A CONVOLUTION-SUBTRACTION TECHNIQUE TO COMPENSATE FOR DOWNSCATTER EFFECTS IN SIMULTANEOUS TL-201 TC-99M CARDIAC SPECT/

One of the major concerns in simultaneous Tl-201/Tc-99m cardiac SPECT is the downscatter counts acquired in the 70+/-10% keV window, contrib uted by the 140 keV Tc-99m photons. In this study, a new method was pr oposed to remove the downscatter counts. The downscatter distribution was obtained by convolving the 140 keV reconstructed image with a spat ially variant 2-D scatter response function (SRF), and was then subtra cted from the 70 keV projection data. The SRF was assumed to be a comb ination of either two Lorentz functions or two Gaussian functions. The ir amplitudes and widths were determined by fitting the Monte Carlo (M C) simulated downscatter distributions. Furthermore, the amplitudes we re assumed to be exponential functions and the widths to be linear fun ctions of the depth of the source. The derived SRF depends on the shap e of the object relative to the detector, so errors may occur when a S RF derived for an object at a particular view is applied to other obje cts or to other views even with the same object. To reduce the errors, a simple correction method was proposed. As a result, the downscatter distribution of a Tc-99m point source obtained using the derived SRF agreed reasonably with the MC results for the same object at all the v iews. The agreement becomes worse as the detector rotates away from th e view where the SRF is derived. When applying to the phantom study, t he quality of the dual-isotope 70 keV images is improved, resulting in a comparable accuracy as that of the Tl-201 only 70 keV images.