The TW3 atlas [7] has been described as being more accurate and reproducible than the GP atlas in white populations [1,2], while the GR atlas has been found to be much more precise and of better quality than the GP atlas [1]. A 2009 study found that, due to the risk of age overestimation in females, the GR atlas \"only seems to be suitable for forensic age diagnostics to a limited extent\" [9]. A review and meta-analysis published in 2019 concludes that the ethnicity or origin of a child can influence the applicability of the GP atlas. It was found to be imprecise and should be used with caution in Asian, certain Hispanic, and African populations, particularly when assessing age for forensic or legal purposes [10]. Endocrine or nutritional factors, certain medications, non-endocrine chronic diseases, or certain medical syndromes can influence bone development [2]. Additionally, differences in bone age development exist for different ethnicities and genders [9,10]. Aside from methods based on radiographs, bone age assessment methods based on MRI of the left hand have also been tested [11], and the predictable ossification pattern of the patella, tibia, fibula, and femur on knee MRI has also been found to enable accurate bone age assessments [12]. However, the acquisition of radiographs requires ionizing radiation, and MRI is an expensive and less accessible technology. To address the shortcomings of MRI or radiographs, the usage of non-invasive quantitative ultrasound (QUS) has been proposed. For example, in the related field of osteoporosis diagnosis, ultrasound (US) measurements of the bones are used. These systems work within a typical US frequency range from 0.2 to 1.0 MHz to measure bone mineral content of the calcaneus and allow the determination of bone loss. The onset and progress of osteoporosis can be classified using conventional signal processing [13] or, more recently, using multi-channel convolutional neural networks based on 1D US radio frequent signals [14]. First attempts in applying QUS to assess bone age by performing B-mode imaging of the femoral head articular cartilage (FHC) were proposed in 1995 [15]. However, clinical  
Figure 1: Bone age radiographs showing different degrees of growth plate ossification of the distal radius and ulna in girls aged 12 years (**a**), 15 years (**b**), and 18 years (**c**). use of methods based on signals stemming from the FHC was strongly discouraged in a publication stating that the sensitivity of these methods was too low [16]. A study evaluated the QUS system BonAgeTM on 37 children aged 5 to 15 years and stated it was accurate at assessing the bone age [17]. This system computes the bone age by combining US propagation speeds and the distance between emitter and receiver and comparing those parameters to a database containing reference data based on sex and ethnicity [18]. In two studies, the BonAgeTM system was evaluated on 152 and 65 subjects, respectively, and demonstrated sufficient accuracy of bone ages [18; 19]. However, a follow-up paper found the potential of US for assessing bone properties to be largely unexploited at the time [20], while another study concluded that US assessment should not yet be considered a valid replacement for radiographic bone age determination [21]. In the latter study, the authors report multiple reasons not to consider US as a valid replacement for radiographs. They found a higher variability of bone age estimates, compromised readings due to diseases that increase the thickness of cartilage, the need for an operator to perform a time-consuming examination, and greater variability in correlation studies [21]. A study examined the QUS device SonicBoneTM and found it to be safe, convenient, and non-painful, with highly reproducible results [22]. The same study stated that the BonAgeTM system was not commonly used due to its lack of consistency and confirmatory data in large groups.