Lead apron artifact

A lead apron artifact is the shadow a protective apron or thyroid collar casts when it ends up in the primary beam. On a panoramic radiograph an apron worn with a thyroid collar produces a radiopaque band along the lower edge, with a triangular opacity rising towards the maxillary arch. The margin belongs to the garment, not to anatomy.

Synonyms: thyroid collar artifact; thyroid shield artifact; lead shield artifact; protective apron artifact

What is a lead apron artifact?

A protective apron is draped over the patient's trunk. A thyroid collar sits over the thyroid gland. Both are lead or lead-equivalent, and both are dense enough to attenuate an X-ray beam — which is the whole point of them, and also the whole problem. Put either one where the beam is going and it stops being a shield. It becomes an object in the picture.

That puts this in the superimposition branch of the radiographic artifact family: something outside the patient's jaws, drawn onto the image because the beam had to pass through it. What makes it worth its own entry is that it is one of the few artifacts produced by doing something protective. Nobody leaves an earring in on purpose. Aprons and collars get put on deliberately, by a trained person, following what they were taught — which is why the artifact keeps recurring long after the technique faults around it have been drilled out.

What does it look like, and where on the radiograph does it appear?

There are two shapes, and which one you get depends on whether the apron carries a collar.

Writing in Decisions in Dentistry in 2020, Kiser describes an apron worn with a thyroid collar as producing a radiopaque artifact along the lower edge of the image, with a triangular opacity projecting upward toward the maxillary arch. That upward triangle is the one that costs you the diagnosis. It climbs off the lower border and into the region a panoramic radiograph is often taken to survey.

The same account describes an apron worn without a collar as producing a smaller triangular radiopacity near the lower edge, in the centre of the image. If you have only ever been shown one of these two pictures, you will miss the other — so hold both.

Both come back to placement. Kiser's account attributes the artifact to the apron being placed above the patient's clavicles, or too high on the patient's back.

The collar can also behave as a ghost image. In a series of 2,629 digital panoramic radiographs taken at a dental institution in Riyadh between August 2020 and June 2021, Lingam and colleagues illustrate a thyroid collar appearing as a ghost image in young children, and note that shield and accessory artifacts of this kind are magnified in the panoramic image.

Why does it happen on panoramic radiographs in particular?

Because of how the image is made. Ramos and colleagues, mapping artifact formation on a dry skull in 2016, put it plainly: panoramic radiography involves synchronised rotation between an X-ray source and an image receptor around a head that stays still. An object sitting outside the zone of focus is not thereby out of the picture. If it has enough density to attenuate the beam, it gets drawn in anyway, distorted.

Where it lands follows a rule. Ramos and colleagues found that an object between the X-ray source and the centre of rotation produces a ghost image: the same shape as the object, distorted, appearing on the opposite side of the radiograph and higher than the object itself, stretched vertically. An object on the other side of the centre of rotation — between it and the receptor — produces a real image instead, sharp when it lies near the middle of the image layer.

"Higher than the object itself" is the phrase to hold on to. And a thyroid collar is one of the objects that rule has been shown to catch: the Riyadh series above illustrates exactly that.

Then there is the field itself. The 2020 second edition of the PHE and FGDP(UK) Guidance Notes for Dental Practitioners on the Safe Use of X-ray Equipment notes that panoramic, cephalometric and CBCT examinations may have a field of view extending well below the mandible, and that in those cases the thyroid may unavoidably be in the primary beam. The collar and the diagnostic field are competing for the same few centimetres of neck.

None of this is a fringe observation. A 2024 simulation study of thyroid shield materials opens by noting that researchers have been looking for an alternative to the lead thyroid shield precisely so that the panoramic image carries minimal metal artifacts. And when the American Dental Association set out its 2024 position, one of its stated reasons was the image: aprons and collars, it said, can block the primary X-ray beam and prevent the dentist from capturing the image they need.

What it's confused with

The shield artifact is partly a ghost phenomenon, so the useful question is rarely "is this a ghost?" — it is "a ghost of what, and is it a ghost at all?"

On the first row, cone cut already carries this comparison from its own side, with a differential row for shielding caught in the beam. Read it there rather than here — that entry rules cone cut out for you; this one tells you what you have instead.

The metal-restoration row is not hypothetical. Ramos and colleagues report a case in which a radiopaque band over the roots of the upper right teeth turned out to be a ghost of an amalgam restoration in the upper left third molar, confirmed by a lateral cephalometric radiograph that showed nothing on the right at all. Two views settled it. One would not have.

If you find yourself weighing a second real image against a ghost more generally, what a doubled panoramic image actually means works through that separation in full.

Should a lead apron — or a thyroid collar — be used at all?

This is where remembered training and current guidance have come apart, so every sentence below carries the body that said it and the year it said it. There is no single "the guidance says" here.

PHE and the FGDP(UK), 2020. Their second-edition Guidance Notes state there is no justification for the routine use of protective aprons for patients undergoing any form of dental radiography or dental CBCT imaging, since the main beam should never be directed towards the abdomen, leaving dose to the patient or the foetus from scatter and leakage negligible. On thyroid shields they say something different, and the difference matters: for intra-oral, panoramic and cephalometric radiography, equipment, exposure factors, technique and collimation have an equal or greater influence on thyroid dose than a shield does, so those measures should be advocated in preference to one — but a thyroid shield should nevertheless be used in any situation where the thyroid may unavoidably be in the primary beam, on the advice of the medical physics expert. The document's own summary row reads: protective aprons and thyroid shields are not recommended for routine use on patients. Not routine. Not never.

The American Academy of Oral and Maxillofacial Radiology, 2023. Its position statement in JADA recommends discontinuing shielding of the gonads, pelvic structures and fetuses during all dentomaxillofacial radiographic imaging procedures, and recommends that thyroid shielding not be used during intraoral, panoramic, cephalometric or cone-beam CT imaging.

The American Dental Association, February 2024. An expert panel established by the ADA Council on Scientific Affairs stated that lead abdominal aprons and thyroid collars are no longer recommended for patients having dental radiographs, and that this applies to all patients regardless of age or health status, pregnancy included. It remains the ADA's stated position: its own topic page, last updated in March 2026, says thyroid collars are no longer recommended for any imaging procedure in dentistry.

All three move away from routine use. They are years apart, and they do not say the same thing — the 2020 UK guidance keeps a thyroid shield in reserve for the case where the thyroid is unavoidably in the beam, and the 2024 ADA position does not.

What goes in the shield's place is the same in each: limit the field rather than block it. The UK guidance puts rectangular collimation at about a 50% reduction in effective dose compared with circular collimation — that figure is for periapical, bitewing and occlusal radiography, not for panoramic — and for panoramic and cephalometric work says the field size should be chosen to image only the area of clinical interest. The ADA points to the same things: rectangular collimation, correct positioning, and dose-reduction procedures.

It was argued on measurements first. A 2017 phantom study by Schulze and colleagues, across five digital panoramic units, found skin-level doses very low throughout but significantly higher at the radiation-sensitive female breast region when no apron was used. It predates the AAOMR and ADA statements, which weighed organ dose and risk rather than skin entrance dose.

And the law may not have moved with the guidance. In California, Title 17 of the Code of Regulations, section 30311(b)(6), still states that each patient undergoing dental radiography shall be draped with a protective apron of not less than 0.25 millimetre lead-equivalent to cover the gonadal area. The California Dental Association wrote in December 2023 that section 30311 still stands and that it was discussing regulatory changes with the state health department. The AAOMR's own statement asks that state and local authorities be contacted to update regulations to reflect its recommendations, and the ADA's recommendations include adhering to all applicable federal, state and local regulations on radiation safety.

So: professional guidance on both sides of the Atlantic has moved away from routine shielding, and at least one state's regulation has not yet followed. That is a real tension, not a misunderstanding, and it is not this page's to resolve. Find out what your own regulator requires.

How to be sure

Start with where the garment was, not with the shadow. Kiser's account ties the artifact to an apron placed above the clavicles or too high on the back, so the first check is physical rather than radiographic.

If a shield was worn and the field extended low, the 2020 UK guidance is explicit that for all extra-oral imaging the operator should ensure, as far as possible, that the thyroid shield is outside the primary beam — and it gives the reason as the diagnostic quality of the image, not just dose. That instruction is also a diagnostic test: if the shield was in the beam, it is in the picture.

For anything you still cannot place, Ramos and colleagues' recommendation applies — remove metal objects from the head and neck region before exposure. An opacity that disappears when the garment is repositioned was the garment.

And where the shield is not being used at all, under whichever guidance applies to you, this particular artifact simply stops arising.

Common errors

Reading the triangle as pathology. An upward-projecting opacity over the posterior mandible or the maxillary arch invites a lesion-shaped reading. Telling an artifact from pathology is the general skill; this is one of its cleanest cases, because the answer is standing in the room.

Calling it a cone cut. The shield's shadow has the garment's shape and the garment's position; the cone cut entry linked above sets out the separation from its own side.

Learning only one of the two shapes. Collar and no-collar give different pictures. A reader who has memorised the band-plus-triangle will not recognise the small central wedge.

Flattening the guidance. "We don't use aprons any more" is not what any of the three documents above says, and two of them are careful about the thyroid specifically. If you are going to quote a position, quote the body and the year.

Under-weighting it in children. In the Riyadh series, ghost images as a class — the category that includes shield, hair-clip and jewellery artifacts — were recorded on 2.20% of radiographs and were significantly more common in children aged 5 to 12 than in other age groups. That figure is for the whole class rather than for aprons alone. The paper's own illustration of the effect, though, is a thyroid collar on a young child.

None of this is a substitute for a system. If you are still building one, a working method for reading a radiograph comes first; artifacts like this one are what a method catches. The panoramic positioning errors entry covers the faults that come from the patient's head being in the wrong place, which is a different problem from an object in the beam.