## 5 The Importance of Head Orientation  
Head positioning plays a major role in the correct performance of a PAN examination. Errors in head positioning can occur on horizontal, vertical, and anteroposterior planes, or may result from a combination of multiple (compound) errors in more than one plane [14; 15; 16; 24; 25; 26]. To simplify our analysis of head positioning, horizontal and vertical head movements are defined as tilting and nodding, respectively, whereas rotational head movements are defined as rolling (Figure 1) [31; 32]. Errors in horizontal positioning can occur when a patient's head is tilted (tilting) or twisted (rolling), causing a wrong alignment on the midsagittal plane. Accordingly, anatomic structures appear asymmetrically enlarged on one side and reduced on the opposite side [14; 15; 16]. To avoid errors in head positioning on the horizontal plane, the midsagittal plane should be placed perpendicular to the floor and correspond to patient's midline. The lateral head supports can help a patient's stabilization and prevent tilting/rolling movement during the exposure [24; 25]. In case of rolling, the mandibular ramus width and teeth size decrease on the opposite side to the head's movement, since the opposite side in which the head is twisted is closer to the detector. When the head is twisted to one side, both mandibular ramus and teeth on the opposite side appear shortened (Figure 2) [14; 15; 16]. Figure 1: Types of movement of patient’s head. N: nodding. R: rolling. T: tilting. Figure 2: Rolling. Head rotation towards the left: (**a**) schematic drawing. Posterior teeth move to the right and do not coincide with the focal trough. Left hemi-arch is close to the X-ray radiation tube with consequent magnification of left hemi-arch teeth and mandibular ramus. On the contrary, right  
hemi-arch is close to the detector with consequent contraction in size of right hemi-arch teeth and mandibular ramus. a: X-ray radiation tube. b: detector. c: focal trough. d: dental arches. e: nose. (**b**) Panoramic radiography. Note the vertical dark band in the paraincisal area on the right side corresponding to the asymmetric position of two strong X-ray-absorbing structures, i.e., cervical spine and occipital bone. Premolar and molar teeth are blurry and increase in size on the left side; in addition, an overlapping of premolar teeth can be observed. In case of tilting, both the occlusal plane and mandibular lower border are inclined towards the tilted side. When the head is tilted to the left, the occupit is closer to the right side, causing an enlargement of the left side due to its closer position to the radiation source and a farther one from the detector. Conversely, the right side will appear reduced in size, as it is farther from the radiation source and closer to the detector (Figure 3) [12; 13; 14]. Errors in vertical positioning (nodding) affect the inclination of the occlusal plane. Vertical errors are related to an incorrect upper or lower position of the chin. If the chin is positioned higher, the hard palate is superimposed over the maxillary teeth apexes and condyles are dislocated posteriorly, or even projected off the image because of the increasing intercondylar distance. The nasal cavity and maxillary sinuses appear blurred and out of focus, and upper teeth are elongated (Figure 4) [12; 13; 14]. Conversely, if the head is nodded down, the upper and lower jaws appear magnified. In addition, the anterior mandibular area is partially out of the focal trough and may be superimposed on the hyoid bone (Figure 5) [14; 15; 16]. Errors on the anteroposterior plane are often related to an incorrect positioning of a patient's teeth on the bite block, which may result in excessive forward or backward positioning. If a patient's position is too anterior, the anterior teeth of both arches are placed out of the focal trough, and thus appear narrowed and blurred; however, the spine, nasal fossa, and maxillary sinuses are included in the focal trough.