Magnetic resonance apparatus with pressure reservoir

A magnetic resonance apparatus comprising a superconducting magnet coil which is disposed in a cryostat, and a refrigerator for cooling the magnet coil, which comprises a compressor for compressing a working gas, and a high-pressure line and a low-pressure line between the compressor and a control valve, which periodically connects the high-pressure line and the low-pressure line to a connecting line from the control valve to a cold head of the refrigerator, thereby generating pressure pulses via the connected working gas, wherein the control valve is mechanically rigidly connected to the cryostat, is characterized in that one pressure reservoir is provided in the high-pressure line and/or in the low pressure line, which is mechanically rigidly connected to the cryostat and which is connected to the compressor via a mechanically flexible line section. This permits supply of a working gas to a cold head using a compressor in such a manner that the artifacts in the NMR signals caused by the pressure pulses of the working gas are minimized.

This application claims Paris Convention priority of DE 10 2005 035 892.6 filed Jul. 30, 2005 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a magnetic resonance apparatus with a superconducting magnet coil which is disposed in a cryostat, and a refrigerator for cooling the magnet coil, which comprises a compressor for compressing a working gas, a high-pressure line and a low-pressure line between the compressor and a control valve that periodically connects the high-pressure line and the low-pressure line to a connecting line between the control valve and a cold head of the refrigerator, thereby generating pressure pulses through the switched working gas, wherein the control valve is mechanically rigidly connected to the cryostat.

A configuration of this type is disclosed by T. Tomaru et al. in Cryogenics 44, pages 309-317 (2004). The compressor is disposed outside of the cryostat and connected to the control valve via the high-pressure or low-pressure line. In order to reduce transmission of vibrations from the surroundings to the cryostat, in particular, to decouple the cryostat from the compressor, the high-pressure line and the low-pressure line are designed as flexible hoses through which a working gas is guided from the compressor to the control valve and finally to the cold head of the refrigerator. However, the vibrations of the pressure wave of the working gas within the lines cause the flexible conduit hoses to vibrate and transmit these vibrations to the control valve and therefore also to the cryostat. The vibrations may produce artifacts in the NMR signals.

It is therefore the underlying purpose of the invention to propose a magnetic resonance apparatus which supplies a working gas to a cold head using a compressor, such that the artifacts in the NMR signals caused by the above-described vibrations are minimized.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in that one pressure reservoir is provided in the high-pressure line and/or low-pressure line, which is mechanically rigidly connected to the cryostat and which is connected to the compressor via a mechanically flexible line section.

The compressor is therefore not directly connected to the control valve via the high-pressure and/or low-pressure line. The pressure reservoir serves as a buffer container from which the refrigerator is fed. The oscillating amplitude of the pressure pulses of the working gases from the compressor to the pressure reservoir can be minimized due to the additional volume of the pressure reservoir, wherein the refrigerator can be simultaneously fed with pressure pulses with the same amplitude. This permits unobstructed operation of the refrigerator. The motion of the high-pressure and/or low-pressure line and therefore transmission of vibrations to the cryostat can be considerably reduced by filling the pressure reservoir using pressure pulses of small amplitude.

The cryostat and the magnet coil are preferably isolated with respect to vibrations from the surroundings. This prevents transmission of vibrations from the surroundings.

In a preferred embodiment of the inventive magnetic resonance apparatus, the pressure reservoir is connected to the control valve via a mechanically rigid line section in order to prevent introduction of further vibrations from moving conduit hoses.

With particular preference, the volume of the pressure reservoir is selected such that the pressure fluctuations in the high-pressure line and/or low-pressure line are minimized upstream of the pressure reservoir on the compressor side. This also minimizes transmission of vibrations from the compressor to the reservoir and the components rigidly connected thereto.

Towards this end, the volume of the pressure reservoir is preferably x times the volume of the high-pressure line or low-pressure line, wherein 1<x<100, preferably 5<x<10.

In a preferred embodiment of the inventive magnetic resonance apparatus, the control valve is a rotary valve.

With particular preference, the refrigerator is a pulse tube cooler, since pulse tube coolers can be operated with particularly low vibration.

The inventive magnetic resonance apparatus is preferably designed for high-resolution NMR measurements. These measurements require high vibration decoupling and for this reason, the properties of the invention can be utilized with particular advantage.

The use of the pressure reservoir in the high-pressure and/or low-pressure line of the inventive magnetic resonance apparatus can improve the pressure fluctuations from the compressor to the pressure reservoir and thereby the transmission of vibrations to the cryostat, thereby improving the MR measurement quality.

Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2shows a refrigerator of a magnetic resonance apparatus according to prior art. A compressor1supplies or discharges a working gas to/from a cold head4of the refrigerator via a high-pressure lie2and a low-pressure line3. The supply of the working gas is controlled by a control valve5which alternately connects high pressure and low pressure to the cold head4via a connecting line6which opens into the cold head4. The magnet coil, cryostat, cold head4and control valve5are thereby rigidly connected to each other to form a system which is isolated with respect to vibrations from the surroundings. For this reason, vibrations introduced in the control valve5e.g. due to pressure waves of the working gas or from the surroundings also have an effect on the magnet coil and therefore also on measurements produced with this magnet coil.

In order to prevent or at least reduce transmission of vibrations of the pressure wave of the working gas, the inventive magnetic resonance apparatus has at least one pressure reservoir7,8in the high-pressure line2and/or the low-pressure line3.

FIG. 1shows one embodiment, wherein a pressure reservoir7is disposed in the high-pressure line2and a pressure reservoir8is disposed in the low pressure line3between the compressor1and the control valve5, such that the high-pressure line2is divided into two line sections2a,2band the low-pressure line3is divided into two line sections3a,3b. The line sections2a,3awhich extend between the compressor1and the pressure reservoirs7,8are flexible in order to prevent or at least minimize transmission of vibrations from the surroundings to the pressure reservoir and the cryostat. The configuration of the pressure reservoirs7,8additionally prevents transmission of vibrations to the cryostat due to movement of the flexible line sections2a,3a, since the working gas can be supplied to or discharged from the pressure reservoir through pressure impacts of smaller amplitude than would be possible with direct supply of the working gas to the control valve5. The working gas can be supplied to or discharged from the control valve5and therefore also to or from the cold head4of the refrigerator via the reservoirs7,8, still using pressure pulses with the same amplitude as in prior art, such that the refrigerator can be operated with the same power as conventional configurations. The pressure reservoirs7,8may also be designed as line sections with correspondingly large line cross-sections compared to the line cross-sections of the line sections2b,3badjacent to the control valve5.

The pressure reservoirs7,8and the control valve5are coupled to the cryostat such that they are fixed relative to the oscillating frequencies transmitted by the working gas, and are therefore part of the system which is isolated with respect to vibrations from the surroundings, but may oscillate relative to the surroundings, wherein the components of the system are rigidly connected to each other. The line sections2b,3bbetween the pressure reservoirs7,8and the control valve5may therefore be rigid without transmitting further vibrations to the overall system.

LIST OF REFERENCE NUMERALS

1compressor2high-pressure line2aflexible line section of the high-pressure line2brigid line section of the high-pressure line3low-pressure line3aflexible line section of the low-pressure line3brigid line section of the low-pressure line4cold head5control valve6connecting line7pressure reservoir8pressure reservoir