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 zero 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.
It is the definition for CT scanners that are calibrated with reference to water.
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.
The HU of common substances
|Lung||-700 to −600|
|Fat||−120 to −90|
|Urine||-5 to +15|
|Bile||-5 to +15|
|Kidney||+20 to +45|
|Lymph nodes||+10 to +20|
|Blood||+30 to +45|
|Muscle||+35 to +55|
|White matter||+20 to +30|
|Grey matter||+37 to +45|
|Liver||+40 to +60|
|Soft Tissue, Contrast||+100 to +300|
|Bone||+200 (craniofacial bone), +700 (cancellous bone) to +3000 (cortical bone)|
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.
CT machines were the first imaging devices for detailed visualization of the internal three-dimensional anatomy of living creatures, initially only as tomographic reconstructions of slice views or sections. Since the early 1990s, with advances in computer technology and scanners using spiral CT technology, internal three-dimensional anatomy is viewable by three-dimensional software reconstructions, from multiple perspectives, on computer monitors. By comparison, conventional X-ray images are two-dimensional projections of the true three-dimensional anatomy, i.e. radiodensity shadows. It was established by Sir Godfrey Newbold Hounsfield, one of the principal engineers and developers of computed axial tomography (CAT, or CT scans).
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