In this study our previously presented reconstruction algorithm was extended and optimised for dual isotope Tl-201/Tc-99m studies in terms of reconstruction speed. The new dual isotope algorithm performs Tl-201/Tc-99m reconstructions by splitting the reconstruction into Tc-99m reconstruction, Tc-99m down-scatter simulation, and Tl-201 reconstruction incorporating the down-scatter estimate. Two scatter update iterations and 105 simulated photons for the Tc-99m and Tl-201 reconstructions and 106 simulated down-scatter photons were found to provide accurate results in clinically acceptable reconstruction times ().
The 2 scatter update iterations and 105
have been found to suffice also for single isotope Tc-99m reconstruction [14
]. Simulating more scatter photons does not lead to improvement in either ventricular or lesion contrast. For the down-scatter estimation, however, we used more photons in order to make the down-scatter estimate less noisy. The contrast values in the down-scatter compensated Tl-201 images are practically identical for 105
, and 107
simulated down-scatter photons () but 105
photons seems to generate much noisier down-scatter estimate than 106
photons. This noise might progress to the reconstructed images if the Tl-201 activity is very low.
The developed down-scatter correction method cannot fully compensate for the Tc-99m/Tl-201 cross-talk as can be seen by comparing the results obtained with the dual isotope data and pure Tl-201 data in Tables and . The pure Tl-201 data contrasts are better than the down-scatter compensation contrasts. This can be explained by the lead X-ray emissions, which occur when Tc-99m photons hit the collimator. These X-rays are emitted at the Tl-201 energy level and they contaminate the Tl-201 data. Currently our reconstruction method corrects only for patient scatter, but more accurate correction might be needed. De Jong et al. [11
] have included lead X-ray correction into their Tl-201/Tc-99m reconstruction method and have shown that it improves image quality. The only problem in including collimator effects into MC-based scatter correction is the increase in reconstruction time. The common Gaussian collimator model, which was also used in this work, is very efficient. Changing that to something more complicated will definitely have a big effect on the reconstruction times and it will also make reconstruction algorithm implementation more challenging.
Our reconstruction method has similar features to the methods published by Kadrmas et al. [10
] and de Jong et al. [11
]. Both of these methods also broke the reconstruction into three parts: Tc-99m reconstruction, Tc-99m down-scatter simulation, and Tl-201 reconstruction. Kadrmas et al. used effective source scatter estimation for scatter modelling, whereas de Jong et al. used Monte Carlo as we did. We focused also on optimising the reconstruction parameters, which is very important for an algorithm to be clinically acceptable. In our case the reconstruction parameter optimisation reduced the reconstruction times to approximately 1/4 of the un-optimised reconstruction times (). At the moment our dual isotope reconstruction algorithm is limited to Tl-201/Tc-99m reconstructions. The same concept could, however, probably be extended to other isotope pairs or to isotopes with several peaks. Reconstruction would always start with the isotope or energy peak, which has the highest energy. This data would be reconstructed and downscattered to lower windows and subsequent reconstructions would run until the isotope or energy peak with the lowest energy is reached.
Dual isotope imaging with simultaneous acquisition is clinically attractive, as it increases the possible patient throughput and reduces patient discomfort. Dual isotope imaging also offers perfect alignment and identical physiological conditions between stress and rest images, which may give additional information to the physician.
This study has limitations. MC-simulated projection data and physical phantoms were used instead of real patient studies. We tried to compensate the lack of real patient data by using realistic phantoms and we also tried to adjust the activity levels to clinically meaningful values. We believe that our optimised reconstruction method works well also with patient data but a large number of patient studies are still required to validate our method and the entire Tl-201/Tc-99m dual isotope SPECT.