Measuring the Effect of CZT Detector Materials on MRI Field Homogeneity

There has been significant interest in the use of semiconductor detectors such as cadmium zinc telluride (CZT) for SPECT/MRI and PET/MRI due to their ability to operate in high magnetic fields. However, the effect of these materials on magnetic field homogeneity must be considered when designing an MR-compatible gamma camera. Field maps were generated for several materials considered for an MR-compatible gamma camera to determine the extent of the shift in magnetic field. The materials used were CZT, AI, carbon fiber, printed circuit boards, thermoelectric cooler, Pb and a composite material consisting largely of tungsten. A lipid phantom filled with a mixture of 50 % canola oil and 50 % water $(\sim 9\ \text{cm x}\sim 9\ \text{cm x}6\ \text{cm})$ was imaged with and without the materials placed adjacent to the phantom/foot ankle coil. Data were acquired with the materials individually and as a combined system with and without power. MRI phase images, at TE = 5 and 8 ms (using a 3T MRI), were used to calculate magnetic field (B0) homogeneity. Tungsten and the thermoelectric cooler resulted in a significant Bo shift near the boundary of the phantom where the materials were placed, whereas the aluminum had the least effect on the field homogeneity. Based on a linear approximation, the material with the largest effect on the magnetic field, tungsten, would need to be placed approximately 0.6 cm outside of the RF receiver coil, making it 3.4 cm in total from the phantom to have negligible effect on the field homogeneity. There was negligible effect on magnetic field homogeneity when high or low voltage power were supplied to the CZT detector system.