Comparison of two different methods for CTDIw calculation in CBCT systems

Introduction: Dental Cone beam CT (CBCT) is an imaging modality which has emerged as a 3D imaging modality of choice for osseous structures in the maxillofacial region. With increased usage of CBCT imaging for diagnosis and treatment planning, it is imperative that stringent dose assessments must be performed for dose estimation to safeguard both the patients and operating staff. In CT scanners, CT dose index (CTDI) measurements are part of standardized quality assurance protocols that uses a 10-cm pencil ionization chamber to approximate dose output for a single axial scan. The weighted CTDI (CTDIw = 1 CTDIc + 2 CTDIp) is used for 3                           3 CTDI calculations. The SEDENTEXCT project introduces a type of dose index calculation method for dental CBCT systems, that is calculating the dose measurements from the central and the peripheral positions to represent a dose index (Sedentex-DI2). In this study we utilize the same method to calculate CTDIw that is used in CT scanners and compare with the Sedentex-DI2 method as shown in: D             =Dc+Dp      to provide insights for developing a robust dose periphery                 2    assessment in dental CBCT systems. Materials and Methods: The CTDI head phantom of PMMA with a diameter of 16 cm and 15 cm long and 10-cm cylindrical pencil-shaped air ionization chamber for CTDI measurements were used in this study. Two different CBCT machines scanned the phantom: CS9300 (Carestream Health, New York, NY), ProMax 3D mid (Planmeca Oy, Helsinki, Finland) and three FOVs was used in both of them. Scans in both of the systems were repeated another two times at the same exposure parameters to check the reproducibility of the measured dose. All dose measurements at off- axis FOVs were performed. Results: The CTDIw from SEDENTEXCT (DI2) method was 3.1245 mGy, 3.2296 mGy and 1.5447 mGy for (ø8×5, ø20×10 and of ø4×5 cm2 FOVs, respectively) in ProMax 3D mid system. In CS9300 system these measurements were 3.104 mGy, 3.942 mGy and 6.648 Gy for FOVs of (ø5×5, ø10×5 and ø10×10 cm2, respectively). But calculated CTDIw by standard CTDI method was 2.923 mGy, 3.308 mGy and 1.763 mGy for (ø8×5, ø20×10 and ø4×5 cm2 FOVs, respectively) in ProMax 3D mid system and in CS9300 system these measurements were 2.969 mGy, 3.794 mGy and 6.502 Gy for FOVs of (ø5×5, ø10×5 and ø10×10 cm2, respectively). Conclusion: The results show that the calculated CTDIw values were very close to each other in both the standard and Sedentex-DI2 methods and did not differ significantly. So, it seems that both methods can be used to measure the dose index in CBCT systems. However, it should be noted that the dose values in the peripheral holes indicate that there is no symmetric distribution for the dose in these systems, and therefore CTDI measurement methods cannot be applied to all different FOVs at different central and off-axis positions. We should introduce a valid dose index based on the dose distribution in each mode of the FOV for each system.