As early as 1967, it was apparent that ultrasound was ideally suited for the detection of pleural effusions.1 In addition, thoracic ultrasonography can also detect less common pleural pathology, guide thoracentesis, and other pleural procedures. As a result, its role for critical care physicians has become increasingly important.
General Considerations in Pleural Ultrasound
Ultrasound examination of the pleura is influenced by the surrounding structures. The ribs block ultrasound waves and prevent deeper structures from being visualized. In contrast, air reflects ultrasound. The surface of aerated lung will reflect most of the ultrasound waves. The point of reflection is immediately below the pleura. However, if the lung is consolidated or atelectatic, it can be readily visualized.
In addition to the artifacts seen in other aspects of medical ultrasonography, there are specific artifacts, such as rib shadowing, that are found commonly in pleural ultrasound. Air reverberation artifacts, which originate below the pleura, are another artifact type that can be commonly observed by clinicians (see Chapter 19). Translational artifacts, due to patient breathing or mechanical ventilation, may also confuse the examiner.2
Obesity and subcutaneous edema can degrade image quality. Significant edema may also present problems in judging the depth for procedures. The presence of subcutaneous air will make visualization of deeper structures problematic. The use of firm pressure on the skin and the use of a coupling medium will reduce some artifacts. Artifacts are often visible in only one scanning plane, so changing the probe angle may cause artifacts to disappear. Further, artifacts usually will not move with the respiratory cycle. The observation of an image throughout several respiratory cycles often helps to clarify the issue.
Ultrasound Machine Requirements and Machine Control
Pleural ultrasonography can be performed with many different types of two-dimensional ultrasound machines. Doppler capability is not needed. A probe with a small “footprint” to easily fit between rib spaces should be used. The preferred ultrasound probe is a phased array transducer with a frequency of 2–5 MHz (typically 3.5 MHz) that may also be used for cardiac ultrasonography. Probes with higher frequencies can visualize the pleural surface, but lack adequate penetration for clinical applications that require visualization of deeper thoracic structures.
In order to standardize image interpretation, the use of uniform probe orientation and a screen marker is required. The machine should be set up, so that the image marker on the screen is in the upper left corner of the screen. When using the longitudinal scanning plane, the probe should be oriented with the probe marker positioned cephalad. If this orientation is maintained, the cephalad direction will always be to the left of the screen.
The gain and depth should be adjusted so that the chest wall, pleural surfaces, and deeper structures, such as the liver or spleen, with overlying diaphragm are ...