Nuclear imaging is a commonly used tool in today's diagnostics and therapy planning. For interventional use however it suffers from drawbacks which limit its application. Freehand SPECT was developed to overcome these limitations and to provide 3D functional imaging during an intervention. It combines a nuclear probe with an optical tracking system to obtain its position and orientation in space synchronized with its reading. This information can be used to compute a 3D tomographic reconstruction of an activity distribution. However, as there is no fixed geometry the system matrix has to be computed on--the--fly, using ad--hoc models of the detection process. One solution for such a model is a reference look up table of previously acquired measurements of a single source at different angles and distances. In this work two look up tables with a one and four millimeter stepsize between the entries were acquired. With the Freehand SPECT system twelve datasets from a phantom with two hollow spheres filled with a solution of Tc99m were obtained. Reconstructions with the look up tables and two other currently used models were performed with these datasets and compared with each other. A comparison between the reconstructions of the look up tables and the models showed that the look up tables yield similar results with low computational times for the reconstruction process. The look up tables have a mean error of 9.05mm and 11.43mm in the position of the spheres while the current models have mean errors of Nuclear imaging is a commonly used tool in today’s diagnostics and therapy planning. For interventional use however it suffers from drawbacks which limit its application. Freehand SPECT was developed to overcome these limitations and to provide 3D functional imaging during an intervention. It combines a nuclear probe with an optical tracking system to obtain its position and orientation in space synchronized with its reading. This information can be used to compute a 3D tomographic reconstruction of an activity distribution. However, as there is no fixed geometry the system matrix has to be computed on–the–fly, using ad–hoc models of the detection process. One solution for such a model is a reference look up table of previously acquired measurements of a single source at different angles and distances. In this work two look up tables with a one and four millimeter step size between the entries were acquired. Twelve datasets of a phantom with two hollow spheres filled with a solution of Tc99wm were acquired with the Freehand SPECT system. Reconstructions with the look up tables and two analytical models currently in use were performed with these datasets and compared with each other. The finely sampled look up table achieved the qualitatively best reconstructions, while one of the analytical models showed the best positional accuracy.
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