Spectral Hounsfield units: a new radiological concept

Objective Computed tomography (CT) uses radiographical density to depict different materials; although different elements have different absorption fingerprints across the range of diagnostic X-ray energies, this spectral absorption information is lost in conventional CT. The recent development of dual energy CT (DECT) allows extraction of this information to a useful but limited extent. However, the advent of new photon counting chips that have energy resolution capabilities has put multi-energy or spectral CT (SCT) on the clinical horizon. Methods This paper uses a prototype SCT system to demonstrate how CT density measurements vary with kilovoltage. Results While radiologists learn about linear attenuation curves during radiology training, they do not usually need a detailed understanding of this phenomenon in their clinical practice. However SCT requires a paradigm shift in how radiologists think about CT density. Conclusion Because radiologists are already familiar with the Hounsfield Unit (HU), it is proposed that a modified HU be used that includes the mean energy used to obtain the image, as a conceptual bridge between conventional CT and SCT. A suggested format would be: HUkeV. Key Points Spectral computed tomography uses K-edge and slope effects to identify element signatures. New visualisation tools will be required to efficiently display spectral CT information. This paper demonstrates HU variation with keV using the Medipix3 chip. HUkeV is a suggested format when stating spectral HU measurements.