Continuing evaluation of an MR compatible SPECT insert for simultaneous SPECT-MR imaging of small animals

The goal of this conference record is to present the experiments conducted to evaluate a magnetic resonance (MR) compatible, simultaneous SPECT-MR insert for small animal (SA) imaging that we have been working on for the last three years. As previously demonstrated, the insert consists of 5 rings of 19 MR compatible CZT detectors connected seamlessly. A multi-pinhole (MPH) collimator with focused pinholes was built using a plastic shell filled with high-density coated tungsten powder and fitted with solid tungsten pinhole inserts. To acquire SPECT and MR data simultaneously, a shielded transmit/receive radio frequency (RF) coil is inserted into the MPH collimator. The insert was evaluated as a standalone SA SPECT system using both a hot-rod resolution phantom (HR-RP) experiment and SA imaging studies. Furthermore, simultaneous SPECT-MR phantom and SA imaging experiments were conducted by placing the SPECT-MR insert inside a 3T clinical MRI system. From acquired experimental data, SPECT images were reconstructed using an in-house developed 3D MPH ML-EM method with pinhole collimator detector response model and compensation. As a standalone SA SPECT system, the insert provided good quality images in phantom and SA studies. The quality of the SPECT images of the HR-RP acquired during the simultaneous SPECT-MR imaging experiment was comparable to the quality of the images acquired during standalone SPECT acquisition. Despite the fact that the SPECT insert degraded the MR image signal-to-noise-ratio (SNR) and caused field distortions, MR imaging was possible within the specific field-of-view of interest. When registered and fused, the SPECT and MR HR-RP images were found to be in acceptable geometric agreement. Data acquired during the simultaneous SPECT-MR SA study was used to obtain a dynamic SPECT image and Time Activity Curve showing initial uptake and later washout of ⁹⁹mTc MAG3 into and out of a mouse's kidneys. Furthermore, a SPECT kidney image was fused with the simultaneously acquired MR image and showed acceptable geometric agreement. Fast dynamic MR imaging was not possible due to severe geometric distortions present when using high speed sequences. These experimental results demonstrate the ability of the insert to obtain SA SPECT dynamic studies and the feasibility of simultaneous SA SPECT-MR imaging. Further engineering improvements remain to be explored to reduce the interaction between the SPECT and MR systems and to eventually enable simultaneous fast dynamic MR studies over the full volume of subject animals.