What Is a PA X-Ray in Dentistry and When Is It Needed?

A periapical (PA) X-ray is a small intraoral radiograph designed to capture an entire tooth from crown to root tip, along with a sliver of the surrounding bone. It is the most commonly used image in endodontics and one of the workhorses of general dentistry, ordered whenever your dentist needs to see what is happening below the gumline around a specific tooth. The name comes from “peri” (around) and “apex” (the tip of the root), and that root-tip view is exactly what sets it apart from other dental X-rays. While the image itself is straightforward, knowing when a PA is genuinely needed and when another type of image would serve you better involves more nuance than most patients realize.

What a PA X-Ray Actually Shows

A PA radiograph produces a two-dimensional, flat image of one to three teeth and the bone immediately surrounding them. Because the sensor or film sits inside your mouth, pressed against the palate or the floor of the mouth behind the teeth, the X-ray beam passes through the jaw at close range. That proximity gives PAs their main advantage: high resolution of fine anatomical detail. You can see the internal canals inside a root, the thin ligament space that cushions the tooth in its socket, and the contour of bone at the root tips. Cysts, abscesses, fractures, and resorption all show up as changes in the density or shape of these structures.

The trade-off is that you are collapsing a three-dimensional structure into two dimensions. Roots that overlap in the image can hide pathology behind them, and a lesion on the cheek side or tongue side of a root may be invisible because it is superimposed over healthy bone. A systematic review of periapical X-ray accuracy in endodontic diagnosis found that only two of sixteen included studies supported the idea that periapical conditions can be reliably assessed through PA imaging alone, underscoring that PAs are powerful but far from infallible.¹

How the Image Is Taken

Two main techniques exist for positioning the sensor and aiming the X-ray beam. The paralleling technique places the sensor parallel to the long axis of the tooth, with a holder keeping it at a fixed distance, and the beam is aimed perpendicular to both. The bisecting-angle technique tilts the beam so it bisects the angle between the tooth and the sensor, which sits closer to the tooth. A systematic review comparing the two found that the paralleling technique produced images closer to the true tooth length, with an average discrepancy of about 1.4 mm versus 2.2 mm for the bisecting-angle method.² That difference matters when your dentist is measuring a root canal or assessing how far bone loss extends.

In practice, the paralleling technique is the standard in most clinics because it produces less geometric distortion. The bisecting-angle method is sometimes used as a fallback when anatomy gets in the way, such as a shallow palate that will not accommodate a sensor holder, or when a patient cannot tolerate the sensor positioned far back in the mouth.

Root Canal Treatment

Endodontics is where PA X-rays earn most of their reputation. Before a root canal, your dentist needs to confirm that infection or inflammation has reached the root tip, to count how many canals the tooth has, and to estimate how long each canal is. During the procedure, PAs are taken with small measuring instruments inside the canals to verify depth. Afterward, a PA confirms that the filling material reaches the right level.

When it comes to detecting infection around the root tip, PAs perform reasonably well but miss a meaningful share of cases. A study using histological examination as the gold standard found that PA radiographs detected apical periodontitis in about 71% of roots, compared with 84% for cone-beam computed tomography (CBCT) and 93% confirmed under the microscope. Sensitivity was 0.77 for PAs and 0.91 for CBCT, though both had perfect specificity, meaning neither falsely flagged healthy roots as diseased.³ So PAs catch most infections, but roughly one in four lesions that are actually present may not show up on the image.

For measuring canal length during the procedure, electronic apex locators have become the primary tool, but research consistently advises using them in conjunction with radiographs rather than relying on either method alone.⁴ The PA gives a visual cross-check that the electronic reading makes anatomical sense.

Gum Disease and Bone Loss

Periodontal disease destroys the bone that holds teeth in place, and you cannot see that bone with a mirror. PA X-rays show the height and shape of the alveolar bone between and around roots, making them a standard part of periodontal assessment. Diagnosis of periodontal and peri-implant diseases relies heavily on radiographic examination for assessing bone levels, bone defect morphology, and bone quality, though clinical probing and intraoral radiography both have limitations when it comes to complex bone defects.⁵

Automated systems are being developed to measure bone loss on periapical images, with the goal of catching early-stage disease and tracking the bone’s response to treatment over time.⁶ For now, though, the standard approach is a series of PAs covering the full mouth, sometimes combined with bitewing X-rays to get the clearest possible view of the bone crest between teeth.

Dental Trauma and Fractures

If you chip, crack, or knock a tooth loose, one of the first things your dentist will do is take a PA. Root fractures, in particular, are invisible to the naked eye, and a PA taken from two slightly different vertical angles can reveal a fracture line running through the root. Research comparing periapical radiographs with CBCT for root fracture detection found that sensitivity and specificity were good for both modalities, and that PA radiographs taken at two different angulations can be considered an accurate method for detecting root fractures.⁷

One practical concern in trauma settings is image storage and compression. Digital PAs are often saved as compressed JPEG files for easier transfer between clinics or insurance systems. A study testing whether JPEG compression affects the ability to spot root fractures found no significant difference in diagnostic accuracy between uncompressed TIFF images and compressed JPEG files, even when file size dropped from 1.77 MB down to 95 KB.⁸ That means the images your dentist emails to a specialist or uploads to your insurance portal retain their diagnostic value.

Implant Monitoring

After a dental implant is placed, the bone needs to grow onto the implant surface, a process called osseointegration. PA X-rays are the go-to tool for tracking this. Evidence-based guidelines recommend conventional imaging such as periapical or panoramic radiography as the first choice for assessing an implant following placement and during the osseointegration period.⁹ PAs can show changes in bone density and trabecular thickness around the implant over time, with measurable differences appearing as early as a few weeks after surgery.¹⁰

The limitation, as with all PA uses, is the two-dimensional view. Periapical and panoramic radiographs are commonly used for implant assessment but remain limited by their flat nature.¹¹ Bone loss on the cheek-facing or tongue-facing side of an implant will not be visible until it becomes severe enough to alter the overall outline. For pre-surgical planning, where the dentist needs to know the exact width and depth of available bone, a three-dimensional CBCT scan is typically ordered instead. But for routine follow-up visits, PAs are cheaper, faster, and deliver far less radiation.

PA X-Rays Versus Other Dental Imaging

Patients often wonder why they sometimes get one type of dental X-ray and sometimes another. The answer usually comes down to what the dentist is looking for and where in the mouth they need to look.

Bitewing X-rays are optimized for catching cavities between teeth, especially in the early stages. When researchers compared bitewing and periapical images for detecting early interproximal cavities, bitewings had significantly higher sensitivity at every level of cavity progression, along with higher predictive values and a larger area under the curve.¹² If your dentist specifically wants to screen for cavities between your back teeth, a bitewing is the better tool. But if the concern is about the root or the bone below the gumline, the PA wins because bitewings crop that area out of the frame.

Panoramic X-rays capture the entire jaw in a single wide image. They are useful for getting a broad overview: wisdom-tooth positioning, large cysts, jaw fractures, or orthodontic planning. For detecting infections at the root tip, a meta-analysis found that PA radiographs averaged about 71% diagnostic accuracy compared with 66% for panoramic images. The advantage was most pronounced in the upper jaw and the lower front teeth, while in the lower premolar and molar regions, the two were similarly accurate.¹³ An older study also found PAs significantly superior for certain lesion types in the upper jaw, though overall the two modalities were not dramatically different by ROC analysis.¹⁴

CBCT scans provide a full three-dimensional view and are the most detailed option. They are increasingly used in complex endodontic and surgical cases. However, for many routine scenarios, the extra information does not change the treatment plan. One clinical study found no significant difference in the treatment decisions clinicians selected when using CBCT versus conventional PA radiographs for periapical lesions.¹⁵ CBCT also comes at an increased cost in terms of radiation dose, equipment, and additional training required to interpret the images.¹⁶ The general principle in dental radiology is to start with conventional imaging and escalate to CBCT only when the two-dimensional picture does not provide enough information.

Radiation Dose

A single PA X-ray delivers a very small dose of radiation. In a study measuring exposure from dental radiographs in children and adolescents, the effective dose for an intraoral dental or bitewing radiograph was about 0.77 microsieverts, a value consistent with other published estimates.¹⁷ To put that in perspective, you receive roughly 8 microsieverts of background radiation just from living on Earth for a single day. A full-mouth series of about 18 to 20 PAs still totals less than a day’s worth of natural background exposure.

Dose reduction strategies have evolved alongside the technology. Rectangular collimation, which narrows the X-ray beam to the size of the sensor rather than using a wider round cone, cuts scatter radiation substantially. A study on thyroid shielding found that switching from round to rectangular collimation reduced thyroid dose by a factor of four for upper canine exposures. In positions where rectangular collimation was already in use, a thyroid shield added only about 10% further reduction, a margin the researchers considered clinically insignificant.¹⁸ Many dental offices have moved away from routine thyroid collar use for most PA positions, relying on rectangular collimation as the primary dose-reduction measure.

Digital Sensors Versus Film

Most dental offices now use digital sensors instead of traditional film. Digital imaging requires roughly half the radiation of conventional film to produce diagnostically equivalent images.¹⁹ Beyond the dose savings, digital images appear on screen within seconds, can be brightened, darkened, or magnified without retaking the shot, and are easy to store and share electronically. The transition has been so thorough that film-based PA radiography is now uncommon in newly equipped practices, though some clinics in resource-limited settings still rely on it.

Digital sensors do have one practical downside: they are rigid and slightly thicker than film packets, which can make them less comfortable for the patient, especially in the back of the mouth or in patients with small jaws. Phosphor plate systems, which use a thin flexible plate that is scanned after exposure, split the difference between film’s flexibility and a wired sensor’s instant display, though they require a separate scanning step.

When Patients Struggle With the Procedure

For something that takes only a few seconds of actual exposure time, PA X-rays can be surprisingly difficult for certain patients. The most common complaint is gagging. The sensor or film needs to sit well back in the mouth for posterior teeth, and for some people that triggers a strong reflex. Films and digital sensors are among the factors that can provoke this response, and patients with severe gag reflexes may be unable to tolerate them during procedures that require multiple PAs, such as root canal treatment.²⁰

Several workarounds exist. Nitrous oxide sedation has been studied specifically for this purpose. Researchers found that administering nitrous oxide at concentrations up to 70% allowed every tested subject with a hypersensitive gag reflex to tolerate a size-2 digital sensor placed in the back of the mouth for the time needed to take the image.²¹ Extraoral techniques, where the sensor is placed against the outside of the cheek and the beam is angled through the jaw, offer another option, though image quality is not quite as sharp. Topical anesthetics sprayed on the palate, distraction techniques, and simply using a smaller sensor size are other strategies dentists employ before resorting to sedation.

PA X-Rays in Children

Pediatric dentists order PAs for many of the same reasons as adult dentists, but with additional considerations. Baby teeth have thinner enamel and shorter roots, and infections can progress faster to the root tip. A retrospective study of primary first molars found that pathological root resorption was strongly associated with age, appearing in roughly 31% of children aged six to eight compared with about 9% of those under six. Deep cavities involving the pulp and the presence of restorative materials were also significantly correlated with both abnormal resorption and periapical infection.²² PAs are often the only way to see whether a deep cavity in a baby tooth has reached the point where extraction, rather than a filling, is the appropriate treatment.

Radiation concerns are slightly more relevant in children because growing tissues are more sensitive. However, the dose per PA remains extremely low, and using digital sensors, rectangular collimation, and appropriate exposure settings minimizes it further. The guiding principle is justification: a PA should be taken only when the clinical question cannot be answered by looking in the mouth or by using an image already on file.

Artificial Intelligence Reading PA X-Rays

AI-assisted interpretation of dental radiographs is moving from research into commercial products. One system tested on periapical radiographs correctly identified periapical lesions with a sensitivity of about 92% and identified healthy teeth with a specificity of roughly 98%, yielding an overall accuracy near 97%.²³ Those numbers are encouraging, though this particular study used a relatively small test set and the algorithm was evaluated on images from a single institution. Larger, multi-center validation is still needed before AI reading can be considered a standalone diagnostic tool.

Where AI is most likely to help in the near term is as a second set of eyes. A dentist reviewing a PA can miss a subtle lesion, especially on a busy day with dozens of images to evaluate. An AI flag that says “this root tip looks suspicious” adds a safety net without replacing clinical judgment. Some software packages already integrate this feature into the image-viewing workflow, highlighting areas of concern on PAs and panoramic radiographs automatically as soon as the image is captured.