Purpose: To evaluate the use of a conversion factor to derive CTDI₁₀₀(center) for dental cone beam CT (CBCT) scanners, as per regulatory requirements and manufacturer-suggested radiation output measurements, from radiation output measured at the input surface of the image receptor (Doseᵣ).
Methods: Four clinically-used Promax 3D (Planmeca, Helsinki, Finland) dental CBCTs were used in this study. Using a calibrated ion-chamber (10x5-10.3CT), controller (9010) and electrometer (9060; Radcal Corporation, Monrovia, CA), and a head CTDI phantom, CTDI₁₀₀(center) was measured as per manufacturer-suggested protocol for 2 dental CBCTs (kV: 90; mA: 4-10; time: 12 s). Using the chamber length (100 mm) to beam width (80 mm) correction factor, CTDI₁₀₀(center) values were computed and verified based on manufacturer-specified tolerance values (at 10 mA: 7.43 – 11.15 mGy). Then, for all 4 dental CBCTs, Doseᵣ was measured using a calibrated solid-state detector (AccuGold AGMS-D with AGDM; Radcal Corporation) at source to detector distance of 51 cm (kV: 90; mA: 6.3; time: 12 s). A conversion factor was calculated to translate Doseᵣ to CTDI₁₀₀(center) measurements using data from 2 scanners. This conversion factor was utilized to compute and validate CTDI₁₀₀(center) for other 2 scanners based on Doseᵣ.
Results: The correlation between mAs and CTDI₁₀₀(center) was 0.99. The calculated CTDI₁₀₀(center) at 10 mA for two dental CBCTs were 8.0 and 8.3 mGy, resulting in the average conversion factor of 2.1 to translate Doseᵣ to CTDI₁₀₀(center). Using this conversion factor, the resulting CTDI₁₀₀(center) for the other two dental CBCTs were 7.9 and 8.3 mGy; within the manufacturer-specified tolerance values.
Conclusion: A conversion factor to translate Doseᵣ to CTDI₁₀₀(center) measurements that are required for regulatory compliance has been derived and validated. Given the relative ease to measure the Doseᵣ, this methodology has potential to obviate the need to perform CTDI₁₀₀(center) measurements for dental CBCT scanners.
Funding Support, Disclosures, and Conflict of Interest: Medical physicist consulting agreement with Rayscan Inc (NJ); Research consulting with CurveBeam; Research support from Philips Healthcare; Contrast agent support from Lantheus Medical Imaging and GE Healthcare
Acceptance Testing, Dental Radiography, Dosimetry