The Hounsfield unit (HU) scale is a linear transformation of the original
linear attenuation coefficient measurement into one in which the
radiodensity of
distilled water at standard
pressure and
temperature (
STP) is defined as 0 Hounsfield units (HU), while the radiodensity of
air at STP is defined as −1000 HU. In a
voxel with average linear attenuation coefficient , the corresponding HU value is therefore given by:
where and are respectively the linear attenuation coefficients of water and air.
Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.[1]: 259
Calibration tests of HU with reference to water and other materials may be done to ensure standardised response. This is particularly important for CT scans used in
radiotherapytreatment planning, where HU is converted to
electron density.[2] Variation in the measured values of reference materials with known composition, and variation between and within slices may be used as part of test procedures.[1]: 283 [3]
Rationale
The above standards were chosen as they are universally available references and suited to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g.humans.
Values reported here are approximations. Different dynamics are reported from one study to another.
Exact HU dynamics can vary from one CT acquisition to another due to CT acquisition and reconstruction parameters (kV, filters, reconstruction algorithms, etc.). The use of contrast agents modifies HU as well in some body parts (mainly blood).
A practical application of this is in evaluation of tumors, where, for example, an
adrenal tumor with a radiodensity of less than 10 HU is rather fatty in composition and almost certainly a benign
adrenal adenoma.[28]
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