The physics of ultrasound

Ultrasound is a safe, non-invasive imaging modality which is increasingly used in anaesthesia to aid placement of central venous cannulae and local anaesthetic blocks. Transoesophageal echocardiography (TOE) is used to assess myocardial and valvular function during anaesthesia or on the intensive care unit. It is essential to understand the underlying principles of how the ultrasound image is created in order to optimize the image, and recognize and prevent artefacts.This article describes the physics of waves and their interactions, and applies these principles to explain how the ultrasound machine produces an image. The Doppler effect and its application to measurement of blood flow and cardiac output is described.