If you take one thing from the quality assurance topic, take this: there are two grades, not three.
Each image is rated A for diagnostically acceptable, or N for not acceptable. That is the whole scale. It came in with the second edition of the FGDP and PHE guidance in 2020, replacing the three-point scale in the first edition, and it applies to every form of dental radiography and to dental CBCT.
Why this is worth a whole article
Because a great many people were taught the three-point version, and half-remembering it is worse than never having heard of it.
Exam questions on this topic are written by people who know the scale changed. So the distractors are not random — they are the superseded system, presented in the forms you are most likely to recognise. A three-point scale written out in words. The same thing expressed as grades 1, 2 and 3. Occasionally a four-point scale, which never existed. If your instinct is to look for a middle grade, the question has already worked.
There is no middle grade to reach for. The image either answers the clinical question or it does not.
What happened to “excellent”
It stopped being a rating, and the reason is genuinely useful rather than administrative.
A CBCT image containing metal artefacts is frequently of entirely adequate diagnostic value. It is not excellent, and it never will be, and grading it against a standard of excellence tells the operator nothing they can act on. What matters is whether the image answers the question that justified taking it. A two-point scale asks exactly that and nothing else.
Once you see the reasoning, the scale stops being a fact to memorise and becomes obvious — which is the point at which it stays learnt.
The screen is part of the rating
A detail that surprises people, and one that gets examined.
When digitally acquired images are being rated, the viewing conditions matter. A suitable grey scale — window level and window width — should be chosen so bone, teeth and other tissues are shown to best advantage, and where required the display should be configured using an SMPTE or AAPM TG-18QC test pattern. Ambient lighting should be optimised too.
The purpose is precise: to avoid rating an image N when the display, not the image, is at fault. Rejecting a diagnostic image because a monitor was badly set up means a patient is re-exposed for nothing, which is the exact opposite of optimisation.
A light box, incidentally, belongs to film viewing. It is not the answer to a question about digital viewing conditions, however clinically familiar it feels.
Where this sits in the certificate
Image quality rating is the single largest block of questions in the Faults and Quality Assurance topic, and it connects to nearly everything else in it. Rating an image N is only useful if you can then say why — which is what the fault taxonomy is for, and why positioning, exposure, processing and receptor faults are each learnt as an appearance-to-cause pair.
Reject image analysis then turns those individual judgements into a pattern. One N is an incident. A run of Ns with the same cause is a process problem, and finding those is the whole point of recording them.
The Faults and Quality Assurance study guide covers the fault types and the QA programme in full, and the Projection and Technique guide covers the technique side that prevents most of them.
As ever: work from the current published guidance for anything you will act on. This scale changed once already, which is rather the point of the article.
- Dental Radiography
- Quality Assurance
- Post-registration
Frequently asked
What is the two-point image quality scale?
Each dental radiograph or CBCT image is rated either diagnostically acceptable ('A') or not acceptable ('N'). It replaced the three-point scale used in the first edition of the FGDP and PHE guidance, and applies to all forms of dental radiography including CBCT.
Why was the three-point scale dropped?
Partly because 'excellent' was not a useful category: a CBCT image containing metal artefacts is often of entirely adequate diagnostic value without being excellent. A two-point scale asks the only question that matters — does this image answer the clinical question?
Can a display screen make an image look 'not acceptable'?
Yes, which is why viewing conditions are part of the rating. Window level and width should be adjusted so bone, teeth and tissues show to best advantage, and the display configured with a test pattern where required, so an image is not rated N when the screen is at fault.
