Room: AAPM ePoster Library
To evaluate several methods for validation of the machine-reported DAP in dental/maxillofacial cone beam CT
Multiple methods for measuring DAP were evaluated on two CBCT scanners of the same model. Three methods were employed to measure x-ray field dimensions: radiochromic film, an electronic beam profile device and storage phosphor radiography. A solid state dosimeter positioned at the x-ray tube exit window and at the surface of the image receptor measured air-kerma at the center of the field of view. Alternatively, air-kerma was measured with a 100mm long CTDI pencil chamber at the tube exit window. Finally, a new commercial DAP meter was evaluated. DAP was measured and calculated using each of these techniques, and results compared with machine-reported values.
The electronic beam profile measurements agreed with radiochromic beam width measurements to within 1.5% on average. Using radiochromic field area and solid-state dosimeter air-kerma, measured DAP agreed with reported DAP to within 16.3% on average (range 10.2% – 31.6%) across a range of field sizes. When air-kerma and field size was measured at the image receptor plane, reported DAP error was 12.4% and 26.3% at two field sizes. CTDI chamber measurements when scaled by beam dimensions were within 5% of solid state dosimeter and the commercial DAP meter values.
Because the CBCT unit under evaluation had no bowtie filter, a single air-kerma measurement at the center of the field was used to determine DAP. Field measurements were in agreement with machine reported values to within 32%. For systems employing a bow-tie beam shaping filter, a point dose measurement is inadequate. Our work shows that an extended integrating dosimeter (100 mm long CT pencil chamber) can be used, and would be advantageous when a bow-tie filter is present.