1).

  In equation (1), Sseq represents the signal strength determined by the signal acquisition sequence after the prepulse, α represents the flip angle of the set prepulse, and TI is the time from the prepulse application to the collection of the k-space center signal. T1 represents the longitudinal relaxation time depending on the tissue.

Similarly, the signal intensity of the target pixel of the image data obtained from the second and third signal acquisition sequences can be expressed by Expression (2) and Expression (3).

here

Equations (1) to (3) can be written as Equations (4) to (6).

By solving the simultaneous equations of Equations (4) to (6), X and Y are obtained from Equations (7) and (8).

Here, by definition, since X = 1−cos (B1 · α), B1 can be calculated by Equation (9).

  In the present embodiment, since the B1 map can be obtained by solving simultaneous equations from images obtained by at least three signal acquisition sequences with different TIs, B1 distribution measurement can be performed in a very short time of several seconds.

  As described above, the B1 map can be obtained accurately by solving simultaneous equations from multiple measurement results (signal intensities) with different TIs. It is also possible to calculate the B1 map by fitting the result.

  In this embodiment, in order to solve the simultaneous equations described above, at least three images (execution of three signal acquisition sequences) are necessary, but if the range of the spin longitudinal relaxation time after applying the prepulse is 3 It may be more than once. In addition, the TI of the plurality of signal acquisition sequences is an integer ratio in the above-described example, but may be different from each other and may be other than the integer ratio.

  The signal acquisition sequence is not limited to the sequence shown in FIG. 7 as long as image data can be acquired in a short time, and various changes can be made.

  For example, the three signal acquisition sequences having different TIs shown in FIG. 7 are not executed at regular intervals, but a plurality of signal acquisition sequences can be executed continuously without providing any intervals. Alternatively, before and after the signal acquisition sequence, an RF pulse (301 in FIG. 7) can be continuously applied with the same TR as the signal acquisition sequence without measuring an echo. In these modified examples, by continuously applying the RF pulse, the spin can be maintained in a steady state, and the occurrence of a contrast difference in the signal acquisition sequence can be suppressed.

  According to this embodiment, a mask is created from an image for creating a mask, and this is used to create a B1 map based only on the data of the signal area where the subject exists. Therefore, the B1 map is accurately obtained. Therefore, there is no risk of artifacts in the obtained MRI image.

  Next, Example 2 of the present invention will be described. First, FIG. 10 shows the internal structure of the mask creation unit 29 in the second embodiment, but the complex addition is performed between the mask creation image generation unit 29-1 and the average / standard deviation calculation unit 29-2. Part 29-1a is provided. The complex adder 29-1a generates an image by performing complex addition of the mask creation images.

  FIG. 11 is a flowchart of the mask creation step 204 in the second embodiment. FIG. 11 is different only in the mask generation step 204-1a in the first embodiment. Specifically, step 204-1a is added between step 204-1 and step 204-2. Hereinafter, only step 204-1a will be described.

(204-1a)
The mask creation image created in step 204-1 is complex-added. Specifically, an image is created by performing complex addition of images composed of complex numbers.

  According to the present embodiment, a threshold value is determined in step 204-2 based on an image obtained by complex addition. In this way, by using an image obtained by complex addition, in the image obtained by complex addition, the signal is large because the phase is aligned in the signal domain, but the signal is small because the phase is not uniform in the noise domain. Therefore, there is an advantage that a signal region and a noise region can be easily distinguished, and a more appropriate mask image can be created.

  The present invention is applied to an MRI apparatus.

  204 Mask creation step

Claims (3)

An RF irradiation coil comprising a plurality of RF coils and irradiating a subject with a high frequency magnetic field for causing nuclear magnetic resonance;
A measurement unit that measures a plurality of echo signals from the subject via the RF irradiation coil;
An arithmetic unit that reconstructs the plurality of echo signals and generates an image for each RF coil;
With
The computing unit is
Generating an image for creating a mask by synthesizing an image for each RF coil;
Based on the pixel value of the area below the bed in the mask creation image, calculate the noise statistic,
Determining a threshold based on the noise statistic;
A mask image is generated using the mask creation image and the threshold value,
Using the image for each RF coil and the mask image, calculate the B1 map of the RF irradiation coil,
Based on the B1 map, the amplitude and phase of the RF pulse applied to each RF coil are calculated,
The measuring unit is
Measuring the plurality of echo signals using the amplitude and phase of the RF pulse calculated for each RF coil ;
A magnetic resonance imaging apparatus. 2. The magnetic resonance according to claim 1, wherein the arithmetic unit performs a complex addition of the plurality of mask creation images, and calculates the noise statistic using the mask creation image after the complex addition. Imaging device. An RF shimming method in a magnetic resonance imaging apparatus comprising a plurality of RF coils and comprising an RF irradiation coil for irradiating a subject with a high frequency magnetic field for causing nuclear magnetic resonance.
Measuring a plurality of echo signals from the subject via the RF irradiation coil;
Reconstructing the plurality of echo signals to generate an image for each RF coil; and
Generating an image for creating a mask by synthesizing an image for each RF coil; and
Calculating a noise statistic based on a pixel value of an area below the bed in the mask creation image;
Determining a threshold based on the noise statistics;
Generating a mask image using the mask creation image and the threshold;
Using the image for each RF coil and the mask image to calculate a B1 map of the RF irradiation coil;
Calculating the amplitude and phase of the RF pulse applied to each RF coil based on the B1 map;
Measuring the plurality of echo signals using the amplitude and phase of the RF pulse calculated for each RF coil ;
An RF shimming method comprising:

Images