Abstract Rationale Previous work has demonstrated basic regional correlations between CT findings and hyperpolarized 129Xe MRI images of gas exchange. However, these studies have conventionally relied on 129Xe images comprised of a single gas exchange compartment (ventilation, membrane, or red blood cell RBC), or a single metric such as the RBC:Membrane ratio. While these measurements are all critical aspects of gas exchange, this approach does not give a “one-stop” ability to simultaneously visualize the presence of abnormalities at all stages of the gas exchange process using a single image. Methods Here, we introduce a new schema for visualizing 129Xe gas exchange function, a composite image called a gas exchange map (GEM). These maps use previously published thresholds for defect/low signal (in all three compartments) as well as abnormally high signal (membrane only), thus quantizing images from each compartment into simple classifiers of low/normal/high. Each combination of normal/abnormal across all three compartments is then assigned a different label and color (see Figure, left), which is used to label each voxel and thereby generate a composite image of gas exchange function. This image is then registered to the CT scan for direct visual comparison. Results from two example subjects are shown in the Figure, with diagnoses of scleroderma (A), and COPD (B). In addition to the GEM, the CT is shown along with a color overlay for voxels with high attenuation (-250 to -700 HU in blue) for subject A and low attenuation (-950 HU in red) for subject B. Note that while regions of abnormality on CT generally have a spatially overlapping functional abnormality on the GEM, this map will also frequently highlight additional areas of functional abnormality that appear normal-looking on CT. Conclusion Three-dimensional composite maps of gas exchange function may provide a practical means of assessing the functional correlates to CT abnormalities, and for highlighting areas of functional impairment at the microstructural level of gas exchange that may presage structural changes on CT. Future work will employ longitudinal data to assess temporal associations between GEM abnormalities and the appearance of findings on CT. This abstract is funded by: Polarean Imaging, NIH NHLBI
Mummy et al. (Fri,) studied this question.