## 3 Current Techniques for OP Study
### Computed Tomographic Imaging Techniques
#### Micro- and Nano-CT  
Micro-CT is the most powerful instrument for bone feature assessment at the laboratory scale, enabling the achievement of high isotropic resolutions of up to 1 \(\upmu\)m. Thanks to these high resolutions, micro-CT equipment can distinguish between cortical and trabecular bone in a very reliable way. However, these high-resolution analyses are performed exclusively on sedated small animals or on ex vivo bone biopsies since the scanning of whole bone would require extremely long durations and high radiation exposures, not applicable to patients [12]. Furthermore, the long scan durations of micro-CT analyses would lead to a higher risk of patient movement and consequently to the generation of image artefacts. Considering the same limitations in terms of scan duration and radiation exposure, nano-CT instruments, able to reach resolutions of up to 350 nm, are only employed in the research field for ex vivo analyses of bone biopsies [9]. It is worth mentioning that the nano-CT scanners actually work between the nano- and the microscale: the pixel size used for the scan, in fact, depends on the sample size. However, in the presence of a sample with reduced dimensions and optimal characteristics, the nano-CT instrument can work at a resolution of less than 1 \(\upmu\)m, thus justifying the prefix nano-. Synchrotron radiation is an alternative radiation source used in CT equipment at the laboratory scale. Thanks to its monochromatic nature, it allows for avoiding the appearance of several image artefacts deriving from the multi-chromatic nature of X-ray radiation employed in common CT scanners. The resolutions achievable with this type of source are significantly lower than 1 \(\upmu\)m [9]. However, the extremely high cost and complexity of synchrotron technology limits its availability to a few specialised research centres [58]. These up-to-date CT instruments are employed at the research level for the analysis of OP-related modifications of cortical and trabecular features and of their composition. As will be described in Section 4.3, this architectural and compositional information can be exploited for the creation of finite element models for the investigation of the mechanical behaviour of bones and for their fracture prediction. In fact, the high resolution achievable with micro- and nano-CT instruments allows for the reproduction of singular trabecula, improving the accuracy of the model and, consequently, of the fracture prediction [4]. Moreover, micro-CT images can also be employed for the accurate replication of bone structure through rapid prototyping techniques to fabricate advanced bone scaffolds. To this aim, the bone surface is approximated through a series of triangles and the coordinates of their vertices are reported in an.stl format file. Subsequently, this file should be converted into a G-code containing the instructions required by the additive manufacturing machine to fabricate the bone replica layer-by-layer. In Table 1 are the principal characteristics of the imaging techniques for bone feature analysis considered in this review, highlighting their advantages and limitations. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline
**Technique** & **Evaluated Parameters** & **Analysis Conditions** & **Dimension** & **Resolution** & **Advantages** & **Limitations** & \\ \hline
**DXA** & \multirow{2}{*}{Areal BMD} & In vivo & \multirow{2}{*}{2D} & \multirow{2}{*}{Up to 1 mm} & Availability & \multirow{2}{*}{Results dependent on bone volume} & \multirow{2}{*}{Results dependent on bone volume} \\  & & In vivo & & & Low radiation dose & & Does not provide 30 structural information \\ \hline
**Histomorphometry** & \multirow{2}{*}{\ \begin{tabular}{} \end{tabular}  } & Structural parameters & \multirow{2}{*}{Ex vivo} & \multirow{2}{*}{2D} & \multirow{2}{*}{Up to 10 μm} & \multirow{2}{*}{High resolution} & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}