**2. Advanced imaging assessments of bone mineral density**

Bone mineral density refers to the amount of bone contained in a unit volume (volume density) or a unit area (area density). There are many methods of bone mineral density measurement, and different methods have different roles in the diagnosis of osteoporosis, monitoring of curative effect and assessment of fracture risk. Plain film absorptiometry (RA) and single-energy X-ray absorptiometry (single x-ray absorptiometry. SXA) two detection methods have been rarely used in clinical practice. X-ray plain film can evaluate changes in bone mineral density, but its sensitivity and accuracy are not high. It's difficult to make a positive diagnosis when bone mineral loss is less than 20%.Only when the bone mass is reduced by more than 30%, or even more than 50%, there are abnormal manifestations [6], thus it is generally not used as a tool for routine evaluation of bone mineral density. Presently, the commonly used bone mineral density measurement methods in clinical and scientific research include dual energy X-ray absorptiometry (DXA), quantitative computed tomography (QCT), quantitative ultrasound (QUS) and MRI, etc.
