Cephalometric X-Ray (Lateral Ceph): Uses, Setup and Dose

A cephalometric x-ray, or lateral ceph, is a standardized side view of the skull taken with the head held in a cephalostat. It is the image on which every cephalometric analysis is traced, and it is one of the classic orthodontic records alongside the panoramic radiograph, photographs and study models. This guide is about the radiograph itself: when it is justified, how the patient is positioned, why the image is magnified and how to calibrate it, what dose it delivers compared with other dental images, and what to look at before you place a single landmark. Tracing and norms are covered in our cephalometric tracing guide.

At a glance

What a cephalometric x-ray is

"Ceph", "cephalogram", "lateral cephalometric radiograph" and "lateral ceph x-ray" all name the same image. The cephalostat (sometimes called a cephalometer) is the head holder: two ear rods enter the external auditory meati, a nasal positioner rests on soft-tissue nasion, and together they fix the head relative to the x-ray source and the sensor. Broadbent introduced the standardized technique; the date usually cited is 1931, while our university course materials give 1926.

The point of the cephalostat is reproducibility: with the head, source and sensor always in the same geometry, two cephalograms taken months apart can be superimposed, and the changes attributed to growth or treatment rather than to a different projection.

When is a cephalometric x-ray indicated?

Our D.U. course materials list the lateral ceph as part of the standard orthodontic records, used to assess:

• the skeletal relationships in the sagittal and vertical planes, and the incisor axes;
• the signs of mandibular rotation described by Björk, which hint at the growth direction;
• head posture, the soft-tissue profile and the airway;
• progress and outcome, by superimposing successive cephalograms.

Two additional situations come from the course materials. In a Class III, a cephalogram in centric relation as well as in maximum intercuspation quantifies a possible anterior functional shift. Before a palatal mini screw or implant, the line seen on the ceph is the nasal floor, and the bone actually available in the anterior and middle thirds rises at least 2 mm higher, hence a 2 mm safety margin (EMC).

Justify it case by case

Every exposure must be justified. Radiation-protection authors put it bluntly: take a radiograph only when there is a patient-specific reason to expect information that will influence diagnosis or treatment (White and Mallya, 2012, Aust Dent J).

This matters because the evidence on how much the ceph changes treatment plans is mixed:

• adding cephalometrics to casts did not change the consistency of 14 clinicians’ decisions on functional appliances, expansion or extractions in 48 cases aged 11 to 14 (Nijkamp et al., 2008, Eur J Orthod);
• for 114 British orthodontists planning 6 cases, the ceph and its tracing changed most decisions very little, but did change the extraction decision for one patient with a Class I incisor relationship on a Class II skeletal base (Devereux et al., 2011, AJODO);
• a systematic review of 17 articles concluded that evidence on the usefulness of lateral cephalometry for treatment planning was still lacking and contradictory (Durão et al., 2013, Prog Orthod).

So the teaching, where the ceph is a standard record, diverges from studies that question its routine use. These studies looked at planning decisions, not at screening or growth follow-up. Know the question the ceph should answer, and follow the prescribing rules where you practice.

Patient positioning: the cephalostat setup

Good positioning is what turns an x-ray into a measurable record. The course materials describe the following conditions:

1. Ear rods in both ear canals, without forcing, so that the head cannot rotate around a vertical axis.
2. Frankfort plane horizontal (porion to orbitale), and the midsagittal plane parallel to the sensor.
3. Central ray perpendicular to the sensor, with a small focal spot, a long source-to-patient distance and the shortest possible patient-to-sensor distance.
4. Teeth in maximum intercuspation, the usual position for the profile. Note any other position (centric relation, open mouth).
5. Lips at rest: tense lips distort the profile. Course materials also describe lips-together views; record which one you used.
6. Calibration ruler visible, usually on the nasal positioner.
7. Thyroid shield and a field limited to the area of interest, as long as they do not hide the structures you need.

One exception: for an airway analysis, the EMC prefers a radiograph in natural head position, ideally without the cephalostat, because head extension or flexion changes the pharyngeal dimensions.

What happens when the head is rotated

Head rotation does not affect all measurements equally. On a dry skull rotated up to 14° around a vertical axis, horizontal linear measurements and angles between horizontal planes on the lateral ceph changed by 16.1% to 44.7%, while vertical linear measurements were more robust (Malkoc et al., 2005, Eur J Orthod). This was a single skull with a large rotation, but the direction of the error is useful to remember.

In clinical cephalograms, the impact of a slight positioning error was smaller. In 104 patients, two observers traced the Tweed-Merrifield measurements twice; head rotations and inclinations had little effect on reliability, and the authors concluded that averaging the positions of bilateral structures allows a reliable analysis despite imprecise posture, so that retaking the image for that reason alone is ethically questionable (David et al., 2017, Radiol Med).

Magnification and calibration

X-rays leave from a point source and diverge, so the image of the head is always slightly larger than the head. Our course materials give about 7.14% magnification with the source at 1.5 m and about 1.9% at 5 m. Moving the sensor closer to the patient, or the source farther away, reduces it.

Three consequences for practice:

• Angles are not affected by magnification. SNA, ANB, FMA or IMPA can be read on an uncalibrated image.
• Millimeter values are. At 7%, a distance of 50 mm on the patient measures about 53.5 mm on the image. U1-NA and L1-NB in millimeters, the Wits appraisal, facial heights and the airway distances all need calibration before they are compared with norms.
• Comparisons need the same setup. To superimpose two cephalograms, use the same machine and the same distances, or calibrate both.

Why bilateral structures look doubled

Structures on the side of the face farther from the sensor are magnified more than those on the near side. That is why the two mandibular borders, the two orbits and the two ear canals often do not superimpose. A small offset is expected. A marked double contour should make you check the head position first, then consider an asymmetry. When the two mandibular contours do not cross, the usual convention is to trace the midline between them and note the finding.

On screen, calibration means clicking the two ends of a known length on the ruler and entering it. If the ruler is not visible, keep to angles and ratios, and say so in the report.

Image quality: what to check before tracing

Before measuring anything, check that the cephalogram is fit for purpose:

• Identity, date and the position used (intercuspation, centric relation, lips at rest or together).
• Sharpness and contrast on bone and on the soft-tissue profile at the same time; a burned-out nose and chin make the esthetic lines impossible.
• Ruler visible, no motion blur, no jewelry over the areas of interest.
• Field of view covering sella, nasion, the whole mandible, the upper cervical vertebrae and the soft-tissue profile, without unnecessary extension.

Field size is often too generous. In 118 lateral cephs of children aged 7 to 12, the anterior limit was always appropriate, but the field was too large posteriorly in 54%, superiorly in 86% and inferiorly in 76%, and a thyroid shield had been used in only 71% of cases (Pakbaznejad Esmaeili et al., 2015, Eur J Orthod).

Radiation dose: how much is a ceph x-ray?

The effective dose of a lateral ceph is low, but published values vary with the device, the sensor, the field and the tissue weighting factors used in the calculation. The figures below are therefore orders of magnitude, not values for your own machine.

The weighting factors matter: in one phantom study, a full orthodontic set (with a complete periapical survey) was 17.5% of the CBCT dose under the 1990 ICRP factors but 47.2% under the 2007 factors (Chinem et al., 2016, Dental Press J Orthod).

Keeping the dose down

• Justify, then optimize: the ALARA principle (as low as reasonably achievable) applies to every image, including low-dose ones.
• Digital sensors: direct digital cephalometry halved the effective dose compared with film-screen in the Visser study.
• Lower exposure settings: with storage phosphor plates at halved milliampere-seconds, landmark visibility did not differ significantly from film-screen at low tube potential (Kaeppler et al., 2007, Dentomaxillofac Radiol).
• Collimation and thyroid shield, adjusted to the child’s size.
• No routine repeat for a minor positioning error if bilateral structures can be averaged (David et al., 2017).

Reading a cephalometric x-ray: look before you measure

The first reading is qualitative. Our course materials, following Boileau, ask for a general screening before any measurement:

• Sella turcica: an abnormal shape or size (pituitary).
• Cranial base and vault: basilar impression, digital impressions of the vault (a possible mild craniosynostosis).
• Cervical vertebrae: malformation or agenesis, and their shape for maturation staging (see our French article on cervical vertebral maturation).
• Any unusual image, such as a possible tumor: it calls for a specialist opinion, whatever the measurements say.

Then the functional and soft-tissue signs:

• Airway: adenoids, tonsils, hyoid position. The absence of contact between the soft palate and the tongue suggests mouth breathing. A ceph is a 2D image of a 3D airway; a single measurement is never enough to conclude.
• Tongue: low (mouth breathing, short frenum) or high and posterior (often seen in Class II division 2).
• Head posture (extension in mouth breathers) and growth pattern (Björk’s structural signs of mandibular rotation).

Only then does the tracing begin: landmarks, planes, measurements, and a final check against the clinical examination. Landmark definitions are detailed in our guide to cephalometric landmarks, the sagittal reading in our ANB angle article and the full analysis in the Steiner analysis guide.

What a lateral ceph cannot tell you

• It flattens three dimensions into two: transverse problems and asymmetries need a frontal view or 3D imaging.
• It localizes impacted teeth poorly, because two hemi-arches overlap; the course materials favor periapical parallax views and, when needed, 3D imaging.
• Its norms come from other populations: a value outside the norm is a sign to explain, not an indication for treatment.
• Landmark error is real, especially at porion, orbitale, gonion and the incisor apices.

In practice

1. Write down the question the ceph should answer before you prescribe it.
2. Position the patient in the cephalostat: ear rods, Frankfort horizontal, intercuspation, lips at rest, ruler visible.
3. Limit the field and use a thyroid shield adapted to the patient.
4. For a Class III, add a view in centric relation; for an airway question, consider natural head position.
5. Check identity, sharpness, ruler, field and double contours before tracing.
6. Read the image qualitatively first: sella, vault, vertebrae, airway, tongue, any unusual image.
7. Calibrate before any millimeter measurement; keep the same machine for superimpositions.
8. Interpret the measurements against the clinical examination, never alone.

Frequently asked questions

What does a cephalometric x-ray show?

It shows the skull, the jaws, the teeth and the soft-tissue profile from the side. From it, the clinician assesses how the maxilla and the mandible relate to the cranial base and to each other, how the incisors are inclined, the vertical facial pattern, the airway and the cervical vertebrae. It is the basis of cephalometric analyses such as Steiner and Tweed.

How much radiation is a ceph x-ray?

Phantom studies report effective doses of about 1 to 5 microsieverts for a lateral ceph, lower with digital sensors (Visser et al., 2001; Ngan et al., 2003). That is of the same order as or lower than a panoramic radiograph and far below a CBCT. The exact value depends on the machine and the settings, and every exposure must still be justified.

Is a cephalometric x-ray needed for every orthodontic patient?

University courses teach it as a standard record, especially for growth assessment and treatment follow-up. Several studies found that it changes few treatment-planning decisions, and a systematic review judged the evidence insufficient (Durão et al., 2013). The decision is case by case, according to the clinical question and local prescribing rules.

Can a CBCT replace a lateral ceph?

A CBCT gives 3D information that a ceph cannot, but standard CBCT scans deliver a higher dose. In one phantom study, even with shielding and a smaller field, standard CBCT scans exceeded the dose of a panoramic plus a lateral ceph; only shielded low-dose protocols fell below it (Ting et al., 2020, Angle Orthod). CBCT is chosen when the question needs 3D, not as a routine replacement.

With GS Ortho

In GS Ortho, AI places 29 landmarks on the lateral cephalogram, soft-tissue profile included, and the Steiner and Tweed analyses follow. You check each landmark, correct it if needed, and validate: the reading stays yours. The ceph sits in the diagnostic studio’s 12 panels, next to the clinical examination, the photographs and the treatment plan of the same patient. To practice, the free online cephalometric analysis lets you calibrate a ceph and place 15 landmarks without sign-up, and the image stays on your computer. The GS Ortho interface is currently available in French.

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