The development of a high-resolution insert for simultaneous SPECT-MR imaging of small animals

2401 Objectives To develop an insert for simultaneous SPECT-MR imaging of small animals. Methods The insert consists of a complete ring-type SPECT subunit that can be inserted inside an MRI system with a minimum 20 cm bore. The SPECT subunit comprises 5 seamless rings of 19 CZT detectors (2.54x2.54 cm2, 16x16 pixels). High-resolution SPECT images are achieved by using optimally designed cylindrical multipinhole (MPH) collimator sleeves (CSs), filled with tungsten powder and fitted with solid tungsten pinhole apertures, for minimum interference with MR imaging. The MPH collimators were designed with the maximum detection efficiency for a given resolution under the system design constraints. Sparse-view reconstruction methods were developed for reduced CS rotation. Detector and system calibration methods were developed to ensure accurate and artifact-free reconstructed images. A shielded birdcage quadrature transmit/receive RF coil for mouse or rat imaging fits inside the corresponding CS to minimize interactions between SPECT and MRI systems. Results Two MPH CSs for mouse imaging, with 18 and 36 pinholes that have 1 and 1.5 mm targeted system resolution, respectively, have been completed. The measured point source detection efficiencies were 0.017% and 0.037%, and system resolutions were 1.1 and 1.5 mm, for the two collimators. They are consistent with design predictions and simulation results. Images from an Ultra-Micro SPECT phantom and mice injected with 99mTc MDP demonstrated high image quality after careful detector and system calibration. A similar designed birdcage RF coil was independently tested to provide high-resolution MRI images of a mouse on an earlier version of the system. Conclusions A high-resolution SPECT-MR insert for imaging of small animals was developed. The SPECT subunit and RF coil have been independently evaluated to provide high quality SPECT and MR images. The combined insert is ready for evaluation for simultaneous SPECT-MR imaging of small animals. Research Support NIH R01 EB8730 and a research contract from Gamma Medica, Inc