Role of thoracic ultrasound in the assessment of pleural and pulmonary diseases
Introduction
Ultrasonography (US) can be used to explore the surfaces of the lungs through the intercostal spaces, but the presence of the ribs and of air in the expanded lung reduces the value of this imaging modality in the examination of deeper thoracic structures. Nevertheless, US is considered a reliable, inexpensive, safe, and reproducible diagnostic method for the work-up of patients with diseases of the diaphragm (neoplasms, paresis), thoracic wall (abscesses, fistulas, neoplasms), lung (atelectasis, pulmonary consolidation), anterosuperior mediastinum (neoplasms, lymphoma, cysts), the region between the thorax and the abdomen, and above all, the pleurae (extrapleural masses, pleural effusions) [1].
Thanks to the recent diffusion of sophisticated US scanners equipped with color and power Doppler technology and special transducers for transesophageal and endobronchial examinations, US can now be used to investigate disorders involving the esophagus, bronchi, bronchial blood vessels, mediastinum, and the large vessels of the heart [2]. Although computed tomography is still the imaging method of choice for the diagnosis of these conditions, thoracic US can be considered an important supplementary tool in this setting [3]. Today, thoracic US is mainly used to guide transthoracic biopsy of peripheral lung lesions and the drainage of pleural effusions [4]. The increasingly widespread use of second-generation ultrasound contrast agents is further expanding the role of thoracic US, and it is producing promising results in the characterization of peripheral lung masses [5].
Section snippets
Technique
The US examination of the chest requires a scanner equipped with a sector or convex small array probe with medium to high frequency (3.5–7.5 MHz). In most cases, conclusive information can be obtained with a 3.5-MHz transducer. However, a high-frequency (8–10 MHz) linear array probe is needed to study the chest wall, the pleurae, and the superficial structures of the lung. Color Doppler technology is not essential during the initial thoracic US examination, but it is a must during minimally
Clinical applications
The thoracic structures that can be explored by US are (starting at the surface) (1) skin, (2) derma, (3) intercostal muscles and endothoracic membrane, (4) extrapleural fat and the parietal and visceral pleurae (Fig. 1). Once the US beam has penetrated the visceral pleura, it is completely dispersed by the air in the lungs. The elevated acoustic impedance generated at the interface between the superficial soft tissues and the air in the lung results in a thin (<3 mm) echogenic line known as the
Pleural and extrapleural diseases
Thoracic US is the “gold-standard” method for studying pleural effusions [9]. It is more sensitive than chest radiography or CT in the detection of small amounts of pleural fluid (less then 10 ml) (Fig. 3). Effusions appear as a sharply demarcated, dark or echo-free zone image associated with downward displacement of the pleural line. The underlying lung may be well aerated, consolidated, or atelectatic [10]. Various formulas have been elaborated to estimate the volume of a pleural effusion and
Pulmonary diseases
Pulmonary disease can also be detected by US as long as there is no air between the probe and the lesion and the beam reaches the pleura. Even a thin layer (1–2 cm) of air can seriously reduce the visualization of solid lesions, regardless of their size. In certain cases, US imaging can also reveal deeper-seated pulmonary lesions, e.g., when the surrounding parenchyma is consolidated, i.e., atelectatic, or when the lesion is surrounded by a pleural effusion, which acts as an acoustic window [16].
Invasive pleural and pulmonary procedures
Thoracic US is widely used to guide needle placement during thoracentesis procedures, reducing the risk of pneumothorax [41]. Pneumothorax reportedly occurs in 7–15% of patients who undergo blind thoracentesis, but the frequency drops to 0.5% when US needle guidance is used [42], [43]. In the past 10 years, we have performed 1480 diagnostic or therapeutic thoracenteses under US guidance. In 270 (18%) cases, the purpose of the procedure was to examine the drainage fluid for tumor cells. There
Conflict of interest statement
None declared.
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