NMTCB Computed Tomography (CT) Board Practice Exam 2025 - Free CT Board Practice Questions and Study Guide

In the context of Hounsfield Units (HU), air is typically assigned a value of -1000 HU. This scale is used in computed tomography (CT) to quantify radiodensity: the lower the value, the less dense the substance. The Hounsfield scale is standardized around water, which has a value of 0 HU.

Air, being significantly less dense than water, is represented by the lowest value on this scale. The negative value indicates a level of radiodensity that is much lower than that of water. This allows for a clear distinction between various tissues and materials seen on a CT scan, aiding in diagnosis and analysis.

Other substances have varying values as well; for instance, fat is around -100 to -50 HU, while muscle has values near +40 to +60 HU. Understanding these values is crucial for interpreting CT images, as they assist in differentiating between different types of tissues and materials within the body.