Determining CTDIvol for cone beam ct with partial rotational coverage and wide beams: Symmetry and reciprocity.

Purpose: The radiation dose to the patient in C-arm cone beam CT (C-arm CBCT) is delivered in a similar fashion to that by standard diagnostic CT. However, the wide beam and limited angular excursion of CBCT may require alternative methods for determining CTDIvol. We have devised a simple methodology to determine a CTDIvol equivalent for C-arm CBCT. Methods: C-arm rotation was constrained from -130° to +130° with 0° defined as the bottom-most position of the tube. The beam was collimated to 10 cm and a 0.6 cc thimble chamber was placed in the center and at the -90° edge positions of the CTDI head phantom. (The experiment was repeated using a beam collimated to 2 cm and a conventional 10 cm CTDI pencil chamber.) X-ray beam parameters were controlled by the AEC. The dose rate waveform was recorded and the dose was integrated from -90° to +90° for all three thimble chamber positions. The sum of the edge position doses and double the center dose provide the equivalent to conventional edge and center doses, respectively. CTDIvol is computed in an analogous fashion and the value scaled to the angular excursion utilized for a particular scan. Results: The CTDIvol for head phantom was 24 mGy for a full 360° rotation (17 mGy for the maximum 260° possible). We have demonstrated the equivalence of this to using a narrow (2 cm) beam and a conventional 10 cm CTDI pencil chamber. The waveforms from the wide beam and thimble chamber were properly proportional to those from the narrow beam and pencil chamber. Conclusion: We have developed a simple technique for determining a value essentially equivalent to the conventional CTDIvol dose metric for a C-arm CBCT system. This provides a wide array of familiar tools for machine comparison and dose determination