Investigation of RF field penetrability of a novel electrically floating PET insert for PET/MR

Combined positron emission tomography (PET) and magnetic resonance imaging (MRI) has shown potential to provide a powerful tool for disease characterization as it enables the simultaneous measurement of molecular, functional and anatomical information of the body. However, the availability of whole-body simultaneous PET/MRI has been limited by its high cost. To address this issue, we have developed an RF-penetrable PET technology that can be inserted into an MRI system without requiring modifications to the MR hardware. The prototype PET insert consists of 16 PET detector modules in a 32 cm ring pattern with 1 mm inter-modular gaps. By using electro-optical coupling technology, and batteries for power, the PET insert is electrically floating relative to the MRI RF ground which allows the RF field transmitted from the built-in body coil to penetrate through the PET ring. We performed experiments with various configurations to study the RF-penetrability of the PET insert in the MR system: No PET insert, with PET insert (powered with batteries), with PET ring gaps blocked with copper conductor but ends open, and with PET inserted, ends blocked with copper conductor but gaps open. With these configurations, we acquired B₁ maps and performed electromagnetic simulations. We then compared the magnitude and homogeneity of the acquired B₁ maps and simulation results. We have found that negligible amount of RF field enters through the gaps with the ends blocked, but this contributes to increasing the uniformity by ∼400% when ends and gaps are both open. Then, the electromagnetic simulations show that by either decreasing the detector height and/or widening the inter-module gaps improve the RF-penetrability by nearly 300%.