CBCT dose maps

Creating dose maps, depicting the amount of scattered radiation at various locations in the vicinity of the CBCT device, plays an important part in the protection of personnel.

Even though it is highly recommended that there is a sufficient amount of shielding material (wall, lead glass) between the patient and the personnel or public, it is occasionaly needed for a person to be in the proximity of the device to make sure the patient is at ease. In those cases, it is important to know the optimal location for this person, at which a minimal scattered dose is received.

This location depends on three factors. Firstly, it is well-known that the amount of radiation decreases at larger distances from the source (which is in this case the patient or object), following the inverse square law:

The intensity of radiation from a point source in free space is inversely proportional to the square of the distance from the source.

Secondly, the exposure geometry of the CBCT device also plays a certain part. Many devices do not scan using a full 360° rotation, and many other have the choice between a 180° or full rotation. The extent of the rotation will affect the distribution of scattered radiation. Also, the angle of exposure differs between devices, which will also have some effect on the distribution of scatter. An obvious, yet extreme example is comparing devices using supine patients positioning with those using standing or sitting positioning.

Finally, in any room used for CBCT scanning, there are a number of absorbing materials present, most notably the device itself. Therefore, the scatter dose distribution will not only be device-specific, but somewhat room-specific as well.