Clinical Procedures - Ultrasound (USS) Indications • Indications are manifold and too numerous to list. See making best use of a department of clinical radiology via www.rcr.ac.uk. Contraindications • For some examinations, the patient must be able to cooperate with the operator and a degree of mobility is often required • Ultrasound becomes increasingly less diagnostic at greater depths. Images of deeper structures in large individuals are often unobtainable and this should be borne in mind when considering who to refer. Technology • The ultrasound probe houses a piezoelectric crystal which both projects and receives high-frequency sound waves. Much like radar, the 'echos' are converted to images by the machine's software • Ultrasound cannot image through gas and requires a semi-liquid 'gel' between the probe and skin surface for optimum imaging. Procedure • This depends on the part of the body examined and the indications for the examination: • As ultrasound is a dynamic examination, any part of the patient that can be reached by the probe may be examined • Time taken will vary depending on part of body examined, patient co-operation and complexity of the findings. Most examinations last between 5-20 minutes. Risks • There is no published evidence that ultrasound has ever directly caused any harm to a patient: • The acoustic output of modern machines, however, is much greater than previously used • Heating: some equipment can produce temperature rises of 4°C in bone. Most equipment in clinical use is unlikely to increase tissue temperature more than the 1.5°C which is considered 'safe • Non-thermal hazard: ultrasound has been demonstrated to produce tiny gas pockets and bubbles in animal models. Neonatal lung is considered vulnerable to this but there is no evidence that diagnostic ultrasound can cause harm to other tissues • Machines have a 'mechanical index' (MI) displayed on screen which acts as a guide to the operator. Patient Preparation Depends on the indication and body part being examined. • Abdomen: patients are usually asked to fast for 6 hours prior to the examination. This ensures distension of the gallbladder and prevents the epigastric structures being obscured by overlying bowel gas • Renal tract/pelvis: a full bladder is usually required. A full bladder creates an 'acoustic window', effectively pushing small bowel aside so that deeper structures (e.g. ovaries) may be seen.
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