Sina Tafti^{1}, William J Garrison^{2}, John P Mugler^{2,3}, Y Michael Shim^{4}, Talissa A Altes^{3,5}, Jaime F Mata^{3}, Nicholas J Tustison^{3}, Kun Qing^{3}, Eduard E de Lange^{3}, Gordon D Cates^{1,3}, and G Wilson Miller^{2,3}

Emphysema index (EI) based on CT provides a quantitative measure of emphysema burden in patients with chronic obstructive pulmonary disease (COPD). Diffusion-weighted MRI of inhaled hyperpolarized gases also provides a method for detecting emphysema, as elevated ADC values indicate airspace enlargement due to emphysematous destruction of alveolar walls. We propose an alternative formulation of EI based on He-3 or Xe-129 ADC measurements and compare their performance in characterizing emphysema severity with that of CT-EI. Our results suggest that ADC-EI may provide a useful quantitative measure of emphysema burden in patients with COPD that is more sensitive to early stages of emphysema than CT-EI.

Figure 1. Histograms of
all ADC voxel values from all healthy (green) and all COPD (red, all stages
combined) subjects. The vertical blue lines depict thresholds of 0.30 and 0.048
cm^{2}/s which correspond to the 95th percentile of all voxel values from
healthy subjects. Both sets of distributions are self-normalized and have equal
areas.

Figure 2. Scatter plot of
He-3 versus Xe-129 ADC-EI showing excellent correlation. The variance among subjects
within each COPD stage is noticeably higher than that of the healthy group,
which leads to larger error bars in Figure 5.

Figure 3. Scatter plots
of CT-EI versus both He-3 and Xe-129 ADC-EI. Numbers 1-4 label the quadrants
defined by vertical and horizontal lines at the data point with highest ADC-EI for
healthy subjects. Thus quadrant 1 spans the apparently “healthy” ranges.

Figure 4. The first row
depicts axial views of healthy lungs, having uniform and homogenous CT tissue
distribution and ADC maps. The second row depicts a subject with severe
emphysema. Elevated ADC values can be seen to correspond to dark regions in CT;
this is an example of both CT and ADC characterizing emphysema in a similar
manner. The bottom row shows relatively homogenous and uniform tissue
distribution in CT, but elevated ADC values for both He-3 and Xe-129. This is a
COPD stage III subject with high ADC-EI (0.65 for He-3 and 0.72 for Xe-129) but low
CT-EI (0.013).

Figure 5. From left to
right: mean CT-EI and He-3 and Xe-129 ADC-EI values with their associated
standard errors of the mean, broken down by COPD severity. The *p*-value
next to each data point corresponds to the *t*-test performed between that
category of COPD (shown by color) and the healthy group. Notice the enhanced
separation between healthy and COPD stage I, and healthy and COPD stage II distributions in
the case of mean ADC-EI as compared to CT-EI.