Characterisation of fan-beam collimators

Fan-beam collimators offer a good balance between resolution and noise. The collimator response may be included in iterative reconstruction algorithms in order to improve single-photon emission tomography (SPET) resolution. T o this end, accurate determination of the focal region and characterisation of the collimator response as a function of the source co-ordinates must b e performed. In this paper, a method to characterise fanbeam collimators is evaluated. First, we calculated the real focal region and the accuracy of the collimator convergence. Then, we confirmed the hypothesis that Gaussian distributions adequately fit the collimator responses, although no individ ualised treatment was performed for the tails of detector response which ar e associated with scattering and septal penetration. Finally, analytical fu nctions were used to model the resolution and sensitivity. The parameter va lues in these functions were obtained from experimental measures by non-lin ear regression fitting. Our findings show differences of 1.43% between nomi nal and real focal length and standard deviations of 2.5 mm in the x-direct ion and 7.1 mm in the y-direction for the focal convergence. The correlatio n coefficients between experimental and predicted values were 0.994 for res olution and 0.991 for sensitivity. As a consequence, the proposed method ca n be used to characterise the collimator response.