Precise accurate mineral measurements of excised sheep bones using X-ray densitometry

Abstract

An Hologic QDR 1000-W dual energy X-ray absorptiometer (DXA) was used to make a densitometric evaluation and to optimize ex vivo scanning and analysis conditions for excised sheep bones. Articulated lumbar spines and femora, removed at autopsy from eight 1–2-year-old cross-bred merino ewes, were studied using standard lumbar spine and hip protocols (version 4.20). Spine data were analysed with the ultra-high resolution (UHR) protocol (version 4.25). Preliminary hip measurements also were made using the lumbar spine (version 4.20) and scoliosis protocols (version 4.25). Bone mineral density (BMD) precision for the total hip (n = 5) was 0.4% using version 4.20, and 0.8% using the scoliosis protocol (version 4.25) but was 1.2–3.4% for femoral sub-regions. Precision for the lumbar spine was 0.8% (version 4.20). There was considerable variation in BMD and bone mineral content (BMC) at the same site between animals. Right and left hip BMD were highly correlated (r = 0.98, P < 0.00001). Measurement variables evaluated to determine the optimal conditions included: Hologic scanning protocols, depth of water immersion of bone and dimensions of the region of interest. Accuracy was determined by ashing. Use of the scoliosis protocol (version 4.25) gave the best results for proximal femur measurement. Measured BMC decreased by 4.4% as the water depth increased from 0 to 22.5 cm. At 5 cm water, BMC accuracy was 99% for the total hip, and BMD precision values (n = 3) were 0.3% (total hip) and 0.4% (femoral neck, trochanteric region and femoral shaft). The much-improved precision was attributed to use of a positioning device and to optimizing of the depth of water immersion. Rib segments (length 2.5 cm) were measured using the ultra-high resolution protocol (version 4.25). BMC accuracy determined using 6 ribs was best (100 ± 2%) at 5 cm water. BMD precision (n = 5) was best (0.3%) at 0 cm water depth and was 0.5% at 4.5 cm water depth. Rib sample BMC (n = 15, 4.5 cm water depth) and ash weight were highly correlated (r = 0.985, P < 0.001). It is concluded that highly precise, accurate densitometry can be performed on excised small and large sheep bones. This technique will support studies evaluating the sheep as an animal model of human osteoporosis.