Use of a Pulse Tube Refrigerator for Cooling a HTS-Antenna for Magnetic Resonance Imaging

Abstract

We report on the first operation of a YBCO receiving antenna for magnetic resonance imaging by use of a specially designed double-inlet pulse tube refrigerator. The pressure oscillation in the cooler is generated by means of a rotary valve in combination with a 2 kW helium-compressor. To reduce noise at the cold head, the oscillating gas flow from the rotary valve is supplied to the cold head via a thin flexible tube with a length of 4 m. Optimum cooler performance is achieved with the pulse tube in vertical position (cold end facing downwards). Then, at an operating frequency of 4.6 Hz, a minimum no-load temperature of 31 K is achieved, and a net cooling power of 2 W is available at 50 K. Tilting of the pulse tube leads to enhanced heat losses through free convection of helium in the tube. By increasing the operating frequency to 8 Hz the orientation dependence of the cooler performance could be reduced. This made possible the testing of the PTR-cooled antenna in a Siemens Magnetom Open 0.2 T MRI-system with a horizontal pulse tube and with the cold block at 55 K. Compared to a standard room temperature antenna the improved signal to noise ratio that is achieved with the superconducting antenna is clearly reflected in the higher quality of MR-images taken from parts of the human body.