TU‐G‐BRA‐02: Can We Extract Lung Function Directly From 4D‐CT Without Deformable Image Registration?

Abstract

Purpose:Dynamic CT ventilation imaging (CT‐VI) visualizes air volume changes in the lung by evaluating breathing‐induced lung motion using deformable image registration (DIR). Dynamic CT‐VI could enable functionally adaptive lung cancer radiation therapy, but its sensitivity to DIR parameters poses challenges for validation. We hypothesize that a direct metric using CT parameters derived from Hounsfield units (HU) alone can provide similar ventilation images without DIR. We compare the accuracy of Direct and Dynamic CT‐VIs versus positron emission tomography (PET) images of inhaled ⁶⁸Ga‐labelled nanoparticles (‘Galligas’).Methods:25 patients with lung cancer underwent Galligas 4D‐PET/CT scans prior to radiation therapy. For each patient we produced three CT‐ VIs. (i) Our novel method, Direct CT‐VI, models blood‐gas exchange as the product of air and tissue density at each lung voxel based on time‐averaged 4D‐CT HU values. Dynamic CT‐VIs were produced by evaluating: (ii) regional HU changes, and (iii) regional volume changes between the exhale and inhale 4D‐CT phase images using a validated B‐spline DIR method. We assessed the accuracy of each CT‐VI by computing the voxel‐wise Spearman correlation with free‐breathing Galligas PET, and also performed a visual analysis.Results:Surprisingly, Direct CT‐VIs exhibited better global correlation with Galligas PET than either of the dynamic CT‐VIs. The (mean ± SD) correlations were (0.55 ± 0.16), (0.41 ± 0.22) and (0.29 ± 0.27) for Direct, Dynamic HU‐based and Dynamic volume‐based CT‐VIs respectively. Visual comparison of Direct CT‐VI to PET demonstrated similarity for emphysema defects and ventral‐to‐dorsal gradients, but inability to identify decreased ventilation distal to tumor‐obstruction.Conclusion:Our data supports the hypothesis that Direct CT‐VIs are as accurate as Dynamic CT‐VIs in terms of global correlation with Galligas PET. Visual analysis, however, demonstrated that different CT‐VI algorithms might have varying accuracy depending on the underlying cause of ventilation abnormality.This research was supported by a National Health and Medical Research Council (NHMRC) Australia Fellowship, an Cancer Institute New South Wales Early Career Fellowship 13‐ECF‐1/15 and NHMRC scholarship APP1038399. No commercial funding was received for this work.