Thoracic ultrasound: a review of the state-of-the-art

Abstract

Thoracic ultrasound (TUS) is a tool that has become increasingly essential in the daily practice of thoracic medicine. Driven by the need to assess patients during the COVID-19 pandemic, there has been an increase in the use of point-of-care TUS, which has demonstrated several benefits, either as a complement to clinical decision-making for diagnosis or as a real-time guide for procedures, whether as a predictor or measure of treatment response. Here, we present a review of TUS, based on the most recent scientific evidence, from equipment and techniques to the fundamentals of pulmonary ultrasound, describing normal and pathological findings, as well as focusing on the management of lung disease and guidance for invasive thoracic procedures at the bedside. Finally, we highlight areas of perspective and potential lines of research to maintain interest in this valuable tool, in order to improve the diagnostic process and expand the treatment arsenal.

Figure 1. Aerated lung in two-dimensional (2D) mode and corresponding motion (M)-mode image. a) Normal findings: the muscles, fascia, and other soft tissues of the chest wall (CW) are in the upper part of the image. The surfaces of the ribs (R) can be seen as two horizontal hyperechoic, white lines with posterior acoustic shadowing (*). The pleural line (PL) is located just below the ribs. The lung tissue is filled with air and therefore cannot be seen. Consequently, the area that can be seen below the PL is not the lung tissue but artifacts, represented by the A-lines (A). The ribs resemble the wings of a bat, whereas the PL mimics the body of the bat, a pattern known as the batwing sign. b) Normal M-mode findings: the M-mode line can be seen running vertically through the PL at the top of the image. In the corresponding M-mode image, the PL is seen as a hyperechoic line placed at the same distance from the transducer as can be seen in the 2D image. Note the seashore sign, which is so named because, in M-mode, the static structures of the CW can appear as horizontal lines above the PL (representing the sea) and, in the presence of lung sliding, the area below the PL will have a grainy appearance (representing the shore).

Figure 2. a) Multiple B-lines (B) can be seen as vertical, hyperechoic lines originating from the pleural line (PL) and stretching all the way to the bottom of the two-dimensional brightness (B)-mode image. b) Idiopathic pulmonary fibrosis: thoracic ultrasound image of the lower lobe of a patient diagnosed with idiopathic pulmonary fibrosis. Multiple B-lines are present, and the PL appears severely thickened and fragmented.

Chart 2. Lung ultrasound score.

Figure 3. Pneumothorax. a) Motion (M)-mode findings in pneumothorax: if lung sliding and the lung pulse are absent, there will be no change in the area below the pleural line in the two-dimensional (2D)/brightness (B)-mode image. In M-mode, horizontal lines will be seen above and below the pleural line (PL), and the seashore sign can no longer be identified. The M-mode pattern has been described as resembling a barcode or a stratosphere and is therefore known as the barcode sign or stratosphere sign, which can be seen when a pneumothorax is present, as well as in conditions in which lung sliding and the lung pulse are absent (e.g., pleural adhesions). b) 2D-mode showing the lung point (LP). The LP is an ultrasonographic sign that is used in order to locate the junction between the pneumothorax and the area with no air between the visceral and parietal pleural and refers to a pattern of repeated transitions between no lung sliding or B-lines (pneumothorax) into a demonstrable area of lung sliding.

Figure 4. Pleural effusion and its varied presentations. a) Simple pleural effusion: a simple, anechoic, pleural effusion (PE) is present. There are no septations or visible structures floating within the effusion. b) Complex nonseptated PE: anechoic fluid with the presence of multiple hyperechoic punctuate foci representing floating debris within the effusion, also known as the plankton sign (PS). c) Complex septated PE: a complex septated PE is present, containing areas of anechoic fluid as well as several septa (S)/loculations. d) Homogeneously echogenic (HE) fluid: a combination of HE fluid with a stratification effect in the costophrenic recesses (the hematocrit sign) may suggest the presence of hemothorax.

Figure 5. Endotracheal intubation. a) Axial image of the cervical region obtained with a linear transducer identifying the trachea and esophagus. b) The same ultrasound window shows the appearance of the esophagus with an orotracheal tube inside, illustrating esophageal intubation (double tract sign).

Figure 6. Percutaneous tracheostomy. a) Sagittal view of the trachea showing the orotracheal cannula represented by a double hyperechoic line with acoustic shadowing. The tip of the endotracheal tube is ideally positioned under the first tracheal ring to ensure a clear path for the tracheal puncture while reducing the risk of accidental extubation. b) Repositioning of the endotracheal tube prior to midline perpendicular tracheal puncture is performed under ultrasound guidance, thus reducing the risk of complications. The patient should be placed in the supine position.

Figure 7. Tube thoracostomy. a) Brightness (B)-mode image, obtained with a linear probe, showing diaphragmatic excursion and a normal lung. The examiner evaluates the range of diaphragmatic excursion during a full cycle of ventilation. This enables the examiner to choose the lowest site for tube insertion while avoiding injury to the diaphragm. If the previously selected site exhibits diaphragmatic movement, a more cranial intercostal space must be scanned. b) Visualization of intercostal vessels using Doppler ultrasound. Once the examiner finds a suitable intercostal space, the insertion site should be scanned with Color Doppler. The intercostal artery most commonly lies on the upper third of the intercostal space. The entire intercostal space should be scanned in order to make sure that there is no blood flow along the insertion path. c) Confirmation of correct positioning of the drain. Axial view of a large-bore chest tube, showing the characteristic hyperechoic arc over a black circle with posterior acoustic shadowing, seen along the subcutaneous plane. Following that image along the drainage site, one should see the drain deepening toward the pleural line.