SU‐GG‐J‐144: Photon Detection Efficiency of TomoTherapy® Array Detectors and Impact On MVCT Image Quality

Purpose: To quantify the photondetection efficiency of the TomoTherapy® X‐ray detector and assess its impact on megavolt CTimage quality. Method and Materials: The signal to noise ratio of two similar X‐ray detector arrays available on TomoTherapy® Hi⋅Art® systems was measured during exposure to the megavolt X‐ray imaging beam. These measurements are compared to calculations of the ideal signal‐to‐noise ratio if all incident photons were detected to determine the detective quantum efficiency (DQE). Similarly, the signal‐to‐noise ratio is measured in CTimagesreconstructed from data collected by these detector arrays and compared to theoretical predictions. Results: The detection efficiency of the out‐of‐focus detector cells are 11% and 18% for the two types of detector arrays studied. The efficiency drops for the central detector cells that are aligned toward the source. The signal‐to‐noise ratio in the resulting CTimages is 19.7 and 28.4 using a dose of 1 cGy to the center of the 30 cm diameter solid‐water phantom. These image signal‐to‐noise ratios are in good agreement with previous theoretical predictions at this dose level, beam energy, and detector efficiency. Conclusion: Compared with conventional flat‐panel detectors that can have detection efficiencies of only 1 – 2% at megavolt energies, the TomoTherapy® detector arrays are an order of magnitude more efficient, resulting in better signal‐to‐noise ratios in the resulting CTimages or a lower dose (or a combination of both).