# CT Density Tables

The use of CT data within a planning system necessitates a conversion from the Hounsfield scale, with units (HU) describing the radiodensity of the imaged tissue (compared to water, for X-ray energies), into density values, which the planning system can use to calculate dose deposition for treatment energies. A HU is defined as:

$HU_{material} = 1000 \times \frac{\mu_{material} - \mu_{water}}{\mu_{water}}$

Where the μ values are the linear attenuation coefficients of the material and water respectively. Thus the scale begins at -1000, where the linear attenuation coefficient is zero (air is approximately -1000) and generally remains below approximately 3000 for dense bone, with higher values for metallic objects such as implants.

The conversion to either mass or electron density values is unique to the CT scanner and operational energy. Physicists acquire scans of objects with known density values and relate those values with the resultant HU values in the image. This embedded spreadsheet (available here) contains CT density tables for 3 machines operating nominally at 120 kVp. Mass density values are expressed in g/cc. Relative electron densities are expressed relative to the value for water. The CB2 30% and 50% values are representative of the calcium carbonate concentration. These values are provided as reference only.