Ultrasound Elastography for Lung Disease Assessment

Abstract

Ultrasound elastography (US-E) is a noninvasive, safe, cost-effective and reliable technique to assess the mechanical properties of soft tissue and provide imaging biomarkers for pathological processes. Many lung diseases such as acute respiratory distress syndrome, chronic obstructive pulmonary disease, and interstitial lung disease are associated with dramatic changes in mechanical properties of lung tissues. Nevertheless, US-E is rarely used to image the lung because it is filled with air. The large difference in acoustic impedance between air and lung tissue results in the reflection of the ultrasound wave at the lung surface and, consequently, the loss of most ultrasound energy. In recent years, there has been an increasing interest in US-E applications in evaluating lung diseases. This article provides a comprehensive review of the technological advances of US-E research on lung disease diagnosis. We introduce the basic principles and major techniques of US-E and provide information on various applications in lung disease assessment. Finally, the potential applications of US-E to the diagnosis of COVID-19 pneumonia is discussed.

Fig. 1.

(a) Three groups (control, mild, and severe) of mouse lungs embedded in gelatin in plastic containers for measuring the lung surface wave speed. (b) Schematic of the experimental setup (adapted from [24]).

Fig. 2.

(a) Eight locations over 8 mm on the mouse lung surface were used to measure the wave speed using US tracking beams. (b) Phase delay of the remaining locations, relative to the first location, is used to measure the surface wave speed. (adapted from [24]).

Fig. 3.

Experimental setup and result of LUSWE. (a) Position of the handheld shaker. (b) Magnified view of the US probe in the intercostal space. (c) US B-mode image; the blue dots are the selected points for wave speed measurements. (d) Wave phase delay relative to the first location (leftmost blue dot) measures the lung surface wave speed (adapted from [31]).

Fig. 4.

Examples of radiologic appearance, quantitative analysis, and clinical severity, along with LUSWE velocity. Case A is a healthy control with a normal chest CT and normal lungs quantitatively color-coded as dark and light green by CALIPER. LUSWE velocity is 3.94 m/s, expectedly slower than the ILD cases. Case B is a clinically and radiologically identified mild ILD case with ILAs involving 35% of the total lung volume (shown in foci of yellow and orange) and significantly higher LUSWE velocity. Case C is a case with both high clinical and visually assessed severity with diffuse disease involving 83% of the lung and high LUSWE velocity (adapted from [32]).

Fig. 5.

B-mode and real-time US-E images of a lung lesion. The pulmonary lesion was invisible on the B-mode imaging. The lesion appeared inelastic (red on color mapping) on the US-E image. A fibrotic string connecting the lesion with the pleura was reproducible by real-time US-E imaging (adapted from [44]).

Fig. 6.

Experimental design for testing the surface wave speed. An ex vivo fresh swing lung was tested and a thick rubber pad was placed under the lung to reduce wave reflection. Water was injected into the lung through the trachea. The surface wave propagation was generated using a small vibrator and measured using an ultrasound probe (adapted from [53]).