Design and simulation of U-SPECT, an ultra-high resolution molecular imaging system

Currently, the majority of small animal SPECT systems are equipped with large gamma-camera´s with a spatial resolution of a few mm. Using pinhole collimation in combination with strong projection image magnification, a reconstructed image resolution of about 0.5 mm has been obtained in vivo in the thyroid of the mouse. For larger organs the resolution degrades rapidly to a few mm. To obtain high resolution, small pinholes are required, resulting in a sensitivity-drop that can inhibit many biological applications. With high resolution detectors, less image magnification is required, so that small detectors can be put closely behind pinholes and the animal is observed with hundreds of camera´s simultaneously. Pinhole imaging combined with high resolution detectors allows for exciting new imaging geometry designs. The projected U-SPECT system presented inhere is such a system that uses a series of detectors placed in a polygonal configuration. To meet the high resolution requirements a novel detector based on scintillating micro-columns is designed for high resolution pinhole imaging. The columns are pointed towards the pinholes in order to eliminate parallax errors due to varying depth-of-interaction. A cylinder containing a total of 180 gold-alloy pinhole apertures (9 rings containing 20 pinholes each) are used for collimation. Non-overlapping elongated projections can be acquired using pinhole shaping and/or shielding. Voxels in the central field-of-view containing a rat brain are observed under 80 up to 115 angles simultaneously through the pinholes. The pinhole placement used in U-SPECT allows for closely approaching the Orlov conditions for sufficient data sampling, and prevents that moving parts are required. U-SPECT simulation was compared with a simulation model of a popular small animal SPECT system equipped with two pinhole scintillation camera´s (A-SPECT). Detector resolution of U-SPECT is assumed to be 150 microns at 140 keV, while the detector resolution of A-SPECT was assumed to be 2.2 mm. For both systems iterative image reconstruction with correction for detector and collimator blurring was employed. Images of a digital rat brain phantom show much more detail in U-SPECT simulations than in A-SPECT simulations. In a resolution phantom spheres - of 0.5 mm are clearly visible with U-SPECT, while with A-SPECT the smallest spheres visible are 1 mm, which is an 8-fold difference in volumetric resolution. Deformations in the Defrise disc phantom that occur in traditional pinhole SPECT are strongly suppressed with the U-SPECT imaging geometry. This geometry does not require moving parts to acquire a sufficient set of suitable projection angles. Results support our believe that advanced many-pinhole systems with high resolution detectors can improve many aspects of molecular imaging in rodents significantly.