Simultaneous 99mTc/201Tl dual-isotope SPET with Monte Carlo-based down-scatter correction

In Simultaneous technetium-99m/thallium-201 dual-isotope (DI) single-photon emission tomography (SPET), down-scatter of Tc-99m photons contaminates the Tl-201 image, which leads to a decrease in lesion contrast and loss of quantitative accuracy. Correction for down-scatter can be achieved by first reconstructing the Tc-99m activity distribution. Subsequently, the Tc-99m down-scatter in the Tl-201 photopeak window is simulated and used for correction during iterative reconstruction of the Tl-201 image. In this work, the down-scatter projections are calculated using a dedicated Monte Carlo simulator which is able to efficiently model the detection of lead X-rays from the collimator. An anthropomorphic torso phantom with a cardiac insert with and without cold lesions was used for evaluation of the proposed method. Excellent agreement in lesion contrast and quantitative accuracy was found between the down-scatter corrected DI-SPET Tl-201 image and the virgin (i.e. separately acquired) Tl-201 image, in particular when the effects of lead X-rays were included. Compensation for the noise added by down-scatter to the Tl-201 image can be achieved by using a 15% lower dose of Tc-99m, a 15% increase in scan time and a 12% increase in Tl-201 dose. In conclusion, the Monte Carlo-based down-scatter correction recovers lesion contrast and quantitative accuracy in DI-SPET Tl-201 images almost perfectly. In addition, degradations due to the added noise of down-scatter in simultaneous DI-SPET can be prevented by slight adaptations to the data acquisition protocol.