*2.3.3 High-resolution peripheral computed tomography (HR-pQCT)*

HR-pQCT is newly developed QCT scanning modality, which can reconstruct multiple 2D slices (most commonly the radius or tibia) into a 3D virtual bone biopsy and provide enhanced spatial resolution beyond that provided by cQCT, pQCT or MRI [37]. The effective radiation dose of standard HR-pQCT in the distal radius or tibia is 3–5 μsv, which is considered to be a low radiation dose examination compared with other common medical imaging techniques [38]. HR-pQCT assessments have been performed in large epidemiological cohort studies such as the MrOs, OFELY, CaMos and Framingham Osteoporosis Study, which notably can be used for in vivo bone microstructural imaging at peripheral bone sites to understand the pathophysiology underlying bone fragility and improve fracture prediction. The pathophysiological is the basis of fragility and improve the prediction of fractures [39, 40]. And HR-pQCT is based on semi-automatic profiling and segmentation of tissue, which provides data from density, morphology, microstructure, and biomechanical (including stiffness and elastic modulus) measurements through finite element analysis. The clinical application and research of HR-pQCT in many other metabolic diseases exceeds osteoporosis, such as drug effects, rare bone diseases, hand joint imaging and fracture healing. It is used in rheumatoid arthritis to assess joint space width and bone erosion, in knee osteoarthritis and in some studies of fracture healing of the distal

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

radius [41, 42]. The unique advantage of HR-pQCT is the high spatial resolution in vivo, which enables the quantification of trabecular and cortical bone microstructure. HR-pQCT has high research value in bone quality, especially microstructure [43]. However, HR-pQCT is expensive and the imaging technology needs to be further standardized. Although recent recommendations for standardization in scanning, analysis, quality control, and result reporting have been given, the prospect of HR-pQCT in clinical practice still needs to be further studied [44].

There are some advantages and disadvantages for the QCT diagnostic measurements. The main advantages included the followings: ①The measurement of true volumetric bone mineral density is not affected by bone size and shape; ② Selective measurement of cancellous bone mineral density, more sensitive to reflect the changes of early bone mass; ③ The 3D geometric measurement parameters can be used to measure the bone mineral density of multiple sites and analyze the bone composition of cross sectional image; ④ It can be used in preoperative evaluation of orthopedics to guide the selection of clinical surgical methods and surgical sites. The disadvantage of DXA is not as common as DXA in clinical application because of its large size, expensive examination, larger dose of radiation received by patients and smaller application range than DXA.

In conclusion, QCT has been widely used in the clinical and health management of osteoporosis in recent years due to its advantages in imaging technology. Although QCT is more accurate in measuring volumetric bone density, it can measure cortical bone density separately bone and cancellous bone density, while the radiation is larger and there is a partial volume effect. In the vast majority of clinical cases, patients are undergoing CT scans for medical reasons, and the QCT bone mineral density analysis system is used to simultaneously scan the patients to obtain bone mineral density values, without additional radiation doses for patients. QCT can also measure intraabdominal fat and liver fat content, and QCT combined with low-dose chest CT has a promising application in health management [45, 46].

### **2.4 Magnetic resonance imaging (MRI)**

MRI uses strong magnetic fields and electromagnetic pulse sequences to obtain three-dimensional images. It has the advantages of sensitive signal display and rich post-processing. It can perform quantitative bone density examination, and can also perform bone microstructure imaging to understand the internal situation of bone structure, especially in judging osteoporotic fractures, it is superior to X-ray and CT examination, and there is no X-ray radiation. In recent years, various MR imaging techniques have gradually highlighted their advantages in the field of osteoporosis research, mainly including the followings [47, 48]. 1) Transverse relaxation time (T2\*) measurement is a quantitative MRI that indirectly reflects the morphological structure of bone tissue through the T2\* value of the bone marrow. Due to the difference in magnetic susceptibility between trabecular bone and bone marrow tissue, the magnetic field at the junction between the two is not uniform, and the morphological and structural changes of bone trabecular bone will affect the relaxation characteristics of the surrounding bone marrow. In the gradient echo sequence, the bone marrow T2\* value changes. And it has a certain order of magnitude relationship with the number of trabecular bone. Studies have shown that MRI T2\* values are moderately inversely correlated with quantitative computed tomography to assess bone mineral density in postmenopausal women with osteoporosis, and have certain potential in assessing the severity of lumbar osteoporosis [49]. A large number of studies have

confirmed that T2\* is closely related to osteoporosis, but its sensitivity, specificity, random type, parameters and many other reasons are different [50]. Currently, there is no standard for the diagnosis of osteoporosis with T2\*. 2) High resolution MR (HRMR) HRMR scanning has been widely used in recent years. The imaging is based on the signal difference between bone marrow and trabecular tissue. In the background of high signal in the bone marrow, trabecular bone appears as a black network structure. Studies have shown that the bone structure parameters of HRMR have a good correlation with the morphological structure parameters of tissue slices at the same site. The HRMR scanning matrix can reach the order of microns, which can better observe the trabecular bone microstructure and diagnose osteoporotic fractures [51–53]. The effect of HRMR in the detection of osteoporosis is positive, while MR examination time is relatively long, the price is high, and the evaluation is relatively complicated. There is still a lot of work to be done, such as sensitivity, specificity, accuracy, and standardized data processing. At present, it is not widely used in clinical practice, but is believed that with the deepening of research and the improvement of MR software and hardware. MR imaging will definitely play an important role in the diagnosis of osteoporosis. 3) Magnetic resonance spectroscopy (MRS) MRS can evaluate the organic matter, inorganic matter and bone matrix density of bone. Currently, there are phosphorus spectroscopy (13P-MRS) and hydrogen proton spectroscopy (1 H-MRS). Among them, phosphorus spectroscopy is to use the echo signal of 13P in bone to determine the content of bone inorganic components [54]. 1 H-MRS uses chemical shift to detect bone marrow water and adipose tissue, analyze its biochemical composition and metabolic changes, and indirectly assess bone quality from the molecular level [55]. Due to high technical requirements and many influencing factors, MRS has not been widely used in clinical evaluation of osteoporosis. 4) Others diffusion-weighted imaging (DWI)reflects the early changes in bone marrow composition and can quantitatively assess bone marrow changes. The apparent diffusion coefficient (ADC) and signal-to-noise ratio (SIR) can better reflect the bone mineral density of vertebral bodies in patients with lumbar spine diseases, and can quantitatively evaluate them, which is important for the diagnosis of lumbar spine osteoporosis [56, 57]. Diffusion tensor imaging (DTI) characterizes the diffusion direction of water molecules, which is helpful in assessing fracture risk in patients with osteoporosis [58]. Perfusion-weighted imaging (PWI) uses paramagnetic contrast agents to induce transient changes in the local magnetic field of perivascular tissue, which can reflect the perfusion and hemodynamic changes in tissue microcirculation, and help to detect early abnormal blood supply in diseased tissue [58].

MRI has a good auxiliary role in the diagnosis and differential diagnosis of osteoporosis by taking advantage of its multi-sequence imaging. Tomography can be used to understand the internal situation of the bone structure, Bone quality can be evaluated quantitatively, noninvasively and without radiation. It can reflect the physiological and pathological changes of bone histologically, and better understand the physiological characteristics of bone, so as to make its diagnosis more early and accurate. Because the image analysis process and parameter thresholds of HR MR and quantitative magnetic resonance (QMR) examinations have not been unified, functional imaging such as diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) are very important for osteoporosis. The significance of the diagnosis is inconclusive, and the MRI examination is expensive and time-consuming. Therefore, QMR in the diagnosis of osteoporosis still needs further research. With the further maturity of MR imaging technology, the further improvement of coils and the
