Beam diameter and scan velocity effects on linear bone mass measurement by photon attenuation

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Abstract
A theoretical analysis is presented of the fundamentals involved in the calculation of the linear bone mass from the observed transmitted photon intensities. Intermittent scanning and continuouscanning methods are considered. The absolute errors caused by finite beam diameters and velocity of scan are calculated. It is demonstrated that only partial correction is possible for errors associated with the beam diameter, whereas those associated with scan velocity can be fully compensated. Although most systems applied to routine measurements of linear bone mass are independently calibrated and are claimed to have correlation coefficients of around 0.96 it is found that conditions exist where the error between two individual measurements, despite calibration, may be greater than 20%.