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

^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.

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.

^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.