  The integration of PET into MRI to form a hybrid system requires often
compromises for both subsystems. For example, the integration might come at the
cost of a reduced PET detector height or a reduced MRI examination volume
diameter. Here, we propose a so-called shared-volume concept to use the volume
required for both subsystems more efficiently, in which the MR transparency of
the scintillator is exploited by integrating the RF shielding into the
scintillator of the PET detector. Semi-monolithic scintillator prototypes (PVC
slabs) of 7 mm and 12 mm height with integrated copper foil between every and
every other slab, respectively, were investigated to evaluate the shielding
effectiveness (SE). The SE was measured with probes on a test bench system. In
addition, the PET detector performance was evaluated by determining the
positioning using gradient tree boosting, energy and timing resolution using
digital SiPM arrays (DPC3200, PDPC) in three LYSO scintillator configurations:
ESR separation (Slab$\mathrm{_{ESR}}$), ESR with copper foil in between
(Slab$\mathrm{_{ESR+Cu}}$), and purely copper foil separation
(Slab$\mathrm{_{Cu}}$). The prototype with shielding between each slab and 12
mm height showed the highest SE with 31 dB (mean) in combination with a
supporting frame. While Slab$\mathrm{_{Cu}}$ was best for positioning, followed
by Slab$\mathrm{_{ESR}}$ and Slab$\mathrm{_{ESR+Cu}}$, the
Slab$\mathrm{_{ESR}}$ had the best energy resolution (10.6 %), followed by
Slab$\mathrm{_{ESR+Cu}}$ (11.2 %) and Slab$\mathrm{_{Cu}}$ (12.6 %). For the
timing resolution, Slab$\mathrm{_{ESR}}$ and Slab$\mathrm{_{Cu}}$ achieved 279
ps and 288 ps, respectively. Slab$\mathrm{_{ESR+Cu}}$ performed worst (293 ps).
The scintillator-based RF shielding shows good RF shielding with similar PET
performance, demonstrating the potential for more effective integration of PET
detectors into MRI systems.
