Cephalometric radiography
Main indications
Orthodontics
• Initial diagnosis – confirmation of the underlying skeletal and/or soft tissue abnormalities
• Monitoring treatment progress, e.g. to assess anchorage requirements and incisor inclination
• Appraisal of treatment results, e.g. 1 or 2 months before the completion of active treatment to ensure that treatment targets have been met and to allow planning of retention.
When considering these indications, it should be remembered that all radiographs must be clinically justified – a legislative requirement in most countries. In the UK, indications and selection criteria for cephalometric radiographs are clearly identified in the British Orthodontic Society’s 2008 booklet Orthodontic Radiographs – Guidelines for the Use of Radiographs in Clinical Orthodontics (3rd Edition) and in the Faculty of General Dental Practice (UK)’s 2013 booklet Selection Criteria for Dental Radiography (3rd Edition). These guidelines are designed to assist in the justification process so as to avoid the use of unnecessary radiographs.
Equipment
Traditional film-based equipment
This either consists of an additional attachment to a panoramic unit as shown in Fig. 14.1, or as a completely separate dedicated unit as shown in Fig. 14.2. The basic components include:
• X-ray generating apparatus that should:
– Be in a fixed position relative to the cephalostat and film so that successive radiographs are reproducible and comparable. To minimize the effect of magnification the focus-to-film distance should be greater than 1 m and ideally in the range 1.5–1.8 m (see Fig. 14.2).
– Include a light beam diaphragm to facilitate the collimation. The beam should be collimated to an approximately triangular shape to restrict the area of the patient irradiated to the required cranial base and facial skeleton, so avoiding the skull vault and cervical spine and thyroid gland (see Fig. 14.2).
– Be capable of producing an X-ray beam that is sufficiently penetrating to reach the film and parallel in nature.
• Cephalostat (or craniostat) (see Fig. 14.3) comprising:
– Head positioning and stabilizing apparatus with ear rods to ensure a standardized patient position. Additional positioning guides can include forehead supports and infraorbital guide rods.
– Optional fixed anti-scatter grid to stop photons scattered within the patient reaching the film and degrading the image. This is not usually included in combined panoramic/cephalostat units.
• Cassette (usually 18 × 24 cm) containing rare-earth intensifying screens and indirect action film.
• Aluminium wedge filter designed to attenuate the X-ray beam selectively in the region of the facial soft tissues to enable the soft tissue profile to be seen on the final radiograph. This is either attached to the tubehead, covering the anterior part of the beam (the preferred position) or it is included as part of the cephalostat and positioned between the patient and the anterior part of the cassette.
Digital equipment
Equipment variations exist depending on the type of digital image receptor chosen.
Using solid-state sensors
Several manufacturers have developed combined panoramic/cephalostat units utilizing specially designed solid-state sensors. An example is shown in Fig. 14.4. Sensor design was discussed in Chapter 4 and illustrated in Fig. 4.18.
The sensor is obviously not the same size as an 18 × 24 cm cassette and the image cannot be captured in the same way. During the exposure, the X-ray beam and sensor move either horizontally or vertically to scan the patient, as shown in Fig. 14.5. The final image therefore takes a few seconds to build up. To ensure that the X-ray beam is the same shape as the CCD array in the sensor and that they are aligned exactly, the beam passes through a secondary collimator, which also moves throughout the exposure, as shown in Fig. 14.5.
Main radiographic projections
True cephalometric lateral skull
As stated in Chapter 12, the terminology used to describe lateral skull projections is somewhat confusing, the adjective true, as opposed to oblique, being used to describe lateral skull projections when:
• The image receptor is parallel to the sagittal plane of the patient’s head
• The X-ray beam is perpendicular to image receptor and sagittal plane.
In addition, the word cephalometric should be included when describing the true lateral skull radiograph taken in the cephalostat. This enables differentiation from the non-standardized true lateral skull projection taken in a skull unit, as described in Chapter 13. It is now an accepted convention to view orthodontic lateral skull radiographs with the patient facing to the right, as shown in Fig. 14.6.
Technique and positioning
This can be summarized as follows:
1. The patient is positioned within the cephalostat, with the sagittal plane of the head vertical and parallel to the image receptor and with the Frankfort plane horizontal. The teeth should generally be in maximum intercuspation.
2. The head is immobilized carefully within the apparatus with the plastic ear rods being inserted gradually into the external auditory meati.
3. The aluminium wedge, if used, is positioned to cover the anterior part of the image receptor.
4. The equipment is designed to ensure that when the patient is positioned correctly, the X-ray beam is horizontal and centred on the ear rods (see Fig. 14.6).
Cephalometric tracing/digitizing
This produces a diagrammatic representation of certain anatomical points or landmarks evident on the lateral skull radiograph (see Fig. 14.7). These points are traced on to an overlying sheet of paper or acetate or digitally recorded. Either method allows precise measurements to be made. As a basic system these could include:
• The outline and inclination of the anterior teeth
• The positional relationship of the mandibular and maxillary dental bases to the cranial base
• The positional relationship of the dental bases to one another, i.e. the skeletal patterns
• The relationship between the bones of the skull and the soft tissues of the face.
Main cephalometric points
The definitions of the main cephalometric points (as indicated in a clockwise direction on the tracing shown in Fig. 14.7) include:
Sella (S). The centre of the sella turcica (determined by inspection).
Orbitale (Or). The lowest point on the infraorbital margin.
Nasion (N). The most anterior point on the frontonasal suture.
Anterior nasal spine (ANS). The tip of the anterior nasal spine.
Subspinale or point A. The deepest midline point between the anterior nasal spine and prosthion.
Prosthion (Pr). The most anterior point of the alveolar crest in the premaxilla, usually between the upper central incisors.
Infradentale (Id). The most anterior point of the alveolar crest, situated between the lower central incisors.
Supramentale or point B. The deepest point in the bony outline between the infradentale and the pogonion.
Pogonion (Pog). The most anterior point of the bony chin.
Gnathion (Gn). The most anterior and inferior point on the bony outline of the chin, situated equidistant from pogonion and menton.
Menton (Me). The lowest point on the bony outline of the mandibular symphysis.
Gonion (Go). The most lateral external point at the junction of the horizontal and ascending rami of the mandible.
Note: The gonion is found by bisecting the angle formed by tangents to the posterior and inferior borders of the mandible.
Posterior nasal spine (PNS). The tip of the posterior spine of the palatine bone in the hard palate.
Articulare (Ar). The point of intersection of the dorsal contours of the posterior border of the mandible and temporal bone.
Porion (Po). The uppermost point of the bony external auditory meatus, usually regarded as coincidental with the uppermost point of the ear rods of the cephalostat.
Main cephalometric planes and angles
The definitions of the main cephalometric planes and angles shown in Fig. 14.8 include:
Frankfort plane. A transverse plane through the skull represented by the line joining porion and orbitale.
Mandibular plane. A transverse plane through the skull representing the lower border of the horizontal ramus of the mandible.
There are several definitions:
Maxillary plane. A transverse plane through the skull represented by a joining of the anterior and posterior nasal spines.
SN plane. A transverse plane through the skull represented by the line joining sella and nasion.
SNA. Relates the anteroposterior position of the maxilla, as represented by the A point, to the cranial base.
SNB. Relates the anteroposterior position of the mandible, as represented by the B point, to the cranial base.
ANB. Relates the anteroposterior position of the maxilla to the mandible, i.e. indicates the anteroposterior skeletal pattern – Class I, II or III.
Maxillary incisal inclination. The angle between the long axis of the maxillary incisors and the maxillary plane.
Mandibular incisal inclination. The angle between the long axis of the mandibular incisors and the mandibular plane.
Cephalometric posteroanterior of the jaws (PA jaws)
This projection is identical to the PA view of the jaws described in Chapter 13, except that it is standardized and reproducible. This makes it suitable for the assessment of facial asymmetries and for preoperative and postoperative comparisons in orthognathic surgery involving the mandible.
Technique and positioning (Fig. 14.9)
This can be summarized as follows:
1. The head-stabilizing apparatus of the cephalostat is rotated through 90°.
2. The patient is positioned in the apparatus with the head tipped forwards and with the radiographic baseline horizontal and perpendicular to the film, i.e. in the forehead–nose position.
3. The head is immobilized within the apparatus by inserting the plastic ear rods into the external auditory meati.
4. The fixed X-ray beam is horizontal with the central ray centred through the cervical spine at the level of the rami of the mandible (see Figs 14.9 and 14.10).
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