## 3 Discussion
### Dual-Energy CT  
DECT is a more recent imaging modality, which concurrently captures images at two distinct energy levels, enabling refined tissue characterization [32; 33]. Widely applied for material decomposition and virtual non-contrast imaging, it also enhances lesion detection and characterization [32; 33]. Its diverse applications make it a valuable tool across medical disciplines, contributing to more precise diagnostic capabilities in scientific research. In contrast to traditional CT, material distinction in DECT can provide novel, pertinent information for several musculoskeletal applications [34; 35]. A DECT-based post-processing algorithm has recently been evaluated, allowing PL volumetric BMD assessment of lumbar trabecular bone. The accuracy of PL volumetric DECT BMD evaluation based on lumbar spine material breakdown, as an indication for the 2-year occurrence risk of osteoporosis-associated fractures, was studied retrospectively by [PERSON] et al. [36]. They first characterized the trabecular VOI in 3D, prior to performing a PL vBMD evaluation of the L1 vertebra. The whole vertebral body's trabecular bone--not cortical bone--was included in the VOI for this analysis. The VOI and the two DECT series (90 and 150 kVp) were loaded into a second measure of the volumetric BMD. The algorithm uses specialized material breakdown to identify between each voxel's collagen matrix, calcium hydroxyapatite, water, fat marrow, and adipose tissue composition, as exposed in a previous work by [PERSON] et al. [37]. They employed a biophysical model-based technique that considered the five main components of trabecular bone. The fraction of the volume occupied by the matrix material (i.e., bone mineral + collagen), VTB, and the volume of fat tissue, VF, are related to the HU intensities of both imaging data sets at 90 and 150 kV. The remaining variables are energy-related constants, and the values for t and g are 0.92 and 1.02, respectively. Therefore, values for VTB and VF can be acquired by calculating the mean intensity for an area of the trabecular bone in both imaging data sets, and the BMD value BM expressed in g/cm\({}^{3}\) can be determined from VTB. Using a similar approach, [PERSON] et al. [38] identified the trabecular VOI of each vertebra, not only L1 like the previous study, but from L1 to L4. The selected VOI was the whole vertebral body's trabecular bone, carefully excluding the cortical bone. The VOI and the two DECT series (90 and 150 KVp) were used as input for the volumetric BMD evaluation. A second step was to distinguish among red blood cells, water, collagen matrix, bone minerals, and adipose tissue for each voxel, was used to perform volumetric BMD evaluation. For PL assessment of the cancellous BMD of vertebral pedicles and to investigate the relationship with pedicle screw pull-out strength, [PERSON] et al. [39] examined the quantitative DECT. First, a template mesh representing each pedicle screw vector in the 3D image data set was automatically added using an automated method for recognizing vertebral anatomy. The user then evaluated this template mesh to determine how to appropriately exclude cortical bone and delineate the cancellous pedicle screw vector and, if necessary, manually deform it. The labeled volume and the two image data sets corresponding to the low-energy and high-energy DECT scans were then imported into the analysis application. According to the calculated spatial cancellous BMD distribution for all indicated locations, each voxel in the bone region considered of each pedicle vector was assigned a unique BMD value. Both a global value for the whole pedicle screw vector and particular values for each pedicle segment were calculated. The 3D representation of the estimated spatial distribution of cancellous BMD values in the resultant pedicle screw vector was shown and overlaid on the vertebral model. Red indicates places with low BMD, white areas with normal BMD, and blue areas with high BMD.