Simultaneous SPECT/MR Imaging with a SiPM-Based Preclinical Insert

The results of a systematic and quantitative experimental characterization of simultaneous operation of a preclinical SPECT insert inside an unmodified standard MR scanner (Philips 3T Achieva) are presented. The static gantry (20 cm diameter, 15 mm field of view) is composed of 10 MRI-compatible gamma cameras based on 8-mm thick CsI(Tl) scintillator (50 mm × 50mm) coupled with arrays of SiPMs. Mutual compatibility is achieved by proper design of the SPECT system: shielding, choice of materials, design of the analog boards. The measured energy resolution is 13% at 122 keV after correction of SiPM gain variability and local crystal luminosity. The intrinsic spatial resolution, after planar image statistical reconstruction by means of maximum likelihood, is 1mm, while the extrinsic resolution is below 0.9 mm across the entire transaxial FoV, measured with capillaries of 290 pm diameter. With the custom-designed 3 × 3 multi-pinhole collimator (whose minimal perturbation of the magnetic field can be reduced below 1ppm thanks to shimming), the measured sensitivity with
^99m
Tc is 1100 cps/MBq. The MR image is not distorted by the operation of the SPECT, as well as SPECT performances are no degraded by MRI (T2W and gradient sequences). Simultaneous imaging of different phantoms (capillaries, Derenzo hot rods of 0.85-1.7 mm diameter) as well as a mouse brain are reported.