November 11, 2009

Hounsfield Unit

Sir Godfrey Hounsfield

The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement in one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU.

For a material X with linear attenuation coefficient μX, the corresponding HU value is therefore given by where is the linear attenuation coefficient of water.

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.

It is the definition for CT scanners that are calibrated with reference to water.

+1000 = Bone
+80 = Liver
+55 = Muscle
0 = water (cyst)
-1000 = Air

value below is slightly different. taken from this website

Bone 1000
Liver 40 - 60
White matter ~20-30 HU
Grey matter ~37-45 HU
Blood 40
Muscle 10 - 40
Kidney 30
Cerebrospinal fluid 15
Water 0
Fat -50 - -100
Air -1000


  1. HU ni sangat berguna jugak untuk kesan ade abnormality tak kat organ, for eg: brain CT.
    just gerak2 kan cursor kat seluruh brain dalam CT images, die akan automatically bg reading for HU at any places where we put the cursor.

    best kan HU ni walaupun sangat physics :)

  2. cik As:
    ya betul tu saudari... i'm sure u're more expert than me since u're involving with radiology a lot

    feel free to give more enlightenment on radiology issue.


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