## 1 Introduction  
Osteoporosis is defined as a systemic skeletal disorder in which a combination of low bone mass and microarchitectural deterioration cause an increased risk of bone fragility [1; 2]. The consequence is that osteoporosis represents a major risk factor for fragility fractures (FFs) and imposes a significant burden on society and healthcare systems. Typical sites of FFs are the dorsal and lumbar vertebrae, femoral neck, proximal humerus, and distal radius: experiencing a fragility fracture is a strong predictor of an increased risk for future fractures. For instance, women with existing vertebral fractures are five times more likely to sustain another vertebral fracture and twice as likely to experience a hip fracture [3]. Early detection of osteoporosis and the initiation of suitable treatment can contribute to FF risk reductions [4].  
In clinical practice, instrumental diagnosis of osteoporosis relies on the assessment of bone mineral density (BMD) at the lumbar spine and hip using dual-energy X-ray absorptiometry (DXA) [5].  
BMD primarily reflects bone quantity and is closely related to bone strength; however, many individuals experiencing FFs (or with prevalent FFs) may exhibit normal BMD values. This controversial aspect can be attributed to the impact of skeletal factors that influence skeletal strength other than BMD, including bone geometry, microarchitecture, and remodeling. These skeletal features are considered to be related to \"bone quality\", and account for about 30% of bone strength [6].  
DXA is a two-dimensional imaging technique influenced by bone size, which cannot differentiate between cortical and trabecular bone or evaluate bone microarchitecture [7]. To address these limitations, the Trapecular Bone Score (TBS) was developed in 2010. This DXA-based software analyzes lumbar spine images to indirectly assess trabecular microarchitecture through image textural analysis: higher TBS values indicate a normal microarchitecture, while lower values reflect a degraded microarchitecture and increased risk of fracture [8; 9].  
Quantitative computed tomography (QCT) is a CT-based cross-sectional technique, which provides three-dimensional volumetric BMD (vBMD) values of mg hydroxyapatite/cm\({}^{3}\) from the lumbar spine and femur [10]. The main advantage over DXA is the ability to analyze trabecular and cortical bone separately, providing vBMD measurements of the trabecular part only, avoiding artifacts typically found at the spine on DXA (e.g., due to degenerative disease) [11]. Moreover, QCT is also less affected by surrounding tissues compared to DXA [12]. A CT scanner calibration, with external phantoms, is necessary to obtain vBMD values from CT images, which typically provides Hounsfield Unit (HU) values. These phantoms are composed of a set of known density values of hydroxyapatite samples. The calibration can be performed synchronously or asynchronously. In the case of synchronous calibration, the phantom is placed within the CT table during the patient scan, while for asynchronous calibration, the phantom is scanned periodically in the absence of the patient [11; 13]. Therefore, asynchronous calibration allows vBMD values to be obtained from routine CT examinations performed for other clinical reasons, setting the stage for an opportunistic osteoporosis screening.  
Despite DXA representing the reference for measuring BMD, a discrepancy is reported between areal BMD from DXA and volumetric BMD from QCT [14]. The common explanation is that DXA is influenced by vertebral degenerative changes or aortic calcifications, while QCT can exclusively evaluate the trabecular region of the vertebral body [14]. In this study, we aimed to compare QCT to DXA including not only BMD but also the TBS, which is generally reported to be less affected by degenerative changes [15]. Therefore, we compared the diagnostic accuracy of these parameters in discriminating between individuals with and without fragility fractures.