SU‐FF‐I‐08: Results of An Optical Fiber‐Based Dosimetry System for Use in Computed Tomography Characterization

Purpose: Modern multi‐detector CT and cone‐beam CT offer wide beams, making the concept of CT dose index (CDTI) no longer valid for CT dosimetry. A real‐time OSL dosimetry system has been developed and is evaluated for CT dosimetry in this study. Comparisons with a pencil ionization chamber were made. Methods and Materials: The system utilizes the optically stimulated luminescence (OSL) of KBr:Eu. The size of the KBr:Eu single crystal dosimeter equals approximately 1 mm³. The dosimeter was affixed to the terminal end of a plastic fiber cable and placed in the center hole of a plastic cylindrical phantom. The distal end of the fiber cable was attached to OSL reader, containing a 658 nm red laser, and photo‐multiplying tube (PMT), and associated optics/ electronics. CT slices of 1 s duration were performed over a range of energies (80–140 kVp) and tube currents (60–350 mA), as well as slice thickness (5 and 10 mm) using a GE LightSpeed Ultra scanner. Gantry tilt dependence was investigated over a range of 40.5° (22° superior to 18.5° inferior). OSL data was obtained before, during, and after the scan at the rate of 10Hz. Results: Performance was determined in part by normalizing both the initial OSL intensity and the background‐subtracted integral OSL to exposure reported previously by an ionization chamber. Good correlation between exposure and OSL data was found. Initial intensity and background‐subtracted OSL normalized to exposure show coefficients of variation of ∼%5 or less. Significant deviation was observed between the ∼10 OSL measurements taken for each slice, presumably as a result of absorption of x‐rays by the patient table. Conclusions: Initial tests have shown that this OSL dosimetry system possesses great potential for faster CT characterization. This system may prove a valuable alternative to CTDI.