Pinhole PET (pPET): a multi-pinhole collimator insert for small animal SPECT imaging on PET cameras

To effectively study small animal models of disease in vivo a wide range of PET and SPECT imaging agents need to be employed. In an effort to achieve high resolution and high sensitivity small animal SPECT imaging we propose the notion of using a multiple pinhole insert in a PET scanner (pPET). Adapting PET scanners to enable pinhole SPECT imaging is an attractive solution for many reasons. Most PET systems are full ring detectors which offer the potential for pinhole configurations that fully sample an object without the need for complex detector rotation. Dual modality studies can be performed without moving the subject providing perfect spatial alignment. Furthermore, enabling SPECT imaging in a PET scanner makes financial sense by reducing the initial investment and maintenance costs. In this study we prove the feasibility of using a pinhole insert to enable SPECT imaging in PET scanners. Simulations performed using the geometry of the Mosaic (Philips), a dedicated small animal PET scanner (Rdet=98.5 mm), show a variety of attractive pinhole configurations offering a blend of resolution (to under 1.0 mm), sensitivity (up to 0.5%), field of view (up to 40 mm) and good image quality. Measurements with the Mosaic demonstrate that by raising the high voltage, to account for the lower light output, 140 keV gammas can be efficiently detected and adequately positioned for both GSO (gadolinium oxyorthosilicate) and LYSO (lutetium yttrium orthosilicate) scintillators. Initial imaging studies with a 12×1 (holes × rows) collimator with 1.0 mm pinholes at a 30 mm radius demonstrate the feasibility and show promising resolution (1.5 mm FWHM), good image quality, and a relatively large field of view.