I was recently asked what values i recommend to use on Morita CBCT devices when using the Navident fiducial marker.
My response is as follows:
"All I can tell you at present is that as far as I am aware, my customers operating another Morita 3D device, do not adjust from normal patient exposure values.
Depending on the model revision of the customers’ Morita CBCT device, the maximum KV will be either 80 or 90 KV.
If kV is too low, then this could give rise to more scatter as the object may deflect more photons. Remember, kV is penetration power of the photons. A higher beam energy may be deflected less than a lower beam energy.
If kV is too high, then delicate structures may be obliterated or burnt out as there is not enough attenuation of the beam to cause a suitable shadow. This effect is typically noticable on devices operating at around 120 KV such as iCat and other products.
However consideration must also be given to the patient anatomy as well as the requried diagnostic yield. If the patient is say 60+ years old, then there will likely be some osteoporosis of the trabecular bone in the mandible. So if you consider the above statement, for high kV and delicate structures, then you can imagine that a high kV will likely burn through what is remaining of the trabecular bone and so it can be difficult to distinguish the inferior dental nerve canal from the surrounding bone. My customers have found that reducing the kV for patients in this category, results in more photons being stopped by the soft bone, causing a higher contrast result against the lower density of the canal and therefore better visualisation of it compared to the surrounding bone.
If mA is too low, then the image typically may be noisy/grainy.
If mA is too high then anatomy may be low in in noise, but if there is scatter generated by an object, then by throwing more photons into the mix, there will possibly be more scatter artefact.
You should always base the exposure parameters on the patient, not the marker. I am quite confident that for standard exposure parameters there is unlikely to be an issue and the marker should be sufficiently detailed for the software to recognise it. Having exposed the patient to radiation, the first responsibility is to have a diagnostically useful image rather than a detailed image of the fiducial.
360 degree exposure angle on Accuitomo will minimise effect of scatter. However you should be able to use 180 degree exposure angle (as standard with other products like Veraviewepocs 3D from Morita) to reduce patient dose. My customers with Veraviewepocs 3D have not reported a problem with the 180 degree exposure angle so far.
However if there are lots of heavy restorations elsewhere in the mouth then scatter from those subjects could potentially interfere with the definition of the marker in the final reconstruction. In that case, a 360 degree exposure angle might be considered
Vital of course that the fiducial is mounted in the same axial plane as the crowns of the other teeth of that jaw, so that any artefact generated is not cast on the area for diagnosis (under the bone). Having said that, if there are restored crowns then a judgement may need to be made if that restoration is likely to interfere with the marker outline."
It is all about understanding the characteristics of the machine being used, the general consequences of making setting adjustments (which any trained radiographer should understand) and then striking the right balance for the individual case.