CHAPTER 2 Rob J. Hallifax, John P. Corcoran and Najib M. Rahman Oxford Centre for Respiratory Medicine, Churchill Hospital, Oxford, UK The standard postero‐anterior (PA) chest radiograph (CXR) has been augmented by the use of thoracic ultrasound and multi‐slice computed tomography (CT) scans in imaging the pleural space. Ultrasound is now routinely used in the assessment of suspected pleural effusions and guidelines strongly recommend its usage in all pleural procedures (pleural aspiration, biopsy and chest drain insertion). Ultrasound can easily distinguish between pleural fluid and collapsed or consolidated lung, which on CXR can both appear as similar areas of opacification. The safety of pleural procedures has been enhanced by routine use of ultrasound. Pleural thickening and pleural plaques are often best visualised on CT. CT provides cross‐sectional images which can also distinguish pleural and chest wall diseases from diseases of the lung parenchyma. The combination of positron emission tomography (PET) with CT scanning allows identification of metabolically active cells which can differentiate malignant from non‐malignant tissue. This chapter describes each imaging modality, highlighting advantages and disadvantages of each technique with respect to pleural disease. In health, the pleura is usually only visible on plain PA CXR in the fissures. In the horizontal and oblique fissures there are double layers of visceral pleura; visible when tangential to the X‐ray beam. The visceral pleura is also visualised in patients with a pneumothorax. Identifying the edge of the pleura as a thin white line on CXR, with no normal lung markings distally, is key to making the diagnosis (Figure 2.1). The parietal pleura are never normally visualised in health. However, pleural plaques and pleural thickening may be seen on CXR. Pleural plaques, which can be idiopathic or related to asbestos exposure, are often calcified which increases radiological visibility (Figure 2.2). The sensitivity of a CXR in the detection of plaques is in the range of 30–80% and the specificity is in the range of 60–80%, depending on the number of plaques, their distribution and the quality of the CXR. Diffuse non‐calcified pleural thickening is more difficult to identify on CXR. CT scanning is the definitive imaging technique for pleural thickening. Figure 2.1 Postero‐anterior (PA) chest radiograph (CXR) showing pneumothorax. The white arrow indicates the edge of the visceral pleural. Figure 2.2 PA‐CXR showing pleural plaques in left midzone (white arrows). Pleural effusions are often first identified on plain CXR. Increased opacification at the lung base or visible fluid level (meniscus) may be seen on CXR. However, CXR is not a sensitive test as tumour, enlarged/dilated left ventricle (on left) or pleural thickening provide a similar CXR appearance. Conversely, a small effusion may not be evident on CXR. Ultrasound is the imaging investigation of choice for pleural effusions. Ultrasound is a low‐cost, portable, radiation‐free imaging option. It is an ideal next step investigation after CXR for investigation of pleural disease. There is a trade‐off between the optimal spatial resolution (often provided by higher frequency probes) and depth penetration to allow visualisation of large effusions. Therefore, variable transducers (3–5 MHz) are often used. Probes with smaller footprints are easiest for intercostal access, to avoid rib ‘shadows’. Ultrasound relies on reflection of high frequency sound waves from tissues. Any interface with air, including normal aerated lung, will produce large reflections and therefore appear white, whereas soft tissue and fluid interfaces do not reflect and so pleural effusions appear dark (black). Ultrasound of the normal lung and pleura reveals a characteristic echogenic ‘stripe’ of the two opposing pleural layers (Figure 2.3
Radiology of Pleural Disease
Plain radiography
Ultrasound
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