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

Abstract

Background: 99mTc-macroaggregated albumin (MAA) imaging is part of the standard work-up procedure for radioembolization using 90Y microspheres. In certain scenarios, it may be warranted to visualize the distribution of 99mTc in co-presence of 90Y, for example when validating intra-procedural 99mTc-MAA imaging after 90Y-therapy to enable single-session radioembolization. Another instance involves additional 99mTc-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 90Y can result in reduced 99mTc image quality and quantitative accuracy. This study investigates the feasibility and optimal method of 99mTc SPECT imaging from combined 99mTc+90Y data using phantom experiments. Results: An anthropomorphic torso phantom with two liver tumor inserts was filled with 99mTc without (single-isotope) and with 90Y (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 90Y crosstalk in the 99mTc 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 99mTc+90Y (ratio 1:20) with LEHR, 29 GBq with ME and > 30 GBq with HE. Compared to the clinical standard (single-isotope 99mTc 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 90Y 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 90Y crosstalk when using LEHR, but could be effectively mitigated using a ME or HE collimator. Conclusion: 99mTc imaging in the presence of 90Y 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 99mTc SPECT from dual-isotope data.