Lung US Surface Wave Elastography in Interstitial Lung Disease Staging

Abstract

Background Lung US surface wave elastography (SWE) can noninvasively quantify lung surface stiffness or fibrosis by evaluating the rate of surface wave propagation. Purpose To assess the utility of lung US SWE for evaluation of interstitial lung disease. Materials and Methods In this prospective study, lung US SWE was used to assess 91 participants (women, 51; men, 40; mean age ± standard deviation [SD], 62.4 years ± 12.9) with interstitial lung disease and 30 healthy subjects (women, 16; men, 14; mean age, 45.4 years ± 14.6) from February 2016 through May 2017. Severity of interstitial lung disease was graded as none (healthy lung [F0]), mild (F1), moderate (F2), or severe (F3) based on pulmonary function tests, high-resolution CT, and clinical assessments. We propagated surface waves on the lung through gentle mechanical excitation of the external chest wall and measured the lung surface wave speed with a US probe. Lung US SWE performance was assessed, and the optimal cutoff wave speed values for fibrosis grades F0 through F3 were determined with receiver operating characteristic (ROC) curve analysis. Results Lung US SWE had a sensitivity of 92% (95% confidence intervals [CI]: 84%, 96%; P < .001) and a specificity of 89% (95% CI: 81%, 94%; P < .001) for differentiating between healthy subjects (F0) and participants with any grade of interstitial lung disease (F1-F3). It had a sensitivity of 50% and a specificity of 81% for differentiating interstitial lung disease grades F0-F2 from F3. The sensitivity was 88% and the specificity was 97% for differentiating between F0 and F1. The highest area under the ROC curve (AUC) values were obtained at 200 Hz and ranged from 0.83 to 0.94 to distinguish between healthy subjects and study participants with any interstitial lung disease. Conclusion Lung US surface wave elastography may be adjunct to high-resolution CT for noninvasive evaluation of interstitial lung disease. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Verschakelen in this issue.

Figure 1:

Experimental setup and image postprocessing for lung US surface wave elastography. A,B, Schematic shows positioning of the handheld shaker and US probe in the intercostal space. C, US B-mode image, with blue dots representing the selected points for wave speed measurements. D, Wave phase delay relative to the first location (leftmost blue dot) to measure lung surface wave speed.

Figure 2:

Flow diagram of study subjects. F0 = no interstitial lung disease (ILD) (healthy control subjects), F1 = mild ILD, F2 = moderate ILD, F3 = severe ILD, HRCT = high-resolution CT, LUSWE = lung US surface wave elastography, NSIP = nonspecific interstitial pneumonia, PFT = pulmonary function test, UIP = usual interstitial pneumonia.

Figure 3:

Bilateral wave speed measurements at the base of the lung. Wave speed was measured at three frequencies—A, 100 Hz, B, 150 Hz, and, C, 200 Hz—in the left and right lungs of healthy control subjects and study participants with different grades of interstitial lung disease (mild, moderate, and severe). Wave speeds were significantly different between healthy control subjects and participants with interstitial lung disease, but no significant differences were observed among participants with different grades of interstitial lung disease.

Figure 4:

Receiver operating characteristic (ROC) curves of lung US surface wave elastography. Cutoff values indicate the wave speed for differentiating between healthy control subjects and participants with mild, moderate, or severe interstitial lung disease at six intercostal spaces and three frequencies (100 Hz, 150 Hz, and 200 Hz). AUC = area under the ROC curve, L1 = left anterior lung zone, L2 = left lateral lung zone, L3 = left posterior lung zone, R1 = right anterior lung zone, R2 = right lateral lung zone, R3 = right posterior lung zone.