## 3 Discussion  
### Single-Energy CT  
Single-energy CT utilizes a single X-ray energy level to generate detailed anatomical images by measuring tissue attenuation [20; 21]. Its applications range from diagnostic imaging to quantitative analysis, providing valuable insights into tissue composition and pathology [20; 21]. Before performing any quantitative analysis, it is important to consider all the factors that may impact CT Hounsfield Unit (CTHU) stability, such as the type of scanner used, the patient's body size, and all the parameters for the scan algorithm. One way to reduce errors is by using the subcutaneous fat and paraspinal muscle as internal calibration standards. [PERSON] et al. [22], by using two different approaches, compared the agreement and precision of BMD measurements: in a cohort of patients with chronic kidney disease, asynchronous PB calibration and PL internal tissue calibration were used. For the latter, a similar approach to the one described for the PB assessment was used to manually place elliptical volumes of interest (VOIs) in the front portion of the mid-vertebral body. On each image, VOIs of muscle and fat were positioned on the right side within the posterior subcutaneous fat, and on the left, within the paraspinal muscle group. The objective was to produce a normal distribution on the HU histogram with a prominent Gaussian fit within the predetermined values. The variable HU distribution inside the muscle and fat ROI could be a drawback. As muscle CT numbers rely on the patient's level of hydration, other variables that might affect accuracy and precision include differences in the patient's vasculature and cellularity. It was found that the measured BMD with phantoms was higher than the measured BMD without phantoms. Despite a negligible absolute difference of 3.3 mg/cm\({}^{3}\) and a negligible relative difference of 5.1%, interindividual variances were substantial. In comparison to the PB approach, the PL methodology exhibits a higher intra- and inter-operator variability. Although there were several connections between the techniques, each person's between-method difference varied greatly. Due to the manual selection of the region of interest (ROI) of body tissues in the conventional PL-QCT system, its repeatability could be rather low. One significant issue with the conventional PL-QCT is solved by the automatic selection of the ROI for fat and muscle. This process also increases the accuracy of the BMD data by computing BMD values using the calibration factor. [PERSON] et al. [23] used this innovative system with automatic ROI selection, which makes BMD testing easier, dramatically increasing accuracy if compared to conventional BMD measuring techniques. In a work of [PERSON] et al. [24], the automated ROI technique was used to calibrate the findings of the BMD measurement using the subcutaneous fat and paraspinal muscle as internal calibration standards. Several methods were used to successfully select body tissue ROIs from CT images. First, to accomplish segmentation of various body tissues in CT images, the HU scale was used. As reported by the authors, the determination of the HU range was based on clinical practices and experts' observations of spinal CT scans: fat (HUmin = \(-\)150; HUmax = \(-\)50) and muscle (HUmin = 20; HUmax = 80). Each patient had two or three vertebrae examined, with L1-L3 of major consideration. In the PB-QCT study, an ROI with a restricted range of adjustment was chosen for each vertebra. The anterior portion of the vertebral body was automatically assigned an elliptical VOI, which was then manually modified as needed. The posterior venous plexus and any specific disease, such as bone islands and calcified herniated disks, were avoided to obtain the most accurate values. On each picture, VOIs of muscle and fat were positioned in almost the same locations: on the right side, in the posterior subcutaneous fat, and on the left, in the paraspinal muscle group.