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 make it difficult to distinguish between the detected distributions of the two isotopes, particularly when the emitted photons are of similar energies. The 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 distinguish the source distributions of the two isotopes by fitting calculated projections of potential sites of activity to the acquired projection data. The initial estimates of the potential sites are assumed to be identified using a reconstruction of the uncorrected data. Once the activities of the sources have been determined, corrected projections, if desired, can be generated allowing for the reconstruction of cross-talk free images. The authors have performed an initial feasibility study of this approach using small ¹²³I and ^99mTc sources in homogeneous and inhomogeneous media filled with ^99mTc. The results show good agreement between the true source activity and the authors´ estimated activities