Sampling considerations for high resolution small animal SPECT

Using the small animal hybrid PET/SPECT scanner YAP-(S)PET we demonstrate how we can improve both image quality and spatial resolution in SPECT modality, by acquiring data with sampling steps smaller than the detector intrinsic sampling pitch. Due to the planar configuration of our pixellated detector we can easily perform this by shifting detector heads from the central position of an arbitrarily small step tangential to the field of view. This acquisition technique, together with the knowledge of the detector response function, allows us to determine the best sampling step as a trade-off between image quality, resolution, and acquisition time. In the real case, the more one samples the FOV with a finer step, the more one reduces aliasing (better image quality). Furthermore one also records higher frequency components in the image spectrum (better spatial resolution). To demonstrate how this technique can enhance the image, images of capillaries and phantoms will be presented. In particular the trend of resolution versus sampling step is reported. In this paper we present two different kinds of results: first we reconstruct images, acquired with different sampling modalities, with the same spatial frequency cut-off; in this way we show how the decreasing of the sampling step enhances image quality for a fixed resolution. Second we reconstruct images by recovering all possible spatial frequencies so as to enhance spatial resolution. All images are deconvolved for the frequency response (transfer function) of our system by implementing deconvolution in a standard filtered back projection (FBP) algorithm. The results show that it is possible to approach the 2 mm resolution limit imposed by the crystal; in fact we can recover resolution down to 2.2 mm (±3.5%).