Mooney, Michael Joseph; (1993) A study of scattered X-ray radiation in bone density measurement. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Various aspects of scatter in bone density measurement are studied. Firstly the detrimental effects of scatter in photon absorption bone mass measurements are assessed, and secondly, the use of scatter as a source of information about bone density is studied and a Compton scatter densitometry (CSD) system described. The detrimental effects of multiply scattered photons in CSD are also investigated. Photon absorption techniques are frequently used to carry out bone mass measurements. The dual energy analysis used to process the data depends upon scatter-free detector signals. In practice, the detector and source collimation produce high levels of scattered radiation in the detected signal. An analysis of this problem using Monte Carlo based photon transport computer studies is presented. The results indicated that current systems are reasonably insensitive to the scatter contribution except where patient changes occur over a long series of measurements, or where new results taken with dual energy x-ray absorptiometry (DEXA) systems are compared with those taken on older systems such as dual photon absorptiometry (DPA). Inclusion of scattered radiation in the calculation of bone mass can lead to a 0.5% to 3.5% reduction between system types. Patient size changes could lead to a reduction in bone mass of 0.5%a to 1.0%b. The use of Compton scatter to determine bone density has been described. Experimental and computer simulated data have been used to determine an optimum operating energy of 120 kVp and an exposure time of 30 s. The detrimental effects of multiple scatter were reduced by the use of energy windows. The remaining effects were corrected by correction factors calculated over a range of measurement sites for the apparatus geometry. Density measurements made over a range of phantoms produced reproducible results with a CV of less than 1%. An effective dose value of 6.9 µSv was found for a femoral neck measurement. Comparisons of CSD measurements were made with other established bone density measurement techniques. These were found to correlate very well with r=0.90 for quantitative computed tomography and r=0.95 for DEXA.

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