SU‐E‐I‐27: CTDI Verification with a Farmer Chamber

Purpose: The goal of our research is to determine the viability of a Farmer ion chamber in verifying the reported CTDIvol from CT scanners. Methods: We used a Siemens Somatom Sensation 16 slice scanner, three CTDI phantoms, a RadCal 100 mm pencil chamber (Model 10X6‐3CT), a RadCal Farmer ion chamber (Model 10X6‐0.6CT), and a RadCal Model 9015 control unit. Exposures were made with our routine adult chest protocol, for a 100 mm scan length of the phantom. Readings were made at the center and 12 o'clock positions. Results: We found the doses measured by the Farmer chamber to be greatly dependent on the pitch. Scans with a pitch < 1 are generally result in a larger CTDIvol than what was reported by the scanner, while scans with a pitch > 1 are generally lower and give very inconsistent results. The dose rate is higher when p >1, but the tube may not pass directly over the detector when at the 12 o'clock position. If the p > 1, an average of all the 3, 6, 9, and 12 clock positions should be made. Conclusions: The CTDI measurements with the Farmer chamber seem most accurate at a pitch less than 1 for longer scans. We plan expand the study of to other scanners with wider beams as well as more pitch values between 0.85 and 1.0.