QCT can be performed on a vast majority of commercially available CT scanners, provided they include densitometry analysis software and a calibration phantom. When interpreting QCT vBMD results, it is important to recognize 2 important differences to DXA. Z- and T-scores can be calculated from the vBMD, but the T-scores do not apply to the WHO definition of osteoporosis or osteopenia [31]. The exclusive application of the WHO classification is inherent to projectional BMD [25]. The ACR QCT cutoff values for low bone mass or osteopenia are 80 to 120 mg/mL and <80 mg/mL for osteoporosis [12]. Another major difference between QCT and DXA is related to monitoring. Spine BMD values measured by QCT demonstrate higher rates of bone loss with advancing age, principally because of the exclusive measurement of cancellous bone. The rate of change in cancellous bone is significantly greater than that of cortical bone. By contrast, the projectional properties of DXA summate the cortically predominant end plates and posterior elements with the cancellous vertebral body measurements, thereby decreasing their rate of change over time [31]. Indications for utilization of QCT as a screening modality are the same as DXA. However, in the setting of screening or initial imaging, QCT is regarded as a secondary tool to DXA. QCT may be considered as a primary imaging modality in certain conditions. Cases in which QCT is considered superior to DXA include extremes in height (very tall and very small patients), patients with obesity (BMI >35 kg/m2), patients with severe degenerative spine disease, and when an increased sensitivity to small changes in trabecular bone density is desired (parathyroid hormone and glucocorticoid treatment monitoring) [34]. It was recently reported that opportunistic QCTs of the lumbar spine was more predictive of spine fractures in neurological and oncologic patients than reference DXA scans, but there were only 84 patients in this study [35].
Osteoporosis and Bone Mineral Density. QCT can be performed on a vast majority of commercially available CT scanners, provided they include densitometry analysis software and a calibration phantom. When interpreting QCT vBMD results, it is important to recognize 2 important differences to DXA. Z- and T-scores can be calculated from the vBMD, but the T-scores do not apply to the WHO definition of osteoporosis or osteopenia [31]. The exclusive application of the WHO classification is inherent to projectional BMD [25]. The ACR QCT cutoff values for low bone mass or osteopenia are 80 to 120 mg/mL and <80 mg/mL for osteoporosis [12]. Another major difference between QCT and DXA is related to monitoring. Spine BMD values measured by QCT demonstrate higher rates of bone loss with advancing age, principally because of the exclusive measurement of cancellous bone. The rate of change in cancellous bone is significantly greater than that of cortical bone. By contrast, the projectional properties of DXA summate the cortically predominant end plates and posterior elements with the cancellous vertebral body measurements, thereby decreasing their rate of change over time [31]. Indications for utilization of QCT as a screening modality are the same as DXA. However, in the setting of screening or initial imaging, QCT is regarded as a secondary tool to DXA. QCT may be considered as a primary imaging modality in certain conditions. Cases in which QCT is considered superior to DXA include extremes in height (very tall and very small patients), patients with obesity (BMI >35 kg/m2), patients with severe degenerative spine disease, and when an increased sensitivity to small changes in trabecular bone density is desired (parathyroid hormone and glucocorticoid treatment monitoring) [34]. It was recently reported that opportunistic QCTs of the lumbar spine was more predictive of spine fractures in neurological and oncologic patients than reference DXA scans, but there were only 84 patients in this study [35].