Advanced Cryocooler Cooling for MRI Systems

Abstract

Advances in cryocooler technology during the past several years have enabled the design of new cooling methods for magnetic resonant imaging (MRI) systems. Open cycle operation of MRI systems using a Gifford-McMahon (GM) cycle cryocooler to cool two thermal shields, one at 20 K and the second at 80 K, has been the standard approach used to minimize helium usage in these systems. The concept has worked very well and enabled the development of an important medical imaging modality. However, the 12 K temperature limit of these cryocoolers has limited the design flexibility of the MRI magnet system by requiring a cylindrical design with two thermal shields and a large helium container to extend the operating time. The development of Gifford-McMahon cycle cryocoolers capable of cooling below liquid helium temperature, or providing larger cooling capacities between 4.2 and 10 K, has removed this design barrier and provided greater overall system design flexibility. The paper describes the impact that new GM cryocooler developments, based on rare earth intermetallic compounds in the second-stage regenerator, has had on MRI designs. By extending the cooling capacity of these units to below 4.2 K with rare earth materials, new MRI products have been developed that operate as closed cycle systems without the need for replenishing liquid helium to maintain the magnet at temperature for long periods of time. The paper describes the evolution of MRI systems at the General Electric Company from open cycle systems to two new developments using conduction cooling and helium recondensing to eliminate the need for refilling with helium. The paper reviews the design of a conductively cooled system developed for an open MRI magnet used for interventional therapy and a helium recondensing system that was incorporated into GE’s product line. In addition to a description of these systems, the operational reliability of cryocooled systems will also be reviewed.