Optimization of 99mTc-SPECT in the presence of 90Y for radioembolization

Background 99m Tc-macroaggregated albumin (MAA) imaging is part of the standard work-up procedure for radioembolization using 90 Y microspheres. In certain scenarios, it may be warranted to visualize the distribution of 99m Tc in co-presence of 90 Y, for example when validating intra-procedural 99m Tc-MAA imaging after 90 Y-therapy to enable single-session radioembolization. Another instance involves additional 99m Tc-MAA administration during the therapeutic procedure itself, e.g. when initial imaging reveals insufficient targeting of a specific liver segment. In these situations, crosstalk from 90 Y can result in reduced 99m Tc image quality and quantitative accuracy. This study investigates the feasibility and optimal method of 99m Tc SPECT imaging from combined 99m Tc+ 90 Y data using phantom experiments. Results An anthropomorphic torso phantom with two liver tumor inserts was filled with 99m Tc without (single-isotope) and with 90 Y (dual-isotope) in various activities and isotope concentrations. Three collimators (low energy high resolution: LEHR, medium energy: ME, and high energy: HE) and three methods to compensate for 90 Y crosstalk in the 99m Tc photo peak window (Monte Carlo-based, dual-energy-window and triple-energy-window correction) were evaluated. No substantial dead-time effects were observed in the clinically relevant activity range, up to approximately 12 GBq 99m Tc+ 90 Y (ratio 1:20) with LEHR, 29 GBq with ME and > 30 GBq with HE. Compared to the clinical standard (single-isotope 99m Tc imaging with LEHR collimator), contrast recovery typically decreased from 70.0 ± 1.3% to 49.0 ± 0.9% (LEHR), 61.2 ± 1.5% (ME) or 62.1 ± 1.4% (HE) due to 90 Y crosstalk. Compensation methods increased contrast recovery, with Monte Carlo-based correction combined with a ME or HE collimator yielding the best recovery at 68.5 ± 1.6% and 68.3 ± 1.5%, respectively. Visual image quality in terms of resolution and scatter contamination was superior when using a ME collimator. Lung shunt fractions were also severely affected by 90 Y crosstalk when using LEHR, but could be effectively mitigated using a ME or HE collimator. Conclusion 99m Tc imaging in the presence of 90 Y leads to substantial image degradation due to crosstalk effects. Monte Carlo-based crosstalk compensation in combination with a ME or HE collimator was identified as the most accurate, robust and visually optimal reconstruction method for 99m Tc SPECT from dual-isotope data.