**2.1 DXA**

Dual X-ray Absorptiometry (DXA) measures two-dimensional bone mineral density (areal BMD), namely the measured bone mineral content (bone mineral content, BMC) divided by the measured bone projection area. DXA bone mineral density measurement is the most commonly used method for bone mineral density measurement in clinical and scientific research. The main measurement site is the axial bone, including: lumbar spine and proximal femur. Lumbar BMD can sensitively reflect the changes of bone metabolism and therapeutic effect, while femoral BMD is one of the most reasonable indicators for predicting femoral fractures. Anterior and posterior lumbar spine measurements are generally selected for lumbar BMD examination, and the region of interest includes the vertebral body and its posterior appendages. The regions of interest for proximal femur measurement were the BMD of the femoral neck, greater trochanter, total hip and Wards triangle, and the regions of interest for the diagnosis of osteoporosis were the femoral neck and total hip. If the measurement of the lumbar spine and proximal femur is limited, especially when secondary osteoporosis (e.g., hyperparathyroidism) is considered, the non-dominant distal forearm third (33%) can be selected. The distal forearm measurement can obtain the bone mass parameters of the radius, ulna, and radius plus ulna at the super-distal end, the distal mid-segment, the distal 1/3, and the total distal part, totaling 12 different regions [7].

BMD measured by DXA is currently a common diagnostic index for osteoporosis. For postmenopausal women and men aged 50 and over, the BMD value according to the diagnostic criteria recommended by WHO is lower than the peak bone value of healthy adults of the same sex and race. The patients with T value less than 1 are considered as healthy; the T value ranging from 1 and 2.5 as osteopenia (or low bone mass); the T value equal to or more than 2.5 are diagnosed as osteoporosis, the patients with severe osteoporosis usually have one or more fragility fractures simultaneously (**Table 1**). Bone mineral density is usually expressed by T-Score, T-value = (measured value - peak bone mineral density in normal young people of the same race and sex)/standard deviation of peak bone mineral density in normal young people of the same race and sex. For children, premenopausal women and men under the age of 50, it is recommended to use the Z value of the same race to judge the level of bone mineral density, z-value = (bone mineral density measurement value - the

*Advances in Clinical Application of Bone Mineral Density and Bone Turnover Markers DOI: http://dx.doi.org/10.5772/intechopen.109074*


#### **Table 1.**

*The diagnostic criteria for osteoporosis based on DXA, BMD, and T values.*

mean bone mineral density of the same race and the same sex and the same age) /the same race and the same age. The standard deviation of bone mineral density among sex peers, z-values below −2.0 were considered as "low cohort expected range" or low bone mass.

The lumbar spine BMD examination generally chooses the anterior and posterior lumbar spine measurement, and the area of interest includes the vertebral body and its posterior appendage structures, so the measurement results are affected by the degenerative changes of the lumbar spine (such as bone hyperplasia and sclerosis of the vertebral body and vertebral facet joints, etc.), abdominal Aortic calcification, intervertebral disc calcification, schmorl node, etc. Literature studies suggested that the choice of lateral lumbar spine BMD measurement can avoid the interference of the above factors [8]. At the same time, about 60% of the vertebral body is cancellous bone, which is also a site prone to osteoporotic compression fractures, while the spinous process, transverse process and pedicle of the posterior 1/3 of the spine are rich in cortical bone, which can be difficult for osteoporotic compression fractures and not play an important role in fractures. Lateral measurement of the lumbar spine can exclude the posterior 1/3 of the spine and detect early vertebral bone loss. In addition, with aging, the bone loss of cortical bone and cancellous bone is different. During a person's lifetime, BMD of the anterior vertebral body decreases by about 50%, while the posterior decreases by about 25%. Therefore, the lateral BMD measurement of the vertebral body can better reflect the actual changes of the spongy bone and the bone mass of the vertebral body itself. The lateral lumbar spine bone mass measurement is paired (accompanied) with the anterior and posterior lumbar spine, that is, combined with the lateral scan measurement on the basis of the anterior and posterior scan measurements, so that the estimated volumetric bone mineral density (vBMD) of the lumbar two-dimensional scan can be obtained at the same time. Also known as widthadjusted BMD (WA-BMD), the bone mass parameters of each vertebral body and the entire vertebral body can be obtained. It also avoids some interference factors and improves the ability of early detection of bone loss, thereby improving the diagnosis of bone loss and susceptibility to loose tissue. The lateral thoracolumbar vertebral images collected by the DXA measuring instrument can also be used for vertebral morphological assessment and vertebral fracture assessment (VFA), but the repeatability of DXA lateral lumbar spine measurement is not as good as the anteroposterior one.

Although BMD measured by DXA is currently recognized as the gold standard for the diagnosis of osteoporosis, there are still some limitations. DXA has the characteristics of high specificity and low sensitivity for the prediction of fracture, and depends on the choice of diagnostic point. A large number of studies have shown that BMD only partially reflects bone strength and cannot effectively evaluate the effect of anti-osteoporosis treatment. It only partially reflects changes in bone structure during aging, metabolic disorders or treatment. More scholars began to pay attention

to how to expand the DXA measurement function: 1) Trabecular bone score (TBS) is a measurement index for evaluating bone microstructure by analyzing image pixels of lumbar spine DXA [9]. 2) Hip structure analysis (HSA) is to evaluate the bone strength by computer analysis of the geometric data obtained from the DXA scan image of the proximal femur [10]. 3) Finite element analysis (FEA) is a twodimensional model for evaluating femoral strength parameters, which can be used as a hip fracture risk assessment [11]. 4) Body composition measurement, which can be used for the evaluation of body composition, and can provide information on BMC, bone density, lean mass and fat content in different regions of the body, but the whole body bone density cannot be used for diagnosis of osteoporosis [12]. 5) Bone density assessment around the prosthesis, DXA can evaluate the stability of the prosthesis by measuring the bone density around the prosthesis [13].

DXA is a currently widely used technology with low radiation dose, and highly recognized as bone mineral density measurement method, while there are still many deficiencies. The regional BMD measured by DXA is a comprehensive measurement of cancellous bone and cortical bone, and the measurement results cannot reflect the early changes in BMD. At the same time, due to the principle of DXA plane projection imaging technology, the area BMD measured by DXA is affected by weight, scoliosis, bone hyperplasia, vertebral fractures and vascular calcification and then reduce the accuracy of BMD measurement. Testing in pregnant women is not yet recommended. As development of osteoporosis percentage increasing and the research of DXA new function in the elderly, further improvement of DXA fan beam scanning technology and application of multidetector, the scope of the application of the low radiation dose DXA is expanding in the assessment of human body bone mineral density measurement. But in addition to the DXA bone mineral density measurement, body composition analysis and evaluation are relatively mature, other functions (such as HAS, TBS, FEA detection, peripheral bone mineral density measurement, etc.) are mostly limited to the preliminary clinical application or the research phase of the trial.
