Hounsfield-Scale

Observation: Attenuation Coefficients are not very handy!

Since the human body is made from up to 70% of water, we introduce a different scale of attenuation that is normalized to water, the so called CT-Value or Hounsfield Units (Hounsfield Unit = HU).

$ CT(Material) = \frac{\mu(Material) - \mu(Water)}{\mu(Water)} \cdot 1000 HU $

From that definition, the CT-Value of various materials and tissue can be derived on the Hounsfield scale:

• According to definition, water has a CT-Value of 0 HU.
• Air does merely absorb X-ray radiatiom, so that it has a CT-Value of −1000 HU.
• Fat tissue absorbs X-rays a little less than water Wasser, havig a CT-Value of about −100 HU.
• Depending on density, bone has CT-Values ranging from 500 to 1500 HU.

A CT Scan yields a HU value for each voxel that relates to a specific material.

But: The assumption of monochromatic radiation, on which the Hounsfield-Scale is based, is not valid in reality, since real X-ray radiation sources always emit a spectrum of wave lengths.

Conclusion: We can deduce the CT-Value from the attenuation coefficient, but only approximately, meaning that X-ray images are not 100% identical and reproducable from X-ray device to device.