Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2003-11-26
2004-12-07
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C324S318000, C062S047100, C062S051100, C505S892000
Reexamination Certificate
active
06828889
ABSTRACT:
BACKGROUND OF THE INVENTION
The present technique relates to medical imaging devices and, more particularly, to imaging devices with cryogenic cooling systems.
A number of important applications exist for superconductive magnet systems. These include imaging systems, as for medical imaging, as well as spectrometry systems, typically used in materials analysis and scientific research applications. The present technique relates to management of cryogenically cooled superconductive magnets, and particularly to the monitoring and servicing of such systems. Although reference is made throughout the following discussion to imaging systems, it should be borne in mind that the technique is applicable to a range of systems that utilize cryogenically cooled superconducting magnets.
Imaging devices are omnipresent in typical medical environments. Medical practitioners, such as physicians, may employ medical imaging devices to diagnose patients. Imaging devices, such as Magnet Resonance Imaging (MRI) devices and Nuclear Magnetic Resonance (NMR) devices, produce detailed images of a patient's internal tissues and organs, thereby mitigating the need for invasive exploratory procedures and providing valuable tools for identifying and diagnosing disease and for verifying wellness.
Typical MRI and NMR devices develop diagnostic images by affecting gyromagnetic materials within a patient via controlled gradient magnetic fields and radiofrequency pulses in the presence of a main magnetic field developed by a superconductive magnet. During an MRI exam, a main magnetic field of upwards of two Tesla may be necessary to produce vivid images. Typically, superconductive electromagnets comprise loops of coiled wire, which are continuously bathed in a cryogen, such as liquid helium, at temperatures near absolute zero. For the example of bathing the coils with a liquid pool of helium, system temperatures are approximately −269° C. (or 4 K) near atmospheric pressure (e.g. less than 5 psig). When cooled to such extreme temperatures, the coiled wire becomes superconductive, i.e., the electrical resistance of the wire falls to essentially zero, enhancing the field strength without requiring significant energy input for continued operation. Advantageously, superconductive electromagnets reduce the electrical load requirements for producing the desired magnetic fields, thereby making the MRI system more economical to operate.
Challenges exist, however, in maintaining the electromagnets at these extreme temperatures which are significantly lower than ambient temperatures. Because of this temperature difference with ambient, a considerable driving force exists for heat transfer from the environment into the magnet system. Accordingly, thermal insulating material and other heat transfer barriers, such as vacuum regions, may insulate the magnet and cryogen to impede heat transfer from the environment. For environmental heat effects that reach the inner workings of the magnet system, the liquid pool of cryogen that surrounds the magnet must absorb the heat to maintain the magnet at desired temperature. Cryogens operating at or near their boiling points typically expend this external heat by vaporizing relatively small amounts of cryogen.
In general, the cryogen liquid pool and its heat of vaporization consume heat while maintaining the magnet at constant temperature. On the whole, cryogen liquid pools in well-insulated systems, such as typical superconducting magnet systems, are able to absorb heat transferred from the environment over relatively long periods of time to maintain the magnet at desired temperature. Other systems with refrigerants operating below their boiling points (i.e., super-cooled) and which primarily absorb heat via sensible heat increases, typically require more refrigerant and processing of the refrigerant. Additionally, for liquid pools relative to other techniques, the magnet temperature, in some circumstances, may be better maintained constant because, in part, the cryogen boils at fairly constant temperatures at moderately fixed pressures.
Furthermore, liquid pools of cryogens may be suitable for superconducting applications because magnet temperature may be controlled by controlling cryogen pressure at a specified pressure that gives a liquid pool boiling temperature that corresponds to the desired magnet temperature. This may prove advantageous over direct control of temperature because pressure measurement may generally be a more economical and reliable application than temperature measurement. Additionally, because cryogens, such as helium, boil in the desired lower temperature ranges at slightly positive pressures (e.g., near atmospheric at 0-5 psig), vacuum operating conditions are generally not needed to induce low boiling temperatures, thus permitting simpler and more economical system design and operation.
Cryogenic liquids, such as liquid helium, however, are relatively expensive to refine and maintain. Therefore, the aforementioned advantages of cryogen in superconducting magnets applications may be offset if cryogen losses are excessive. Accordingly, older approaches of “open” systems which have no recondensing capability and where cryogen vapor is normally vented to the atmosphere, have generally fallen out of favor in the industry. In these systems, as the liquid cryogen absorbs environmental heat in maintaining the desired magnet temperature, vaporized cryogen is normally vented to limit pressure increases and thus to limit temperature increases. The simplicity, however, of relying solely on a vent or relief device to control the high end of cryogen pressure is usually offset by additional costs and downtime of servicing and refilling the cryogen system.
While the economic operation of the system is desirable, there is the environmental consideration driven by the reality that helium is a finite natural resource.
Once extracted from the ground (helium is a refined by-product of natural gas extraction) it is not replenished. The utmost care must be employed to ensure closed systems remain closed. Once vented by the relief valve in an overpressure condition OR accidentally by an unintentional plumbing leak those molecules are gone forever into the atmosphere. Helium's density being lighter than most other elements rises, and because of this does not remain at ground level in sufficient concentration for atmospheric extraction and re-processing.
Therefore, to conserve cryogen, such as helium, and to support cryogen pressure control, magnet systems in typical MRI devices may now include a cryogen condensing system, which recondenses volatilized cryogen back into its liquid phase. That is, cryogen is maintained in a sealed cryogen vessel (or cryostat) that provides cryogen vapor (i.e., gaseous helium) to the condensing system and receives liquid cryogen (i.e., liquid helium) from the condensing system in a closed loop process. The condensing system condenses cryogen vapor, thus recovering the vapor, as well as, maintaining the cryogen pressure below the set point of the vent or relief device. On the flip side, as discussed more below, a heater may be used to prevent the cryogen pressure from dropping too low. In sum, for the older open systems, a loss in cryogen level is expected and the timing of service intervals is typically based on this loss of level. In contrast, for recondensing magnets systems which recover the vaporized cryogen, losses in cryogen liquid level are not expected during normal operation. Thus, recondensing magnet systems generally retain cryogen level and reduce the requirement of periodic refilling of cryogen.
Recondensing magnet systems, however, from time to time, require maintenance, for example, when the cryogen condensing system may require repair or replacement. In particular, the performance of the condensing system components will degrade due to wear, thereby reducing the efficacy of the condensing system and overall magnet cooling system (cryogenic cooling system). Moreover, leaks within the cryogen (helium) vessel and/or condensi
Barrera Ramon M.
Fletcher Yoder
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