JP5280022B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus for reducing noise generated by vibration of a gradient magnetic field coil.

  In recent years, there is an increasing need for shortening the time required for imaging in magnetic resonance imaging apparatuses (MRI apparatuses). In response to this requirement, a pulse sequence involving high-speed inversion of a gradient magnetic field pulse such as single shot or multi-shot EPI (echo planar imaging), which is a technique for ultra-high-speed imaging, has been developed and put into practical use.

  When capturing an image using such a pulse sequence, a pulsed large current is supplied to the gradient coil to give position information to each signal emitted from the measurement target, so that a Lorentz force acts on the gradient coil. Cause mechanical distortion. Since this phenomenon repeatedly acts, the gradient magnetic field coil vibrates and generates a large noise. Further, when the gradient magnetic field pulse is reversed at a high speed, the vibration increases, so that the noise increases as the imaging speed increases.

  This noise causes discomfort and anxiety to the subject arranged in the imaging space, and techniques for reducing this noise are described in Patent Document 1 and Patent Document 2.

  The technique described in Patent Document 1 reduces the sound wave radiated to the imaging space by sealing the gradient magnetic field coil fixed to the static magnetic field generating means with a sound insulation cover and blocking the sound wave generated by the gradient magnetic field coil. Noise is reduced.

  Moreover, the technique described in Patent Document 2 blocks the air propagation of noise generated by the gradient magnetic field coil by providing a vacuum space while keeping the airtight inside the pole piece where the gradient magnetic field coil is installed.

JP 2001-299719 A JP 11-137535 A

  However, in the sound insulation structure according to the technique described in Patent Document 1, since the sound insulation cover is also fixed to the static magnetic field generation means, the vibration of the gradient coil is transmitted to the sound insulation cover via the static magnetic field generation means. . Furthermore, an eddy current flows in the static magnetic field generating means by the gradient magnetic field pulse, and Lorentz force acts to vibrate the static magnetic field generating means. This vibration is also transmitted to the sound insulation cover fixed to the static magnetic field generating means.

  In the sound insulation structure described in Patent Document 2, the gradient magnetic field coil is attached to the pole piece, and the sound insulation cover is attached to the pole piece. Is transmitted to the sealing cover. Further, an eddy current flows in the static magnetic field generating means due to the gradient magnetic field pulse, Lorentz force acts on the static magnetic field generating means, and the static magnetic field generating means vibrates. This vibration is also transmitted to the sound insulation cover connected to the static magnetic field generating means.

  As described above, in the techniques described in Patent Documents 1 and 2, since vibration is transmitted to the sound insulation cover from a plurality of paths, the sound insulation cover itself becomes a sound source, and noise reduction in the imaging space is insufficient.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of reducing transmission of noise generated by vibration of a gradient coil to an imaging space.

In order to achieve the above object, the present invention is configured as follows.
The present invention uses the exterior cover as a sound insulation cover, and further the exterior cover is configured so as to be in non-contact with the static magnetic field generating means, the gradient magnetic field means, and the high frequency irradiation means, and these static magnetic field generating means, gradient magnetic field means, and high frequency Reduces the transmission of vibration from the irradiation means to the exterior cover.

  A magnetic resonance imaging apparatus that can reduce transmission of noise generated by vibration of the gradient magnetic field coil to the imaging space can be realized.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is an external configuration diagram of a superconducting magnet type horizontal MRI apparatus to which an embodiment of the present invention is applied. In FIG. 1, an MRI apparatus includes a static magnetic field generating means 1 so as to surround an imaging space, an irradiation coil 3 that irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in an atomic nucleus constituting a biological tissue of a subject, The scanner gantry includes a gradient magnetic field coil 2 for giving position information to each signal emitted from the specimen, and a magnetic field generator having the static magnetic field generating means 1, the gradient magnetic field coil 2, and the irradiation coil 3. Has been. An exterior cover 7 is disposed so as to cover the scanner gantry.

  In addition, the MRI apparatus receives a signal that is generated from the subject, a receiving coil 4, a bed 5 on which the subject is loaded, and an image reproduction device that obtains an image representing the physical properties of the subject using the received signal. A configuration calculating means 6 and means (not shown) for controlling the inspection conditions are provided.

  The static magnetic field generating means 1 is a permanent magnet type, constant electric magnet type or superconducting magnet type magnetic field generating means around the imaging space into which the subject is inserted, and the body axis direction or body axis around the subject. A uniform static magnetic field is generated in the direction perpendicular to the line.

  The transmission system of the MRI apparatus irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in atomic nuclei constituting the biological tissue of the subject, and is arranged in close proximity to the subject inside the gradient magnetic field coil 2. The high-frequency magnetic field transmitting irradiation coil 3 and the high-frequency magnetic field transmitting irradiation coil 3 are provided with a high-frequency transmitter, a modulator, a high-frequency amplifier, and the like.

  The receiving system of the MRI apparatus detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the living body tissue of the subject, and is based on the high frequency magnetic field irradiated from the irradiation coil 3 for high frequency magnetic field transmission. In order to detect a response echo signal (NMR signal) from the subject, the receiving high-frequency coil 4 is disposed in the vicinity of the subject, and the echo signal is A / D converted via an amplifier and a quadrature detector. The signal is input to a machine, converted into a digital quantity, and the signal is sent to a signal processing system.

  The signal processing system performs an image reconstruction operation using the echo signal detected by the reception system. The image data obtained from the echo signal is Fourier-transformed to perform processing such as correction coefficient calculation and image reconstruction. The image reconstruction calculation means 6 (central processing unit (CPU)), a recording device such as a magnetic disk and a magneto-optical disk, and a display such as a CRT for displaying the obtained tomographic image data of an arbitrary cross section are provided. Become.

  A scanner gantry using the sound insulation structure, which is the main part of the present invention, in the MRI apparatus configured as described above will be described.

  As shown in FIG. 1, an exterior cover 7 is provided around the scanner gantry. The exterior cover 7 is in non-contact with the static magnetic field generating means 1, the gradient magnetic field coil 2, and the irradiation coil 3 and covers them, and the leg 7f is self-supporting on the floor surface or via a support member. The structure is supported from the floor.

  The exterior cover 7 includes an interior cover portion 7 i that covers the imaging space in which the subject is disposed in a non-contact manner with the static magnetic field generating means 1, the gradient magnetic field coil 2, and the high-frequency signal irradiation coil 3.

  In addition, the space | interval of the exterior cover 7 and the static magnetic field generation means 1 shall be 40 mm-50 mm, for example.

  As shown in FIG. 2, a gradient magnetic field coil 2 is installed in the bore of the scanner gantry and is supported from the floor or a vacuum vessel. In the case of being supported from a vacuum vessel, it is desirable that the gradient magnetic field coil 2 is supported via a vibration damping material so that the vibration of the gradient magnetic field coil 2 is not transmitted to the vacuum vessel.

  Inside the gradient coil 2, the irradiation coil 3 and the bed 5 are installed so as not to contact the gradient coil 2 so that vibration of the gradient coil 2 is not transmitted.

  The bed 5 is supported from the floor or a vacuum vessel. In particular, in order to reduce vibration transmission from the vacuum vessel, the bed 5 is preferably supported from the floor so as not to contact the vacuum vessel.

  In that case, as shown in FIG. 2, the bed 5 is fixed to the support leg 8 installed on the floor. Further, the irradiation coil 3 is integrated with the bed 5 so as to be in non-contact with the gradient coil 2. The interior cover 7i is installed inside the irradiation coil 3 that is closest to the imaging space so as not to contact the gradient coil 2 and the scanner gantry and to cover the imaging space.

  Here, the bed 5 is supported by a rail so as to be movable in the long axis direction of the imaging space, and this rail is integrated with the interior cover portion 7i.

  Further, the exterior cover 7 is configured to cover the front and rear surfaces of the scanner gantry except for the end surface of the imaging space.

  Thus, since the exterior cover 7 completely covers the scanner gantry including the gradient coil 2 and the vacuum vessel, there is an effect of reducing transmission of sound generated from these to the imaging space.

  The sound insulation effect by the exterior cover 7 is generally determined by the mass rule shown in the following equation (1).

TL = 18 × log (f × ρ × t) −44 [dB] (1)
In the above equation (1), TL represents a transmission loss representing a sound insulation effect, f represents a sound frequency, ρ represents a density of the sound insulation material, and t represents a thickness of the sound insulation material.

  The above equation (1) means that the sound insulation performance increases as the mass of the sound insulation material increases, and in order to enhance the sound insulation effect, the outer cover 7 is made of a material having a high density ρ and the plate thickness t is increased. It is desirable to do.

  Therefore, in order to enhance the sound insulation effect by the exterior cover 7 as a sound insulation material, a material having a high density ρ, for example, a material such as a fiber reinforced resin, is used, and the thickness t thereof should be increased to such an extent that the workability is not impaired. .

  Further, as described above, the exterior cover 7 and the scanner gantry are installed so as not to contact each other. In the prior art, since the scanner gantry and the exterior cover are connected to each other by a metal fitting, vibration is transmitted from the metal fitting to the exterior cover, and noise is radiated to the imaging space.

  However, by installing the exterior cover 7 and the scanner gantry so as not to contact each other as in the present invention, the vibration transmitted from the scanner gantry to the exterior cover 7 is reduced, so that the sound insulation effect by the exterior cover 7 is further improved. To do. According to the analysis, the structure according to the present invention reduces the noise in the imaging space by about 10 dB compared to the structure in which the exterior cover and the scanner gantry are brought into contact with each other.

  As described above, the outer cover 7 has a structure that can stand independently from the scanner gantry by the legs 7f. However, the outer cover 7 may have another structure as long as it has sufficient rigidity to be able to stand by itself. . Specifically, the plate thickness t is increased, and the rigidity is improved by installing ribs on the support portion in the vicinity of the floor surface. Alternatively, a means for improving the rigidity by providing the exterior cover 7 with a reinforcing member made of an aluminum frame or the like may be used. Alternatively, instead of improving the rigidity of the exterior cover 7, it is also effective to suspend the exterior cover 7 from the ceiling and install it so as not to contact the gantry.

  It is also conceivable that the vibration of the scanner gantry is transmitted to the floor surface on which the MRI apparatus is installed, and transmitted from the floor surface to the exterior cover 7. Therefore, in order to reduce the vibration transmitted to the exterior cover through the floor surface, a vibration damping material 9 is installed between the exterior cover 7 and the floor surface as shown in FIG. As a result, the vibration transmitted from the floor surface can be reduced, so that noise in the imaging space can be further reduced.

  Even when the vibration damping material 9 is installed between the exterior cover 7 and the floor surface, as described above, the exterior cover 7 may be suspended from the ceiling.

  As described above, according to the present invention, attention is paid to the fact that vibration is transmitted from the gradient magnetic field coil or the like to the exterior cover of the MRI apparatus, and the exterior cover is not in contact with the scanner gantry. It is configured to be able to stand on its own.

  Therefore, it is possible to reduce the vibration from the gradient magnetic field coil or the like from being transmitted to the imaging space via a member that directly or indirectly contacts the gradient magnetic field coil or the like.

  Further, since there is no additional member that covers only the gradient magnetic field coil, noise can be reduced at low cost.

1 is a cross-sectional view of a magnetic resonance imaging apparatus to which an embodiment of the present invention is applied. 1 is a longitudinal sectional view of a magnetic resonance imaging apparatus to which an embodiment of the present invention is applied. It is a modification of one Embodiment of this invention, and is a figure which shows the example which supported the exterior cover with the vibration damping material.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generation means, 2 ... Gradient magnetic field coil, 3 ... Irradiation coil, 4 ... Reception coil, 5 ... Bed, 6 ... Image reconstruction calculation means, 7 ...・ Exterior cover, 7f ... Leg part, 7i ... Interior cover part, 8 ... Supporting leg, 9 ... Vibration damping material

Claims (5)

A static magnetic field generating means;
A gradient magnetic field generating means and a high-frequency magnetic field irradiating means arranged outside the static magnetic field generating means ;
Receiving means for receiving a nuclear magnetic resonance signal from the subject;
An exterior cover that covers the static magnetic field generating means, the gradient magnetic field generating means, and the high-frequency magnetic field irradiation means in a non-contact manner ;
Equipped with a,
The magnetic resonance imaging apparatus, wherein the exterior cover is supported from a floor surface via a vibration absorbing member and is suspended from a ceiling in a room where the exterior cover is installed . The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the outer cover has auxiliary legs. The magnetic resonance imaging apparatus according to claim 1 or 2,
2. The magnetic resonance imaging apparatus according to claim 1, wherein the exterior cover includes an interior cover portion that covers an imaging space in which the subject is disposed in a non-contact manner with the static magnetic field generation unit, the gradient magnetic field generation unit, and the high-frequency magnetic field irradiation unit. The magnetic resonance imaging apparatus according to claim 3.
A bed that supports the subject in the imaging space, and a rail that movably supports the bed,
The magnetic resonance imaging apparatus, wherein the interior cover part is integral with the rail. The magnetic resonance imaging apparatus according to claim 4.
The magnetic resonance imaging apparatus, wherein the bed is supported from the floor so as not to contact the static magnetic field generating means.

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