**2.3 Quantitative computer tomography (QCT)**

QCT is a method of bone mineral density measurement based on CT scan data after QCT phantom calibration and professional software analysis [14]. QCT uses CT three-dimensional volume data for analysis, and measures the true volumetric bone mineral density (vBMD), which can more sensitively reflect changes in bone BMD. Compared with DXA, QCT measurement is not affected by spinal hyperplasia and regression. The influence of factors such as changes and vascular calcification can avoid the false negative results of planar projection bone mineral density measurement technology caused by the above factors [28]. At the same time, the raw data of QCT can also be used for complex image processing to analyze and study bone changes and structural features [29].QCT includes central QCT, peripheral QCT and high-resolution peripheral bone quantitative CT (HR-pQCT), and micro-CT.

#### *2.3.1 Central QCT (central computed quantitative tomography, cQCT)*

cQCT is a pattern that uses multiple two-dimensional slices, the central delineation area of the pattern is the lumbar spine (especially the L1–3 vertebral bodies), the proximal femur, and central QCT also provides a measure of muscle mass [30]. Compared with DXA, central QCT is a measure of mean volumetric BMD (mg/cm3), which

improves the sensitivity and accuracy of BMD measurement and can assess the biological properties of interosseous BMD, bone geometry, and bone strength [31]. However, its disadvantage is that it increases the load of ionizing radiation, and due to the fact that most scanners are single-energy devices, which will lead to the potential problem of bone marrow fat changes. Studies have shown that the sensitivity of lumbar spine QCT to determine BMD is better than that of lumbar spine and hip DXA measurement, and it can more accurately reflect the changes in bone metabolism [32]. Clinical needs to choose to do spine or hip. Hip CT scans can be used for QCT, and the measured BMD results are equivalent to DXA areal BMD [33, 34]. According to the diagnostic criteria of the International Society for Clinical Bone Densitometry (ISCD) and the American College of Radiology (ACR), studies have found that QCT is more sensitive than DXA to detect osteoporosis [35]. This diagnostic criterion applies to postmenopausal women and older men. Lumbar vertebra QCT diagnostic criteria for osteoporosis: taking the average value of cancellous BMD of 2 lumbar vertebrae (usually the first and second lumbar vertebrae), and using the absolute value of lumbar spine QCT BMD for diagnosis. The evidence of BMD larger than 120 mg/cm3 usually is classified as normal, the absolute value of BMD in the range of 80–120 mg/cm3 as the group of low bone mass, the absolute value of BMD less than 80 mg/cm3 being considered as osteoporosis [36].
