Comparison of image registration based measures of regional lung ventilation from dynamic spiral CT with Xe‐CT

Abstract

Purpose: Regional lung volume change as a function of lung inflation serves as an index of parenchymal and airway status as well as an index of regional ventilation and can be used to detect pathologic changes over time. In this paper, the authors propose a new regional measure of lung mechanics-the specific air volume change by corrected Jacobian. The authors compare this new measure, along with two existing registration based measures of lung ventilation, to a regional ventilation measurement derived from xenon-CT (Xe-CT) imaging. Methods: 4DCT and Xe-CT datasets from four adult sheep are used in this study. Nonlinear, 3D image registration is applied to register an image acquired near end inspiration to an image acquired near end expiration. Approximately 200 annotated anatomical points are used as landmarks to evaluate registration accuracy. Three different registration based measures of regional lung mechanics are derived and compared: the specific air volume change calculated from the Jacobian (SAJ); the specific air volume change calculated by the corrected Jacobian (SACJ); and the specific air volume change by intensity change (SAI). The authors show that the commonly used SAI measure can be derived from the direct SAJ measure by using the air-tissue mixture model and assuming there is no tissue volume change between the end inspiration and end expiration datasets. All three ventilation measures are evaluated by comparing to Xe-CT estimates of regional ventilation. Results: After registration, the mean registration error is on the order of 1 mm. For cubical regions of interest (ROIs) in cubes with size 20 mm × 20 mm × 20 mm, the SAJ and SACJ measures show significantly higher correlation (linear regression, average r 2 = 0.75 and r 2 = 0.82) with the Xe-CT based measure of specific ventilation (sV) than the SAI measure. For ROIs in slabs along the ventraldorsal vertical direction with size of 150 mm × 8 mm × 40 mm, the SAJ, SACJ, and SAI all show high correlation (linear regression, average r 2 = 0.88, r 2 = 0.92, and r 2 = 0.87) with the Xe-CT based sV without significant differences when comparing between the three methods. The authors demonstrate a linear relationship between the difference of specific air volume change and difference of tissue volume in all four animals (linear regression, average r 2 = 0.86). Conclusions: Given a deformation field by an image registration algorithm, significant differences between the SAJ, SACJ, and SAI measures were found at a regional level compared to the Xe-CT sV in four sheep that were studied. The SACJ introduced here, provides better correlations with Xe-CT based sV than the SAJ and SAI measures, thus providing an improved surrogate for regional ventilation.

Evaluation of the ΔV 4D CT ventilation calculation method usingin vivoxenon CT ventilation data and comparison to other methods

“…The majority of the errors were within 3 mm, which is comparable to other studies 15 , 37 and consistent with our previous study (30) . Based on the evaluation results, DM DIR was selected for the ventilation calculations in this study.…”

“…Ventilation differences introduced by registration errors can be reduced by smoothing. This evaluation study and previous reports 15 , 18 support the use of ventilation calculated using 4D CT in clinical applications such as radiation treatment planning, 12 , 23 , 24 , 40 , 42 , 43 , 44 , 45 pulmonary function, 15 , 18 and lung disease (22) …”

“…Ventilation can also be calculated using the Jacobian of the deformation transformation (13) or using the transformation directly (geometrically) (14) . Recently, promising results have been reported on the agreement between ventilation results obtained using the Jacobian method and xenon‐enhanced CT (XeCT) in a large animal model (15) . The latter measures regional ventilation by observing the contrast gas, xenon (Xe), wash‐in or wash‐out rate on serial CT images and is considered a gold standard for regional ventilation imaging (16) .…”

Clinical Validation Of 4DCT-Ventilation With Pulmonary Function Test Data

“…Another possibility to explain the lower correlation values for our study compared to Murphy et al (15) is that the relationship between 4DCT-ventilation and PFT metrics may not be linear (a saturation effects appears in Figure 2) and other mathematical relationships need to be investigated in future work. Our data can be also be compared to other validation work comparing 4DCT-ventilation imaging with other forms of ventilation imaging modalities (6,(16)(17)(18)(19)(20)(21). For example, Ding et al (19) and Reinhardt et al (6) 76.…”

“…Studies have attempted to validate 4DCT-ventilation by comparing it against other ventilation imaging modalities such as nuclear medicine ventilation-perfusion (VQ) imaging (16)(17)(18), xenon-CT (6,19), positron emission tomography (20), and magnetic resonance imaging (21). The studies generally found good agreement on a global level with worsening results locally.…”