Theoretical feasibility of dual-energy radiography for structural and functional imaging of chronic obstructive pulmonary disease.

PURPOSE: Chronic obstructive pulmonary disease (COPD) affects ∼200 million people worldwide. We propose two-dimensional (2D) dual-energy (DE) x-ray imaging of lung structure and function for the assessment of COPD, and investigate the resulting image quality theoretically.

METHODS

We investigated xenon-enhanced DE (XeDE) radiography for functional imaging of COPD and unenhanced DE radiography for structural imaging of COPD. We modeled the ability of human observers to detect ventilation defects in XeDE images and emphysema in (unenhanced) DE images using the detectability index ( d ' ) as a figure of merit. We accounted for the extent of emphysematous destruction and functional impairment as a function of disease severity, defect/lesion contrast, spatial resolution, x-ray scatter, quantum noise, anatomic noise, and the efficiency of human observers. Whether or not disease was detectable was determined based on a detectability threshold of two. For (unenhanced) DE imaging of emphysema, we compared detectability with that of single-energy (SE) imaging. Models of signal and noise were compared to published data.

RESULTS

Models of signal and noise agreed well with published data, and model predictions of the detectability of emphysema by SE radiography were consistent with poor sensitivity (i.e., d ' < 1 ) to mild to moderate COPD but moderate sensitivity (i.e., d ' > 1.5 ) to severe COPD. The detectability of emphysema by DE radiography was greater than that of SE radiography, but did not cross the threshold of detectability for mild to moderate COPD. The detectability index for XeDE imaging exceeded the detectability threshold for mild, moderate, and severe COPD.

CONCLUSIONS

Dual-energy radiography may offer modest improvements in the detection of emphysema relative to SE imaging, but will unlikely enable detecting mild and moderate COPD. However, XeDE radiography may enable detection of functional abnormalities associated with mild, moderate and severe COPD at x-ray exposures typical of those used in conventional chest radiography, thus warranting further investigation as a low-dose, low-cost alternative to CT- and MRI-based approaches for functional imaging of COPD.