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An investigation into dose optimisation for imaging root canal anatomy using cone beam CT.

OBJECTIVES
To identify a dose as low as diagnostically acceptable and a threshold level of image quality for CBCT imaging root canals, using maxillary first molar (M1M) second mesio-buccal (MB2) canals of varying complexity for two CBCT scanners.


METHODS
Dose-area product (DAP) and contrast-to-noise ratio (CNR) were measured for two scanners at a range of exposure parameters. Subjective-image-quality assessment (SIQA) at the same exposures was performed for 3 M1M's of varying MB2 complexity, positioned in a anthropomorphic phantom. Nine raters (three endodontists, three dental radiologists and three junior staff) assessed canal visibility, using a five-point confidence scale rating (CSR).


RESULTS
Identification of simple-moderate MB2 canal complexity was achieved at a range of protocols, with DAP values of ≥209.3 mGy cm² and ≥203.2 mGy cm² and CNRs of 3 and 7.6 for Promax®3D and Accuitomo-F170® respectively. For complex canal anatomy, target subjective image quality was not achieved, even at the highest DAP values for both scanners. Junior staff classified significantly more images as undiagnostic compared with senior staff (p = 0.043).


CONCLUSIONS
In this first study to address optimisation of CBCT imaging of root canal anatomy, a similar threshold dose for both scanners was identified for M1Ms with simple-moderate MB2 canal complexity. Increasing dose to enhance visualisation of more complex canal anatomy was ineffective. Selection of standard protocols (while avoiding lower kV/mA protocols) instead of high-resolution scans was a practical means of reducing patient dose. CNR is not a transferable measure of image quality.