Simultaneous information of MR and molecular Positron Emission Tomography (PET) imaging, especially in the pre-clinical small animal field, is strongly being currently investigated. This implies the working compatibility of both modalities, MR and PET. On one hand, the homogeneity of the B0 and B1 MR fields should not be compromised by either ferromagnetic components in the PET electronics such as Nickel or eddy currents generated in the possible RF screen. On the other hand, PET electronics have to properly be RF shielded. In this work we present a novel RF shielding design for a new development of a PET insert for pre-clinical MR systems. We propose a hybrid approach merging Carbon Fiber (CF) structures to shield the B1 field and slotted copper layers (SCL) to reduce eddy currents produced by the switching gradient coils.

A single PET ring made out of 8 detectors blocks using monolithic crystals and arrays of 16x16 SiPMs has been designed. The inner and outer diameters are 116 mm and 196 mm, respectively. An inner tube to the PET together with lateral rings has been made out of CF using aluminum molds. Packs of two or three unidirectional CF layers of 200 um each have been tested. The unidirectional CF layers are always at 90o one to each other. In one case one CF layer was aligned to the axial axis of the PET and MR, and in another case at 45o (see Figure 1). The CF structures are prepared in out-of-autoclave curing process (16h at 70oC), using an exhaustive control of compaction levels through LVDT (Linear Variable Differential Transformer), as a quality control to ensure the best possible mechanical (Tensile Strength: 54 MPa Flexural Strength: 80 MPa Compressive Strength: 142 Mpa), electrical and toughness properties. The SCL screens are only used on the outer to the PET cylinder. The copper layers are of 35 um thickness. Slits of 2 mm wide are traced along the axial axis of the cylinder at 30 mm gaps. A second identical copper layer separated by 100 um FR4 from the first and 16 mm shifted, avoids the penetration of the RF field, minimizing eddy currents.

These structures have been tested in the Bruker BioSpec 70/30 system equipped with a BGA 12S gradient system. The standard 112/86 RF coil was also used for reference measurements.

EPI ghosting acquisitions using 300 kHz bandwidth, TR = 2000ms, TE = 37 ms, SpinEcho mode, automatic ghost correction, 16 slices with 1 mm thickness and 2 mm gap were carried out on both the CF and SCL structures.

Concerning RF shielding, tests were carried out on the two CF layers directions (0 and 45o) using a network analyzer. They are comparable with standard SCL shields and there is almost no difference between the two CF tubes types. Although the RF shielding of CF tubes is sufficient for the use in PET/MR systems, a direct comparison of the RF shielding properties of CF tubes with the SCL structure has shown that there is small difference due to the lower electrical conductivity of CF layers (compared to the copper).

The EPIs on the SCL have been acquired using the standard 112/86 resonator with a ghosting uniform along all slices. On the CF screens there was a variation visible along the slices.

The appearance of eddy currents while using the different RF shields surrounding the coil has also been inspected by PRESS tests. The lowest influence is visible for the reference coil. A slight influence could be detected for both CF shields, which was larger for the 45o configuration.

This work shows a novel RF shielding for PET inserts developments based on a hybrid approach. The optimum RF shielding cavity resulted of CF laminates and slotted copper layers. The inner tube of the cavity and the lateral rings are made out of CF, which shielded the RF as good as simply copper. However, because this material showed some eddy currents originated by the gradient switching coils, the outer cylinder of this structure uses a double SCL separated by 100 um FR4. Lateral CF rings and exterior SCL tube are in contact using conductive rubbers and foams.

In addition to these results, the PET performance has already been tested outside the MR with unprecedented results.¹ Tests inside the MR using the proven hybrid RF shielding are scheduled for late November 2015.