Farquharson, Michael James; (1996) Characterisation of bone tissue using coherently scattered X-ray photons. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An energy dispersive x-ray diffractometer was designed and built to measure bone mineral density in the trabecular region of the bone and to assess the suitability of the technique as a clinical in-vivo method. Trabecular bone has a higher turnover rate than that of cortical bone, and to detect excessive bone mass loss, with the prevention of osteoporosis in mind, it would be advantageous to be able to measure trabecular bone mineral density in isolation from cortical bone density. At present the only method capable of achieving this in-vivo is quantitative computerised tomography. Initially measurements were made of trabecular bone mineral density on dry excised bone samples consisting of femurs, vertebrae and radii. These measurements were compared to measurements made using dual energy x-ray absorptiometry (DEXA) and photodensitometry techniques. The energy dispersive x-ray diffraction (EDXRD) measurements were the most accurate when correlated to the actual trabecular densities of the femur and vertebrae with correlation coefficients of r=0.84 and r=0.92 respectively. This compares with r=0.64 and r=0.74 for the DEXA measurements and r=0.77 and r=0.85 for the photodensitometry measurements. For the radii samples the correlation coefficients for all the methods were approximately the same at r=0.75. In-vivo measurements were simulated using a specially designed phantom. The results were analysed using multivariate calibration techniques and the radiation dose to the patient estimated using thermoluminescent dosimetry (TLD). Two main clinical sites were targeted, the calcaneus (heel) and the radius (forearm). It was found that the technique was capable of producing results with the required accuracy and precision (approximately 1[percent] of peak bone mass) but the radiation dose to the patient was high compared to other diagnostic radiographic procedures. The calcaneus measurements produced an effective dose of 270 [mu]Sv. for an accuracy of between 1[percent] and 2[percent] fractional bone mass loss, and the radius measurement resulted in an effective dose of 3.3 mSv. for an accuracy of approximately 2[percent] fractional bone mass loss. Measurements were made on recently excised femoral heads from total hip replacements, with 5 cm of tissue equivalent material. Predictions of bone mineral density were made from a calibration model created using phantom measurements. The predictions were compared to CT numbers obtained from QCT measurements and the correlation coefficient was calculated to be 0.94 significant to the 0.1 [percent] level. It is concluded that EDXRD is a technique that, with suggested refinements, has the potential to be able to make clinical measurements giving good accuracy and precision at an acceptable radiation dose to the patient.

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