Sampling considerations for high resolution small animal SPECT

Using the small animal hybrid PET/SPECT scanner YAP-(S)PET we demonstrate how oversampling is important to improve both image quality and spatial resolution in SPECT modality. Due to the planar configuration of our pixellated detector we can easily perform oversampling by shifting detector heads from the central position of an amount smaller than the detector pixel size. Oversampling, together with the knowledge of the detector response function, allows us to determine the best sampling step as a trade-off between image quality and acquisition time. In the real case, the more one oversamples the field of view, the more one reduces aliasing (better image quality). Furthermore one also introduces high frequency components in the image spectrum (better spatial resolution). To demonstrate how oversampling gives evident enhancements, studies on capillaries and phantoms will be presented. In particular the trend of resolution vs sampling step are shown. In this paper we present two different kinds of results: first we reconstruct images that have different sampling modalities, with the same spatial frequency cut-off; so we show how oversampling enhances image quality for a fixed resolution. Secondly we have reconstruct images by recovering all frequencies so as to enhance spatial resolution, deconvolving the frequency response of our system using a standard FBP algorithm. The results show that increasing the sampling from 2 mm to 0.5 mm improves the spatial resolution form 3.5 to 2.2 mm (FWHM).