Transthoracic Ultrasonography for the Respiratory Physician

Abstract

Transthoracic ultrasonography is still not utilized to its full potential by respiratory physicians, despite being a well-established and validated imaging modality. It allows for an immediate and mobile assessment that can potentially augment the physical examination of the chest. Ultrasound (US)-assisted procedures can be performed by a single clinician with no sedation and with minimal monitoring, even outside of theatre. The main indications for the use of transthoracic US are: the qualitative and quantitative description of pleu-ral effusions, pleural thickening, diaphragmatic dysfunction and chest-wall and pleural tumours. It may also be used to visualise lung tumours and other parenchymal pulmonary processes provided they abut the pleura. It is at least as sensitive as chest radiographs as far as the detection of a pneumothorax is concerned. It is the ideal tool to assist with thoracocentesis and drainage of effusions. The US-assisted fine-needle aspiration and/or cutting-needle biopsy of extrathoracic lymph nodes, lesions arising from the chest wall, pleura, peripheral lung and mediastinum, are safe and have a high yield in the hands of chest physicians. US may also guide the aspiration and biopsy of diffuse pulmonary infiltrates, consolidations and lung abscesses, provided the chest wall is abutted. Advanced applications of transthoracic US include the diagnosis of pulmonary embolism.

Transthoracic Ultrasonography for Clinicians

“…Ultrasound generates digital images by transmitting sound waves from a transducer to human tissues and recapturing the reflected sound waves (or echoes)that are then converted back into electrical impulses and processed into images (8,9). The frequency of diagnostic ultrasound is in the millions of Hertz (MHz) and ranges from 2 to 10 MHz for transthoracic US (8)(9)(10).…”

“…The frequency of diagnostic ultrasound is in the millions of Hertz (MHz) and ranges from 2 to 10 MHz for transthoracic US (8)(9)(10). The distribution and intensity of the US image on the screen is determined by three characteristics: 1) the direction and 2) intensity of the captured echoes, and 3) the time elapsed from emission to capture (8)(9)(10).…”

“…Echogenicity is the ability to reflect an echo, and the echogenicity of any tissue or lesion is defined relative to that of the normal liver, which is arbitrarily considered isoechoic (9). Sound waves propagate well though liquids (e.g., pleural effusions) and through tissues with a high fluid content (e.g., consolidated lung, tumors, and liver) (8,10,11). Tissues that have higher echogenicity are called "hyperechoic" and are usually represented with lighter colors.…”

“…This probe is ideal for initial screening of superficial and deeper structures. A high frequency linear probe (5-10 MHz) provides excellent image resolution but is unable to penetrate deeply into tissues and is best used for refined assessment of an abnormal chest wall or pleura (8)(9)(10). Some US units have default settings: "abdominal" being low frequency and "thyroid" being high frequency.…”

“…Some US units have default settings: "abdominal" being low frequency and "thyroid" being high frequency. The images should be optimized using the "depth" and "gain" functions on the US unit in real time (8,9). The depth function is comparable with a digital zoom and defines the section of the scanned image displayed on the monitor at a certain magnification, with the scale displayed on a vertical axis.…”