The Hounsfield scale is used in computed tomography (CT) to describe the attenuation of X-rays in tissue and represent it in gray-scale images. The values can be assigned to tissue types and pathological deviations can be recognized. The underlying tissue property is often referred to physically imprecisely as (X-ray) density.
The CT number or CT value is given in Hounsfield units (HE) or Hounsfield units (HU) on the Hounsfield scale proposed by the English electrical engineer Godfrey Hounsfield (1919-2004). The CT number is based on the linear attenuation coefficient , which describes how monochromatic X-ray radiation is attenuated when penetrating matter along the irradiated path, but is not identical to it.
When the accelerated electrons hit the anode plate of the X-ray tube , bremsstrahlung is generated. The electrons are slowed down at different depths on the surface of the plate, so that no monochromatic X-rays but an electromagnetic spectrum is created. The measured attenuation values therefore relate to this spectrum. The maximum photon energy occurring in the spectrum is equal to the acceleration voltage of the tube, i.e. the voltage that is applied between the cathode and anode. X-ray tubes also have different radiation qualities due to their design. These are changed by device-specific and manufacturer-specific, different pre-filtering, in that the pre-filtering removes low-energy components of the spectrum, as these primarily increase the radiation exposure of the patient, but make little or no contribution to the imaging.
In the medical application of computed tomography, tube voltages between 80 and 140 kV are used . By normalizing to the attenuation coefficients of water and air, the CT number is approximately independent of the spectrum of the X-ray radiation used. In this way, the image results remain comparable despite varying beam qualities. However, since the Hounsfield scale is only defined via a two-point calibration, it is not completely free from the influence of changing radiation qualities. When working with low tube voltages, higher CT numbers are determined for bones and contrast media than with higher tube voltages.
With the attenuation coefficient of the tissue under consideration and of water , the CT number is defined as:
Theoretically, the scale is open at the top. In practice, the range from −1024 HU to 3071 HU has prevailed; these 4096 gray levels can be represented with a twelve-digit binary number (2 12 = 4096). However, metals can cause even stronger absorption , up to and including total absorption (which then cannot be represented). Since significantly more attenuation values can be clearly separated on the scale than the human eye can distinguish on a gray scale, only the part of the Hounsfield scale that represents the image content to be examined is shown in the image by windowing .
The definition gives the number of CTs for different substances and tissues:
- Air hardly absorbs X-rays at all and by definition has a CT number of −1000 HU. When calibrating a CT, the attenuation of air is simply set to. This is not correct, but in practice it would only be possible with great effort to create a vacuum for the detector calibration. The error that arises from working with is always present, but very small and completely irrelevant for the image quality.
- According to the definition, water has 0 HU.
- Adipose tissue absorbs X-rays a little less than water and has about −100 HU.
- Bones have values from 500 to 1500 HU, depending on their density.
- Contrast medium has values between 100 and 300, depending on its type and concentration
Two-Spectra-CT (= Dual-Energy CT)
By measuring a layer with two different X-ray spectra, the so-called "two-spectra CT", which is also called "Dual-Energy CT" (DECT for short), the CT numbers on the Hounsfield scale can be converted into values that would be obtained using monochromatic X-rays. The images are free of image artifacts such as those caused by beam hardening when passing through the patient. With the help of two-spectra CT, the material density, in clinical use especially the calcium and soft tissue density, as well as the effective atomic number of the tissue can be determined.
Clinically relevant, in particular, is the possibility of differentiating fresh bleeding from old calcium deposits with the help of two-spectra CT and of calculating the contrast agent used from the image or displaying it separately. A bone density measurement can also be carried out.
- Thorsten M. Buzug: Introduction to Computed Tomography: Mathematical-physical basics of image reconstruction. Springer, Berlin / Heidelberg / New York 2002; ISBN 3-540-20808-9 , p. 404 ( limited preview in Google book search).
- Willi A. Calendar: Computed Tomography. Basics, device technology, image quality, applications. 2., revised. and exp. Edition. Publicis Corporate Publishing, Erlangen 2006; ISBN 3-89578-215-7 .
- Rodney A. Brooks: A quantitative theory of the Hounsfield unit and its application to dual energy scanning. In: J. Comput. Assist. Tomogr. 1, No. 4, 1977, pp. 487-493 ( PMID 615229 ).