Analytical calculations of cross-talk for dual-isotope SPECT projections

Simultaneous dual-isotope SPECT scans are clinically desirable because they require less acquisition time than sequential scans, and provide perfect spatial co-registration of the acquired images. However, they often suffer from extensive contamination caused by cross-talk, the detection of photons from one isotope in the energy window of the second. This contamination can be reduced by using asymmetric windows but doing so does not completely remove the effect of cross-talk and there is a corresponding loss in sensitivity due to the reduction in size and/or the change in location of the energy window. The authors propose to correct for cross-talk using an accurate method of analytically calculating the photon distributions. An initial estimate of the source distributions from the two isotopes can be formed by assuming that there is no cross-talk. Fitting the projections corresponding to these source distributions to the experimental data will allow for cross-talk correction and permit an accurate evaluation of the relative source activities. The authors have performed an initial feasibility study of this approach using small /sup 123/I and /sup 99m/Tc sources in an inhomogeneous medium filled with /sup 99m/Tc. The results show good agreement between the experimentally measured cross-talk distribution and the authors' calculations.