Calibrating and Measuring CT Density ASTM E1935

Scope:

ASTM E1935 is an excellent method for measuring density variations; these measurements can evaluate parts, compare different systems, or characterize a particular system. This method is helpful for the determination of the adequate X-ray energy of a CT system. The ASTM E1935 test method is more precise and less susceptible to errors than other CT-based approaches.

Test Procedure:

In ASTM E1935, measured mean CT values of the known standards are determined from an image evaluation, and their linear attenuation coefficients are calculated by multiplying their measured physical density by their published mass attenuation coefficient. The density calibration is accomplished by applying linear regression to the data. Once calibrated, the linear attenuation coefficient of an unknown feature in an image is measured by determining its mean CT value. Its density is then extracted from a knowledge of its mass attenuation coefficient.

Data:

The linear attenuation coefficient of the region of interest is obtained using the equation from the known calibration parameters.

NCT = a·μ + b 

Here μ is the linear attenuation coefficient and N CT is the CT number.