Radiographs, or “x-rays,” have been used in dentistry for nearly as long as the profession has existed. They look at the roots of teeth for abscesses, assess bone levels around the teeth, and look for cavities between the teeth. The technologies available have significantly improved from high-dose film, to high-speed film, to digital sensors, and now to conebeam computed tomography (CBCT).

dental x rays radiography

A brief overview of conventional vs. digital radiography

In general, conventional radiographs require higher doses of radiation (measured in microsieverts) than do digital x-rays to get the same quality of image, but there is debate over this concept.

Digital radiographs may theoretically provide less radiation for a variety of reasons. The initial push for the “less radiation” benefit came from marketing that was done to promote digital sensors, with some questionable evidence provided by the sensor manufacturer. While some sensors may be more sensitive than others (just like E-Speed film is faster than D-speed), the fact remains that the tissues that the radiation must penetrate are the same as they are for conventional radiographs.

Digital sensors are not all the same, and phosphor plates (which some consider a form of digital radiographs) require similar doses of radiation as E-speed conventional film. Radiation exposure with intraoral radiographs can be minimized by using a rectangular collimator.1

Advantages of digital radiography

One of the biggest advantages of digital radiographs is convenience and efficiency. I can’t imagine having to wait for film to develop again! 

Whether intraoral radiographs are digital or conventional, they are currently considered standard for evaluation of interproximal decay, crestal bone levels, and periapical pathology. 

Digital panoramic radiographs have similar advantages over conventional film. One of the benefits of digital images, be they phosphor plate or sensor-captured, is the ability to manipulate the image for brightness, gray scale and colorization. To some extent, this can also be done if a conventional image is scanned with a flat-bed scanner that enables scanning negative images.

Panoramic radiographs are appropriate for screening to rule out gross pathology but not for ruling out interproximal caries or periapical pathology (although, gross periapical lesions can be identified on a panoramic image).  At a minimum, panoramic radiographs should image the TMJs, part of the nasal spine, and both jaws entirely. 

Limitations of digital radiography in dentistry

Many of the latest digital panoramic machines yield stunning images.  It is important to remember, however, that panoramic and intraoral radiographs are still limited to the evaluation of hard tissues in two dimensions. For example, when the TMJs are imaged on a panoramic radiograph, the medial aspect of the condyles is what can be visualized, while the middle and lateral aspects are superimposed.

conventional panoramic radiograph dentistry
FIG. 1. An example of a conventional panoramic radiograph.

The many uses and benefits of the conebeam CT scan

This century ushered in another imaging modality for the general dentist: the conebeam CT scan, which offers a three-dimensional image of hard tissues at similar radiation levels to a full-mouth intraoral series of radiographs. 

Of course, medical CT units have been around for many years and are preferred for some oral surgery procedures. However, the cost, dosage and referral challenges of medical CT images to image the structures of interest to the general dentist make them difficult to use routinely.

Conebeam CTs capture the target tissues in one pass by shooting a cone-shaped beam of radiation which is then captured digitally into “sections” of data, whereas medical CT images require multiple horizontal passes of a specific anatomic region.

While CBCT scans are accurate typically up to within .5mm, medical CBCTs are much more accurate and allow for differentiation of multiple tissue types due to radiation differences.2

Depending on the size of CBCT image taken, the radiation dose of a CBCT image that will give the dentist the ability to see the jaws and teeth in 3D can be comparable or less than the dosage needed for a full series of individual dental x-rays, while enabling far superior diagnostic capabilities. 

For example, a full series of digital x-rays taken with a round collimator requires about the same amount of radiation as a person gets just from living for 21 days under normal conditions; a CBCT image of the jaws and four digital bitewing x-rays (needed to look for cavities between the teeth) requires a dosage comparable to about 8 ½ days of background radiation, depending on the CBCT unit utilized.

The difference is that the diagnostic potential of the CBCT image is far superior to a 2D full mouth series of x-rays! For example, within the first week of using a CBCT in our office, a compound endodontic lesion was discovered that appeared as a single periapical lesion around one tooth.

 

periapical lesion conventional radiograph
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FIG. 2 and 3: Two periapical lesions were superimposed on the conventional radiograph, appearing as involving the lateral incisor, but they are clearly separate lesions on this CBCT image.

 

cbct imaging implant placement
FIG. 4: CBCT images can be very helpful in planning for difficult implant placement.

Pros and cons of CBCT imaging

Some items that can more readily be visualized with CBCT images than conventional digital radiographs are:

  • Position of the nerve to teeth and for implant placement
  • Accurate lengths of teeth
  • “Inside out” visualizing of abscesses
  • 3D bone assessment to evaluate for appropriate gum disease treatment
  • Sinus position as they relate to the teeth
  • Measurement of the narrowest part of the airway for sleep apnea risk assessment
  • 3D evaluation of the jaw joints (TMJs) to look for bone changes
  • Evaluation of impacted tooth position

Currently, some of the limitations of CBCT images are:

  • Training is limited to the manufacturers, some of which offer none.3
  • Continuing education about interpreting CBCT images is limited in availability.
  • Return on investment of purchasing a unit is limited to non-monetary benefits of better diagnostics.
  • Standards for use have not been universally established.
  • Many clinicians continue to trust intraoral radiographs over CBCT images for periapical pathology assessment.
  • Image quality is not standard from one manufacturer to another.
  • Learning curve for software utilization is typically significant.
  • Each image taken must be reviewed and documented.
  • The scope of information that the dentist is responsible for interpreting remains debatable.
  • Oral maxillofacial radiologists are limited in number.
  • Insurance companies have not recognized the value of CBCT images and refuse to reimburse at this time.

        As with implementing any new technology, hurdles must be overcome.  Despite the challenges, adapting to the use of digital imaging modalities opens many doors to excellence in dental diagnosis. For more information, please visit this SpearTalk thread on CBCT and TMJ imaging.    

References

  1. Langlais, R. Rectangular collimation: no longer a matter of choice.  Accessed 2/21/2017.
  2. Miles, D. Atlas of cone beam imaging for dental applications. 2nd edition. Quintessence. 2013. 1-14.
  3. Huff, K. Interviews conducted with Prexion, Planmeca, Carestream, Owandy, and Kavo representatives. February 2017.