Imaging Studies of An Ultrahigh Resolution Simultaneous SPECT/MRI System for imaging Neural Stem Cell in a Mouse Brain

106 Objectives There is a strong interest in utilizing neural stem cells (NSCs) as carriers for targeted delivery of therapeutics to brain tumors due to their intrinsic ability to preferentially migrate towards both local and disseminated disease. The development of NSC-based brain tumor therapies would benefit from the knowledge of behavior of NSCs in various preclinical and clinical settings[1]. Non-invasive imaging approaches for tracking of stem cells in the brain with high sensitivity and high spatial resolution can provide invaluable information of the dynamics and distribution of stem cells in the normal brain and malignant tissue. An ultrahigh resolution stationary MRI compatible SPECT (MRC-SPECT-I) system for small animal imaging using second generation Energy Resolved Photon Counting (ERPC) detectors has been developed in our lab[2]. The objective of this study was to evaluate feasibility of in vivo imaging of NSCs in the mouse brain using the new MRI/SPECT system. Methods a) MRC-SPECT-I consisted of 40 small-pixel CdTe detector modules (10 detectors, each had 4 modules) assembled in a fully ring SPECT geometry, coupling with 40 inserted pinhole (diameter 300 μm or 500 μm) aperture. Each module had an overall size of 2.2 cm × 1.1 cm, divided into 64 × 32 pixels with size of 350 μm. The system was installed on a 25 cm diameter non-metal gantry constructed with 3D printing using nylon powder. b) MRC-SPECT-I was integrated with Siemens 3T Trio-I scanner and a dedicated specially designed RF hybrid transmit/receive coil using thin copper foil with thickness of 65 μm[3]; c) To derive precise system response function, we calibrated the system with 390 parameters and modeled system response including attenuation effects of aperture, detector charge collection process inside strong magnetic field. d) To evaluate the system performance, Jaszczak phantom filled with 1.5 mCi Tc-99m (1h acquisition) was imaging by both MRC-SPECT-I alone and simultaneous MRI/SPECT; e) two groups of 111In-labeled NSCs, with a 5 times difference in NSC concentrations, were injected into the left and right striatum of the mice and simultaneous MRI/SPECT imaging was carried on. Results The preliminary imaging results as shown in Fig. 1 shows that the 350 µm hot rods in a Jaszczak phantom were clearly resolved when the MRC-SPECT system operated outside the magnet, and 500 µm hot rods could be resolved in simultaneous SPECT/(3T)MR imaging; We have also carried out imaging studies of mouse bearing 111In labeled NSCs in the brain using the combined MEC-SPECT and Siemens Trio 3T MR scanner (for simultaneous SPECT-MR studies), and an Oxford Instrument 14.1T scanner (for sequential imaging studies). Some preliminary results are also shown in Fig. 1. Further imaging results will be discussed in this presentation. Conclusions Sub-500μm-resolution SPECT/MR imaging system was developed. The results demonstrate that this unique imaging system could be used for in vivo assessing the dynamics and distribution of stem cells in the mice brain. The combining of high sensitivity from SPECT and high soft tissue contrast provided by the MR could provide an interesting platform for functional and molecular imaging of a wide range of disease models, such as cancer and neural degenerative diseases.