Comparative study between electrically ground and electrically floating PET inserts using MRI built-in RF coil at 3 T

A modular radiofrequency (RF) shielded PET insert, electrically floating from the RF ground, enables the RF field from outside to penetrate through the inter-modular gaps to the imaging region (ROI) with less attenuation compared to the electrically grounded PET insert. In this study, we developed an electrically floating PET insert by implementing conventional electric data transmission and power source cables and, performed a comparison with the same PET ring as a grounded PET insert inside a 3 T MRI system. An existing electrical data transmission capable RF shielded PET detector ring is modified to perform the study. The PET inserts, with the inner diameter of 240-mm, had eight RF shielded PET detector modules with intermodular gap of 5 mm. Here the term ground PET is used to mean that both the RF shields of cables and the PET detector modules are together connected to the RF ground. On the other hand, the term floating PET is used to mean that the RF shield boxes of PET detector modules kept electrically isolated from the cable shield that remained connected to RF ground. The MRI system built-in body RF coil (700-mm in diameter) is used both as RF transmitter and receiver. A homogeneous phantom study revealed an approximate 10% and 20% reduction in mean RF field attenuation for the front 125 mm axial-FOV of respectively floating and ground PET insert. Compared to the front of PET insert, an approximate 20% and 40% field attenuation was seen for an imaging region close to the PET insert cable-end. However, RF field homogeneity for both the front and center of the PET insert was comparable to the MRI-only values.