Effective use of the Hounsfield unit in the age of variable energy CT

Background: The increasing use of dual-energy computed tomography (CT) and of reduced kVp protocols exposes the marked energy dependence of the Hounsfield unit (HU) as a measure of X-ray attenuation. This dependence of HU on energy limits its use as a quantitative diagnostic parameter, potentially leading to inaccurate diagnoses. Purpose: To describe a method for annotating the HU using superscripts and subscripts that inform the observer of the origin of the unit. Methods: First, we review the HU and describe its use and limitations with variable energy CT. Examples of energy dependence and measureable curve effects are then discussed. The significance of HU value variation and its effects on diagnostic criteria will be presented. Improved annotation based on superscript and subscript methods that are derived from international nomenclature guidelines is proposed. Conclusion: The increasing use of variable energy CT allows for improved diagnostic techniques, and reduced radiation exposure, but at the expense of increasing the complexity of interpreting the meaning of HU values in various diagnostic contexts. A method for precise HU description is presented, streamlining diagnostic criteria.