Total Linear Attenuation Coefficients Research Articles

Computed Tomography Scanning for the Measurement of Bone Mineral in the Human Spine

An investigation into the feasibility of using X-ray computed tomography (CT) to measure disease induced changes in bone mineral content of the human spine is described. A theoretical study of this type of measurement has been made using a mathematical model of osteomalacia. The measured EMI number changes linearly with the mineral content, and the sensitivity is shown to be 1.2 EMI units (EU: 500 scale)/1% change in mineral content in vertebral bone. The physical sensitivity to an equal mineral change in cortical bone is found to be 8.6 times greater. The mineral selectivity of the CT method is such that only about half the change in the EMI number arising from progressive osteomalacia reflects change in actual mineral content, while half is due merely to changes in bone density that accompany demineralization. In addition, the perturbing effects of beam hardening on measurement accuracy are evaluated and shown to be significant. Finally, an experimental measurement indicates that in practice the reproducibility of such measurements would be about 1 EU, and it is shown that the measured parameter correlates well with the calculated total linear attenuation coefficients.

Approximate Methods for Calculation of Intrinsic Total-Absorption-and Double-Escape-Peak Efficiencies for Ge(Li) Detectors

Approximate methods are presented for calculation of intrinsic total-absorption-and double-escape-peak efficiencies of Ge(Li) detectors. These methods utilize characteristics of gamma-ray photon interactions with matter, and, while developed in this paper for specific application to the Ge(Li) detector efficiency problem, should be applicable to any gamma-ray detector or absorptive medium. The technique for total-absorption-peak efficiency determination includes the use of an energy-degradation and of the Dirac chord method for calculation of collision probabilities. The average-photon energy-degradation curve is obtained from differential Compton collision cross sections and detector material total-crosssection characteristics; it is independent of detector size and shape. The Dirac chord method is then used to determine the probability of further collisions for photons generated within the detector volume. In the model these probabilities are functions only of a characteristic detector dimension, s?, where s is the average chord length and ? is the total linear attenuation coefficient. Double-escape-peak efficiencies are determined through the calculation of the average probability of occurrence of the following three phenomena: (1) pair production, (2) electron-positron absorption, and (3) annihilation-photon double escape. Complete working curves and expressions are included to enable convenient utilization of these techniques. Example results are compared with efficiency determinations made experimentally and by Monte Carlo calculations.