## 3 Discussion
### Single-Energy CT
For the best match, the VOI's size and form for muscle and fat were modified to produce a normal distribution on the HU histogram with a dominating Gaussian fit within the predetermined values. Similar PL-QCT was used by [PERSON] et al. [25], with the automatic function of selecting vertebrae, hip, fat, and muscle ROIs and using fat and muscle ROI CT values to calibrate BMD results with high precision. For the five different mineral compositions of the reference phantom that were captured in the same CT scans, [PERSON] et al. (2018) employed five ROIs. The multiple regression models were used to create the PL HU-to-BMD conversion equations using a stepwise regression technique. The dependent variables were the HU value, circumference, and bone area; the BMD value was the independent variable. The images of the spine and hips were initially segmented. As post-processing, holes were filled, and surfaces were smoothed. The segmented pictures were then instantly transformed from each voxel to a matching 8-node solid element. Using the BMD modulus connection for the spine and femur, the elastic moduli of each finite element were converted from the estimated and reference BMD values of each voxel. Since the attenuation factor of the patient's soft tissue (psoas and fat) is significantly different from that of bone, using body anatomical markers may affect accuracy. For most contemporary scanners, the calibration factor (ratio in actual density phantom rod and CTHU, mg/cm\({}^{3}\)/CTHU) was developed, and it could be helpful for assessing BMD. [PERSON] et al. (2018) discussed the integration of PL-BMD devices into non-dedicated QCT scans of thoracic and abdominal areas, in a head-to-head PB-BMD vs. PL-BMD calculation. With a 9 mm high cylinder to provide a constant VOI, the program automatically placed the elliptic ROI into the trabecular bone compartment of each vertebra. Each tissue ROI's CT number was computed under the assumption of a normal distribution. The program defined the histogram to represent the major tissue component, considering the variable levels of interstitial fat inside the muscle ROI. They concluded that, although PL-BMD's accuracy is less accurate than that of PB-BMD systems, it is a reliable clinical tool for detecting decreased BMD over a wide patient group. In the study by [PERSON] et al. (2018), oval ROIs are positioned on the vertebral body, parasigninal musculature, and subcutaneous fat at each level from T12 to L5, respectively, during PL-QCT. The transverse plane of section is angled to be parallel with the end plate at each level using a sagittal reconstruction. The basivertebral venous plexus posteriorly, the surrounding cortical bone, and any localized lytic or sclerotic lesion are all avoided while placing the vertebral body ROI in the anterior trabecular area. Software generates a BMD measurement (in gm/cc), which was utilized as the primary QCT outcome measure for comparison with the DXA T-score. At L1, a trabecular ROI attenuation cut-off of 160 HU was 100% sensitive (29/29) and 46.4% specific (104/24) for osteoporosis. Area under the curve for osteoporosis using trabecular ROIs at single lumbar levels was 0.888 for PL-QCT and ranged from 0.825 to 0.853. In a study by [PERSON] et al. (2018) L1-L5 HU values were converted to mg/cc using linear regressions between the mean HU values obtained from the phantom ports and the known mg/cc values. To produce a conversion for L1-L5 HU measurements to mg/cc, a PL calibration approach was established, where the fat and muscle HU values were linearly regressed against the ground truth values for fat (69 mg/cc) and muscle (77 mg/cc). For the 50 participants, the mean lumbar volumetric BMD (vBMD) estimated using the two methods (QCT vs. PL) was compared to determine agreement. Excellent agreement was seen in the linear regression of lumbar vBMD produced from the QCT vs. PL calibrations (\(p=0.0001\)).