 At the spine, QCT is used to measure the bone mineral density of only the spongy interior bone separately from the dense cortical bone that forms the exterior walls of the vertebrae.The trabecular bone has much higher metabolic activity than the cortical bone and so is affected by age, disease and therapy-related changes earlier and to a greater degree than cortical bone.This means that QCT of the spine has an advantage compared to other bone density tests because earlier changes in bone mineral density may be detected .