Glossary Of Terms - U

Ultrasound Urticaria

Ulna is the lower arm bone on the little finger side, see Elbow.

Ultrasound. Ultrasound is sound waves above the highest frequency that can be heard by a human ear, about 20 kHz – ie ultrasonic waves. Ultrasound has two applications.

1 Ultrasonic imaging is a method of checking for blood clots or other diagnosis by high-frequency sound waves and the echoes that come back forming a picture – see factsheet on Echocardiogram.

2 Ultrasound is used as a treatment, producing a high-frequency massage, eg for chest pain following heart surgery where the cartilage between the sternum and a rib has not recovered.

A piezoelectric transducer is a crystal – eg quartz – that converts alternating electric voltage and current into sound. The opposite also occurs, so a crystal vibrated by reflected sound produces an electric voltage – ie a detector. These effects are biggest when the frequency equals the natural resonance frequency of the crystal.

The frequencies used in diagnosis and treatment are from 1 to 15 MHz. (Hz = Hertz = beats per second, M = million) Lower frequencies go deeper into body tissues. After a coronary artery bypass graft, the treatment of chest tissues to heal wounds and relieve pain coming from where the ribs join the sternum might be at 1 MHz and 1 Watt per sq cm – producing high-frequency massage of the cartilage. Sometimes the reflected pulse that is detected is a harmonic, ie a multiple frequency, of the transmitted pulse – eg transmit 1.7MHz, detect received 3.4MHz.

For producing an image the pulse repetition frequency is typically a few kHz – low enough for all the reflected echoes from one pulse to be received before the next pulse. A line of 100 transducers triggered one after another gives an image. Beam width affects lateral resolution. Pulse duration affects axial resolution. The advantages are: non-invasive, safe (less than 0.1 W per sq m for diagnosis), and more effective than X-rays. Gas and soft tissue reflect 99.9% of ultrasound energy, so structures on the far side of lungs cannot be imaged. Bone is not easily penetrated by ultrasound.

Unique is a trading name of Heath Lambert Limited. See insurance.

Unguis means nail – eg finger nail, toe nail.

Unstable angina. See Angina.

Urticaria is a skin condition with itchy reddish or whitish raised patches, usually caused by an allergy. Urtication is a burning or itching sensation; or another name for urticaria; or an obsolete way of beating the skin with nettles to produce a counter-irritation. To urticate is to perform urtication.


This information was created and edited by Richard Maddison for the BCPA.
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First published in this form 2002, and updated 2005, 2007, 2008, 2011, 2012, 2013, 2017.
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Authors, sources and acknowledgements

The main sources are BCPA Journal published articles, other information from authors, and publicly available documents and websites. In many cases the journal articles give sources and further information than the Glossary entries.

Parts of the wordings under ECG and Echocardiogram are adapted with permission from BUPA's health information resources, available at

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Richard Maddison

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