A comparison of ventilation methods using 4D-CT and different deformable image registration algorithms

Abstract

Ventilation imaging using 4D-CT is a convenient and cost effective functional imaging methodology which might be of value in radiotherapy treatment planning to spare functional lung volumes. To calculate ventilation imaging from 4DCT we must use deformable image registration (DIR). This study investigates the dependence of calculated ventilation on DIR methods. DIR of the normal end expiration and inspiration phases of the 4D-CT images was used to correlate the voxels between the two respiratory phases. Three different DIR algorithms, Optical Flow (OF), Diffeomorphic Demons (DD) and Diffeomorphic Morphons (DM), were retrospectively applied to 10 esophagus and 10 lung cancer cases with 4D-CT image sets that encompassed the entire lung volume. The three ventilation extraction methods were used based on either the Jacobian, the change in volume of the voxel derived from the volume change of a voxel (ΔV) or directly calculated from the Hounsfield Unit (HU). The images were compared using the Dice similarity coefficient comparison. Dependence of ventilation images on the DIR was greater for the Δ V and the Jacobian methods than for the HU method. The Dice similarity coefficient for 20% of low ventilation volume for Δ V was 0.33 ± 0.03 between OF and DM, 0.44 ± 0.05 between OF and DD and 0.51 ± 0.04 between DM and DD. The similarity comparisons for Jacobian was 0.32 ± 0.03, 0.44 ± 0.05 and 0.51 ± 0.04 respectively, and for HU 0.53 ± 0.03, 0.56 ± 0.03 and 0.76 ± 0.04 respectively. Ventilation calculation from 4D-CT demonstrates some degree of dependency on the DIR algorithm employed. © 2013 Springer-Verlag.