KEY POINTS
- The retrospective study evaluated thoracic 4DCT data from 30 patients: 20 with the most irregular breathing patterns selected from 465 clinical scans and 10 randomly selected verification cases with less pronounced respiratory irregularity.
- ZeeFree uses deformable registration between overlapping CT detector stacks during reconstruction to reduce discontinuities caused by irregular respiratory amplitudes. Each dataset was reconstructed with both standard and ZeeFree algorithms using amplitude-based and phase-based binning.
- Six experienced reviewers assessed image quality. Among physician ratings, 76.7% of challenging amplitude-binned ZeeFree scans were rated nearly artifact-free versus 48.3% with standard reconstruction.
- The difference was even larger with phase-based binning: 70.0% of ZeeFree scans versus 23.3% of standard scans received artifact scores indicating unrestricted clinical usability. Mean physician scores were 4.0 versus 2.5, respectively.
- Across all physician assessments, 91.1% of ZeeFree reconstructions were considered anatomically correct versus 58.9% with standard reconstruction, with both amplitude- and phase-based comparisons statistically significant.
- Medical physicists examining individual breathing phases also favored ZeeFree. 97.3% of ZeeFree phases versus 78.7% of standard phases were rated as having no or only minimal artifacts in the challenging cohort.
- Importantly, ZeeFree did not materially degrade scans that were already of good quality. Only about 2% of previously artifact-free images shifted into a worse artifact category after reconstruction, and no systematic deterioration was seen in the verification cohort.
- The method corrects images retrospectively and therefore does not remove the underlying respiratory irregularity. The study also lacked voxel-level ground truth, and deformation fields may conceal residual registration uncertainty; planning and motion-management decisions still need to consider the patient's true breathing variability.
CLINICAL TAKEAWAY
Residual stack artifacts remain clinically relevant even with modern breathing-adaptive 4DCT. ZeeFree markedly improved anatomical continuity without additional imaging dose and could reduce repeat scanning, but corrected images should not be mistaken for evidence of regular respiratory motion during treatment.