Conventional dental radiographs and cone beam scans are both produced with x-rays, yet they answer different questions. The difference lies in how the image is assembled.
A radiograph is a shadow
In conventional imaging, x-rays pass through the head from one direction and are recorded on a sensor behind it, producing a single projection.
Everything along a given path is superimposed into one point on that image, so structures in front of and behind each other overlap.
Depth is therefore lost. A radiograph shows the height of a structure reasonably well and gives no direct information about its position front to back.
Cone beam imaging reconstructs a volume
The source and detector rotate around the head, capturing many projections from different angles during a single sweep.
Software then reconstructs the internal densities that would produce that set of projections, generating a three-dimensional dataset.
The volume can be sliced in any plane afterward, so structures can be viewed separately rather than superimposed, and distances measured in any direction.
The differences that matter clinically
Implant planning needs bone width and the position of the nerve canal or sinus, neither of which a flat image supplies reliably.
Root canal anatomy, root fractures and the relationship of unerupted teeth to their neighbours are similarly three-dimensional questions.
Routine decay detection between teeth remains well served by conventional images, which are quicker to take and deliver a lower dose. The two are therefore used for different purposes rather than one replacing the other.
Dose is the constraint on use
A cone beam scan delivers a higher radiation dose than a conventional dental radiograph, though the amount varies considerably with the size of the volume captured.
Limiting the field of view to the region of interest reduces dose and also improves resolution, which is why small-volume scans are preferred where they suffice.
Justification is a formal requirement in most jurisdictions, meaning a scan is taken only where the information will change what is done.
Interpretation extends beyond the teeth
A volume includes structures outside the immediate area of interest, and findings unrelated to the reason for the scan appear on a proportion of them.
That places a responsibility on the clinician to review the whole volume or refer it for reporting, rather than examining only the region of interest.
Whether any imaging is appropriate for an individual is a clinical decision based on history and examination, and it is made case by case.