The PRIMA collaboration: Preliminary results in FBP reconstruction of pCT data

A first prototype of proton Computed Tomography (pCT) scanner, made of four planes and a calorimeter, has been developed by the PRIMA (PRoton IMAging) Italian collaboration and first results concerning tomographic image reconstruction of experimentally acquired data are discussed in this paper. The Filtered Back-Projection (FBP) algorithm was used to reconstruct projections of a phantom acquired with a 62 MeV proton beam. Image noise and spatial resolution were assessed for different parameters of the filter used, with and without selection strategies on proton directions. A satisfactory image quality (0.88 mm resolution and 2.5% noise) was achieved even when the backprojection line was defined using only the line connecting the impact points on the second and third planes and all the data were used, irrespective of the proton direction and residual energy. Probably due to the specific detector-phantom arrangement used in this experiment and due to the substantial reduction of the number of useful events, cuts on proton directions did not increase the image resolution significantly. The results confirm the good performances of the PRIMA scanner prototype. They also demonstrate that FBP can produce images of sufficient quality to be used for patient positioning and to initialize iterative pCT reconstruction methods.