Standardized method to quantify the variation in voxel value distribution in patient-simulated CBCT data sets.

objectivesTo suggest a standardized method to assess the variation in voxel value distribution in patient-simulated CBCT data sets and the effect of time between exposures (TBE). Additionally, a measurement of reproducibility, Aarhus measurement of reproducibility (AMORe), is introduced, which could be used for quality assurance purposes.

methodsSix CBCT units were tested [Cranex(®) 3D/CRAN (Soredex Oy, Tuusula, Finland); Scanora(®) 3D/SCAN (Soredex Oy); NewTom™ 5G/NEW5 (QR srl, Verona, Italy); i-CAT/ICAT (Imaging Sciences International, Hatfield, PA); 3D Accuitomo FPD80/ACCU (Morita, Kyoto, Japan); and NewTom VG/NEWV (QR srl)]. Two sets of volumetric data of a wax-imbedded dry human skull (containing a titanium implant) were acquired by each CBCT unit at two sessions on separate days. Each session consisted 21 exposures: 1 "initial" followed by a 30-min interval (initial data set), 10 acquired with 30-min TBE (data sets 1-10) and 10 acquired with 15-min TBE (data sets 11-20). CBCT data were exported as digital imaging and communications in medicine files and converted to text files containing x, y and z positions and grey shade for each voxel. Subtractions were performed voxel-by-voxel in two set-ups: (1) between two consecutive data sets and (2) between any subsequent data set and data set 1. The mean grey shade variation for each voxel was calculated for each unit/session.

resultsThe largest mean grey shade variation was found in the subtraction set-up 2 (27-447 shades of grey, depending on the unit). Considering subtraction set-up 1, the highest variation was seen for NEW5, between data sets 1 and the initial.

conclusionsDiscrepancies in voxel value distribution were found by comparing the initial examination of the day with the subsequent examinations. TBE had no predictable effect on the variation of CBCT-derived voxel values. AMORe ranged between 0 and 64.