Purpose: To develop methods for measuring free-breathing ventilation using free-breathing CT scans
Methods: A multi-scan free-breathing CT protocol was used to scan ten patients. The CT scans were acquired with a simultaneous breathing surrogate (abdominal pneumatic bellows). To reduce calculation time, the image registration was conducted on regions-of-interest (ROIs) from the upper right lungs. The ROIs were first registered using a published registration technique (pTV), which was followed up with a secondary registration that used a two-term objective function; 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 ventilation patterns showed different patterns and magnitudes. The mean ventilations calculated from the DVFs of the pTV and secondary registration were nearly identical, but the standard deviations of the voxel-to-voxel differences were approximately 0.1. The mean of the 90th percentile values of ΔΓ were reduced from 0.153 to 0.079, 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 were less than 0.1. This improvement occurred in spite of the average of the 90th percentile changes in the DVF magnitudes being only 0.58 mm.
Conclusion: 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, including not having 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.