Geometry optimization of electrically floating PET inserts for improved RF penetration for a 3 T MRI system
Authors: Akram Md Shahadat Hossain , Levin Craig S. , Obata Takayuki , Hirumi Genki , Yamaya Taiga 
Published: August 2018

Purpose

An electrically floating radio frequency (RF) shielded PET insert with individual PET detectors shielded by separate Faraday cages enables the MRI built‐in body RF coil to be used at least as an RF transmitter, in which the RF field penetrates the imaging region inside the PET ring through the narrow gaps between the shielded PET detector modules. Because the shielded PET ring blocks more than 90% of the imaging region for the transmit field from the body RF coil, it is very challenging to obtain the required RF field inside a full‐ring floating PET insert. In this study, experiments were performed on the dependence of RF penetrability on different geometric aspects of the shielded PET modules and PET rings to optimize the design parameters to obtain the required RF field inside the PET ring.

Methods

We developed several prototype cylindrical full‐ring PET inserts using completely enclosed empty RF shield boxes (considered as dummy PET modules). Considering the RF shield box, we conducted studies for different axial lengths (240 and 120 mm) and heights (30 and 45 mm) of the shield boxes. On the other hand, considering the PET ring geometry, we also performed studies on three different categories of PET rings: a long‐ring insert (longer than the MRI phantom), a short‐ring insert (shorter than the MRI phantom), and a two‐ring insert that combined two short‐rings. In each ring category, two different inter‐shield box gaps (1 and 3 mm) were considered. In the case of the two‐ring insert, three different ring‐gaps (5, 10, and 20 mm) were studied. In total, 21 PET inserts were studied with an inner diameter (i.d.) of 210 mm. To study the effect of ring diameter, another long‐ring insert was studied for the 270 mm i.d. Experiments were conducted for the transmit RF (B1) fields and signal‐to‐noise ratios of spin‐echo and gradient‐echo images using a homogeneous phantom in a 700 mm bore‐diameter 3 T clinical MRI system. RF pulse amplitudes generated automatically by the MRI system were recorded for comparison.

Results

A PET insert with a 3 mm inter‐box gap was found to perform the best, at a level which is acceptable for PET imaging. In the case of an insert of multiple short‐rings instead of one long‐ring insert, the 5 and 10 mm ring‐gaps provided higher RF field penetration. Increasing the inter‐box gap improved the RF field penetration, whereas a ring‐gap that was too wide concentrated the field near the ring‐gap region. Relatively reduced RF power was required for wider inter‐box gap or ring‐gap or larger shield box height. Moreover, the rectangular shield box outperformed the trapezoidal shield box. On the other hand, when we changed the inner or outer diameter of the PET ring by keeping the same transaxial width of the shield boxes, we did not see any noticeable variation.

Conclusions

Our study results provide comprehensive guidance on the geometrical design aspects of RF‐penetrable PET inserts for efficient RF penetration inside the PET ring. By choosing proper geometric design parameters, we could get the RF field that was similar to the MRI‐only case.