Different values of X-ray energy result in different attenuation by matter, and thus in different pixel/voxel values in the reconstructed images. Subsequently, scaling to Hounsfield units is done. So, CT scanners with different X-ray energies, all yield the same calibrated output: Hounsfield units.

Hi Balsam,

HU is a linear transformation of the original linear attenuation coefficient (which characterizes how easily a volume of material can be penetrated by a beam of X-rays), and this coefficient is different for every voltage. So we can say that different values of energy (80 or 120 kVp for example) have slightly different values in HU; but the difference is small, and every software in the CT scanner calculates and adapts these value in the common standard table of HU.

The table in the attached image correctly represents the standard values used in clinical routine

Vincenzo Alfano

Thank you very much for this demonstrative answer.

Different values of X-ray energy result in different attenuation by matter, and thus in different pixel/voxel values in the reconstructed images. Subsequently, scaling to Hounsfield units is done. So, CT scanners with different X-ray energies, all yield the same calibrated output: Hounsfield units.

Just a small remark:

As Vincenzo Alfano

said,

the HU units depend a bit on the voltage, thus they do not yield the 'same' calibrated output. However the HU dependence on the voltage is much smaller than that of the mean linear attenuation coefficient. Thus the HU scale can be regarded as some kind of 'calibrated' output scale as Lambert Zijp states.

See for example:

http://www.upstate.edu/radiology/education/rsna/ct/technique.php

By the way:

definition of HU unit scale is:

HU = 1000*(µ-µH2O)/µH2O

i.e. the per-mill deviation of the mean linear attenution coefficient µ of the sample from that of the water µH2O