Distilled water at standard temperature and pressure has 0 HU, whereas air under the same conditions has -1000 HU. These values are frequently expressed as Hounsfield units (HU). The extent to which a material can be penetrated by an X-ray beam is described in terms of an attenuation coefficient which assesses how much a beam is weakened by passing through a voxel of tissue (voxel = volumetric pixel). Some tissues will allow the passage of X-rays without influencing them much, whilst other tissues will exert a more significant effect. This is possible as different tissues interact with X-rays in different ways. Underlying principles and terminologyĬomputed tomography (CT) scanning involves the use of X-rays to take cross-sectional images of the body. This article will cover some of the underlying principles of CT head studies, and discuss a method for their interpretation. The CT head scan is one of the most common imaging studies that you can be faced with and the most frequently requested by A&E.
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