Answer to Question #6578 Submitted to "Ask the Experts"

There are two answers to this question, depending on what aspect of the scattered radiation we are talking about: (1) scattered radiation with regard to radiation protection or (2) scattered radiation with regard to image quality. I will assume that you are referring to the latter, although I cannot be entirely sure since protection from radiation is obviously important when using any kind of ionizing radiation, and CBCT is also used in radiotherapy for patient alignment in OBI systems (on-board imaging). Fortunately, it is a fairly straightforward answer from either perspective.

From the radiation protection standpoint, it is fairly intuitive that there is less scattered radiation because there is less radiation used to produce an image. Most CBCT dental units require little or no shielding. There are several factors that play into why less radiation can be used in dental imaging. One of the most significant factors is that CBCT is used in dentistry for imaging high-contrast anatomy (i.e., the interfaces between bone, soft tissue, and air). Typically, there is not a need to differentiate between different types of soft tissue in dental imaging. Better low-contrast (i.e., soft tissue) imaging typically requires more radiation to improve the contrast-to-noise ratio (CNR). The CNR in dental imaging can be relatively low and still get very good high-contrast images.

In oncological applications it is extremely important to have good low-contrast resolution to identify lesions from normal tissue which can have only subtle differences in contrast. CBCT uses some sort of 2-D detector, whether it is a flat-panel digital detector or an image intensifier system. Scattered radiation will inherently be collected in the 2-D detector, adding a false signal. Scatter-reduction grids could be used to prevent detection of the scatter, but more radiation (higher patient dose) would be used to improve the signal-to-noise ratio, and the grid would degrade one of the characteristics that make the 2-D detectors so great—the high spatial resolution. The scatter is ultimately a low-frequency signal (i.e., it does not change much across the 2-D detector) that will degrade the resolution of low-contrast anatomy, such as the contrast between a tumor and the surrounding healthy tissue. For this reason, there is research being done to overcome the scatter in CBCT for oncological applications.

In dental imaging, the scatter will still add the low-frequency signal to the image. However, the scatter signal does not affect the resolution of the high-contrast anatomy. So the image degradation from scatter is essentially ignored in dental CBCT. There are ways to correct for the scatter signal through sophisticated reconstruction algorithms, but it would do little to improve the ultimate clinical utility of dental CBCT images.

Jonathan D. Alspaugh, MS