Cascade removal and microPET imaging with76Br

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Abstract
PET imaging with non-standard nuclides has limitations due to the reduced spatial resolution from the positron range and from the concurrent emission of other gamma rays during the decay process. Particularly in high-resolution small animal PET imaging, these factors become detrimental for accurate quantitation of the activity concentrations from the reconstructed images. This paper presents an evaluation for imaging with such a nuclide, (76)Br, through imaging of specifically designed phantoms in a high-resolution small animal PET scanner. A model is presented for the calculation and removal of fortuitous cascade gamma ray coincidences based on estimation of the activity distribution and the attenuation correction file. In this evaluation, it is shown that 2 mm spatial resolution can be achieved with (76)Br while 1.7 mm was achieved with (18)F using a filtered back projection algorithm despite the much higher end-point energy of the positrons from (76)Br. A detailed evaluation of the point spread function for this nuclide was fitted by a double Gaussian function and explained the long tail from the high-energy positrons. These evaluations are crucial for accurate correction for partial volume effects and to provide accurate measurement of the activity concentrations in small animal PET imaging.