Evaluation of A-SPECT: a desktop pinhole SPECT system for small animal imaging

Pinhole collimation of gamma rays to image distributions of radiolabeled tracers is considered promising for use in small animal imaging. The recent availability of transgenic mice, coupled with the development of /sup 125/I and /sup 99m/Tc labeled tracers, has allowed the study of a range of human disease models while creating demand for ultra-high resolution imaging devices. We have developed a compact gamma camera that, in combination with pinhole collimation, allows for accessible, ultra-high resolution in vivo SPECT imaging of small animals. The system is based on a pixilated array of NaI(Tl) crystals coupled to an array of position sensitive photomultiplier tubes (PSPMTs). Interchangeable tungsten pinholes with diameters ranging from 0.5 mm to 3 mm are available, allowing the camera to be optimized for a variety of imaging situations. We use a 3D MLEM algorithm to reconstruct projection images, allowing for 3D positioning of isotope distributions. Our evaluation indicates that high quality, sub-millimeter spatial resolution images can be achieved in living mice. Reconstructed axial spatial resolution was measured to be 0.53 mm and 0.74 mm FWHM for rotation radii of 1 cm and 2 cm, respectively, using the 0.5 mm pinhole. In this configuration, sensitivity is comparable to that of a standard high resolution parallel hole collimator. SPECT images of hot and cold rod phantoms and a highly structured monkey brain phantom illustrate that high quality images can be obtained with the system. Images of living mice demonstrate the ability of the system to obtain high resolution images in vivo.