Internationally, the dual-energy dual-line bone mineral measuring instrument (DEXA) was developed and produced after the 1980s, and there are about 30 units in China at present, and quantitative CT (QCT) has also begun to be applied in the clinic. Other measurement techniques such as ultrasound measurement (USD) and MRI are still in the initial exploration stage, and Japanese scholars are in the leading position in ultrasound BMD measurement technology. The main methods for BMD measurement are described below. 1.Single Photon Absorption (SPA) This method is the earliest applied BMD detection method, which adopts the photon absorption method of radioactive isotopes. It has the advantages of good repeatability and small amount of radiation, but it can’t measure cortical bone and trabecular bone separately, and it can’t measure the bone parts with non-constant soft tissues, and it is commonly used in the determination of BMD of radial bone cortex, and it can be used in the heel bone, and it is mostly used in the census of large samples of people, and the census and screening of various age groups, genders, regions and nationalities. 2.Dual Photon Absorption (DPA) The DPA measurement method uses two kinds of nuclides, high and low energy, as the radiation source. When the photon beam passes through the subject, two different attenuation curves can be obtained, and the BMD value can be obtained after processing and calculation. However, this method has a long scanning time, high radiation dose, and the need to frequently replace the radioactive source, has gradually reduced the application. 3, dual-energy X-ray absorption method (DEXA) DEXA is the development and continuation of the DPA, it is two different energy X-ray as a radiation source for BMD measurement. DEXA machine photon flow, scanning time is short, than the DPA has a higher accuracy. It can measure the BMD of lumbar vertebrae, proximal femur, whole body bone and adipose tissue content. Its image spatial resolution is 1.5 mm, which is almost 10 times higher than that of DPA, especially the ability to evaluate vertebral BMD from lateral position can even be compared with QCT, but it still can not measure cortical and trabecular bone with different bone conversion rates separately. The orthostatic lumbar spine often results in high measurements due to more posterior attachments of the vertebral body and calcification of the aorta, so the correlation between DEXA measurements in the lateral lumbar spine and QCT measurements is greater than the correlation between orthostatic DEXA values and QCT values. Not only that, the BMD values measured by DEXA also correlated well with the BMD values measured by all other methods. This method is highly reliable. Now there are C-arm DEXA scanners, so that the measurement accuracy can be further improved. 4, quantitative CT (QCT) can be divided into single-energy quantitative CT (SEQCT) and dual-energy quantitative CT (DEQCT) two kinds of DEQCT can improve and correct the effect of fat on the measured value, improve accuracy. DEQCT can improve and correct for the effect of fat on the measurement, but in daily practice, SEQCT is usually sufficient, as it has a higher resolution and is the only method that can measure true bone density in a three-dimensional spatial distribution, as well as the only method that can measure the BMD of cortical bone separately from that of trabecular bone. Modern QCT can measure not only spinal BMD but also peripheral bone (PQCT). It has been reported that the sensitivity and reliability of PQCT measurements of peripheral bones, such as the forearm, in predicting osteoporosis and vertebral fracture is similar to that of lumbar spine 2-4 DEXA, and is superior to other measurement methods. 5.Ultrasonic diagnostic technology (USD) Ultrasonic diagnosis is a new method to determine BMD in recent years. It mainly applies ultrasound to pass through the heel bone and patella when the speed of sound (SOS) and sound volume attenuation (BUA) changes to derive the BMD value, which has its own unique form of expression, and a large range of normal values of the population has not yet been carried out to determine the work. Modern ultrasound BMD measuring instrument can clearly display the bone morphology and trabecular structure on the screen, which can be used by the examiner to select the region of interest (ROI), and study the BMD from both numerical value and image, which can more accurately reflect the changes of bone quantity and quality. Ultrasound is not harmful to the human body, and can also be used for the examination of pregnant women and pediatrics. However, the narrow range of application (only for the heel bone and patella) is a deficiency of USD. It has been reported that changes in bone structure and quality are firstly manifested as changes in ultrasound conduction velocity, so USD has its unique significance in the measurement of changes in bone strength, which can show changes in bone volume at an early stage and predict fatigue fracture and pathologic fracture. However, the credibility of the ultrasound parameters and the uncertainty of the relationship between the bone volume and the elasticity of the bone, as well as the correlation with other methods, need to be further researched. In clinical practice, BMD is mainly used to understand the presence of osteoporosis and the prediction of fracture, and can be based on the image to directly understand the changes and abnormalities of the bone quality in the detection area (such as bone disease, fracture, tumor, etc.). Indirectly, it can be used to understand the skeletal manifestations of other systemic diseases. One of the trends in BMD research today is to define bone strength by combining the BMD values of DPA, DEXA, and QCT with the imaging (CT, US) changes in the bone structure, and to use this index to determine the degree of osteoporosis and the risk of fracture. Lin Ya-min defined bone strength = ultrasound amplitude attenuation (BUA)× bone fixation vibration number (fc), which is more reflective of bone strength than a single application of the BUA value. a decrease in BMD is associated with an increase in the risk of fracture. M.C. Broom believes that the combination of QCT and imaging in the determination of vertebral BMD is one of the most valuable methods for determining vertebral compression strength and predicting fracture trends. There are many articles about BMD measurement of osteoporosis in women after the age of 45~50 years, and it is now believed that it is related to the decrease of estrogen level, which increases the sensitivity of bone to parathyroid hormone (PTH) and aggravates the bone resorption. There is a significant difference between postmenopausal BMD values in women and those who are not menopausal (P < 0.01). And foreign studies have shown that there are significant differences even among people of different races and nationalities; it is believed that the BMD of vegetarians is higher than that of omnivores, the BMD of radial bone of blacks is higher than that of whites, and there are even differences in the BMD of Americans and Britons, which need to be further explored. A growing number of studies have shown that absolute values of BMD have a strong ability to predict fracture, and fracture thresholds have been derived for a variety of measurements.Anderson, Mazess, Melton, et al. have all concluded that BMD falls below the fracture threshold for osteoporotic fracture, and that there is a significant difference between the BMD of persons with and without vertebral compression deformities in the same age group; the Dwelling Clans have used the DEXA measurements of BMD of the proximal femur in women, the mean BMD values of the femoral neck, trochanter, and Ward's triangle in the fracture group were lower than those of healthy non-fracture individuals in the same age group in all age groups. BMD measurement can also be performed to indirectly understand the manifestation of systemic diseases in the skeletal system, and is also an important indicator to monitor the effectiveness of treatment.Ettnger.B, on the other hand, has achieved better results by grouping osteoporosis patients into groups to be given different treatments based on different BMD values. Especially in chronic kidney disease, BMD has been studied more, in addition, in a variety of endocrine diseases or tumors also often cause bone loss or osteoporosis. For osteoporosis in postmenopausal women, it has been suggested that estrogen therapy should be administered as soon as possible after menopause to avoid osteoporotic fractures. Kasai's believed that hormone-induced ischemic necrosis of the femoral head (ANF) was mainly caused by osteoporosis, and the degree of bone reduction was significantly lower than that of the control group. The methods of BMD measurement have gradually tended to be simple, convenient, fast, and less damaging, and there has been a gradual tendency to use the end bones (heel and distal radius) instead of the spinal bones, and it has been demonstrated many times that the measurements of the end bones are in agreement with the spine. At present, SPA, DEXA, and QCT are all playing a significant role, while USD and MRI are still in the developmental stage and have a promising application. In conclusion, BMD values, images, laboratory indicators, and clinical characteristics must be integrated and judged in clinical work to understand the real situation of bone metabolism.