RF-transmissive PET detector insert for simultaneous PET/MRI

Positron emission tomography (PET) and magnetic resonance imaging (MRI) have revolutionized the disease characterization as they enable the simultaneous measurement of functional and anatomical information of the body. However, the whole body simultaneous PET/MRI has been limited by its high cost. To address this issue, we are developing an RF-transmissive PET insert system that can be inserted into an MRI system without requiring modifications to the MR hardware system. Our PET system prototype consists of 16 PET detector modules in a 32 cm ring pattern with small gaps between them. By using electro-optical signal transmission technology, the PET insert is electrically floating relative to the MRI RF ground which allows the PET detector insert to be RF-penetrable. The inter-modular gaps and the electrical floating enable the RF fields of MRI body coil to pass through with some attenuation. We performed 2D electromagnetic simulations and experiments in a 3-T MR system to understand the degree to which a ring of electrically floating Faraday cages facilitates the RF transmissivity. The electromagnetic simulation results showed that the grounded PET insert blocks the RF field while the electrically floating PET insert allows the RF field to uniformly transmit through the gaps with some attenuation. The MRI attenuation experiments showed that the transmit attenuation was -3.47 dB and similarly the receive attenuations were -3.40 and -3.94 dB for GE and SE sequences, respectively. We have shown from both simulations and experiments that the RF field of the MRI body coil can penetrate a PET ring through small inter-modular gaps, when the PET ring is electrically floating with respect to the MR system.