SU‐E‐T‐93: High Dynamic Range Scanning for Optical‐CT in 3D Dosimetry

Purpose: The recent emergence of powerful, fast and high‐resolution 3D dosimetry techniques brings exciting potential to radically strengthen the foundations of quality‐assurance in radiation therapy, and hence treatment efficacy. This work presents a new acquisition method for 3D dosimetry by optical‐computed‐tomography (optical‐CT), which can improve accuracy when imaging strongly attenuating dosimeters (e.g. large or heavily dosed). Methods: The method involves acquiring optical‐CT projection images where the intensity of the light source is increased as much as possible without causing detector saturation within the dosimeter. This improves signal‐to‐noise in dark regions of the dosimeter, but flood‐field and regions outside the dosimeter are unuseable due to saturation. The problem of acquiring a useable flood is solved by acquisition at a reduced shutter opening time. The non‐saturated flood is then scaled up for use with projections, by the ratio of shutter times. The method relies on linearity of signal with shutter time, which is investigated here. The method is evaluated by application to a range of dosimeters with varying degrees of strong attenuation. Results: The relationship between signal and shutter time in the flood was found to be highly linear, a key enabling result for this method. When applied to moderately attenuating dosimeters, the new method agrees with the standard method to a high degree (<1% deviation on average). For very dark dosimeters, the new method was found to have greatly improved signal‐to‐noise (a factor of 2‐3 times better) in the darkest regions and eliminated streak artifacts present in reconstructions using the standard acquisition method. Conclusions: A new optical‐CT acquisition method is presented which yields improved signal‐to‐noise for dosimetry measurements in strongly attenuating dosimeters. A further important advantage is the method does not require any tinting of the refractively‐matched fluid in the optical‐CT water‐bath, which greatly increases convenience and practicability.