X-ray CT dose in normoxic polyacrylamide gel dosimetry

This study reports on the effects of x-ray CT dose in CT imaged normoxic polyacrylamide (nPAG) gel dosimeters. The investigation is partitioned into three sections. First, the CT dose absorbed in nPAG is quantified under a range of typical gel CT imaging protocols. It is found that the maximum absorbed CT dose occurs for volumetric imaging and is in the range of 4.6 +/- 0.2 cGy/image. This does scales linearly with image averaging. Second, using Raman spectroscopy, the response of nPAG to CT imaging photon energies (i.e., 120-140 kVp) is established and compared to the well known dose response of nPAG exposed to 6 MV photons. It is found that nPAG exhibits a weaker response (per unit dose) to 140-kVp incident photons as compared to 6 MV incident photons (slopes m(6) (MV) = -0.0374 +/- 0.0006 Gy(-1) and m(140) (kVp) = -0.016 +/- 0.001 Gy(-1)). Finally, using the above data, an induced change in CT number (Delta N-CT) is calculated for nPAG imaged using a range of gel imaging protocols. It is found that under typical imaging protocols (120-140 kVp, 200 mAs, similar to 16-32 image averages) a Delta N-CT < 0.2 H is induced in active nPAG dosimeters. This Delta N-CT is below the current limit of detectability of CT nPAG polymer gel dosimetry. Under expanded imaging protocols (e.g., very high number of image averages) an induced Delta N-CT of similar to 0.5 H is possible. In these situations the additional polymerization occurring in nPAG due to the imaging process may need to be accounted for. (c) 2007 American Association of Physicists in Medicine.