Hounsfield scale

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.

[3]: 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 radiotherapy treatment planning, where HU is converted to electron density.

[3]: 283 [5] 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.

[citation needed] HU-based differentiation of material applies to medical-grade dual-energy CT scans but not to cone beam computed tomography (CBCT) scans, as CBCT scans provide unreliable HU readings.

[citation needed] 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).