The Role of Transthoracic Ultrasound in the Study of Interstitial Lung Diseases: High-Resolution Computed Tomography Versus Ultrasound Patterns: Our Preliminary Experience

Abstract

Transthoracic ultrasound (TUS) is a readily available imaging tool that can provide a quick real-time evaluation. The aim of this preliminary study was to establish a complementary role for this imaging method in the approach of interstitial lung diseases (ILDs). TUS examination was performed in 43 consecutive patients with pulmonary fibrosis and TUS findings were compared with the corresponding high-resolution computed tomography (HRCT) scans. All patients showed a thickened hyperechoic pleural line, despite no difference between dominant HRCT patterns (ground glass, honeycombing, mixed pattern) being recorded (p > 0.05). However, pleural lines' thickening showed a significant difference between different HRCT degree of fibrosis (p < 0.001) and a negative correlation with functional parameters. The presence of >3 B-lines and subpleural nodules was also assessed in a large number of patients, although they did not demonstrate any particular association with a specific HRCT finding or fibrotic degree. Results allow us to suggest a complementary role for TUS in facilitating an early diagnosis of ILD or helping to detect a possible disease progression or eventual complications during routine clinical practice (with pleural line measurements and subpleural nodules), although HRCT remains the gold standard in the definition of ILD pattern, disease extent and follow-up.

Keywords: high-resolution computed tomography; hyperechoic pleural line; interstitial lung diseases; screening tool; transthoracic ultrasound.

Figure 1

(A) Thickness of the hyperechoic pleural line between honeycombing (HC) and no honeycombing (no HC) HRCT pattern (p > 0.05). (B) Thickness of the hyperechoic pleural line between different HRCT patterns (GG: ground glass, HB: honeycombing, Mix: mixed patterns) (p > 0.05). (C) Thickness of the hyperechoic pleural line between patients with different grades of fibrotic involvement (p < 0.0001). (D) Correlation between the thickness of the hyperechoic pleural line and the Nadir value of SaO₂% reached during 6-min walking test (6mWT) (r −0.38).

Figure 2

TUS images of the hyperechoic pleural line in a patient with an HRCT pattern of Usual Interstitial Pneumonia (UIP). (A) The hyperechoic pleural line (white arrow) appears thickened (5.1 mm) when measured with a middle/low-frequency convex probe (3.5–5 MHz). (B) The thickened (2.4 mm), irregular, fragmented and blurred appearance (yellow arrows) becomes more evident when using a high-frequency linear probe (8–12.5 Mhz).

Figure 3

HRCT and TUS scan of a 54-year-old woman with familiarity for pulmonary fibrosis. In (A), HRCT scan shows a diffuse bilateral peripheral increase in pulmonary density (ground glass opacity) with widespread parenchymal nodular lesions (maximum size: 7 mm), in greater numbers in the lower lobes. In (B), TUS scan with a low-frequency convex probe (3.5–5 MHz) (in the chest area corresponding to the blue box in (A) shows a thickened pleural line (white arrow)). In (C), TUS scan with a high-frequency linear probe (8–12.5 MHz) at the same level does not show a clearly blurred and fragmented aspect.

Figure 4

In (A), HRCT scan shows bilateral honeycombing, more severe in the right postero-basal area (blue box); In (B), TUS scan with convex probe (3.5–5 MHz), in the chest area corresponding to the blue box in (A), shows a subpleural hypoechoic area (yellow rhombus) of 4.5 mm (probably a traction cyst). In (C), HRCT scan passing through the postero-basal segments shows honeycombing pattern, larger on the left (blue box). In (D), TUS scan performed with a linear probe (8–12.5 MHz) in the chest area corresponding to the blue box in (C) shows a thickened (>2 mm), irregular, fragmented and blurred hyperechoic pleural line (white arrow), but subpleural nodules are not appreciated. In (E), HRCT scan of the thorax passing through the postero-basal segments shows a typical fibrotic distortion of the pulmonary parenchyma, with ground-glass opacity superimposed on a predominant reticular pattern with subpleural and basal distribution and with associated traction bronchiectasis in the absence of a frank honeycombing (UIP “probable”). (F) TUS examination performed with a convex probe (3.5–5 MHz) probe in the chest area corresponding to the blue box in (E) shows a subpleural hypoechoic nodule of 5.9 mm.