JPH08126625A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE: To efficiently eliminate noise having random properties of the inclined magnetic field coil of an MRI apparatus. CONSTITUTION: A plurality of conversion elements, especially, piezoelectric elements capable of converting electrical energy to mechanical energy are arranged to an inclined magnetic field coil 9 consisting of a coil conductor 50 and the holding member 51 thereof in order to cancel the electromagnetic force generated at the time of driving. The plezoelectric elements 40a-40b are arranged so that the gap between them in the direction crossing the axis of the moment forces generated by the piezoelectric elements at a right angle becomes narrow, for example, becomes below three times the thickness of the inclined magnetic field coil. By this constitution, the secondary moment generated in the vicinity of the coil by the moment forces generated by the piezoelectric elements is suppressed and electromagnetic force is efficiently cancelled without amplifying the deformation of the coil. Further, the vibration and noise in an MRI apparatus can be reduced and the sensation of fear and unpleasant feeling of an examinee can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to an MRI that reduces noise and vibration generated by a gradient magnetic field generator.
It relates to the device.

[0002]

2. Description of the Related Art An MRI apparatus irradiates an inspection target placed in a static magnetic field with electromagnetic waves to cause a nuclear magnetic resonance phenomenon in atomic nuclei in the inspection target, thereby generating a magnetic resonance signal from the inspection target. An image representing the physical properties of the inspection object is obtained based on the magnetic field generating means of the static magnetic field and the gradient magnetic field, and is arranged inside the magnetic field generating means to irradiate the inspection object with electromagnetic waves or from the inspection object. A high frequency coil for detecting a magnetic resonance (NMR) signal and an image reconstructing means for reconstructing an image using the detected NMR signal are provided.

The gradient magnetic field is applied so as to be superimposed on the static magnetic field in order to add position information to the NMR signal. The gradient magnetic field coil and its holding member are positioned in the magnetic field generated by the static magnetic field generator. And is driven by passing a pulsed current through the gradient coil. In this case, an electromagnetic force acts according to Fleming's left-hand rule by passing a pulse current in a magnetic field. Then, the electromagnetic force tries to deform the gradient magnetic field coil, and noise and vibration are generated. Such noises and vibrations are not preferable because they give a fear or discomfort to the patient to be inspected, and it is preferable that they be soundproofed or silenced.

Therefore, in the conventional MRI apparatus, a noise absorbing member or the like is provided inside a decorative cover that covers the outer periphery of the apparatus to reduce the noise of the gradient magnetic field, and a damping member is used as a holding member for holding the gradient magnetic field coil. The damping characteristic of the member is used to reduce the absolute value of the vibration amplitude and to shorten the decay time. However, when the sound absorbing material is arranged inside the decorative cover, noise is not sufficiently attenuated, but good sound damping cannot be performed, although there is some sound damping effect. Further, the control by the vibration damping member basically obtains a damping effect by mixing a rubber-based material with the holding member, so that the rigidity of the holding member decreases and the displacement of the gradient magnetic field coil increases. Such displacement of the gradient magnetic field coil changes the generated gradient magnetic field and causes image deterioration. Particularly, in recent years, the imaging method used in the MRI apparatus requires high precision of the phase of the NMR signal. Therefore, the displacement of the gradient magnetic field coil must be on the order of several microns to several tens of microns. Vibration control methods are no longer available.

On the other hand, as a method of reducing various noises,
An active muffling method is known in which a sound wave having the same amplitude as that of the noise is generated from an additional sound source to muffle the noise. In this noise reduction method, a device that detects a signal related to noise (microphone) and a device that minimizes the acoustic energy from the noise source (speaker) in the vicinity of the place where you want to mute are always located near your ears. I can't. Therefore, when this muffling method is applied to the MRI apparatus, the position of the subject changes depending on the imaged site in the MRI apparatus, and therefore a microphone and a speaker must be attached to the subject, and the subject feels uncomfortable and uncomfortable. Give a pleasant feeling.

Although there is no application example to the MRI apparatus, there is a method of canceling the vibration by detecting the vibration of the apparatus by using a piezoelectric element and further generating the vibration having a phase opposite to the detected vibration. No. 5,022,272.

[0007]

However, the method disclosed in this US patent is difficult to be directly applied to the gradient coil of the MRI apparatus with complicated deformation. First, in order to detect the deformation of a gradient magnetic field coil having a complicated deformation pattern, a large number of vibration detecting piezoelectric elements (hereinafter, referred to as sensors) are required, and the number of piezoelectric elements (hereinafter, referred to as actuators) that generate the deformation at the same time. Also increases.
Generally, the gradient magnetic field coil has a cylindrical shape, and when an actuator is simply applied to the gradient magnetic field coil having such a shape, the gradient magnetic field coil is deformed at the same time as a force that cancels the deformation generated in the gradient magnetic field coil. Amplifying force (secondary force or secondary deformation) is generated. Therefore, the conventional technique described in the above-mentioned U.S. Patent cannot effectively suppress the deformation of the gradient coil.

An object of the present invention is to provide an MRI apparatus capable of suppressing complicated deformation occurring in a gradient magnetic field coil of the MRI apparatus and effectively canceling vibration and noise.
In particular, it is an object of the present invention to provide an MRI apparatus that can efficiently cancel the deformation generated in a cylindrical gradient magnetic field coil.

[0009]

An MRI apparatus of the present invention that achieves the above-mentioned object comprises a coil conductor for generating a gradient magnetic field,
In order to cancel the electromagnetic force generated in the coil conductor of the gradient magnetic field generating means having the holding member for holding the coil conductor, a plurality of conversion elements for converting electric energy into mechanical energy are provided. The conversion elements are arranged with a predetermined gap so that the secondary deformation caused by one conversion element is suppressed by the main deformation force of the other conversion element.

When the conversion element generates a moment force, it is arranged with a predetermined gap at least in the direction orthogonal to the moment axis generated by the conversion element.
Further, in one aspect of the present invention, the predetermined distance between the elements is 3 times or less the plate thickness of the holding member. Further, the present invention is preferably applied to an MRI apparatus provided with a gradient magnetic field generating means having a substantially cylindrical shape, in which case a piezoelectric element laminated in the polarization direction is used as the conversion element, and the lamination direction is substantially cylindrical. It is arranged so as to be in the radial direction of the mold or in a direction substantially orthogonal to the radial direction.

[0011]

By providing a plurality of conversion elements for converting electric energy into mechanical energy in the gradient magnetic field generating means, the electromagnetic force generated in the gradient magnetic field coil is canceled out, and vibration and deformation due to the electromagnetic force are suppressed. At this time, in addition to the main force that cancels the deformation of the coil, the conversion element generates a weak force in the opposite direction to that force, which causes the auxiliary coil that amplifies the deformation due to electromagnetic force in the coil around the conversion element. Secondary deformation may occur. By arranging the other conversion element so that the main deformation force of the other conversion element is applied to the range where the secondary deformation is generated by such one conversion element, the secondary deformation is canceled. .

Such a secondary deformation becomes a problem when the moment force is applied by the conversion element, and in that case, it is accompanied by the main moment force for suppressing the electromagnetic force, and it is accompanied by it on both sides thereof. A reverse moment force is generated. Therefore, by making the gap of the conversion element a predetermined gap at least in the direction orthogonal to the moment axis,
The moment force can be canceled in the opposite direction.

The magnitude and range of the secondary deformation generated by the conversion element 1 vary depending on the rigidity and thickness of the gradient magnetic field coil, and are preferably 3 times or less the plate thickness of the holding member. When a piezoelectric element stacked in the polarization direction is used as the conversion element and is arranged in the radial direction of the cylindrical gradient magnetic field or in the direction orthogonal thereto, a large voltage is applied to the polarization axis of the piezoelectric element. Moment force,
Since the in-plane force can be generated, the effect of suppressing the secondary deformation is also high.

[0014]

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. FIG. 5 is a block diagram showing an entire MRI apparatus to which the present invention is applied. This MRI apparatus is roughly classified into a static magnetic field generating magnet 2, a gradient magnetic field generating system 3, a transmitting system 4, and a receiving system 5. , Signal processing system 6, sequencer 7,
And a CPU 8.

The CPU 8 controls each of the sequencer 7, the gradient magnetic field generation system 3, the transmission system 4, the reception system 5, and the signal processing system 6 according to a predetermined program. The sequencer 7 is configured to operate based on a control command from the CPU 8, and sends various commands necessary for collecting tomographic image data of the subject 1 to the gradient magnetic field generation system 3, the transmission system 4, and the reception system 5. send.

The static magnetic field generating magnet 2 is for generating a uniform static magnetic field around the subject 1 in the body axis direction or in a direction orthogonal to the body axis. Inside the static magnetic field generating magnet 2, in addition to the irradiation coil 14 a of the transmission system 4, the subject 1
Gradient magnetic field coil 9 of the gradient magnetic field generation system 3 for setting the slice plane and the like, and a receiving coil 14 of the receiving system 5 described later.
b and are installed. The gradient magnetic field generation system 3 has three axes (X,
It has a gradient magnetic field coil 9 wound in each of the (Y, Z) directions and a gradient magnetic field power supply 10 controlled by a sequencer 7 for applying the respective gradient magnetic field coils 9.

When the subject 1 is irradiated with electromagnetic waves by the irradiation coil 14a, a gradient magnetic field coil 9 in the three-axis (X, Y, Z) directions is applied to apply a gradient magnetic field to the subject 1. Electromagnetic waves from the specimen 1 are radiated, and the radiated NMR signal is emitted by the receiving coil 14b of the receiving system 5 to extract a tomographic image of a predetermined portion of the subject 1. At this time, the slice position of the subject is determined according to the magnitude of the gradient magnetic field generated by the gradient magnetic field coil 9 in the three-axis directions.

As will be described later in detail, the gradient magnetic field coil 9 is provided with a plurality of piezoelectric elements 40 for canceling vibration and deformation during driving. The transmission system 4 has a high-frequency oscillator 11, a modulator 12, a high-frequency amplifier 13, and an irradiation coil 14a that is a high-frequency coil, and a high-frequency pulse from the high-frequency oscillator 11 is modulated by the modulator 1 according to a command from the sequencer 7.
2 is amplitude-modulated, and the amplitude-modulated high-frequency pulse is amplified through the high-frequency amplifier 13 and supplied to the irradiation coil 14a, thereby irradiating the subject 1 with a predetermined pulsed electromagnetic wave.

The receiving system 5 has a receiving coil 14b which is a high frequency coil, an amplifier 15 connected to the receiving coil 14b, a quadrature phase detector 16 and an A / D converter 17, and the irradiation coil 14b. When the receiving coil 14b detects the NMR signal emitted from the subject 1 during the irradiation of the subject 1 with the electromagnetic wave by the gradient magnetic field coil 9, the receiving coil 14b detects it, and the amplifier 15, the quadrature detector 16, the A / D converter 17 are provided. Through the CPU, and at the same time as the instruction from the sequencer 7, the quadrature detector 16 converts the sampled data into two series of collected data, and the CPU
Send to 8.

The signal processing system 6 includes a magnetic disk 18 which is an internal recording device and a magnetic tape 19 which is an external recording device.
When the data from the receiving system 5 is input to the CPU 8, the CPU 8 executes processing such as Fourier transform, correction coefficient calculation, and image reconstruction, and the like. By transmitting the data, a cross-sectional image of a predetermined portion of the subject 1 is displayed on the display and recorded in the recording device.

In FIG. 1, the high frequency coils 14a and 14b on the transmitting side and the receiving side and the gradient magnetic field coil 9 are the static magnetic field generating magnetic circuit 2 arranged in the space around the subject 1.
It is located in the magnetic field space. Next, the configuration of the gradient magnetic field coil according to the present invention will be described in more detail. FIG. 1A is a cross-sectional view of a cylindrical gradient magnetic field coil, and a part thereof is shown as enlarged perspective views (b) and (c) for explanation. In the present embodiment, the gradient magnetic field coil 9 generates an X, Y and Z gradient magnetic field coil conductor 50 that generates a magnetic field that linearly changes in the X, Y and Z directions, an FRP bobbin 51 that holds them, and electrical energy. Piezoelectric elements 40a, 40b, 40c, 40d, 40 as conversion elements for converting to mechanical energy
e, or 40f, 40g, 40h, 40i, 40j. Each of the gradient magnetic field coil conductors 50 is bonded to the bobbin 51 with an adhesive or screwed, and each of the piezoelectric elements is bonded to the gradient magnetic field coil 50 with an adhesive that causes less damping. Although only five piezoelectric elements are shown in the drawing, it is assumed that the piezoelectric elements are arranged over the entire gradient magnetic field coil or at positions corresponding to the deformation of the gradient magnetic field coil due to electromagnetic force expected in advance.

Each of the piezoelectric elements 40a to 40j generates a simple compressive pulling force in the direction of the arrow in the drawing by an applied voltage. In FIG. 1 (b), a deforming force (simple compression) mainly in the axial direction (Z direction) is generated. The arrangement that causes the pulling force)
Although FIG. 1 (c) shows an arrangement in which a deforming force (simple compressive pulling force) is generated mainly in the circumferential direction, the direction of the force generated by the piezoelectric element is not limited to the cylindrical direction and the axial direction, and the gradient magnetic field is used. Any direction can be used as long as it can cancel the noise caused by the coil, and the axial and cylindrical arrangements can be appropriately mixed. To this end, it is preferable that the gradient magnetic field coils are appropriately combined and arranged so as to generate a deforming force that cancels the deformation corresponding to the deformation of the gradient magnetic field coil due to the electromagnetic force.

When the piezoelectric element is arranged on one surface of the gradient magnetic field coil 50 as described above, the piezoelectric elements 40a to 40e generate moments about the coil 50 in the circumferential direction due to the applied voltage, and the piezoelectric element 40f. At 40j, a moment about the axis of the coil 50 is generated. This moment force is equal to the product of the simple compressive pulling force of the piezoelectric element and about 1/2 of the plate thickness of the gradient magnetic field coil, which cancels the deformation of the coil of the same size in the opposite phase, but actually When a moment force is applied by a piezoelectric element, a weak opposite moment is secondarily generated in the vicinity of the moment force due to its main moment force.
Therefore, when canceling the electromagnetic force by the piezoelectric element, such a secondary force must be taken into consideration.

Below, one piezoelectric element is arranged at an arbitrary position on the outer diameter of the cylindrical bobbin 51, and the result of analysis in the case where moment force is generated in the circumferential direction and the axial direction is shown in FIG. This will be described with reference to FIG. As shown in FIG. 2A, when a voltage is applied mainly to the piezoelectric element 40 that produces a simple compressive tensile force in the axial direction 61, a moment is applied to two opposing sides in the axial direction 61 from the inside of the piezoelectric element 40 to the outside. Power is generated. In this case, the moment axis is in the circumferential direction. Further, as shown in FIG. 2B, when a voltage is applied mainly to the piezoelectric element 40 that produces a simple compressive tensile force in the circumferential direction, the piezoelectric element 4 is formed on two opposing sides in the circumferential direction 71.
Moment force is generated from 0 inside to outside. The moment axis in this case is the axial direction.

FIG. 3 shows the displacement of the bobbin on the line 62 when a moment force is generated about the circumferential direction (FIG. 2 (a)).
FIG. 3B shows the amount of displacement on the line 72 when a moment force is generated around the axis in FIG. 3A (FIG. 3B). In FIG. 3, the horizontal axis represents the distance, and the vertical axis represents the displacement amount. The solid line indicates the case where the moment force is applied, and the dotted line indicates the case where nothing is added. As can be seen from FIG. 3, regardless of whether the moment force is in the axial direction or in the cylindrical direction, the displacement amount by the piezoelectric element 40 has a peak at the central portion of the piezoelectric element, and a displacement amount opposite to the peak is present on both sides thereof. generate. The magnitude of this displacement amount and the distance at which the displacement occurs vary depending on the size of the piezoelectric element, the applied voltage of the piezoelectric element, the bobbin plate thickness, the bobbin radius, the rigidity of the bobbin and the coil, etc. The opposite displacement is generated outside the part where Therefore,
If a large gap is created in the direction in which a moment force is generated when arranging multiple piezoelectric elements, the displacement of the piezoelectric element position can be canceled, but the regions that generate the opposite amount of displacement overlap and the deformation of the coil is prevented. Amplification sushi will result in the displacement of unevenness being left as a whole. On the other hand, when the gap is narrowed, it is possible to cancel the generation of the opposite displacement amount by the original displacement of the adjacent piezoelectric element. For this purpose, it is preferable to arrange the piezoelectric elements so that there is almost no gap between them, and the present inventors have found that a gap of 1.5 to 3 times the plate thickness is suitable for a practical gradient coil. Confirmed analytically.

Therefore, in the arrangement of the piezoelectric elements shown in FIG. 1B, it is necessary to reduce the gap between the piezoelectric elements in the direction orthogonal to the moment axis, that is, in the axial direction. The circumferential gap may be large. Further, in the arrangement of FIG. 1C, it is necessary to reduce the gap in the cylindrical direction. In this case, the axial gap may be large. The size and the number of piezoelectric elements are not particularly limited as long as the vibration of the gradient magnetic field coil can be canceled, and can be appropriately selected according to the characteristics of deformation of the gradient magnetic field coil. For example, when the displacement due to the shape of the gradient magnetic field coil is local, it is desirable to cancel the vibration with a small piezoelectric element and when the displacement is smooth over a wide range, with a large piezoelectric element.

As the piezoelectric element, as shown in FIG. 4, a piezoelectric element laminated in the polarization direction 60 can be used. By using the laminated piezoelectric element, it is possible to generate a force larger than that generated by a single piezoelectric element.
In the illustrated example, the piezoelectric element 40k arranged so that the stacking direction becomes the circumferential direction and the piezoelectric element 40l arranged so that the stacking direction becomes the radial direction are used. The piezoelectric element 4
0k allows the individual piezoelectric elements constituting the laminate to be laminated such that the polarization directions thereof are alternately opposite to each other, thereby enabling deformation in the same direction even if the boundary surface voltage is the same.

The piezoelectric element 40l can generate a moment in the axial direction by laminating piezoelectric elements having the same polarization direction in the radial direction. That is, when the voltages on the boundary surfaces of the two piezoelectric elements are the same, the directions of the voltages applied to these piezoelectric elements are opposite, so that one of them expands and the other deforms in the direction of contraction to generate a moment. Although only one laminated piezoelectric element is shown in the figure, the laminated piezoelectric elements are arranged in the embodiment at a predetermined interval in the direction orthogonal to the axis of the moment generated by each laminated piezoelectric element. As a result, it is possible to suppress the secondary force generated outside the element and effectively suppress the deformation generated in the gradient magnetic field coil.

In each of the above embodiments, the piezoelectric element 40
Although (a to j, k, l) has been described as being fixed to the outer diameter side of the coil conductor 50, the piezoelectric element 40 is not included in the coil conductor 50.
The inner diameter of the bobbin 51, that is, the outer diameter of the bobbin 51 may be fixed. In addition, if each piezoelectric element generates a large force, it may be arranged only on one side of the gradient magnetic field coil as in the above-mentioned embodiment, but it may be arranged at a position where a bobbin and a gradient magnetic field coil are sandwiched therebetween. When the pair of piezoelectric elements 40 are arranged so as to face each other with the coil conductor and the bobbin sandwiched therebetween, the applied voltage to each piezoelectric element can be reduced.

When the piezoelectric elements are arranged facing each other, if the characteristics, the polarization directions, and the applied voltages of the two facing piezoelectric elements are the same, simple compression or tensile force is generated in both piezoelectric elements. However, when the respective polarization directions are opposite, one becomes a compression force and the other becomes a tensile force, so that a moment force which is the sum of the applied voltages is generated. Furthermore, if the applied voltage is not the same even if the polarization direction is the same,
Approximately ½ of the sum of the respective voltages produces a simple compressive pulling force, and approximately ½ of the difference produces a moment force.

Therefore, also in this case, as in the case of arranging on one side, the gap in the direction orthogonal to the axis of the generated moment is set to a predetermined gap, so that the outside of the element is analyzed as in the case of the above embodiment. Suppresses the secondary force generated in
The deformation generated in the gradient magnetic field coil can be effectively suppressed. As a method of driving the piezoelectric element in the present invention, the piezoelectric element is used as a sensor to detect deformation when the gradient magnetic field coil is driven, and the piezoelectric element as an actuator is driven so as to cancel the detected deformation. However, preferably, the piezoelectric element is driven based on the gradient magnetic field drive information from the sequencer 7 as shown in the block diagram of FIG. In the control method shown in the figure, the piezoelectric element includes a control device 53 that takes in information from the sequencer 7, and a power supply 54 that is driven by a signal from the control device 53. As the information of the sequencer 7, the gradient magnetic field strength, the application timing, and the application axis are used.

The controller 53 obtains the ratio (weighting amount) of the voltage applied to each piezoelectric element in advance and stores it in the memory.
The voltage applied to each piezoelectric element is determined based on this weighted amount. The weighting amount can be determined as follows.
First, the voltage of each piezoelectric element generated when driving only the X axis with a certain gradient magnetic field strength (G ₀ ) is A / D converted, and the value is stored in the memory of the control device 53. Similarly, the Y axis,
With respect to the Z-axis gradient magnetic field, the voltage value of the piezoelectric element at each part when it is driven is also obtained and stored in the memory. Then, the ratio of the voltage applied to each piezoelectric element due to the driving of the gradient magnetic field of each axis is calculated, and this ratio is stored in the memory as the weighted amount (kx, ky, kz) for each piezoelectric element.

At the time of actual photographing, the control device 53 takes in the gradient magnetic field driving information from the sequencer 7, obtains the entire gradient magnetic field strength from the information of the applied axis and the gradient magnetic field strength, and obtains the value for each piezoelectric element. Weight. Then, the power source 54 of each piezoelectric element is driven by the signal weighted in this way to apply a voltage to the piezoelectric element. The timing for driving the piezoelectric element is set in accordance with the application timing information from the sequencer 7. For example, assuming that the gradient magnetic field strength at the time of imaging is G and three axes are simultaneously applied, the voltage applied to one piezoelectric element is -G (kx + ky + kz), where the weighting amounts of the piezoelectric elements are kx, ky, and kz. / G ₀ .

[0034]

As is apparent from the above embodiments, according to the present invention, the electromagnetic force of the gradient magnetic field coil serving as the vibration source and the noise source is canceled by the conversion element for converting mechanical energy into electric energy. Then, in order to prevent vibration and noise, by arranging the conversion element in a specific arrangement, it is possible to efficiently suppress the force that promotes the deformation of the coil that occurs around the conversion element due to its characteristics. Electromagnetic force can be canceled. In particular, when a moment force is generated by the conversion element, by arranging the conversion elements densely in the moment axis direction, it is possible to suppress the reverse moment force generated around the element.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a gradient magnetic field coil according to the present invention.

FIGS. 2A and 2B are views for explaining a moment force generated by piezoelectric elements arranged so that their polarization directions are different from each other.

FIGS. 3A and 3B are views for explaining moment forces generated in the axial direction and the circumferential direction by a piezoelectric element, respectively.

FIG. 4 is a view showing another embodiment of the gradient magnetic field coil to which the present invention is applied.

FIG. 5 is an overall configuration block diagram showing an embodiment of the MRI apparatus of the present invention.

FIG. 6 is a block diagram showing an example of driving a piezoelectric element according to the present invention.

[Explanation of symbols]

7 ... Sequencer 9 ... Gradient magnetic field coil 40, 40a-40l ... Piezoelectric element (energy conversion element) 50 ... Gradient magnetic field coil conductor 51 ... .... Bobbin (holding member) 53 ... Control device 60 ... Polarization direction

Claims (4)

[Claims] 1. A static magnetic field generating means for generating a static magnetic field in a space where an inspection object is placed, a gradient magnetic field generating means for generating a gradient magnetic field in the space, and an electromagnetic field for irradiating the inspection object or from the inspection object. A magnetic resonance imaging apparatus comprising: a high-frequency coil for detecting a nuclear magnetic resonance signal, and an image reconstructing unit that obtains an image representing the physical property of the inspection target by using the nuclear magnetic resonance signal. The means includes a coil conductor that generates a gradient magnetic field, a holding member that holds the coil conductor, and a plurality of conversion elements that convert electrical energy into mechanical energy in order to cancel an electromagnetic force generated in the coil conductor. Where the plurality of conversion elements are such that secondary deformation caused by one conversion element is suppressed by the main deformation force of the other conversion element. A magnetic resonance imaging apparatus characterized by being arranged with a constant gap. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the conversion element is arranged at least with a predetermined gap in a direction orthogonal to a moment axis generated by the conversion element. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the predetermined interval is not more than 3 times the plate thickness of the holding member. 4. The gradient magnetic field generating means has a substantially cylindrical shape, and the conversion element is a piezoelectric element stacked in a polarization direction, and the stacking direction is a radial direction of the substantially cylindrical shape or with respect to a radial direction. The magnetic resonance imaging apparatus according to any one of claims 1 to 3, wherein the magnetic resonance imaging apparatus is arranged so that the directions are substantially orthogonal to each other.