Periapical radiograph

A periapical radiograph is an intraoral view showing a whole tooth — crown, root and apex — together with the bone immediately around the root tip. That field of view is what it's for: questions about the root and the bone around it. Two techniques produce it, paralleling and bisecting-angle.

Synonyms: PA radiograph; intraoral periapical; periapical x-ray

What does a periapical radiograph show?

The whole tooth, and the bone at the end of it. A periapical radiograph visualises the tooth up to the root apices and the peri-radicular region — which is why a 2023 systematic review calls it intuitively suitable for many indications in endodontics, those needing the complete root system in one image.[5]

That's the useful half. The other half is what it doesn't show, and you should know the size of it before you trust a clear apex.

In one retrospective study of 339 teeth (796 roots) diagnosed with irreversible pulpitis or pulp necrosis and scheduled for primary root canal treatment, two calibrated expert endodontists read periapical radiographs against CBCT as the reference standard. They found 65.3% of the apical radiolucencies CBCT identified. Their specificity was 97.7%.[2]

Read those two figures together, because they say different things. The specificity says they rarely called a radiolucency on an apex CBCT showed to be sound. The sensitivity says something weaker: when they called an apex clear, roughly a third of the lesions CBCT found were not visible to them on the periapical view.

What are the paralleling and bisecting-angle techniques, and how do they change the image?

They differ in where the receptor sits and where the beam is aimed.

Paralleling. The receptor is placed parallel to the long axis of the tooth, and the central beam is directed perpendicular to both the receptor and the tooth.[5]

Bisecting-angle. The receptor rests as close to the palatal or lingual surface of the tooth as it will go. The beam is then aimed perpendicular to an imaginary line bisecting the angle formed between the long axis of the tooth and the receptor.[5]

The received view, which the literature has carried since its earlier works, is that paralleling is more reproducible and produces less distortion.[5] The 2023 systematic review of the two techniques found most of its included studies advocating paralleling for general practice, endodontics and implantology, with bisecting-angle advocated for patients with a low palatal vault — in that group, paralleling had the higher retake rate.[5]

Worth knowing how thin the ground under that preference is. The same review found only 26 studies comparing the techniques, published between 1976 and 2021, and judged all of them to sit at Oxford CEBM Level 3 or Level 4.[5] The preference is consistent. The evidence behind it is old and low-level, and the review says so itself.

Why do the teeth look too long or too short?

Because the geometry between tooth, receptor and beam decides the length of the shadow, and neither technique reproduces it exactly.

The clearest measurement of that comes from one study within the 2023 review, comparing image length against actual tooth length. Paralleling gave a mean difference of +1.4 mm; bisecting-angle gave +2.2 mm.[5]

Vertical angulation is the lever. As the vertical angulation of the central beam increased, the error in judging lesion size increased for both techniques.[5] So a steep beam doesn't only distort the tooth; it distorts your reading of what's at the apex.

The two techniques also fail in different places, and the literature doesn't fully agree on where. In one included study, paralleling produced far fewer vertical-angulation errors (1.1–2.6% against 11.8%) and far fewer cone cuts (1.0–0.7% against 10.2%), but a higher rate of improper receptor positioning (25.6–28.4% against 18.5%).[5] A second included study found the opposite on that last point, reporting that paralleling produced significantly fewer placement errors as well as fewer angulation errors and cone cuts, with a diagnostic unacceptability rate of 16.7% against 48.6% for bisecting-angle.[5]

Take the disagreement at face value. Paralleling's advantage on beam angulation is consistent across both. Its effect on receptor placement is not settled.

What it's confused with

Those first two rows come from a post-treatment recall population — teeth followed up after endodontic retreatment or endodontic microsurgery, where apices are healing and the bone is remodelling. Don't carry the percentages onto an untreated tooth. Carry the shape of it: the periapical view errs in both directions — it misses lesions CBCT finds, and it shows lesions CBCT does not.

When do you take a periapical?

The decision follows the patient, not a schedule.

The 2026 ADA and AAOMR consensus recommendations state that a thorough evaluation of the patient history and clinical findings should precede radiographic examinations, and that clinicians should base imaging decisions on the patient's medical and dental histories, clinical examination findings, disease risk assessment, and the presence of specific clinical conditions.[1] Previously obtained images should be reviewed, and all modalities — cone-beam CT especially — used judiciously to limit cumulative exposure.[1]

Two things follow for a student. The examination comes first and the radiograph answers a question it raised. And if you were taught the 2012 ADA/FDA patient-selection tables, those have been replaced: the 2026 document is an update to them.[1] Note that the panel issued consensus recommendations rather than formal guidelines, because the available evidence was limited.[1]

Beyond that principle, the periapical's own territory is the root system: the review judges it intuitively suitable for endodontic indications that need the complete root visible in one image.[5] Where the question is about the crown or the bone crest, a bitewing radiograph is answering a different question — and how early proximal caries shows on a bitewing is a separate skill with its own thresholds.

Common errors

Reading a clear apex as a healthy one. The specificity figure is the reassuring one; the sensitivity figure is the one that should change your behaviour.[2] A negative periapical view is weak evidence of absence.

Reading every apical radiolucency as a lesion. Not every dark area at an apex survives confirmation.[3] The habit of asking what else could produce this shadow is the same habit that keeps cervical burnout from being read as caries, and it's worth building on both.

Chasing length with vertical angulation. Steepening the beam to fit the apex on the receptor costs you accuracy at the apex, which is the thing you were trying to see.[5]

Treating "paralleling is better" as universal. It isn't, in the one place the review names: for a patient with a low palatal vault, paralleling had the higher retake rate, and bisecting-angle was the technique advocated.[5]