Simultaneous Tc-99m/Tl-201 imaging using energy-based estimation of the spatial distributions of contaminant photons

The advantages of simultaneous acquisition of Tc-99m and Tl-201 myocardial perfusion SPECT images can be fully realized only if the effects of the Tc-99m agent can be accurately removed from the Tl-201 image. The authors and others have previously reported simultaneous dual-isotope techniques for cardiac studies which make use of a third energy-window to estimate the Tc-99m scatter to be subtracted from the Tl-201 window. The authors have recently demonstrated, however, using a Monte Carlo program which simulates all details of the photon transport, that lead X-rays produced in the collimator may also contribute significantly to contamination in the Tl-201 window. The spatial distribution of the Tc-99m scattered photons differs from that of the lead X-rays. Therefore, the authors modified their correction technique so that, at each projection angle, the contaminant image to be subtracted from the image in the Tl-201 window was estimated as a linear combination of a scatter-window (90-110 keV) image, blurred by a 2D Gaussian filter, and the Tc-99m photopeak image, blurred by a different Gaussian filter. For simulated data which included `liver´ activity and non-uniform `lung´ attenuation, the improved dual-window subtraction technique provided a more accurate estimate of the true Tl-201 image, with less image noise, than did the single-window correction