WO2016178413A1

WO2016178413A1 - Magnetic resonance imaging device

Info

Publication number: WO2016178413A1
Application number: PCT/JP2016/063421
Authority: WO
Inventors: 眞次黒川; 陽谷口; 由香里山本; 久晃越智
Original assignee: 株式会社日立製作所
Priority date: 2015-05-07
Filing date: 2016-04-28
Publication date: 2016-11-10
Also published as: JP6590514B2; JP2016209252A

Abstract

Provided are a gradient magnetic field pulse in a new shape and having an effect in suppressing noise, and an MRI device that executes a pulse sequence including the gradient magnetic field pulse. The MRI device of the present invention is provided with: a magnetic field application unit that applies high frequency magnetic field pulses and gradient magnetic field pulses in accordance with a pulse sequence describing the timing and the applied strength of the high frequency magnetic field pulses and the gradient magnetic field pulses; and a receiving unit for receiving nuclear magnetic resonance signals generated from a test object because of the magnetic field applied by the magnetic field application unit. The shape of at least one type of the gradient magnetic field pulses included in the pulse sequence is a shape that minimizes noise and is obtained under a condition that the shape satisfies the slew rate of the gradient magnetic field at the magnetic field application unit.

Description

Magnetic resonance imaging system

The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to a technique for reducing noise during imaging.

One of the problems in the MRI apparatus is the reduction of noise generated by switching the gradient magnetic field pulse at high speed. In general, there is a trade-off relationship between noise reduction and image quality improvement (high-speed sequence adopted for that purpose), and it is important to reduce noise while suppressing deterioration in image quality as much as possible.

One approach to this problem is to change the parameters of the gradient magnetic field pulse so as to minimize noise in a given imaging condition or imaging sequence. For example, Patent Document 1 discloses internal repetition of a gradient magnetic field. A technique is described that determines the slew rate and gradient magnetic field strength to achieve low noise with a fixed time (intra-repetition-interval). Another approach is to determine an imaging sequence that can be realized within an acceptable noise level, as opposed to the above approach. For example, in Patent Document 2, an acceptable noise level is set and a selected pulse is selected. It is disclosed that the noise level when the sequence is executed is calculated and the pulse sequence is changed when the sequence exceeds an allowable value. Patent Document 3 proposes a technique related to a UI that calculates a noise level of a selected imaging sequence, displays the result, and allows an operator to change the imaging sequence according to an expected value of the noise level. ing.

In addition to the techniques described above, several attempts have been proposed to devise the shape of the gradient magnetic field pulse itself. For example, Patent Document 4 discloses a technique for changing the shape of a gradient magnetic field pulse by providing a filter before the gradient magnetic field pulse generator. Non-Patent

Documents

1 and 2 propose changing the shape of the gradient magnetic field pulse to a shape typified by a sine wave instead of a trapezoid.

US Patent Application Publication No. 2013/0200893 US Pat. No. 6,407,548 US Patent Application Publication No. 2013/0275086 International Publication No. 98/13703

In each of the above-described conventional techniques, the gradient magnetic field pulse shape is based on a known model waveform such as a trapezoid or a sine wave, and the optimization is within the range of the model waveform. .

The conventional noise reduction approach is to optimize the imaging sequence when the noise level is kept within an allowable range, or to suppress the noise level under the conditions of the given imaging sequence. In doing so, it is necessary to determine the priority of noise suppression and image quality improvement in a trade-off relationship in consideration of the tolerance for the sound to be inspected and the image quality that is the purpose of the inspection.

An object of the present invention is to obtain a shape of a gradient magnetic field pulse having a noise suppressing effect, to provide an MRI apparatus that executes a pulse sequence including such a gradient magnetic field pulse, and further, an inspector has a degree of freedom. An MRI apparatus that can determine the priority of noise suppression and image quality improvement is provided. The subject of the present invention is further to present the relationship between the parameters such as the bandwidth and the noise level to the inspector in the MRI apparatus adopting the gradient magnetic field pulse whose waveform is not trapezoidal, and to set the imaging condition of the inspector. Including providing useful information.

In order to solve the above problems, the present inventor searches for a waveform of a gradient magnetic field pulse that can suppress the noise most under the condition of the shape considering the output of the MRI apparatus, and the waveform having a predetermined characteristic suppresses the noise most. I found out that I can do it. The MRI apparatus of the present invention comprises a pulse sequence including a novel gradient magnetic field pulse having this specific waveform.

The MRI apparatus according to the present invention further includes means for providing an operator with imaging parameters related to noise in relation to noise when performing imaging with noise reduction. Further, the MRI apparatus of the present invention comprises means for allowing the operator to select the priority for noise reduction and image quality improvement.

According to the present invention, by using a gradient magnetic field pulse having a specific waveform as a gradient magnetic field pulse applied for imaging, noise caused by application of the gradient magnetic field pulse can be minimized. Further, according to the present invention, it is possible to provide information that makes it easy for an operator to determine imaging conditions, and it is possible to simplify trial and error and complicated procedures for noise reduction.

1 is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied. Functional block diagram of control unit The figure which shows an example of the pulse sequence integrated in the MRI apparatus of this invention Diagram showing an example of gradient magnetic field pulse shape that minimizes noise Flow showing the procedure for obtaining the gradient magnetic field pulse shape of the first embodiment The figure explaining the step of FIG. The figure which shows an example of the frequency response function (FRF) of an apparatus The figure which shows an example of the gradient magnetic field pulse shape calculated | required by the procedure of FIG. The figure which shows the other example of the gradient magnetic field pulse shape calculated | required by the procedure of FIG. Graph showing the noise level when the rephase time is varied for gradient magnetic field pulse shapes that minimize noise The figure explaining the procedure which calculates | requires the gradient magnetic field pulse shape of the example 1 of a change The figure which shows an example of the gradient magnetic field pulse shape calculated | required by the procedure of the modification 1 The figure which shows an example of the gradient magnetic field pulse shape calculated | required by the procedure of the modification 2 The figure which shows an example of the gradient magnetic field pulse shape calculated | required by the procedure of the modification 3 The figure explaining the idea of the method of calculating | requiring the gradient magnetic field pulse shape by 2nd embodiment The figure explaining the idea of the method of calculating | requiring the gradient magnetic field pulse shape by 2nd embodiment The figure which shows an example of the model for calculating | requiring the gradient magnetic field pulse shape by 2nd embodiment. The figure which shows an example of the gradient magnetic field pulse shape calculated | required by the method of 2nd embodiment The figure which shows the shape after filtering the shape of FIG. The figure which shows the other example of the model for calculating | requiring the gradient magnetic field pulse shape by 2nd embodiment. Flow showing procedure for obtaining gradient magnetic field pulse waveform according to third embodiment The figure explaining the procedure by 3rd embodiment. Flow showing procedure of modified example of third embodiment The graph which shows the relationship (an example) between the imaging parameter used with the MRI apparatus of 4th embodiment, and a noise level The figure which shows the example of a display screen in the MRI apparatus of 4th embodiment. Graph showing another example of relationship between imaging parameters and noise level The figure which shows the other example of the display screen in the MRI apparatus of 4th embodiment. The figure which shows the other example of the display screen in the MRI apparatus of 4th embodiment. The figure which shows the other example of the display screen in the MRI apparatus of 4th embodiment.

Hereinafter, embodiments of an MRI apparatus to which the present invention is applied will be described.

<First embodiment>
The MRI apparatus of the present embodiment includes a magnetic field application unit that applies a high frequency magnetic field pulse and a gradient magnetic field pulse according to a pulse sequence that describes the application intensity and timing of the high frequency magnetic field pulse and the gradient magnetic field pulse, and a magnetic field applied by the magnetic field application unit. And a receiving unit that receives a nuclear magnetic resonance signal generated from the inspection object. In the MRI apparatus of this embodiment, at least one type of gradient magnetic field pulse included in the pulse sequence used for controlling the magnetic field application unit satisfies the slew rate of the gradient magnetic field in the magnetic field application unit and is symmetric. It has the required shape that minimizes noise. One aspect of the shape of the gradient magnetic field pulse that minimizes noise is a shape depicted by a curve having three or more inflection points.

The shape of the gradient magnetic field pulse that minimizes the noise is, for example, a shape formed by sequentially stacking a plurality of minute rectangles until a predetermined area is reached, and until the final gradient magnetic field pulse shape is formed by stacking the minute rectangles. Is a shape obtained by updating the provisional shape so as to be a shape that minimizes noise. Alternatively, the initial value is a model combining a plurality of waveforms specified by the shape parameter, and the gradient magnetic field pulse shape obtained when the shape parameter is changed becomes a shape that minimizes noise. It is the shape obtained by repeating the update.

Hereinafter, the configuration of the MRI apparatus of the present embodiment will be described in detail with reference to FIGS. 1 and 2.
In all the drawings for explaining the embodiments of the present invention, those having the same function are denoted by the same reference numerals, and repeated explanation thereof is omitted.

An MRI apparatus 100 shown in FIG. 1 obtains a tomographic image of a subject 101 by using an NMR phenomenon, and a static magnetic field generation unit 120 that generates a static magnetic field and a subject 101 disposed in the static magnetic field. A gradient magnetic field generator 130 for applying a gradient magnetic field, a transmitter 150 for transmitting a high-frequency magnetic field pulse for exciting the magnetization of the subject 101 at a predetermined flip angle, and a receiver for receiving an echo signal generated by the subject 101 160, a controller 170 that reconstructs an image from the echo signal received by the receiver 160, and controls the operation of the gradient magnetic field generator 130, the transmitter 150, and the receiver 160 according to the imaging sequence, a sequencer 140, Is provided. The gradient magnetic field generation unit 130 and the transmission unit 150 correspond to the magnetic field application unit of the present invention.

The static magnetic field generator 120 generates a uniform static magnetic field in the space around the subject 101. A static magnet type, normal conduction type, or superconducting type static magnetic field generation source arranged around the subject 101 is used. Prepare. There are a vertical magnetic field method and a horizontal magnetic field method depending on the direction of the generated static magnetic field. In the vertical magnetic field method, a static magnetic field is generated in a direction perpendicular to the body axis. In the horizontal magnetic field method, a static magnetic field is generated in the body axis direction.

The gradient magnetic field generation unit 130 is a gradient magnetic field coil 131 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (apparatus coordinate system) of the MRI apparatus 100, and a gradient magnetic field power source that drives each gradient magnetic field coil. 132. The gradient magnetic field coil 131 drives the gradient magnetic field power supply 132 of each gradient magnetic field coil 131 in accordance with a command from the sequencer 140, thereby generating gradient magnetic field pulses Gx, Gy, Gz in the three axis directions of X, Y, and Z. Apply. Each of the gradient magnetic field pulses Gx, Gy, and Gz is applied in a direction orthogonal to the slice plane (imaging cross section) at the time of imaging, and sets the slice plane for the subject 101 and is orthogonal to the set slice plane. In addition, there is a role of applying the information in the remaining two directions orthogonal to each other and encoding position information in two directions into the NMR signal (echo signal). The MRI apparatus of this embodiment is characterized in that a pulse having a shape that minimizes noise is prepared as a pulse applied by the gradient coil 131.

The transmitter 150 irradiates the subject 101 with a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 101. And a high-frequency oscillator (synthesizer) 152, a modulator 153, a high-frequency amplifier 154, and a high-frequency coil (transmission coil) 151 on the transmission side. The high frequency oscillator 152 generates an RF pulse and outputs it at a timing according to a command from the sequencer 140. The modulator 153 amplitude-modulates the output RF pulse. The high-frequency amplifier 154 amplifies the amplitude-modulated RF pulse and supplies the amplified RF pulse to the transmission coil 151 disposed close to the subject 101. The transmission coil 151 irradiates the subject 101 with the supplied RF pulse.

The receiving unit 160 detects a nuclear magnetic resonance signal (echo signal, NMR signal) emitted by nuclear magnetic resonance of the nuclear spin constituting the living tissue of the subject 101, and receives a high-frequency coil (receiving coil) on the receiving side. 161, a signal amplifier 162, a quadrature detector 163, and an A / D converter 164. The reception coil 161 is disposed in the vicinity of the subject 101 and detects an NMR signal of the response of the subject 101 induced by the electromagnetic wave irradiated from the transmission coil 151. The detected NMR signal is amplified by the signal amplifier 162 and then divided into two orthogonal signals by the quadrature phase detector 163 at the timing according to the command from the sequencer 140, and each is digitally converted by the A / D converter 164. The amount is converted and sent to the controller 170. Although the receiving coil 161 may be composed of a single coil, a multiple array coil (phased array coil) in which a plurality of small coils are combined is often used.

The sequencer 140 functions as an imaging control unit together with the control unit 170, operates in accordance with instructions from the control unit 170, sends various commands necessary for collecting tomographic image data of the subject 101, the transmission unit 150, and the gradient magnetic field. The data is transmitted to the generation unit 130 and the reception unit 160. Thereby, the sequencer 140 repeatedly applies the RF pulse and the gradient magnetic field pulse according to a predetermined pulse sequence. The pulse sequence describes (determines) the high-frequency magnetic field, gradient magnetic field, signal reception timing and intensity, and there are various pulse sequences (basic pulse sequences) that differ depending on the imaging method. Stored in the storage device 172 in advance.

The control unit 170 controls the entire MRI apparatus 100, performs operations such as various data processing, displays and stores processing results, and includes a CPU 171, a storage device 172, a display device 173, and an input device 174. The storage device 172 includes an internal storage device such as a hard disk and an external storage device such as an external hard disk, an optical disk, and a magnetic disk. The storage device 172 stores data necessary for the calculation of the CPU 171 and data in the middle of calculation or as a calculation result. In this embodiment, a gradient pulse shape that minimizes noise or a pulse sequence using the gradient pulse shape is stored together with the basic pulse sequence. As will be described in detail later, when the gradient magnetic field pulse shape that minimizes the noise is obtained under the conditions of a predetermined imaging parameter, the pulse shape is the imaging parameter used as the condition. Stored together.

The display device 173 is a display device such as a CRT or a liquid crystal. The input device 174 is an interface for inputting various control information of the MRI apparatus 100 and control information of processing performed by the control unit 170, and includes, for example, a trackball or a mouse and a keyboard. The input device 174 is disposed in the vicinity of the display device 173. The operator interactively inputs instructions and data necessary for various processes of the MRI apparatus 100 through the input device 174 while looking at the display device 173. Further, it is possible to input imaging parameters necessary for executing the pulse sequence according to the target imaging method.

The CPU 171 implements each process of the control unit 170 such as control of the operation of each unit of the MRI apparatus 100 and various data processing by executing a program stored in advance in the storage device 172 according to an instruction input by the operator. . For example, when data from the receiving unit 160 is input to the control unit 170, the CPU 171 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 101 as a result on the display device 173. At the same time, it is stored in the storage device 172.

The data from the receiving unit 160 is normally data arranged in a data space called k-space. The coordinates (position) of the k space are determined by the application amount of the gradient magnetic field pulse given to the echo signal, and the points (k space matrix) to be acquired are determined according to the image size and the visual field. The control unit 170 controls the pulse sequence so as to measure points to be acquired, obtains data, and performs predetermined image processing on the data set obtained by measurement to obtain an image.

The CPU 171 has a function for creating a pulse sequence that minimizes noise. For example, as illustrated in FIG. 2, the CPU 171 can include a pulse sequence creation unit 180, a noise level calculation unit 181, and a noise level comparison unit 182. Further, a display control unit 183 is provided for causing the display device 173 to display information necessary when the operator creates or designs a pulse sequence. Some or all of these may be omitted depending on the embodiment of the present invention.

The pulse sequence creation unit 180 reads the basic pulse sequence and the imaging parameters stored in the storage device 172, and creates a pulse sequence (hereinafter also referred to as an imaging sequence to distinguish from the basic sequence) executed by the sequencer 140. At this time, if imaging that minimizes noise is selected by the operator, an imaging pulse sequence is created using the gradient magnetic field pulse shape stored in the storage device 172.

The noise level calculation unit 181 calculates the noise level of the created imaging pulse sequence using a frequency response function (FRF) unique to the apparatus. When the MRI apparatus includes the noise level calculation unit 181, the FRF measured in advance is stored in the storage device 172. The noise level comparison unit 182 compares the noise levels calculated by the noise level calculation unit 181 for a plurality of imaging pulse sequences, and causes the display device 173 to display the comparison result.

These functions are executed by a program incorporated in the CPU 171. That is, each of these units corresponds to a program including a predetermined algorithm mounted on the CPU 171. However, a part of this function may be replaced by hardware such as a well-known ASIC (Application Specific Integrated Circuit).

When imaging, the operator selects an arbitrary imaging method via the input device 174, so that the pulse sequence creation unit 180 reads a predetermined pulse sequence from the storage device 172 and stores the gradient magnetic field pulses stored in the storage device 172. A predetermined waveform is selected from the shape. The pulse sequence creation unit 180 selects these pulse sequences, gradient magnetic field pulse shapes, and imaging parameters selected by the operator via the input device 174, for example, echo time (TE), repetition time (TR), inter-echo time (IET). ), An imaging sequence executed by the sequencer 140 is created using an imaging field of view (FOV) or the like. When imaging that minimizes noise is set during imaging, a specific gradient magnetic field pulse shape that minimizes noise is selected, the lowest noise pulse sequence is executed, and imaging is performed.

Next, the gradient magnetic field pulse shape that minimizes noise will be described.
The gradient magnetic field pulses generally used for imaging include a slice selection gradient magnetic field pulse, a phase encode gradient magnetic field pulse and its rephase or dephase pulse, a frequency encode gradient magnetic field pulse and its rephase or dephase pulse, a spoiler pulse, etc. The specific gradient magnetic field pulse can be used for any of these various gradient magnetic field pulses. However, in the following description, the frequency encoded gradient magnetic field pulse that is essential for the pulse sequence and has a large influence on noise. The case where it applies to is demonstrated.

The type of the basic pulse sequence is not particularly limited. As an example, the FSE (Fast Spin Echo) sequence shown in FIG. 3 will be described. In FIG. 3, the horizontal axis RF indicates the RF pulse application timing, and Gs, Gp, and Gf indicate the slice gradient magnetic field, phase encode gradient magnetic field, and frequency encode gradient magnetic field, respectively. Although omitted in FIG. 3, the sampling time of the echo signal is set within the application time of the frequency encode pulse.

In the basic form of the FSE sequence, the frequency encode pulse 301 and its rephase pulse 302 are each trapezoidal as shown in FIG. On the other hand, the gradient magnetic field pulse employed in this embodiment is a gradient magnetic field pulse that has a special waveform 400 as shown in FIG. 4 and minimizes noise under given conditions. These gradient magnetic field pulses are obtained in advance by a greedy method using frequency response characteristics (FRF) of noise inherent to the device, and are stored in the storage device 172.

A method for obtaining a gradient magnetic field pulse shape that minimizes noise will be described.
First, regarding the shape, a target waveform area is set, and a shape condition is determined in consideration of feasibility by the MRI apparatus. The area of the waveform is determined by the encoding time and the gradient magnetic field strength (in other words, the reception bandwidth BW), but here, a provisional value may be set to determine the shape. Furthermore, conditions relating to imaging parameters such as the number of echo trains and inter-echo time IET may be set. Since these imaging parameters affect the frequency spectrum of the gradient magnetic field, and the noise level generated changes as it changes, these imaging parameters may also be parameters for minimizing noise. Then, to make the explanation simple, it is fixed.

First, as a shape condition, in this embodiment, a slew rate of the gradient magnetic field (Slew Rate) is limited, and a shape condition that only one extreme value is included in a section in which the symmetric and positive / negative signs are the same is added. The slew rate of the gradient magnetic field is a change amount per unit time of the gradient magnetic field strength T / m generated by the apparatus, and is determined by the performance of the gradient magnetic field amplifier that drives the gradient magnetic field coil. In a gradient magnetic field waveform with the horizontal axis representing time and the vertical axis representing gradient magnetic field strength, the gradient is limited by the slew rate. The condition of symmetry is intended to simplify the calculation, and the condition of having only one extreme value in the interval where the sign of the sign is the same is intended to prevent large fluctuations in the gradient magnetic field. This is because if the gradient magnetic field greatly fluctuates during reception, the substantial bandwidth (BWeff) at each sampling frequency fluctuates greatly, and it is considered that noise characteristics become unnatural and a desirable image quality is not obtained.

Incidentally, the substantial bandwidth is defined by the following equation (1).
BWeff = (γ / 2π) × G × FOV (1)
In the above equation, γ is the magnetic rotation ratio, G is the gradient magnetic field strength, and FOV is the imaging field of view. Although noise is proportional to the square root of the bandwidth (√ (BW)), in the case of non-uniform sampling where the gradient magnetic field strength is not constant during application, the sampling density must also be considered. It can be said that BWeff includes the influence of sampling density in BW.

The procedure for obtaining the gradient magnetic field pulse waveform that minimizes noise under the above conditions will be described with reference to the flowchart of FIG. First, a state in which no gradient magnetic field is applied is set as an initial state (S101), and a step of adding a small gradient magnetic field pulse shape (S102, S103) is repeated until the target area is reached. Adding a small gradient magnetic field pulse shape The rectangle added in step S103 indicates that the gradient magnetic field pulse that is formed when it is added to the gradient magnetic field pulse shape that is being updated depends on the set condition (only one extreme value in the same symmetrical and positive / negative sign interval). (S102).

The condition of symmetry can be satisfied by, for example, arranging one rectangle in the center. As shown in FIG. 6, the small gradient magnetic field pulse shape is composed of various rectangles having the same area which is sufficiently small, for example, a size of 1 / 10,000 of the final area. The slew rate may be limited by the maximum slew rate (T / m / s) of the line connecting the vertices of the superimposed rectangles.

The selection of a rectangle that minimizes noise is performed by multiplying the frequency spectrum of the waveform when the candidate rectangle is added to the gradient magnetic field pulse shape being updated by the frequency response function (FRF) of the sound pressure. This is done by calculating the level and selecting the rectangle that minimizes the noise level.

As the frequency response function, a value obtained in advance as a value unique to the apparatus is used. Here, a weighted A-weighted characteristic that takes into account the human hearing that is the inspection target of the MRI apparatus is used. A specific example is shown in FIG. Using the FRF weighted with the A characteristic and the frequency spectrum of the gradient magnetic field waveform, the time-average noise level LA _eq is calculated by the following equation.

In equation (2), f is the sampling frequency, fH−fL is the integration interval, G (f) is the frequency spectrum of the gradient magnetic field waveform, FRF _A (f) is the FRF including the weighting of the A characteristic, and P0 is the reference sound. Pressure. Specifically, G (f) is, for example, Fourier transform of the gradient magnetic field pulse shape of Gs, Gp, and Gf in FIG. IET, rephase pulse time, encoding time, etc. will affect G (f).

The step S103 selects a rectangle that minimizes the noise level in this way, and obtains a waveform having a target area while repeating the accumulation of rectangles that satisfy the same condition on the rectangle. As a result, a gradient magnetic field pulse waveform 410 as shown in FIG. 8 is finally obtained. The waveform of FIG. 8 shows the maximum slew rate = 135 [(T / m) / S], the area of frequency encoding = 3.268 × 10 ⁻⁵ [(T / m) × S], and the rephase pulse time = 3. It is a waveform obtained with 276 [ms], encoding time = 7.224 [ms], and IET = 14.8 [ms].

As can be seen from the equation (2), since the frequency of the FRF is censored at the upper limit value fH in the calculation of the noise level, there is no effect of suppressing the frequency beyond that, so the waveform finally obtained has a high frequency. It is not a smooth curve with corresponding irregularities. In order to realize the gradient magnetic field pulse with the gradient magnetic field amplifier, post-processing (for example, filtering processing such as moving average filtering) is performed on the waveform 410 to obtain a smooth waveform 400 as shown in FIG. Is preferred.

The waveforms 400 and / or 410 may be obtained for each condition by changing conditions such as the rephase pulse time / echo time, the number of echo trains, and the inter echo time IET. FIG. 9 shows gradient magnetic field pulse shapes obtained by varying the rephase pulse time and the echo encoding time. The gradient magnetic field pulse shape of FIG. 9 is rephase pulse time = 2.276 [ms], encoding time = 9.224 [ms], and other conditions are obtained under the same conditions as the waveform of FIG.

In this case as well, the method of obtaining is the same as that shown in the flow of FIG. 5, but the obtained waveform has a shape in which a convex waveform is superimposed on a substantially trapezoidal waveform. Each of the waveforms shown in FIGS. 8 and 9 has the following characteristics. First of all, it is clear from the premise of how to find out, but it has only one extreme value in the same interval where the sign is symmetric and has the same sign. In addition, there are a number of inflection points in both cases, considering fine irregularities. In particular, in FIG. 9, even if only the section in which the gradient magnetic field strength is positive is considered, the shape is such that two trapezoids overlap each other, and there are four inflection points even if fine irregularities are ignored.

The gradient magnetic field pulse shape calculation by the greedy method described above may be performed by a computer different from the MRI apparatus shown in FIG. 1, or may be performed by the pulse sequence creation unit 180 of the MRI apparatus.

However, the above-described calculation of the gradient magnetic field pulse shape employs a method of determining the brute force by the greedy method, and therefore is not suitable for calculation with an MRI apparatus every time imaging is performed. Therefore, in practice, the optimum gradient magnetic field pulse shape is calculated in advance under some conditions (for example, rephase pulse time, encode pulse time, reception bandwidth), and the optimum gradient for each condition is calculated. It is preferable to store the magnetic field pulse shape and select the optimum gradient magnetic field pulse shape according to the conditions set or selected during imaging.

In the MRI apparatus of the present embodiment, the waveform obtained by another computer is stored in the storage device 172. When the basic pulse sequence is selected for imaging, the pulse sequence creation unit 180 reads the gradient magnetic field pulse shape stored in the storage device 172 and the imaging parameter conditions for obtaining the gradient magnetic field pulse shape, and is read for the basic pulse sequence. A pulse sequence to be actually executed is created by applying shapes and conditions.

The operation of the MRI apparatus of this embodiment is the same as that of the conventional MRI apparatus, and detailed description thereof is omitted. However, when starting imaging, the operator selects a basic pulse sequence and sets imaging when setting imaging conditions. Set the parameters. At this time, an imaging for minimizing noise is selected, or selection of a gradient magnetic field pulse for minimizing noise is instructed. As a result, the pulse sequence creation unit changes the basic pulse sequence using the gradient magnetic field pulse waveform stored in the storage device 172 and passes it to the sequencer 140. After that, under the control of the sequencer 140, the application of the RF pulse and the gradient magnetic field pulse and the measurement of the echo signal are repeatedly performed, and the tomographic image and spectrum image of the subject are reconstructed based on the measured echo signal. The display on the display device is the same as that of the conventional MRI apparatus.

According to the present embodiment, by using a pulse sequence including a gradient magnetic field pulse that minimizes noise, it is possible to perform imaging with less noise than conventional MRI apparatuses.

The effect of noise suppression according to the present embodiment will be described with reference to FIG. FIG. 10 shows a gradient magnetic field pulse that minimizes noise at each rephase pulse time by changing the rephase pulse time while keeping the total application time of the rephase pulse and the frequency encode pulse constant. It is a graph showing calculated values. For reference, FIG. 10 shows noise levels (calculated values) when trapezoidal gradient magnetic field pulses are used with the same rephase pulse time (conventional method).

As is clear from FIG. 10, the gradient magnetic field pulse of this embodiment can obtain a noise reduction effect of about 10 dB compared to the conventional gradient magnetic field pulse at any rephase pulse time. That is, the sound pressure of noise can be reduced to about one third. It can also be seen that the noise reduction effect is greatest when the rephase pulse time is about 4 ms.

In the present embodiment, the gradient magnetic field pulse that minimizes noise is a frequency encode pulse combined with a rephase pulse, and the frequency encode pulse is obtained under the condition that it has only one extreme value in the same positive / negative sign interval. However, the conditions for the shape are not limited to those described in the first embodiment, and can be added or changed as appropriate. The present invention can also be applied to a phase pulse itself, a slice selective gradient magnetic field pulse other than a frequency encode pulse, a phase encode gradient magnetic field pulse, or a spoiler pulse.
In this embodiment, the pulse sequence and the gradient magnetic field waveform are stored in the storage area. However, the pulse sequence generation unit 180 may generate the pulse sequence every time imaging conditions are input.

Hereinafter, a modification example of the first embodiment will be described.

<Modification 1>
In the first embodiment, the condition that the frequency encode pulse shape has only one extreme value in the section where the signs of the positive and negative signs are the same is used. However, this modified example is characterized in that this condition is excluded. Other conditions are the same as in the first embodiment, and are symmetry, maximum slew rate, area, rephase pulse time, and encode pulse time.

Also in this modified example, the method for obtaining the gradient magnetic field pulse shape is the same as the procedure shown in FIG. 5 except that the initial setting conditions are different. By removing the condition that the positive and negative signs have the same extreme value in the same section, the rectangle added in step S102 is an axis passing through the center along the time axis of the frequency encoding pulse as shown in FIG. It becomes a set of rectangles arranged symmetrically. Since a set of rectangles may touch at the center, in this case, it is substantially the same as arranging one rectangle. In step S102, a noise level is calculated when such a rectangular group is arranged, and a rectangular group that minimizes the noise level is determined. FIG. 12 shows the final gradient magnetic field pulse shape obtained by setting the rephase pulse time to 2.276 ms.

When the shape of FIG. 12 is compared with the gradient magnetic field pulse shape (FIG. 9) obtained with the same rephase pulse time in the first embodiment, each has an independent convex shape at the center, and in the frequency encoding section. It has the feature of having three or more inflection points. In addition, the present modified example has fewer conditions than the first embodiment, and the gradient magnetic field pulse shape obtained in the present modified example can be said to be a shape that prioritizes the noise reduction effect.

<Modification 2>
This modification is characterized in that a condition for the maximum gradient magnetic field strength is further added to the condition for the frequency encoding pulse shape in the first embodiment. Other conditions are the same as in the first embodiment, and only one extreme value exists in the section where the positive and negative signs are the same, the symmetry, the maximum slew rate, the area, the rephase pulse time, and the encode pulse time.

Also in this modified example, the method for obtaining the gradient magnetic field pulse shape is the same as the procedure shown in FIG. 5 except that the initial setting conditions are different. When determining the rectangle to be added in step S102, it is a condition that the gradient magnetic field pulse formed by the addition of the rectangle does not exceed the maximum gradient magnetic field strength.

FIG. 13 shows the gradient magnetic field pulse shape obtained with the rephase pulse time of 2.276 ms in this modification. When the shape of FIG. 13 is compared with the gradient magnetic field pulse shape (FIG. 9) obtained at the same rephase pulse time in the first embodiment, each of them has a width smaller than that at the center of the trapezoid having the same width as the pulse width. It has a shape in which narrow convex portions overlap each other and has three or more inflection points including two inflection points at both ends of the narrow convex portion. In the shape of FIG. 13, the height of the waveform is lower than the waveform of FIG.
In this modification, by adding the condition for the maximum gradient magnetic field strength, a waveform that can be output as it is by the apparatus can be obtained, and further, it is not necessary to adjust the pulse shape in consideration of the condition for the maximum gradient magnetic field strength. In addition, a pulse shape that minimizes noise in consideration of the maximum gradient magnetic field strength can be obtained.

<Modification 3>
In this modified example, a gradient magnetic field pulse having a specific shape that minimizes noise is used as a gradient magnetic field pulse not accompanied by a rephase pulse or a reverse polarity pulse. Also in the present embodiment, the imaging sequence to be employed is not particularly limited. For example, in the case of the FSE sequence illustrated in FIG. 3, the imaging sequence can be applied to the rephase pulse 302 and the slice selection gradient magnetic field pulse 305.

The method for obtaining the gradient magnetic field pulse shape in this modified example will be described. The procedure is the same as the procedure shown in FIG. 5. First, gradient magnetic field pulse shape conditions are set as the initial setting (S101). In this modified example, the conditions are a rephase pulse time, a maximum slew rate, a symmetric and positive / negative sign having only one extreme value, and an area. The initial value of the shape is the smallest rectangle whose side length is the rephase pulse time. After that, when a rectangle was added, a rectangle that satisfies the two conditions of having only one extreme value in the same slew rate and symmetric, positive and negative signs in the same interval was determined and added. The shape is updated (S102, S103). The determination as to whether or not the noise is minimized is made for the maximum slew rate and all the rectangles that can be added that satisfy the two conditions of having only one extreme value in the same symmetric and positive and negative signs in the same interval (1 ) To calculate the noise level and compare the noise levels.

These steps S102 and S103 are repeated until the area of the pulse shape reaches the area given by the initial conditions (S104), and the final gradient magnetic field pulse shape is obtained.

FIG. 14 shows the rephase pulse obtained by such a method. This waveform also has fine discontinuities due to the upper limit of the frequency in the FRF. However, as in the first embodiment, post-processing (filtering) is performed to obtain a smooth waveform, which is converted into a pulse sequence. Incorporation is preferred.

<Second embodiment>
The present embodiment is characterized in that a gradient magnetic field pulse shape that minimizes noise is obtained from a combination of a plurality of waveform models. The use of the obtained gradient magnetic field pulse shape for the pulse sequence selected during imaging is the same as in the first embodiment. The apparatus configuration is also the same as that of the first embodiment shown in FIGS. 1 and 2, and thus the description thereof will be omitted and different points will be described.

First, the basic concept of modeling in this embodiment will be described.
From the gradient magnetic field pulse shape obtained in the first modification of the first embodiment (FIG. 12), noise reduction is high when a reverse pulse is applied in the same shape as the rephase pulse adjacent to the rephase pulse. It turns out that an effect is acquired. The effect of a reverse pulse with the same shape as the rephase pulse can be confirmed by fixing the rephase pulse to the final shape and searching for the frequency encoding. FIGS. 15 and 16 respectively show a case where there is no condition having only one extreme value in the same positive / negative sign interval (modified example 1 of the first embodiment) and a single extreme value in the same positive / negative sign interval. In the case where there is a condition (first embodiment), a halfway progress is shown when the shape of the frequency encoding gradient magnetic field pulse is searched with the rephase pulse fixed. Other conditions are the same for both. FIG. 16 shows the change of the frequency spectrum and the result of multiplying it by FRF and A characteristics as the shape progresses.

In the middle of the process shown in FIG. 15, first, reverse shapes having the same shape appear next to the rephase pulse, and it can be seen that the pulses having these shapes contribute to noise reduction. In the middle of the process shown in FIG. 16, there is a condition that only one extreme value exists in the same section of the positive and negative signs. A convex waveform appears first, and it can be seen that this convex pulse also contributes to noise reduction. In the frequency spectrum shown in FIG. 16, the frequency characteristic shifts more peakly due to the convex pulse. As a result, in the graph obtained by multiplying the FRF and the A characteristic, a component on the high frequency side having a large influence on noise is shown as indicated by an arrow. It can be seen that noise is reduced and noise is reduced.

In the present embodiment, based on the above-described concept, a gradient magnetic field pulse shape that minimizes noise is obtained using a model in which a pulse having a reverse polarity in the same shape as a rephase pulse is added to a frequency encoding gradient magnetic field pulse. .

FIG. 17 shows an example of a plurality of waveform models used in this embodiment. Here, a model is formed by superimposing five trapezoids. Of the five trapezoids, two are rephase pulses G1 and G1 ', and the central pulse is a normal frequency encoding pulse G3 included in the basic pulse sequence. The pulses G2 and G2 'adjacent to the rephase pulses G1 and G1' have the same application time, rise time, and reverse polarity as the rephase pulses G1 and G1 ', respectively.

In this waveform model, when the parameters are changed with the rise time of the rephase pulse G1, the rise time of the frequency encode pulse G3, and the area ratio of one frequency encode pulse G3 with respect to the entire frequency encode pulse as a parameter, the noise reduction effect is the greatest. Determine the shape determined by high parameters. The parameter search may be performed by using a known optimization technique or by brute force.

FIG. 18 shows a gradient magnetic field pulse shape obtained with IET = 14.8 ms, rephase pulse time = 3.276 ms, and encode time = 7.224 ms. This shape is the same as the gradient magnetic field pulse shape (FIG. 8) obtained by the same IET, rephase pulse time, and encoding time in the first embodiment, and it can be seen that this modeling is appropriate.

The waveform (FIG. 18) obtained by modeling may be smoothed by filtering as in the first embodiment. As an example, FIG. 19 shows a waveform obtained by applying a moving average filter of 0.6 ms to the waveform of FIG. The noise level (calculated value) of this frequency encoding gradient magnetic field is −51.9 dB, which is equivalent to the noise level (−52.4 dB) of the frequency encoding gradient magnetic field of the first embodiment obtained under the same conditions. It can be seen that the effect of the optimized shape can be obtained even with the filter alone.

FIG. 17 shows a model in which two pulses having the same shape as the rephase pulse and opposite polarity are added to the two rephase pulses and the frequency encode pulse, but the model is not limited to this. For example, as shown in FIG. 20, it is also possible to adopt a model in which a pulse G2 having the same shape as the rephase pulses G1 and G1 'and having a reverse polarity is superimposed on the center of the frequency encode pulse G3. Also in this case, the rise time of the rephase pulse G1, the rise time of the frequency encode pulse G3, the rise time of the frequency encode pulse G3, and the area ratio of one frequency encode pulse G3 to the entire frequency encode pulse are used as parameters. A set of parameters that can obtain a noise reduction effect is searched.

In FIGS. 17 and 20, the shape of the pulse constituting the model is a trapezoid. However, the shape is not limited to a trapezoid as long as the parameter can be searched analytically. For example, a sine wave (from 0 to π) or a sine square is used. It is also possible to use waves, quadratic functions, etc.

Also in this embodiment, calculation of the gradient magnetic field pulse shape using the model may be performed by a computer different from the MRI apparatus, or may be performed by the pulse sequence creation unit 180 of the MRI apparatus. According to the present embodiment, by using a model, it is possible to calculate the optimum gradient magnetic field pulse shape in a shorter time than in the first embodiment.

<Third embodiment>
In the first embodiment, the gradient magnetic field pulse shape that minimizes the noise by fixing the imaging parameter is obtained, but in this embodiment, the imaging parameter itself is used as a condition for obtaining the gradient magnetic field pulse shape that minimizes the noise. It is a feature to add. Imaging parameters to be added as conditions are not particularly limited, and examples include repetition time (TR), echo interval (IET), rephase pulse time, and the like. Add as a condition. The predetermined width is a range of imaging parameter values allowed in a general pulse sequence.

This embodiment is different from the first and second embodiments only in the method for obtaining the gradient magnetic field pulse shape, and the other configuration and imaging method are the same, so the description thereof will be omitted. Hereinafter, the gradient magnetic field pulse shape according to the present embodiment The procedure for obtaining is described with reference to FIG.

First, set the initial value of the imaging parameter as the initial setting. The initial value may be an arbitrary value such as a minimum value, a maximum value, or a median value of a predetermined width of each imaging parameter. Further, the same conditions as those of the first embodiment or the modified example 1 or 2 are provided for the shape (S101). That is, for example, the maximum slew rate, a symmetric and positive / negative sign in the same section, has only one extreme value or simply a symmetric shape, and an area of a waveform (pulse).

After the initial setting, for example, the shape of the gradient magnetic field pulse that minimizes the noise is obtained (S102 '). This step S102 'may employ either the above-described first embodiment (including a modified example) or the method employed in the second embodiment (step S102 in FIG. 5). If it is the method of 1st embodiment, the procedure of determining the rectangle which minimizes a noise when the rectangle of a unit area is piled up sequentially is repeated. At that time, if the shape condition includes only one extreme value with the same sign, the maximum gradient magnetic field strength does not include only one extreme value with the same sign. There may be cases where additional conditions are added. In the case of the method of the second embodiment, a modeled waveform is used as an initial value, and a parameter that gives a shape that minimizes noise is searched from among parameters that define the model, and finally a waveform that minimizes noise. To decide. As a model, a model combining trapezoids as shown in FIGS. 17 and 20 or a model combining shapes other than trapezoids can be adopted.

In step S102 ′, for example, as shown in FIG. 22, when calculating the noise level when a rectangle of unit area is added, the noise level is taken into account when the imaging parameter is changed, that is, by taking into account the repeating pattern of the gradient magnetic field pulse. Is calculated, and the shape and imaging parameters that minimize the noise level are obtained. In the case of modeling, a combination of parameters that minimizes the noise level is obtained for all combinations of the shape parameter and the imaging parameter.

Alternatively, as shown in FIG. 6, a noise level is calculated when a provisional gradient magnetic field pulse obtained by a combination of one imaging parameter is used as another imaging parameter. A combination of imaging parameters that minimizes may be obtained. The combination of imaging parameters being updated is provisional, and in the next iteration, the updated gradient magnetic field pulse is updated to a combination of imaging parameters determined to minimize noise.

In step S103, the gradient magnetic field pulse shape obtained in step S102 'is set as a new gradient magnetic field pulse shape. That is, the gradient magnetic field pulse shape to be processed in step S102 'is updated. Steps S102 'and S103 are repeated until the updated gradient magnetic field pulse shape reaches the set gradient magnetic field pulse area (S104).

The combination of the gradient magnetic field pulse shape and the imaging parameter that ultimately minimizes the noise is finally determined by the above processing. The gradient magnetic field pulse shape and the imaging parameters thus obtained are stored in the storage device 172. When the basic pulse sequencer is selected for imaging, the pulse sequence creation unit 180 reads the gradient magnetic field pulse shape and imaging parameters stored in the storage device 172, changes the basic pulse sequence, and is actually used for imaging. An imaging sequence is created and passed to the sequencer 140.

In general, in an MRI apparatus, an imaging parameter can be set when an operator selects a pulse sequence. In the MRI apparatus of this embodiment, when imaging that minimizes noise is selected, the imaging parameters determined when the gradient magnetic field pulse shape that minimizes noise is obtained on the imaging parameter setting screen are as follows. The value may be displayed by default. Further, another imaging parameter may be set by the operator. In that case, the pulse sequence creation unit 180 uses the gradient magnetic field pulse shape read from the storage device 172, and a new one set by the operator. An imaging sequence is created based on the basic pulse sequence using the imaging parameters. Also in this case, since the shape of the gradient magnetic field pulse is a form that reduces noise, noise can be greatly reduced compared to the case where a pulse sequence using a basic trapezoidal gradient magnetic field pulse is executed.

<Modification example of the third embodiment>
Also in this modified example, when obtaining the gradient magnetic field pulse shape that minimizes noise, adding the imaging parameter width as a condition, and obtaining the optimum gradient magnetic field pulse shape within this width are the same as in the third embodiment. It is. However, in this modified example, the imaging parameter condition is handled in the same way as the shape condition as in step S102 ′ of the third embodiment, and the gradient magnetic field pulse shape is not searched but applied step by step.

The procedure for this modification is shown in FIG. First, imaging parameters are initially set, and a gradient magnetic field pulse shape that minimizes noise is determined based on the imaging parameters. The imaging parameter may be one or a plurality of combinations. The noise level at this time is calculated and recorded (S201). Next, change the imaging parameters or a combination thereof, determine the gradient magnetic field pulse shape that minimizes the noise in the same way, calculate the noise level at this time, and compare it with the noise level in the conditions of the imaging parameters recorded before that. Then, it is updated to one having a lower noise level (S202).

Step S202 is repeated while changing the imaging parameters (S203), and finally the gradient magnetic field pulse shape when noise can be reduced most among all the imaging parameters or combinations is determined. As the update convergence condition in step S203, methods such as the steepest descent method and the bisection method can be applied. Or you may perform step S202 about all the imaging parameters by the brute force method. In the case of the round robin method, since all changes are examined in one step S202, step S203 is omitted.

In the third embodiment and its modified example, the gradient magnetic field pulse for minimizing noise and the imaging parameter as the condition at that time may be calculated by the MRI apparatus (pulse sequence creation unit 180). What is calculated in advance by the computer may be stored in the storage device 172 of the MRI apparatus. In addition, when calculating in advance, gradient magnetic field pulses that minimize noise obtained under a plurality of conditions with different imaging parameters or combinations thereof are stored, and those closest to the imaging parameters selected by the operator at the time of imaging are stored. May be read from the storage device 172 and executed. Alternatively, some candidates may be presented to the operator via the display device 173 and selected. The GUI for causing the operator to make a selection will be described in detail in the next embodiment.

According to the third embodiment and the modified example thereof, a low noise gradient magnetic field pulse that takes imaging parameters into consideration is used, so that the burden on the subject due to noise can be reduced.

<Fourth embodiment>
The present embodiment is characterized in that it generates information for an operator to select an image that minimizes noise and an image that prioritizes image quality, and includes a GUI that presents the information to the operator. . That is, the MRI apparatus according to the present embodiment, for example, includes a display control unit (183) that displays on the display device (173) the reception bandwidth that minimizes noise and the relationship between the reception bandwidth that minimizes noise and generated noise. ). Further, the MRI apparatus of the present embodiment is based on the display control unit (183) that displays a UI for selecting the priority for noise reduction and image quality improvement on the display device (173), and the priority selected via the UI, An imaging control unit (control unit 170) that controls imaging parameters is further provided.

In the above-described first to third embodiments and their modifications, when imaging that minimizes noise is selected, the pulse sequence creation unit uses a combination of gradient magnetic field pulse shape and imaging parameters that can minimize noise. Create an imaging pulse sequence that minimizes noise. In other words, the operator can select whether or not to select an image that minimizes noise, but the image quality is slightly higher than that of an intermediate image, that is, an image that minimizes noise. When trying to capture an image with a good image quality, it may be difficult to know how much and what imaging parameter should be changed. In general, the image resolution, SNR, imaging time, and the like change monotonically with respect to the imaging parameters. Therefore, the operator can easily determine how to change the imaging parameters when trying to change them. Although it can be predicted, the noise level does not necessarily change monotonously with changes in the imaging parameters, so how much is changed to achieve the desired noise level while considering the trade-off with image quality. It is difficult to know what to do.

The MRI apparatus of the present embodiment includes means for providing the operator with the relationship between the imaging parameter and noise or information derived therefrom, thereby facilitating the setting of the imaging parameter by the operator. The relationship between the noise and the imaging parameter is the noise level when the imaging parameter is changed in the process of obtaining the gradient magnetic field pulse shape that minimizes the noise with the predetermined imaging parameter by the method of the first to third embodiments. It can be derived by calculating the change. Alternatively, in the method of the modified example of the third embodiment, it is possible to derive the relationship between the noise level calculated when obtaining the gradient magnetic field pulse shape that minimizes the noise while changing the imaging parameter and the imaging parameter.

FIG. 24 shows the relationship between the average reception bandwidth BWave and the noise level as an example of the relationship between the imaging parameter and the noise level. BWave is defined by the following equation (3) using the average gradient magnetic field strength Average (G) when the gradient magnetic field strength being received is not constant.

BWave = γ / (2π) × Average (G) × FOV (3)
The graph shown in FIG. 24 is obtained by plotting the noise level when the BWave is changed with respect to the pulse shape after obtaining the gradient magnetic field pulse shape that minimizes the noise with a predetermined BWave. The parameters are fixed.

As can be seen from this graph, the change in noise level is not monotonous with respect to the change in BWave. In general, the larger the BWave, the better the image quality. However, when attempting to lower the noise level at the expense of the BWave to some extent, the change in the noise level is not known unless the BWave is changed. Therefore, the operator changes the BWave to grasp the tendency of the noise level change, for example, when the BWave is automatically determined by the MRI apparatus of the first embodiment and the gradient magnetic field pulse shape and the pulse sequence based on it are created. The operator needs to remember the BWave, then manually determine the BWave and start again, which complicates the procedure.

In the MRI apparatus of the present embodiment, such a relationship between noise and imaging parameters is obtained in advance and stored in the storage device 172, and is presented when the imaging parameters are set by the operator. Thus, the operator can determine how to change BWave, for example.

The information presented by the MRI apparatus can take various forms. For example, a graph as shown in FIG. 24 may be displayed on the imaging parameter setting screen of the display device 173, or as shown in FIG. A display block of the minimum noise BWave may be provided on such an imaging parameter setting screen, and the value of BWave that minimizes the noise may be displayed. The BWave for minimizing noise is the minimum value of the graph shown in FIG. 24 and can be obtained by a normal optimization process. By displaying the value of the minimum noise BWave, when the operator wants to reduce the noise, it is only necessary to change the BWave so as to approach the BWave that minimizes the noise. It can also be seen that the displayed value is the BWave limit that minimizes noise.

24 and 25 exemplify BWave as an imaging parameter related to the noise level, the same applies to imaging parameters whose noise level change is not monotonous with respect to the imaging parameter change. Can be applied to. For example, the relationship between the rephase time and the noise level shown in FIG. 10 can also be used.

Also, instead of the average reception bandwidth BWave, the reception bandwidth BWk0 at the center of the k space may be used. FIG. 26 shows the relationship between BWk0 and the noise level. This relationship is also obtained by the same method as the relationship shown in FIG. 24, and it can be seen that the change in noise level is not monotonous with respect to the change in BWk0, that is, has a minimum value. The graph and / or the minimum value of this relationship is displayed on the display device 173.

As information displayed on the display device 173 for noise, in addition to the information based on the relationship between the noise and the imaging parameter described above, for example, the noise level itself, the noise level when the imaging parameter is varied (maximum value) , Minimum values, imaging parameters that take those values), and combinations of recommended imaging parameters. Such information can be created by the functions of the noise level calculation unit 181 and the noise level comparison unit 182 of the CPU 171.

According to the present embodiment, the imaging parameter (for example, reception bandwidth) that minimizes the noise and / or the relationship between the noise and the imaging parameter are displayed on the display device 173, so that the imaging parameter can be changed. It is possible to obtain information such as whether the noise level can be reduced by changing the imaging parameters, and taking appropriate images in consideration of the required image quality and subject noise tolerance Can do.

Further, the noise level calculated by the noise level calculation unit 181 may be displayed by adding a display block for displaying the noise level, as shown in FIG. Further, in setting the imaging parameters, an “undo” function and a “redo” function may be added as shown in FIG. 28 in order to set the parameters and the noise level before and after the change, as shown in FIG. As shown in the figure, two parameter setting screens may be prepared, set while comparing the two, and finally, an imaging parameter may be determined by pressing a decision button.
By providing such a GUI, the operator can select and execute an optimal imaging method while considering the noise level.

According to the present invention, an MRI apparatus with reduced noise is provided. In addition, an MRI apparatus is provided that makes it easy for an operator to capture desired noise in consideration of the image quality for noise that has a trade-off relationship with image quality.

120: Static magnetic field generation unit, 130: Gradient magnetic field generation unit, 140: Sequencer, 150: Transmission unit, 160: Reception unit, 170: Control unit (imaging control unit), 171 ... CPU, 172 ... storage device, 173 ... display device, 180 ... pulse sequence creation unit, 181 ... noise level calculation unit, 182 ... noise level comparison unit, 183 ... -Display control unit.

Claims

A magnetic field application unit that applies a high-frequency magnetic field pulse and a gradient magnetic field pulse according to a pulse sequence that determines the application intensity and timing of the high-frequency magnetic field pulse and the gradient magnetic field pulse, and the shape of at least one kind of gradient magnetic field pulse included in the pulse sequence is A magnetic resonance imaging apparatus having a shape that minimizes noise, obtained under the condition of satisfying a slew rate of a gradient magnetic field in the magnetic field application unit.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the shape of the gradient magnetic field pulse is a shape depicted by a curve having three or more inflection points in a section where the gradient magnetic field strength is always positive or negative. A magnetic resonance imaging apparatus.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the gradient magnetic field pulse has a shape formed by sequentially stacking a plurality of minute rectangles until a predetermined area is reached. A magnetic shape obtained by renewing the provisional shape so that the provisional shape formed until a gradient magnetic field pulse shape is obtained is a shape that minimizes noise. Resonance imaging device.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the waveform of the gradient magnetic field pulse is a shape obtained by combining a plurality of waveforms specified by a shape parameter.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the waveform of the gradient magnetic field pulse is obtained when the shape parameter is changed with a model obtained by combining a plurality of waveforms specified by the shape parameter as an initial value. A magnetic resonance imaging apparatus having a shape obtained by repeatedly updating the shape parameter so that a gradient magnetic field pulse shape to be a shape that minimizes noise is obtained.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the gradient magnetic field pulse having a shape that minimizes noise is a frequency-encoded gradient magnetic field pulse.
The magnetic resonance imaging apparatus according to claim 1, further comprising a pulse sequence creation unit that creates the pulse sequence,
The pulse sequence creating unit creates a pulse sequence including a gradient magnetic field pulse having a shape that minimizes the noise by adding a restriction determined by an imaging condition input via an input device. Resonance imaging device.
The magnetic resonance imaging apparatus according to claim 1, further comprising a display control unit that causes a display device to display a reception bandwidth that minimizes the noise.
The magnetic resonance imaging apparatus according to claim 8, further comprising a display control unit that displays a relationship between a reception bandwidth that minimizes the noise and generated noise on a display device. apparatus.
The magnetic resonance imaging apparatus according to claim 1, wherein the display control unit displays a UI for selecting a priority for noise reduction and image quality improvement on the display device, and imaging is performed based on the priority selected via the UI. A magnetic resonance imaging apparatus, further comprising: an imaging control unit that controls a parameter.
The magnetic resonance imaging apparatus according to claim 10, further comprising a noise calculation unit that calculates a noise level in an imaging parameter set in advance or via an input device, wherein the display control unit includes the noise calculation unit. A magnetic resonance imaging apparatus, wherein a calculated result is displayed on a display unit.
12. The magnetic resonance imaging apparatus according to claim 11, wherein the display control unit calculates the noise level calculated for the first imaging parameter together with the noise level calculated for the second imaging parameter changed via the input device. A magnetic resonance imaging apparatus characterized by displaying on a display device.