Slit-slat collimator equipped gamma camera for whole-mouse SPECT-CT imaging

A slit-slat collimator was developed for a gamma camera intended for small animal imaging (mice). The gamma camera contains a 10 cm × 10 cm pixelated NaI(Tl) crystal array of individual 1.3 mm × 1.3 mm × 6 mm crystal elements, distanced at 0.2 mm. The housing of the collimator was made of 5 mm tungsten alloy, forming a slit with a 1 mm opening and a 90 degree acceptance angle. The slats were made of lead foils with 0.2 mm in thickness to match the crystal spacing, and possess a length of 40 mm. Individual slats are separated by polyurethane material with a thickness of 1.3 mm corresponding to the size of the crystal elements. Alignment of the collimator with the camera´s crystal pattern was performed by adjusting a micrometer screw while monitoring a Co-57 point source for maximum signal intensity. For SPECT, the collimator forms a cylindrical FOV enabling whole mouse imaging with a magnification of 1.9 at COR over the entire AOR (magnification = 1.0). As the gamma camera is part of a bimodal imaging system incorporating also x-ray CT capabilities the collimator had to be co-registered with the other components to yield intrinsically co-registered reconstruction results of both modalities. For this, five parameters corresponding to the geometrical displacements of the collimator as well as the mechanical co-alignment between the gamma camera and the CT subsystem were estimated by means of a dedicated measurement setup involving a number of bimodal calibration sources. To illustrate the performance of the slit-slat collimator and compare its performance to a single pinhole collimator a Derenzo phantom study has been performed. Transversal resolution along the entire long axis is comparable to a pinhole collimator of same pinhole diameter. Axial resolution of the slit-slat collimator is comparable to that of a parallel beam collimator. In addition, a mice study has been performed, as well, with data being presented.