JP6944816B2 - Magnetic resonance imaging device - Google Patents

Description

The present invention measures magnetic resonance imaging (Nuclear Magnetic Resonance: hereinafter referred to as "NMR") signal from hydrogen, phosphorus, etc. in a subject, and images the density distribution, relaxation time distribution, etc. of the nucleus. (Magnetic Resonance Imaging: hereinafter referred to as "MRI"), in particular, a technique for suppressing noise due to a gradient magnetic field.

The MRI apparatus measures the NMR signal generated by the nuclear spins constituting the tissue of the subject, especially the human body, and images the morphology and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. It is a device. In imaging, the NMR signal is frequency-encoded with different phase encoding depending on the gradient magnetic field, and is measured as time-series data. The measured NMR signal is arranged in a measurement space (hereinafter referred to as k-space) and reconstructed into an image by two-dimensional or three-dimensional Fourier transform. As a method of arranging the NMR signal in the k-space at this time, the Cartesian Scan method is generally used. The Cartesian scan method is a method of scanning a plurality of NMR signals (echo signals) having different phase encodings so as to be arranged in parallel.

As a specific example, the gradient echo sequence will be introduced. In a gradient echo sequence, a phase-encoded gradient magnetic field pulse is usually arranged in a sequential order from the + side to the-side (or from the-side to the + side) of the k-space in one shot. To control.

In addition to the Cartesian scan method, there is a method of arranging signals in k-space. One of them is a measurement method called the Radial Scan method. This measurement method is a method in which signals having different rotation angles are measured so as to be arranged radially with the center of k-space as the origin. The signal for each angle is called a blade. In the radial scan method, since the signals are arranged radially, the artifacts in the phase direction are dispersed, and the body movement artifacts are inconspicuous as compared with the Cartesian scan.

Further, when a moving substance such as blood or a moving tissue is present in the imaging region, for example, as disclosed in Patent Document 1, a pulse sequence to which a rephase gradient magnetic field is added causes movement or movement. The phase of the spin is reconverged (rephased) regardless of the speed or acceleration of movement or motion to eliminate or reduce the effect on image quality (that is, an artifact) (hereinafter referred to as the rephase effect). ing. This rephase pulse is applied in the slice direction, the phase encoding direction, and the frequency encoding direction in a primary or secondary form according to the purpose.

In addition, there is an imaging method in which an RF pulse is applied before the main measurement in order to selectively suppress signals of protons in a specific region or protons of a specific molecule to improve image quality. The RF pulse applied before this measurement is called a prepulse. Prepulses include, for example, RF pulses for suppressing fat, saturation pulses that enhance arterial discrimination by suppressing inflowing venous blood signals, and magnetization transfer contrast that suppresses muscle and brain parenchymal signals ( MTC: Magnetization Transfer Contrast) There are pulses and the like. MTC pulses can improve the ability to visualize blood vessels by suppressing signals in the muscle and brain parenchyma. Therefore, MTC pulse, saturation pulse and the like are used for blood flow imaging such as 3D TOF imaging. Such an imaging method is disclosed in Patent Document 2.

WO2011 / 034004 Japanese Unexamined Patent Publication No. 2011-110086

Patent Document 1 and Patent Document 2 disclose control of applying a magnetic field from the viewpoint of image quality, but do not disclose control of applying a magnetic field from the viewpoint of making sound quiet.

The sound pressure level generated by the application of the gradient magnetic field changes the magnetic field strength of the encode pulse when the signal is acquired, regardless of the measurement method such as Cartesian scan or radial scan. In addition, since the sound characteristics of the gradient magnetic field coil differ depending on each axis of the stationary coordinate system, the sound pressure also changes depending on the angle at the time of measurement. Therefore, the sound pressure level changes depending on the order in which the encode pulses are applied and the order in which the measurement angles are acquired. Further, even in the measurement to which the additional pulse such as the rephase pulse is applied, the sound pressure level differs depending on the measurement point due to the rephase pulse. On the other hand, in the measurement in which the prepulse is applied to a specific region in the k-space, not only the sound pressure level but also the image contrast changes depending on the measurement order.

As described above, there are many factors that affect the sound pressure level in the measurement of the MRI apparatus, and a region where the sound pressure level becomes high appears depending on the measurement order. Moreover, the measurement order may be important in determining the contrast of the obtained image, and in that case, the measurement order cannot be determined only by the sound pressure level.

An object of the present invention is to provide an MRI apparatus that reduces the sound felt by a subject during an examination without deteriorating the image quality.

In order to achieve the above object, the magnetic resonance imaging apparatus includes a static magnetic field generation system that generates a static magnetic field in the space where the subject is placed, a gradient magnetic field generation system that generates a gradient magnetic field superimposed on the static magnetic field, and a subject. A transmission system that irradiates a high-frequency magnetic field pulse, a reception system that detects an echo signal emitted from a subject in response to the application of a high-frequency magnetic field pulse by the transmission system, and a gradient magnetic field generation system based on a predetermined pulse sequence. A sequencer that controls the generation of a gradient magnetic field, the irradiation of high-frequency magnetic field pulses by the transmitting system, and the detection of echo signals by the receiving system, and the application of a gradient magnetic field by the gradient magnetic field generation system during echo signal measurement when executing a predetermined pulse sequence. The echo signal at the k space position where the generated sound pressure is relatively high and the echo signal at the k space position where the sound pressure generated by the gradient magnetic field application by the gradient magnetic field generation system at the time of echo signal measurement are alternately measured. It has a processing device that determines the measurement order.

By optimizing the measurement order based on the sound pressure, the sound pressure level is smoothed.

It is a block diagram which shows the whole structure of an MRI apparatus. This is an example of arranging an echo signal in k-space in a Cartesian scan. This is an example of arranging an echo signal in k-space in a radial scan. It is a figure which shows the arrangement example of the echo signal in k space (kp−ks space). It is a figure which shows the gradient magnetic field strength in a predetermined voxel in k-space. It is a figure which shows the example of the k space filling order when the noise reduction is on. It is a flowchart which shows the operation flow of Example 1. FIG. It is a figure explaining the application pattern of the rephase pulse to the frequency axis. It is a flowchart which shows the operation flow of Example 2. It is a figure explaining the voxel division of k-space in Example 2. FIG. It is a figure explaining the application pattern of the MTC pulse to the frequency axis. It is a flowchart which shows the operation flow of Example 3. FIG. It is a figure explaining the voxel division of k-space in Example 3. FIG. It is a figure which shows the example of the k space filling order when the noise reduction in Example 3 is on. It is a flowchart which shows the operation flow of Example 4. FIG. It is a figure for demonstrating the sound reduction effect by changing the measurement order.

Hereinafter, preferred embodiments of the MRI apparatus according to the present invention will be described in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiment of the invention, those having the same function are designated by the same reference numerals, and the repeated description thereof will be omitted.

First, an overall outline of the MRI apparatus will be described with reference to FIG. The MRI apparatus obtains a tomographic image of a subject using an NMR phenomenon, and 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 central processing. It is configured to include a device (CPU) 8.

The static magnetic field generation system 2 is uniform in the space around the subject 1 in the direction orthogonal to the body axis of the subject 1 in the case of the vertical magnetic field method, and in the direction of the body axis of the subject 1 in the case of the horizontal magnetic field method. Generates a static magnetic field. Therefore, a permanent magnet, a normal electromagnet, or a superconducting magnet as a static magnetic field generation source is arranged around the subject 1.

The gradient magnetic field generation system 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 (resting coordinate system) of the MRI apparatus, and a gradient magnetic field that drives each gradient magnetic field coil. It has a power source 10. By driving the gradient magnetic field power supply 10 of each coil according to the instruction 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 two directions 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 to encode the position information in each direction in the echo signal.

The sequencer 4 is a control means for repeatedly applying a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence. It operates under the control of the CPU 8 and sends various commands necessary for collecting tomographic image data of the subject 1 to the transmitting system 5, the gradient magnetic field generating system 3, and the receiving system 6.

The transmission system 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 has 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 commanded by 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 the high frequency coil 14a, an RF pulse is applied to the subject 1.

The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (reception coil) 14b and a signal amplifier 15 on the receiving side. , A orthogonal phase detector 16 and an A / D converter 17. The NMR signal of the response of the subject 1 induced by the electromagnetic wave emitted from the high frequency coil 14a on the transmitting side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15. The signals are divided into two orthogonal systems 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 sent to the signal processing system 7.

The signal processing system 7 performs various data processing and displays and saves processing results, and includes a storage device such as a RAM 22 and a ROM 21, an external storage device such as an optical disk 19 and a magnetic disk 18, and a display such as a CRT and an FPD. Has 20 and. When the 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 resulting tomographic image of the subject 1 on the display 20 and an external storage device. Record on a magnetic disk 18 or the like.

The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed by the signal processing system 7, and has a trackball, a mouse 23, and a keyboard 24. The operation unit 25 is arranged close to the display 20, and the operator interactively controls various processes of the MRI apparatus 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 transmitting 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, if the vertical magnetic field method is used. As described above, in the case of the horizontal magnetic field method, the subject 1 is installed so as to surround the subject 1. Further, the high frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

Currently, the nuclides to be imaged by the MRI apparatus are hydrogen nuclides (protons), which are the main constituents of the subject, as those that are widely used clinically. By imaging the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state, the morphology or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

FIG. 2A shows an example 201 of arranging the echo signal in the k-space (the kp-ks space is shown) in the Cartesian scan. The echo signals 202 of each phase encode of the Cartesian scan are arranged so as to be arranged in parallel. Further, FIG. 2B shows an example 203 of arranging the echo signal in the k-space (the kp-ks space is shown) in the radial scan. Each blade (echo signal) 204 of the radial scan is arranged so as to be arranged in a radial pattern. Reference numeral 205 denotes a measurement angle at the time of acquisition of each echo signal of the radial scan.

In this embodiment, the sound pressure characteristics due to changes in phase encoding and slice encoding and the sound pressure characteristics depending on the axis of the rest coordinate system are analyzed, and the measurement is performed in consideration of each characteristic and the desired image contrast. The sound pressure level is smoothed by determining the order. As a result, the sound pressure level based on human hearing (so-called A-characteristic sound) is reduced while maintaining the image quality.

In the first embodiment, a method of reducing the sound pressure will be described by taking as an example the case of controlling the measurement order of the phase encoding and the slice encoding in the Cartesian scan of the gradient echo system sequence of 3D imaging.

Before imaging, the operator measures the sound pressure characteristics λx, λy, and λz by applying a gradient magnetic field for each of the three axes X, Y, and Z, and stores them in the ROM 21. In measuring the sound pressure characteristic λx, for example, a sound collecting microphone is installed near the head of the subject 1 at the time of imaging, a gradient magnetic field Gx is applied to the X-axis at a repeating interval TR, and the generated sound pressure Ax is measured. Then (Equation 1) is used. Similarly, in measuring the sound pressure characteristic λy, a gradient magnetic field Gy is applied to the Y-axis at a repeating interval TR, and the generated sound pressure Ay is measured and obtained by (Equation 2). In measuring the sound pressure characteristic λz, a gradient magnetic field Gz is applied to the Z axis at a repeating interval TR, and the generated sound pressure Az is measured and obtained by (Equation 3). The repetition interval TR is defined for each sequence.

The sound pressure characteristics λx, λy, and λz are uniquely determined by the static magnetic field generation system 2 and the gradient magnetic field generation system 3, and once they are measured, they do not have to be measured again, and the same device. If the model has a configuration, it is not necessary to measure for each device if the measurement is performed with one device.

The processing flow at the time of imaging will be described with reference to the flowchart shown in FIG. The CPU 8 displays an imaging condition input screen on the display 20, and the operator inputs the imaging conditions (S601). The imaging conditions include a parameter "silence" (having an on or off value) that selects whether or not to determine the data measurement order based on the sound pressure level, and the operator inputs the data in the same manner as other parameters. For example, a selection screen 610 may be provided on the imaging condition input screen so that the operator can select on / off. The imaging conditions of this example are 3D imaging, gradient echo sequence, Cartesian scan, frequency encoding Freq # is 256, phase encoding Phase # is 256, slice encoding Slice # is 32, and the desired imaging section is desired. It is assumed that the position is input.

When step S601 is completed, imaging is started, and based on the input imaging conditions, the CPU 8 has a k-space data area for the Freq # × Phase # × Slice # = 256 × 256 × 32 matrix indicated by the number of encodings in the RAM 22. (S602). In the following description, the frequency encoding axis is kf, the phase encoding axis is kp, the slice encoding axis is ks, and the center of each axis is 0, −Freq # / 2 ≦ kf <Freq # / 2, −Phase # / 2. Let ≤kp <Phase # / 2, -Slice # / 2 ≤ ks <Slice # / 2. FIG. 3 shows an example of the kp−ks space. 301 is the center of k-space, and the arrangement (encoding amount) of each echo signal is defined in the form of (kp, ks) according to the amount of the gradient magnetic field.

Next, in step S603, when the parameter "silence" is off (first measurement mode), the process proceeds to determine the data measurement order (S604), and when it is on (second measurement mode), the sound pressure level is reduced. The data measurement order is determined so as to be performed (S605).

Step S604 corresponds to the conventional determination of the data measurement order. The CPU 8 calculates the data measurement order by the following calculation and stores it in the RAM 22. In the case of the kp-ks space shown in FIG. 3, for the phase encoding axis kp and the slice encoding axis ks in the k space, starting from the maximum of each axis toward the minimum (or conversely, starting from the minimum toward the maximum). Measure one point at a time sequentially, and measure the slice encoding axis ks before the phase encoding axis kp. Therefore, the measurement order is determined so that the encoded amount of the echo signal gradually decreases or gradually increases (that is, monotonously changes) along each axis. The k-space coordinates kp (i) and ks (i) of the echo signal measured at the number of repetitions i are obtained by (Equation 4) and (Equation 5), respectively. However, RoundOFF in (Equation 4) indicates rounding down after the decimal point, and i Mod j in (Equation 5) indicates the remainder of i divided by j. Also, i is 1 to Phase # × Slice #, that is, 1 to 8192.

On the other hand, a procedure (S605) for determining the data measurement order based on the sound pressure level will be described. The CPU 8 calculates the data measurement order by the following calculation and stores it in the RAM 22. FIG. 4 shows the phase encode pulse and slice encode pulse 401 to 404 applied to the voxels 301 to 304 (see FIG. 3) arranged at predetermined positions in the k space (kp-ks space). It is a thing. The intensity Gp of the phase-encoded pulse and the intensity Gs of the slice-encoded pulse are determined by using the encoding amount kp (Equation 4) or the encoding amount ks (Equation 5), its application time t, and the gyromagnetic ratio γ (Equation 6), (Equation 6). It is determined in 7).

The phase-encoded pulse Gp and the slice-encoded pulse Gs are gradient magnetic fields applied to the axes of k-space (phase-encoded axis and slice-encoded axis), respectively, and the gradient magnetic fields in a predetermined k-space in FIG. 3 are as shown in FIG. Become. The magnetic field strengths 401 to 404 indicate the magnetic field strengths of Voxel 301 to 304 in FIG. 3, respectively. That is, in voxel 301 ((kp, ks) = (0, 0)), the magnetic field strength is 0 for both the phase-encoded pulse Gp and the slice-encoded pulse Gs. In voxel 302 ((kp, ks) = (l, 0)), the phase-encoded pulse Gp has a magnetic field strength L, and the slice-encoded pulse Gs has a magnetic field strength of 0. In voxel 303 ((kp, ks) = (m1, m2), | l | <| m1 |), the phase encode pulse Gp is the magnetic field strength M1 (where L <M1), and the slice encode pulse Gs is the magnetic field strength M2. be. At voxel 304 ((kp, ks) = (n1, n2), | m1 | = | n1 | and | m2 | <| n2 |), the phase-encoded pulse Gp has a magnetic field strength of N1 (where M1 = N1) and a slice. The encoded pulse Gs has a magnetic field strength of N2 (where M2 <N2). In this way, the magnetic field strength applied to the axis of the k-space increases as the distance from the center of the k-space increases on the axis of the k-space.

Projection is performed on the gradient magnetic fields Gx, Gy, and Gz from (Equation 8) using the matrix O (f, p, s) indicating the imaging cross-sectional angle. However, the frequency-encoded pulse is not taken into consideration, and Gf is set to 0.

Using the gradient magnetic fields Gx (kp, ks), Gy (kp, ks), Gz (kp, ks) and the sound pressure characteristics λx, λy, λz, the sound pressure A (kp, ks) at the k-space position (kp, ks) kp, ks) is calculated for each k-space position from (Equation 9).

The sound pressures A (kp, ks) at each k-space position obtained by (Equation 9) are arranged in descending order to form an array Aasc (j), and the k-space data measurement order, that is, the k-space coordinates kp (i) in the number of repetitions i. ) And ks (i) are determined by (Equation 10). Here, Aasc (j) .kp refers to the k-space position kp in Aasc (j), and Aasc (j) .ks refers to the k-space position ks in Aasc (j). (Equation 10) indicates that the k-space position where the sound pressure is relatively high and the k-space position where the sound pressure is relatively low are measured alternately, and the data measurement order shall be such an order. Therefore, the sound pressure received by humans is averaged regardless of the k-space position.

FIG. 14 shows the sound reduction effect by changing the measurement order. FIG. 14 shows the actual measurements according to the measurement order described above, and the graph 1401 shows the noise level generated when the parameter “silence” is turned off, that is, when the echo signal is filled in the k-space in step S604. Graph 1403 shows the sound pressure experienced in that case. Further, the graph 1402 shows the noise level (normalized by the maximum noise) generated when the parameter “silence” is turned on, that is, when the echo signal is filled in the k-space in step S605. It shows the sound pressure experienced in that case. In this way, by alternately measuring the k-space position where the sound pressure is high and the k-space position where the sound pressure is low, the sound pressure perceived by a person becomes gentle and the peak is also reduced. The measurement order according to (Equation 10) is an example, and the measurement order is determined so as to alternately measure the k-space position where the sound pressure is relatively high and the k-space position where the sound pressure is relatively low. It is possible to reduce the perceived sound pressure.

Next, the sequencer 4 executes a pulse sequence using the k-space coordinates kp (i) and ks (i) in the k-space data measurement order stored in the RAM 22 from the CPU 8, that is, in the number of repetitions i (S606). FIG. 5 shows an example of the measurement order when the parameter “silence” is turned on (second measurement mode). Measure from number 50001 to 58192. As shown in FIG. 5, according to the example according to (Equation 10), the echo signal is filled so as to start from the outermost part of the k-space at the odd-numbered number and toward the center of the k-space as a whole at the odd-numbered number, and to the k-space at the even-numbered number. The echo signal is filled from the central part toward the outermost part of the k-space as a whole.

Next, the signal processing system 7 arranges the acquired signal in the k-space data area according to the order of data measurement in the k-space stored in the RAM 22, and executes various data processing and display and storage of the processing result (S607). ).

In this embodiment, the frequency response function (FRF) may be used instead of the sound pressure characteristics λx, λy, and λz. The FRF is a sound pressure when a gradient magnetic field that changes with a sine wave is applied, and the FRF for each frequency is acquired in advance and stored in the ROM 21 in the same manner as in the case of the sound pressure characteristic. As a result, the sound pressure can be predicted by frequency-converting an arbitrary gradient magnetic field pulse shape and applying FRF.

Further, in this embodiment, the Cartesian scan of the gradient echo system sequence of 3D imaging has been described, but it may be applied to 2D imaging, spin echo system sequence, and high-speed spin echo system sequence.

Next, Example 2 will be described. The difference from the first embodiment is that the formula for obtaining the sound pressure A includes the weight related to the sound pressure related to the additional function. Hereinafter, different parts will be mainly described, and the same description will be omitted. As an additional function, the rephase function will be described as an example. The effect of applying the rephase function is strong in the low region of the k-space where the strength of the gradient magnetic field is relatively weak, and weak in the high region of the k-space where the strength of the gradient magnetic field is relatively strong. Further, in the rephase function, since a gradient magnetic field pulse is generally applied as an additional pulse more than usual, the sound pressure at the time of application tends to be high. Therefore, for example, by applying the rephase pulse in the low region of k-space and not applying the rephase pulse in the high region of k-space, it is possible to suppress the sound pressure while obtaining the rephase effect.

An application pattern when a rephase pulse to the frequency axis is applied as an additional pulse will be described with reference to FIG. 7. 701 is the RF axis, 702 is the frequency axis, 703 is the echo axis, 704 and 711 are 90 ° RF pulses, and 705 and 712 are 180 ° RF pulses. 706 indicates a rephase pulse, 707 and 713 indicate a dephase pulse, 708 and 714 indicate an encode pulse, and 709 and 715 indicate an acquired echo signal. 710 indicates a case where a rephase pulse is applied, and 716 indicates a case where a rephase pulse is not applied.

The processing flow at the time of imaging will be described with reference to the flowchart shown in FIG. Also in the second embodiment, when the parameter “silence” is selected on / off in step S803 to determine whether to change the measurement order, and the “silence” is off (first measurement mode). The determination of the data measurement order (S804) is the same as in the first embodiment.

When the parameter "silence" is on (second measurement mode), when the rephase pulse function is applied, the k-space is divided into a low range in the center and a high range in the periphery in step S809. do. FIG. 9 shows an example of k-space. The central region 901 indicates a low region and the intensity of the gradient magnetic field is relatively weak, and the peripheral donut-shaped region 902 indicates a high region and the intensity of the gradient magnetic field is relatively strong. The boundary can be arbitrarily set as an imaging condition.

In step S810, the changes in sound pressure ζs, ζp, and ζf depending on the presence or absence of the rephase function are written to the RAM 22. ζs indicates the amount of change depending on the presence or absence of the slice direction rephase, ζp indicates the amount of change depending on the presence or absence of the phase direction rephase, and ζf indicates the amount of change depending on the presence or absence of the frequency direction rephase. The amount of change ζ can be obtained from (Equation 11) to (Equation 13) from the sound pressure A_on with the rephase function and the sound pressure A_off without the rephase function.

Alternatively, the amount of change may be calculated as ζs, ζp, ζf from the value calculated by FRF. When the rephase function is applied, Gx, Gy, and Gz are calculated by (Equation 14), and the sound pressure is calculated by (Equation 9) (S810).

Based on the calculated sound pressure, the measurement order is determined in the same manner as in Example 1 (S805).

Subsequent steps S806 and S807 follow the same procedure as steps S606 and S607 of the first embodiment. By changing the measurement order in this way, the sound reduction effect as shown in FIG. 14 can be obtained as in the first embodiment while obtaining the rephase effect.

Next, Example 3 will be described. The difference from Examples 1 and 2 is that the effect of the additional function is taken into consideration in the method of determining the data measurement order in the k-space. Hereinafter, different parts will be mainly described, and the same description will be omitted. The MTC function will be described as an example as an additional function.

The application pattern of the MTC pulse to the frequency axis will be described with reference to FIG. 1001 is the RF axis, 1002 is the frequency axis, 1003 is the echo axis, 1004 is the 90 ° RF pulse, and 1005 is the 180 ° RF pulse. 1006 indicates a strongly applied MTC pulse, 1011 indicates a weakly applied MTC pulse, 1007, 1014 indicates a dephase pulse, 1008, 1015 indicates an encode pulse, and 1009, 1016 indicates an acquired echo signal. 1010 indicates a case where MTC is strongly applied, and 1017 indicates a case where MTC is applied weakly.

The effect of applying the MTC function is strong in the low region of the k-space where the strength of the gradient magnetic field is relatively weak, and weak in the high region where the strength of the gradient magnetic field is relatively strong. Further, when the MTC pulse is strongly applied at every repetition interval TR, the SAR (specific absorption rate) becomes high, which limits the imaging in a high magnetic field. When a substance is irradiated with electromagnetic waves, the molecules in the substance vibrate and RF is absorbed inside to generate heat. Therefore, SAR is defined as an index indicating the degree to which RF is absorbed by the human body. SAR has an upper limit, and imaging conditions beyond this upper limit cannot be set. Therefore, the SAR is generated by applying a strong MTC pulse in the low region of the k-space where the effect of applying the MTC function is high, and weakly applying the MTC pulse in the high region of the k-space where the effect of applying the MTC function is low. It becomes possible to obtain the MT effect while suppressing it. On the other hand, when the application interval of the MTC pulse is long, the magnetization is restored and the MT effect is weakened, and by continuously applying the MTC pulse, the magnetization is saturated and the MT effect is enhanced. Therefore, in the low frequency range of the k-space, the MT effect can be effectively obtained by setting the interval between the MTC pulses to be short and setting the measurement order so as to saturate near the center of the k-space.

The processing flow at the time of imaging will be described with reference to the flowchart shown in FIG. Also in the third embodiment, when the parameter “silence” is selected on / off in step S1103 to determine whether to change the measurement order, and the “silence” is off (first measurement mode). The determination of the data measurement order (S1104) is the same as in the first embodiment.

When "silence" is on (second measurement mode), how to determine the measurement order when the MTC function is applied will be described with reference to FIGS. 12A and 12B. The region (low region) 1201 to which the MTC pulse is strongly applied and the region (high region) 1202 to which the MTC pulse is weakly applied are divided (S1109). The boundary of division can be arbitrarily set as an imaging condition. The sound pressure A1 (kp, ks) of the strongly applied region (low range) 1201 is determined by (Equation 15), and the sound pressure A2 (kp, ks) of the weakly applied region (high range) is determined by (Equation 16). calculate.

The array Aasc1 (j) in which the sound pressures A1 (kp, ks) obtained by (Equation 15) are arranged in descending order, and the array Aasc2 (j) in which the sound pressures A2 (kp, ks) obtained by (Equation 16) are arranged in descending order. ). In step S1105, the measurement order is first determined based on the sequence Aasc2 (j) in the high frequency range 1202 (Equation 10), and then in the low frequency range 1201 based on the sequence Aasc1 (j) (Equation 10). By separating the k-space into a high region and a low region in this way and determining the measurement order so as to reduce the sound pressure in each region, it is possible to reduce the sound pressure while sufficiently obtaining the MT effect. FIG. 12B shows an example of the measurement order. For example, in the measurement in the high region, the measurement in the outer edge region 1206, which is the higher region, and the measurement in the inner edge region 1205, which is the lower region, are alternately performed in the high region 1202. For example, in the odd-numbered order, the echo signal is filled so as to start from the outermost part of the outer edge region 1206 and toward the inner edge region 1205 as a whole, and in the even-numbered number, it starts from the innermost part of the inner edge region 1205 and head toward the outer edge region 1206 as a whole. The echo signal is filled as follows. Similarly, in the measurement in the low frequency range, the measurement in the peripheral region 1204, which is the higher frequency band, and the measurement in the central region 1203, which is the lower frequency band, are alternately performed. For example, in the odd-numbered position, the echo signal is filled so as to start from the outermost part of the peripheral area 1204 and toward the central area 1203 as a whole, and in the even-numbered number, it starts from the central part of the central area 1203 and head toward the peripheral area 1203 as a whole. The echo signal is filled as follows.

When the repetition interval TR is short, the MT effect can be obtained by alternately measuring the high frequency 1202 and the low frequency 1201 as in the first embodiment, but the high frequency 1202 and the low frequency 1202 as in the third embodiment. If the measurement order is determined separately from the region 1201, a high MT effect can be obtained even when the repetition interval TR is long. Further, in the low frequency range 1201, the strength of the applied gradient magnetic field is relatively weak, and in the high frequency range 1202, the k-space position where the sound pressure is high and the k-space position where the sound pressure is low are alternately measured. However, in the low frequency range 1201, it is possible to perform the measurement in the conventional measurement order (measurement order in which the encoding amount of the echo signal arranged in the k space is gradually decreased or gradually increased).

Regarding steps S1101, S1102, S1104, S1106, and S1107, the same procedure as in the flowchart S601, S602, S604, S606, and S607 of the first embodiment is taken. By changing the measurement order in this way, the sound reduction effect as shown in FIG. 14 can be obtained as in the first embodiment while obtaining the MT effect.

Next, Example 4 will be described. The difference from Examples 1, 2 and 3 is that the case where a plurality of different angles are measured with respect to the gradient magnetic field coil is taken into consideration. Hereinafter, different parts will be mainly described, and the same description will be omitted. Examples of performing measurement of a plurality of different angles include multi-stack measurement and radial scan measurement. A radial scan measurement will be described as an example.

The processing flow at the time of imaging will be described with reference to the flowchart shown in FIG. Even when the radial scan is selected, it is the same as in the first embodiment until it is determined whether to change the measurement order by selecting on / off of “silence” in step S1303, and when the radial scan is selected, the parameter “ When "Silent" is selected (second measurement mode), the process proceeds to on in step S1303 and yes in step S1308. Since the radial scan measurement measures while rotating a plurality of different echo signals, the sound pressure caused by the gradient magnetic field coil changes depending on the angle (blade angle) at the time of acquiring each echo signal. The gradient magnetic fields Gx, Gy, and Gz are calculated by (Equation 17) using the matrix O (Angle) indicating the angle (Angle) that changes in the measurement.

Sound using the gradient magnetic fields Gx (kp, ks, Angle), Gy (kp, ks, Angle), Gz (kp, ks, Angle) calculated by (Equation 17) and the sound pressure characteristics λx, λy, λz. The pressure A (kp, ks, Angle) is calculated by (Equation 18).

In step S1305, the array Aasc (j) in which the sound pressures A (kp, ks, Angle) are arranged in ascending order is obtained, and the measurement order is determined in the same manner as in the first embodiment.

Note that steps S1301, S1302, S1304, S1306, and S1307 are the same as steps S601, S602, S604, S606, and S607 in the flowchart of the first embodiment (FIG. 6). By changing the measurement order, the sound reduction effect as shown in FIG. 14 can be obtained.

1: Subject 2: Static magnetic field generation system 3: Diagonal magnetic field generation system 4: Sequencer, 5: Transmission system, 6: Reception system, 7: Signal processing system, 8: Central processing device (CPU), 9: Diagonal magnetic field coil, 10: Diagonal magnetic field power supply, 11: High frequency oscillator, 12: Modulator, 13: High frequency amplifier, 14a: High frequency coil (transmission coil), 14b: High frequency coil (reception coil), 15: Signal amplifier, 16: Orthogonal phase detector, 17: A / D converter, 18: magnetic disk, 19: optical disk, 20: display, 21: ROM, 22: RAM, 23: trackball or mouse, 24: keyboard.

Claims (11)

A static magnetic field generation system that generates a static magnetic field in the space where the subject is placed,
A gradient magnetic field generation system that generates a gradient magnetic field superimposed on the static magnetic field,
A transmission system that irradiates the subject with a high-frequency magnetic field pulse,
A receiving system that detects an echo signal emitted from the subject in response to the application of a high-frequency magnetic field pulse by the transmitting system, and a receiving system.
A sequencer that controls generation of a gradient magnetic field by the gradient magnetic field generation system, irradiation of a high-frequency magnetic field pulse by the transmission system, and detection of an echo signal by the reception system based on a predetermined pulse sequence.
In executing the predetermined pulse sequence, the echo signal at the k space position where the sound pressure generated by the application of the gradient magnetic field by the gradient magnetic field generation system at the time of measuring the echo signal is relatively high, and the gradient magnetic field generation system at the time of measuring the echo signal. A magnetic resonance imaging device including a processing device for determining a measurement order so as to alternately measure an echo signal at a k space position where the sound pressure generated by applying a gradient magnetic field is relatively low. In claim 1,
It has a memory that stores the sound pressure characteristics for the gradient magnetic field applied to each axis of the rest coordinate system in advance.
The processing device calculates the sound pressure at the k-space position based on the encoding amount of each echo signal arranged in the k-space and the sound pressure characteristic, and determines the measurement order based on the calculated sound pressure at the k-space position. Magnetic resonance imaging device. In claim 1,
It has a memory that stores in advance a frequency response function that indicates the sound pressure when a gradient magnetic field that changes with a sine wave is applied to each axis of the rest coordinate system for each frequency.
The processing device calculates the sound pressure at the k-space position based on the shape of the gradient magnetic field pulse of each echo signal arranged in the k-space and the frequency response function, and determines the measurement order based on the calculated sound pressure at the k-space position. A magnetic resonance imaging device to determine. In claim 1,
It has a first measurement mode and a second measurement mode.
When the first measurement mode is selected, the processing device determines the measurement order so that the encoding amount of the echo signal arranged in the k-space is gradually decreased or gradually increased, and the second measurement mode is selected. In this case, a magnetic resonance imaging device that determines the measurement order so as to alternately measure the echo signal at the k-space position where the sound pressure is relatively high and the echo signal at the k-space position where the sound pressure is relatively low. .. In claim 1,
The predetermined pulse sequence includes additional pulses generated by the gradient magnetic field generation system.
The processing device is a magnetic resonance imaging device that determines a measurement order based on the sound pressure at the k-space position calculated including the amount of change in the sound pressure due to the additional pulse when calculating the sound pressure at the k-space position. In claim 5,
The sequencer applies the additional pulse in the k-space low region where the intensity of the gradient magnetic field is relatively weak, and controls so that the additional pulse is not applied in the k-space high region where the intensity of the gradient magnetic field is relatively strong. Resonance imaging device. In claim 5 or 6,
The additional pulse is a magnetic resonance imaging device that is a rephase pulse. In claim 1,
The predetermined pulse sequence includes a prepulse generated by the transmission system.
The sequencer controls so that the prepulse is strongly applied in the k-space low region where the intensity of the gradient magnetic field is relatively weak, and the prepulse is weakly applied in the k-space high region where the intensity of the gradient magnetic field is relatively strong.
In the processing device, the measurement of the echo signal included in the k-space high region precedes the measurement of the echo signal included in the k-space low region, and the sound pressure is relatively high in the k-space high region in the k-space. A magnetic resonance imaging device that determines a measurement order so as to alternately measure a position echo signal and an echo signal at a k-space position where the sound pressure is relatively low in the k-space high region. In claim 8.
The processing device alternately measures the echo signal at the k-space position where the sound pressure is relatively high in the k-space low range and the echo signal at the k-space position where the sound pressure is relatively low in the k-space low range. A magnetic resonance imaging device that determines the measurement order. In claim 8 or 9,
The prepulse is a magnetic resonance imaging device that is any one of an MTC (Magnetization Transfer Control) pulse, a fat suppression pulse, and a saturation pulse. In claim 1,
The echo signal to the k-space is a magnetic resonance imaging device arranged by the Cartesian scan method or the radial scan method.

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