Evaluation of different CT maps for attenuation correction and segmentation in static ^99mTc‐MAA SPECT/CT for ⁹⁰Y radioembolization treatment planning: A simulation study

Abstract

Conventional 99m Tc-macroaggregated albumin ( 99m Tc-MAA) planar scintigraphy overestimates lung shunt fraction (LSF) compared to SPECT/CT. However, the respiratory motion artifact due to the temporal mismatch between static SPECT and helical CT (HCT) may compromise the SPECT quantitation accuracy by incorrect attenuation correction (AC) and volume-of-interest (VOI) segmentation. This study aims to evaluate AC and VOI segmentation effects systematically and to propose a CT map for LSF and tumor-to-normal liver ratio (TNR) estimation in static 99m Tc-MAA SPECT/CT. Methods: The 4D XCAT phantom was used to simulate a phantom population of 120 phantoms, modeling 10 different anatomical variations, nine TNRs (2-13.2), nine tumor sizes (2-6.7 cm diameter), eight tumor locations, three axial motion amplitudes of 1, 1.5, and 2 (cm), and four LSFs of 5%, 10%, 15%, and 20%. An analytical projector for low-energy high-resolution parallel-hole collimator was used to simulate 60 noisy projections over 360°, modeling attenuation and geometric collimator-detector response (GCDR). AC and VOI mismatch effects were investigated independently and together, using cine average CT (CACT), HCT at end-inspiration (HCT-IN), mid-respiration (HCT-MID), and end-expiration (HCT-EX) respectively as attenuation and segmentation maps. SPECT images without motion, AC, and VOI errors were also generated as reference. LSF and TNR errors were measured as compared to the ground truth. Results: HCT-MID has slightly better performance for AC effect compared with other CT maps in LSF and TNR estimation, while HCT-EX and HCT-MID perform better for VOI effect. For a respiratory motion amplitude of 1.5 cm and a LSF of 5%, the LSF errors are 19.56 AE 4.58%,

Technical note: Respiratory impacts on static and respiratory gated 99mTc‐MAA SPECT/CT for liver radioembolization: A simulation study

“…15 Previously, we evaluated different CTs as AC and segmentation maps for static 99m Tc-MAA SPECT in LSF and TNR estimation, where a mismatched CT could cause substantial estimation errors especially for large motion amplitudes. 16 This may also influence the LSF and TNR estimation in RG 99m Tc-MAA SPECT/CT. In this study, we aim to systematically evaluate and compare the respiratory impacts on static and RG 99m Tc-MAA SPECT in terms of RM, AC, and VOI segmentation.…”

“…Previously, we evaluated different CTs as AC and segmentation maps for static 99m Tc‐MAA SPECT in LSF and TNR estimation, where a mismatched CT could cause substantial estimation errors especially for large motion amplitudes 16 . This may also influence the LSF and TNR estimation in RG 99m Tc‐MAA SPECT/CT.…”

Dosimetric impact of 3D motion-compensated SPECT reconstruction for SIRT planning

“…The exact impact of this assumption is currently unknown. 3Dcomp reconstruction using other breathing phases is feasible, but they will be marred by errors because of a mismatched CT, as also observed in [ 36 ]. In both studies, VOI (liver, lungs and tumors) were delineated on CT to obtain anatomical contours independent of the distribution of the radiopharmaceutical.…”

“…However, the realism of simulations and phantoms is limited (perfect respiratory cycle and simplifications in the movement, characteristics of each patient, assumption of a perfectly compensated scatter [ 24 ]). In contrast to our method based on list-mode data, Santoro et al [ 25 ] corrected for respiratory motion by realigning the barycenters of the lesions in the projections with each other before realigning them on the CT to improve attenuation correction and VOI definitions [ 36 ]. The latter registration consisted to have the same distance between the hepatic dome defined on the CT and the top of the lesion on two imaging modalities (SPECT and CT).…”

“…The estimated absorbed doses to the lungs are overall higher with 3Dcomp reconstruction than with 3D reconstruction. Lu et al [ 36 ] proposed an explanation of over- and underestimation: Counts in the liver that border on the lungs can be reconstructed in the lungs and not in the liver. Similarly, counts in the lungs can be reconstructed outside the lungs.…”