First results from the high-resolution mouse SPECT annular scintillation camera

High resolution SPECT imaging in small animals is often limited by poor sensitivity, leading to prolonged imaging times, very large injected doses, or both. To increase sensitivity while maintaining spatial resolution, we designed and constructed a multi-pinhole collimator array to replace the parallel hole collimators of a Ceraspect™ human SPECT brain scanner. The Ceraspect scanner is comprised of an annular NaI(Tl) crystal within which the eight pinhole collimators (1 mm diameter holes) rotate while projecting non-overlapping images of the object onto the stationary annular crystal. In this manner, only 1/8^th of a collimator rotation is required to acquire a complete tomographic data set. The imaging field of view (FOV) is 2.56 cm, which is sufficient to encompass a mouse. Data is currently acquired in step-and-shoot mode, however the system is capable of list mode acquisition with the collimator continuously rotating. Images are reconstructed using a cone-beam OSEM method. The reconstructed spatial resolution of the system is 1.7 mm and the sensitivity at the centre of the FOV is 13.8 cps/microCi. A whole-body bone scan of a mouse injected with Tc-99 m MDP clearly revealed skeletal structures such as the ribs and vertebral bodies. These preliminary results suggest that this approach is a good tradeoff between resolution and sensitivity and, with further refinement, may permit dynamic imaging in living animals.