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Is it time to set a Hounsfield standard? [Letter]

Bryant, Jacqueline A., Drage, Nicholas and Richmond, Stephen ORCID: 2009. Is it time to set a Hounsfield standard? [Letter]. Dentomaxillofacial Radiology 38 (5) , p. 306. 10.1259/dmfr/30842473

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From a formal and academic point of view, the Hounsfield number definition is only truly independent of energy for water and vacuum. The X-ray attenuation coefficients of other substances do not vary with energy in the same way as water and indeed they are often very different. Thus before even considering energy spectra and beam hardening effects, the definition is already ambiguous because the mono-energetic X-ray beam is undefined. Many papers have been published on the compensation of beam hardening and among those papers there is a subcategory that proposes some form of integration over the energy spectrum for practical X-ray sources. Usually, there is some mention of the “effective beam energy”1 or mapping values to “some mono-chromatic energy”,2 which in the case of Reference 2 is 70 keV. Thus, both academically and practically, there is a need for defining the conditions under which the Hounsfield number definition is valid. This requires not only the energy to be defined but also the attenuation coefficient. For the former, an alternative might be to define an energy spectrum, but this appears more difficult, at least at first sight. For the latter, it would mean specifying the attenuation coefficient for either scattering plus the photo-electric effect or for just the photo-electric effect. When measuring uniform samples for a standard, it would be possible to adapt the detector to include or exclude scattered photons to a high level of accuracy. Of course, the adoption of a rigorous definition would not cure all CT problems overnight, but it would create a firmer foundation and would open the way to publishing standard Hounsfield values for common materials. Communities other than the medical community working with CT scanners in different energy ranges may require definitions adapted to their particular needs, but the existence rather than the absence of standards can only be positive.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Dentistry
Publisher: British Institute of Radiology
ISSN: 0250-832X
Last Modified: 18 Oct 2022 13:53

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