## 4 Discussion  
The ultrasound scanner showed trueness that was comparable with that of the commercial optical techniques used for taking impressions of an implant at tooth position 11. True values showed that the implant scanned with ultrasound approximated the reference data set values. To be considered accurate, experimental sets should show not only trueness (closeness to the experimental data set) but also precision (agreement among repeated measurements). However, in this study, the accuracy of ultrasound was not assessed. The results suggested the potential of this high-frequency intraoral ultrasound scanner for taking digital impressions of dental implants. A mean discrepancy of 57.40 \(\pm\) 17.44 \(\upmu\)m was found, values considered acceptable from the clinical point of view [12]. Ultrasound scanning is a sensitive technique, in which acoustic waves are transmitted through the object and are reflected back, providing data to the image reconstruction software [23, 24]. Due to its non-ionizing nature, it might be used regularly to scan dental implants for different purposes. Previous studies have summarized the use of ultrasound in dentistry [25, 26, 27, 28, 29, 30, 31]. Although its clinical use is not yet a standard procedure, literature reports have cited the extra benefit, in addition to impression, of assessing and monitoring hard and soft tissues around dental implants [15, 16, 25, 26]. To the best of the authors' knowledge, this is the first study using an ultrasonic technique for taking digital implant impressions. The scanning of dental implants using ultrasound would be especially effective in cases of impression, including teeth prepared at subgingival level. An ultrasound-based technique is able to penetrate soft tissues and scan the subgingival margin of the preparation without being subject to the influence of oral fluids [20, 21]. As the disadvantages of this technology, the following can be highlighted: (1) the need for a medium, such as water or gel, to enable ultrasound wave propagation, and (2) its susceptibility to noise and artifacts that would hamper 3D image reconstruction [22]. In addition, when compared with optical scanners, at present, the ultrasound technique requires more time to scan an object. The main advantage of ultrasound-based scans is the wave penetration into soft tissues [14]. [PERSON] et al. (2019) showed that a 75 MHz ultrasound scanner was able to reproduce subgingival margins of the prepared tooth [20]. Scanning with a high-frequency ultrasound scanner can be beneficial when scanning dental implants together with prepared teeth since the main advantage of scanning with ultrasound is that it is capable of scanning the subgingival region of a prepared tooth. Furthermore, its insensitivity to oral fluids makes the technique suitable when it is necessary to take dental implant impressions with an open flap. For digital implant impression taking, the gingival phenotype and reduced height of the scan body could possibly be limiting factors for the use of ultrasound scanners [11, 21]. A frequency of 42 MHz was chosen to provide the resolution required for intraoral scanning to the detriment of the penetration depth [14]. The resolution of the transducer selected was to the order of 20 \(\upmu\)m axial and 70 \(\upmu\)m lateral, required for robust matching of the complex surface geometries within the oral cavity. By using a 42 MHz transducer, a maximal penetration depth of 2.14 mm was achieved [32]. [PERSON] et al. (2019) showed that a similar ultrasound scanner with a 75 MHz probe was able to scan gingiva with a thickness of up to 1 mm [20]. These parameters were considered sufficient for the clinical purposes of this study. Previous studies evaluating the use of ultrasound in implant dentistry were based on B-mode images for recognizing anatomical structures [17, 27]. Nonetheless, the aim of implant impression is to reproduce the spatial position of dental implants in relation to the adjacent structures, thus requiring a 3D image of surface structures [30].