Introduction
It must be well appreciated and realised that a cephalogram is an X-ray taken in a standard orientation of the head and obtained through a cephalostat. A cephalogram is a two-dimensional X-ray picture of three-dimensional structures of the dentition, face and head. A cephalogram provides highest possible projection resolution in which structures smaller than 0.1 mm can be discerned. This projection resolution with cephalogram is superior to that of conventional computed tomography (CT).
It is presumed that structures of the right and left side of face/head would be exactly overlapping each other when X-rays traverse perpendicular to the mid-sagittal plane at the trans meatal axis of the head. However, the fact that the structures of head on left side of the face are closer to the X-ray source than the right side, some amount of magnification of right side structures is technically unavoidable. Therefore, the left side structures show less magnification compared to the right side structures and this is the reason for not having a perfect overlap of the right and left anatomy.
The appearance of double shadows on a cephalogram should be a routine and not an exception. Most of the cephalometric machines accept 5% enlargement as an acceptable limit. Magnification is an inherent limitation of a cephalogram.
Five features of a good lateral cephalogram
Besides the blend of sharpness, contrast and density of an X-ray film, the other essential features of a cephalogram are those related to accuracies of head positioning in the cephalostat. Besides hard tissue features, a cephalogram should also exhibit a clear soft tissue shadow of the facial contour. The soft tissue is substantially recorded by limiting the radiation intensity in the region with the use of soft tissue filters built in the film holder cassette or provisioned in the sensor holder of the digital cephalogram apparatus.
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The smooth curve of the cervical spine: A cephalogram that is taken without straining the neck would show 6th–7th vertebra of the cervical spine in a smooth curve.
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Concentricity of ear rods: The radiopaque metallic rings of the ear rods present as two shadows in the region of the external auditory meatus (EAM). The ring of the right side appears slightly larger in diameter compared to left side due to the magnification factor. However, it is the concentricity which is important. Lack of the concentricity would be caused by axial rotation of the head to the left or right or vice versa. Larger ring (right side ear rod shadow) being more anterior than the left indicates axial rotation of neck to the left and reverse location will suggest axial neck rotation to the right.
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Some cephalometric systems have a small metal palette embedded in the centre of plastic ear rod which would appear as radio-opaque rounded shadows in the centre of the EAM. Ideally, these two shadows should perfectly coincide, but it is a rare phenomenon to be seen in day-to-day life.
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Overlap of the right and left side structures: There should be a minimal discrepancy in a horizontal and vertical overlap of the right to left side structures. A difference of the condylar heads coupled with a marked discrepancy or significant separation of the posterior border of the ramus of the mandible and lower border of the body of mandible suggest improper head positioning. A cephalogram with a large discrepancy may have to be discarded, and a repeat radiograph is advised. However, while asking for a repeat radiograph, consideration of risks and cost associated with radiation vis-à-vis, the actual benefit of a more accurate cephalogram in treatment decision should be kept in mind.
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A slightly distal placement of the posterior border of the ramus and superior placement of the lower border of the mandible on the right side compared to the left side structures is a normal finding.
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Other structures that can help in determining the accurate positioning of the head while recording a cephalogram are shadows of orbital rims and the pterygopalatine fissure.
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Teeth in centric occlusion: A child may quite often move his/her jaw from centric occlusion position, or open the mandible. It is always better to check the occlusion clinically/on study models and reconfirm on his cephalogram. A simpler and practical technique would be to match the overjet seen on a cephalogram with the one measured in a mouth or on dental study models mounted into correct occlusion. Vertical separation of teeth would appear as double shadows of the cusp tips of molars in the vertical direction or a radiolucent shadow onto the occlusal surfaces of teeth.
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Unstrained lips: A cephalogram taken in a relaxed posture of lips would show a natural outline of the face from forehead down to the region of the Adam’s apple. Similarly, a cephalogram taken with care would also exhibit shadows of the soft palate and anatomical contours of the upper airway ( Fig. 32.1 ).
Figure 32.1 A good quality lateral cephalogram of a case of class II malocclusion.
Features of a good PA cephalogram
It is important to mark (L) or (R) side of the head while taking a PA view radiograph. A cephalogram taken in a correct position in the head holder should exhibit EAM shadows (either rings or around radio-opaque marker) in a horizontal plane. However, this aspect may have to be ignored in children with gross craniofacial deformities. PA cephalogram would exhibit temporal bones, orbits, frontal and ethmoid sinuses, maxilla and its antrum, nasal cavity, palatal floor and the mandible from the condyle to the symphysis ( Fig. 32.2 ).
A good quality lateral cephalogram of a case of class III malocclusion.
A posteroanterior cephalogram is indicated for the evaluation of symmetry of right and left sides. Efforts are made to distinguish any apparent difference from true deformity from left to right side. PA cephalogram is much more technique sensitive compared to lateral cephalogram. An imprecise orientation of the head could exhibit as right to left asymmetry or cant of occlusal plane. While axial rotation of head may appear as asymmetry of left to right tilt (or vice versa) leads to transverse cant of occlusal plane. The rotation of head around a transverse axis may exhibit itself as facial asymmetry.
Errors during taking a cephalogram
Sources of errors at random in taking a cephalogram are mostly associated with improper positioning or orientation of head in cephalostat.
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A strain on the neck: Cephalostat machine needs to be adjusted to patient’s height whether the patient is standing/sitting in a relaxed posture. The ear rods should gently be placed in the EAM.
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Strain in the neck could be due to the error in vertical positioning of the ear rods. In either case, the up and down position of the machine, that is lower or higher than the comfortable for the height of the patient will lead to strain on the neck. To adjust a discrepancy in the height of the machine, either the child has to strain up the neck or bent himself down ( Fig. 32.3 ).
Figure 32.3 Strain on the neck (marked with arrow ), poor positioning of the head in cephalostat.
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Axial rotation of the head: The rings apart in anteroposterior direction suggest that radiations are not passing perpendicular to the mid-sagittal plane. It signifies rotational error of head positioning in a cephalostat ( Fig. 32.4 ).
Figure 32.4 Effect of axial rotation of the head to the left seen as double shadow in the posterior border of the ramus.
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Head tilt:
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Similarly, there may be a superoinferior discrepancy in the concentricity of rings which would suggest a right or left side tilt of the head. A perfect overlap of the left to right mandibular lower border though desired is rare. A slight gap is acceptable however a big gap is suggestive of head tilt ( Fig. 32.5 ).
Figure 32.5 Head tilt to right/left side seen as double shadow in lower border of the mandible.
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Head tilt forward or backward, that is not parallel to FH plane. The head tilt forward or backwards will affect the airway and tongue and hyoid position ( Fig. 32.6 ).
Figure 32.6 The head is tilted upwards leading to FH plane not parallel to the floor.
Also, a poor contrast of the film makes it unsuitable for cephalometric measurements.
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Teeth not in occlusion: Teeth apart not in occlusion or patient biting on incisal edges will give an erroneous reading of the parameters related to the mandible ( Fig. 32.7 A–C).
Figure 32.7 Common error while taking a cephalogram with posterior teeth out of occlusion.
(A) Class II division 2 patients tend to bite in incisal edge during exposure. (B) Same patient with buccal teeth in centric occlusion. Occlusion on cephalometry should always be double checked with centric occlusion relation in the mouth. (C) Improper buccal teeth interdigitation due to slight mouth opening.
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The strain on lips: Excessive strain on lips in an effort to close the lips is a common error while taking a cephalogram. A cephalogram in centric occlusion should be taken with lips in a relaxed posture. The way patient poses himself/herself during exposure would affect soft tissue measurements ( Fig. 32.8 ).
