# Dental Microstructural Imaging: From Conventional Radiology to In Vivo Confocal Microscopy
## 2 Conventional In Vivo Radiological Imaging  
The in vivo imaging of oral hard tissues has deep-rooted origins [19]. Straight after [PERSON] discovered X-rays in 1895, the application of radiology in medical fields immediately found widespread diffusion [20]. Only 14 days after [PERSON]'s intuition, [PERSON] experienced the first application of X-rays during the making of dental radiographs [21]. Oral hard tissue radiographic techniques have been integral to clinical and diagnostic practice for over 100 years [22]. The most used and typical intraoral radiographs are periapical and bitewing radiographs: the former capture an image of the whole tooth, including its root apex and the alveolar bone; the bitewings, on the other hand, provide radiographs of dental arches, interproximal contacts, and the alveolar crest, approximately up to the coronal third of the root [23]. To date, intraoral radiographs have reached very high image definition levels to guarantee elevated sensitivity and specificity values for identifying dental tissue disorders [24]. Apart from intraoral radiographs, dental panoramic radiography, also called orthopontomography (OPG), constitutes a cornerstone in oral radiology [25]. OPG offers two-dimensional dynamic range visualization, providing complete imaging of the maxillary and jaw bones, the maxillary sinuses, and dental arches [26]. The large availability of the technique associated with the vast imaging information has made OPG an irreplaceable weapon for clinicians' daily practice [27]. In any case of intraoral radiography and OPG, X-rays pass through the anatomical structure to evaluate it and are then captured behind the object by a detector, usually a photographic film. In this case, the shadow of the structures is impressed on the film and visualized as negative images on a scale of greys after a precise and complex process of developing, fixing, washing, and drying the impressed analogical pellicle, then applied onto a lightbox to be fully appreciated by the clinician. However, during recent decades, digital detectors have increasingly replaced classical films, thus simplifying and shortening the imaging procedure. In this case, reusable X-rays-sensitive plates directly capture data during X-ray irradiation and immediately transfer it to a computer system that instantly displays the digital radiograph on a monitor. This avoids chemical processing, thus eliminating the cost and production of chemical waste to be disposed of, enabling the ability to apply special image processing techniques that enhance the overall display quality of the image, and is significantly time-saving. Finally, not to be overlooked, less radiation is used to produce an image of similar contrast to conventional radiography [28]. Recently, 3D radiography has increased in popularity to overcome the limitations of traditional two-dimensional dental imaging. It accurately imagines the superimposable details of maxillofacial bone structures and surrounding soft tissues [29]. Cone beam computed tomography (CBCT) has been introduced as a digital device capable of volume tomography depiction, aiming to reproduce imaging for the three-dimensions anatomy of dental and maxillary structures [30]. CT technology is based on the rigid connection between the X-ray tube and the detector placed on the other side of the subject to scan. The tube and detector move over the subject, sending a narrow X-ray beam through one thin slice at a time [31]. Then, the attenuation values of the transmitted X-rays are instantly processed by integrated specific software algorithms that rebuild the image of adjacent slices of the imaged volume [32]. This imaging technology offers applicability in almost all branches of dentistry and clinicians have several evidence-based guidance notes available [33].