Ventilation measurements using fast‐helical free‐breathing computed tomography

Abstract

Purpose To examine the use of multiple fast-helical free breathing CT (FHFBCT) scans for ventilation measurement Methods Ten patients were scanned 25 times in alternating directions using a FHFBCT protocol. Simultaneously, an abdominal pneumatic bellows was used as a real-time breathing surrogate. Regions-of-interest (ROIs) were selected from the upper right lungs of each patient for analysis. The ROIs were first registered using a published registration technique (pTV). A subsequent followup registration employed an objective function with two terms, a ventilation-adjusted Hounsfield Unit difference and a conservation-of-mass term labeled ΔΓ that denoted the difference between the deformation Jacobian and the tissue density ratio. The ventilations were calculated voxel-by-voxel as the slope of a first-order fit of the Jacobian as a function of the breathing amplitude. Results The ventilations of the 10 patients showed different patterns and magnitudes. The average ventilation calculated from the DVFs of the pTV and secondary registration were nearly identical, but the standard deviation of the voxel-to-voxel differences were approximately 0.1. The mean of the 90^th percentile values of ΔΓ were reduced from 0.153 to 0.079 between the pTV and secondary registration, implying first that the secondary registration improved the conservation-of-mass criterion by almost 50% and that on average the correspondence between the Jacobian and density ratios as demonstrated by ΔΓ were less than 0.1. This improvement occurred in spite of the average of the 90^th percentile changes in the DVF magnitudes being only 0.58 mm. Conclusions This work introduces the use of multiple free-breathing CT scans for free-breathing ventilation measurements. The approach has some benefits over the traditional use of 4DCT or breath-hold scans. The benefit over 4DCT is that FHFBCT does not have sorting artifacts. The benefits over breath-hold scans include the relatively small motion induced by quiet respiration versus deep-inspiration breath hold and the potential for characterizing dynamic breathing processes that disappear during breath hold. This article is protected by copyright. All rights reserved