Evaluation of cross-modality electromagnetic interactions in a shielded PET/MRI system

A small-animal PET/MRI scanner was developed previously that was integrated in a Bruker 9.4 T microMRI system, with which simultaneous PET/MR images of a rat's brain and of a gated mouse-heart were obtained. To minimize electromagnetic interactions in our PET/MRI system, viz., the effect of radiofrequency (RF) pulses in the PET, our modular front-end PET electronics were surrounded with variously configured shields. These included a solid aluminum shield and thin segmented layers of copper shielding. It was noted that the gradient-echo RF pulses had no impact on PET data when the PET electronics were shielded with either the aluminum or copper shields. However, we observed spurious counts in the PET data resulting from high-intensity fast spin-echo RF pulses; compared to the unshielded condition, they were suppressed effectively by the aluminum shield (∼97%) and the double-layer copper shield (∼90%). Using the solid aluminum shield yielded a poorer signal in the MR images than compared to segmented copper shields. Our initial results on shielding demonstrate that we can obtain interference-free PET data during gradient-echo pulses and obtain good-quality MR images with thin copper layers covering the PET detector housing.