# Quantitative Ultrasound Analysis of Oral Mucosa: An Observational Cross-Sectional Study
## 1 Introduction  
Ultrasonography (US) is a diagnostic medical tool that uses high-frequency sound waves and their echoes to create images of the internal body structures with frequencies between 1.6 and 50 MHz [1,2]. Ultrasound imaging is based on the waves echoing off the body structures being examined, resulting in different grades of grey, from white to black [2]. The shades are based on tissue density: the denser the tissue is, the lighter it will be. Bone appears bright white, and fluid appears black. Wave reflection or transmission depends on the acoustic impedance of the different tissues: this varies according to the density of the material ultrasound passes through. It is the product of the tissue's density and the sound's velocity through it [3]. Other non-invasive imaging techniques have been used in the oral cavity to study the anatomical and pathological characteristics of the oral mucosa [4, 5, 6]. Noninvasive imaging procedures performable chairside and that show more information in real-time without harming the patient could remarkably enhance the quality and the speed of diagnosis and enable the patient's compliance [7]. These include optical coherence tomography (OCT--which transforms the infrared backscatter into grayscale), reflectance confocal microscopy (RCM--which allows analysis at the cellular and subcellular levels of hard and soft oral tissues), and virtual chromoendoscopic magnification and narrow-band imaging (NBI) [8; 9; 10; 11]. The properties of ultrasound have enabled us to obtain different images, of which the most common are B-mode (brightness mode) and A-mode (amplitude mode). A-mode represents the amplitude of reflected ultrasound plotted against the transit time or, if the speed of sound in the tissue is known, against the transit distance. Therefore, A-mode sonography is increasingly being replaced by B-mode sonography, which provides a tomographic image by representing the amplitude of reflected sound linearly encoded in shades of gray as a function of axial distance from the sound source [12]. The echo returning to the transducer is converted to dots. Their width corresponds to the echo at its base and the brightness to the received amount of energy. Thus, the more the waves were reflected, the brighter the spot. Each point is bright (hyperechoic) or dark (hypoechoic), or completely black (anechoic), corresponding to the intensity of echoes from the corresponding anatomic structure. In this way, a B-mode scan provides an image that resembles an anatomical cross-section of the tissue being scanned [13]. In head and neck surgery, ultrasonography has been used to diagnose neck masses, salivary gland tumors, tongue carcinoma, and other oral lesions. In examining oral lesions, ultrasonography was carried out by placing a transducer on an extraoral site, but it was difficult to obtain detailed data. An extraoral approach has some limitations: it does not provide satisfactory images, particularly of the tongue, palate region, and oral floor, because air spaces within the oral cavity attenuate acoustic waves, and ultrasound does not penetrate the bone well. Furthermore, the acoustic attenuation generated by the distance from the lesion to the skin could become an issue [14]. Nevertheless, the extraoral approach is crucial for some applications, such as temporomandibular joint disorders [15]. The intraoral ultrasonography examination is non-invasive, rapid, and easily repeated; the number of possible applications has thus increased [16]. This technique has been used in the oral cavity for various purposes: from the study of periodontal tissues to the clinical and presurgical characterization of benign and malignant oral mucosa lesions [17; 18; 19; 20; 21]. One of the possible usages of ultrasonography is exposing an impacted tooth/teeth, and to establish if the tooth is covered by bone or not, as this can determine whether one can use less invasive methods, such as a diode laser or the conventional scalpel exposure [22].