Room: Exhibit Hall | Forum 1
Purpose: The glow curve (GC) of thermoluminescent dosimeter CaFâ‚‚:Tm (TLD-300) shows a remarkable sensitivity to the energy of incident photons. For X-rays up to 65 keV, GC analyses have enabled to determine photon energies with 1 keV accuracy. In this work, we investigate the radiation field effective energy within a phantom, exposing TLD-300 chips to CT beams.
Methods: TLD-300 chips (3.2x3.2x0.89 mmÂ³) were exposed to 80 and 120 kV CT beams, in-air (for calibration of the method) and within a 16 cm diameter cylindrical phantom. A rigorous GC deconvolution was applied to obtain 7 peaks, following our laboratory protocol. The integral TL signal (TLR) was evaluated, as well as the GC high-to-low-temperature ratio (HLTR), the observable that correlates with effective energy. The possible dependence of TLR and HLTR on the relative chip orientation with respect to monodirectional beams was also evaluated, in-air and in-phantom. The energy evolution of the field inside the phantom was simulated using the MCNP6 1.0 Monte Carlo code.
Results: Glow curves as a function of phantom depth reflect the radiation field energy evolution, both in topogram (single beam) as in CT (360 degree rotation) modes. At 120 kV and topogram mode, the effective energy increases by about 5 keV from the entrance to the center of the phantom, and then decreases by about 3 keV towards the exit. At 120 kV and CT mode, the beam hardens by about 10 keV fron surface to center. MC calculations describe well the observations. For in-air exposures, TLR is a strong function of the direction of incidence of unidirectional beams, and this effect disappears inside the phantom at 120 kV. HTLR is not sensitive to the relative orientation between incident beam and chips.
Conclusion: CT energy evolution within phantoms can be determined by the analysis of TLD-300 glow curves.