Calculation of attenuation factors from combined singles and coincidence emission projections

We have developed a simple method for determining attenuation-correction factors C (the inverse of the total attenuation factors) from collimated singles (SPECT) and coincidence (PET) projections without the need for transmission data. Attenuation-correction factor estimates are determined for individual lines of response (LOR´s) independently. The data needed to use this technique can be acquired using a gamma-camera system with coincidence capabilities. A first order approximation (R) of C for an LOR is given by the product of the singles count rates taken at each end of the LOR divided by the square of the coincidence count rate. The method has been tested using simulated singles and coincidence projections starting with emission and attenuation-coefficient maps from PET scans of patients. A scatter plot of the values C versus the corresponding values R for all LOR´s produces a well-defined trajectory with little dispersion. Values of InR were reconstructed into good quality attenuation maps that compare favorably with the originals