The criteria for measuring average density by X-ray attenuation: The role of spatial resolution

The attenuation of X-rays as they pass through a material can be used to quantify the amount of matter in the X-ray path. This technique is the basis for industrial gamma ray densitometers and medical imaging of tissue density and composition. While spatial resolution is directly related to the detection of small features, it can also be a critical factor in the accuracy of quantitative density measurements of nonhomogeneous material due to the nonlinear nature of the X-ray transmission. In computed tomography the accuracy of the reconstruction can suffer as it depends on the accuracy of the transmission data. The error depends on the degree of density uniformity, which is defined relative to a scale size set by either the size of a resolution element in the detector, or the size of a collimated X-ray ram. If the transmission data is compromised due to density variations that occur over a distance that is smaller than the system's spatial resolution or scale size, subsequent processing can not correct the errors. A method is presented for quantifying the error in measured density using the spatial variation of X-ray intensity on the face of the detector and the total attenuation in the material