Chest ImagingChest ultrasonography
sonographic examination mainly indicated in
pleural disease. It is usually performed with a 3.5 - 7.5 MHz linear or curvilinear
probe placed in the line of the intercostal space. Normally it is possible to see the visceral
pleura sliding with respiration, behind which is a highly reflective line with distal reverberation echoes, created by the pleuralung
interface.
Pleural fluid usually produces an anechoic space beneath the chest wall which has a well-defined posterior acoustic enhancement unlike other fluid collections due to underlying aerated lung. Most effusions are anechoic but may contain internal echoes due to septation or loculation. Anechoic solid
pleural masses from which they must be distinguished include neurogenic tumours or
lymphoma. Some effusions are echogenic.
Chest ultrasonography may help in the differentiation of a transudate from an exudate. Anechoic effusions may be caused by transudates and exudates with approximately equal incidence, but septa or other internal echoes within the effusion indicate an exudate. The presence of associated pleural thickening, pleural nodules (Fig.1) or a pulmonary mass also indicates an exudate. Multiple small internal echoes which move in a swirling fashion with respiration usually indicate an empyema or haemothorax. Septations, which are better demonstrated by chest ultrasonography than CT, indicate likely difficulties with aspiration, particularly when profuse, and are an indication for the use of fibrinolytic agents.
Chest ultrasonography can identify small quantities of pleural fluid, quantify the volume of pleural fluid, identify pleural fluid in unusual sites such as a subpulmonary location and help differentiate a peripheral pulmonary mass from a loculated pleural fluid collection. Chest ultrasonography may also identify extrathoracic causes for the pleural effusion such as a subphrenic or hepatic abscess. Chest ultrasonography is particularly useful for diagnosing pleural fluid in severely ill and immobile patients in whom an erect chest radiograph is not possible, as it can be performed at the bedside with the patient supine.
Pleural thickening may be identified with chest ultrasonography if greater than 1 cm in depth as a homogeneous and echogenic lesion. Its size, shape and position with respiration or change in patient position enables it to be identified in over 90% of cases.
Chest ultrasonography may provide useful information about the underlying lung such as consolidation with fluid bronchograms, or evidence of a peripheral lung mass. Chest wall invasion by tumour may be seen as disruption of the pleura, extension into the chest wall or fixation of the mass during respiration. Chest ultrasonography is frequently used to guide chest interventions such as thoracocentesis, biopsy or chest tube insertion.
It can be used to ascertain the optimal site for closed pleural biopsy if the position of the diaphragm is unclear from the chest radiograph or if there is only a small pleural effusion. Also, see ultrasonography.
CF - HM