Feasibility of a dual wavelength laser optical CT scanner with in-air gel readout

A fast dual wavelength laser beam fluid-less optical CT scanner for radiotherapy 3D gel dosimetry I: design and development

“…Ray paths for the 2 wavelengths need to be coincident for the scanner to work well. Differences in ray paths of up to 0.13 mm (at the gel/PMMA exit interface) were previously determined by ray tracing calculations (Ramm et al 2015). Ray tracing results shown in figure 2 were calculated in a similar manner with the same refractive indices, using first principles of geometrical optics, computed using Matlab (The MathWorks Inc.).…”

“…The peak dose response of FXG occurs at around 590 nm and beyond that, there is a steep drop in absolute optical attenuation and dose response spectra (Kelly et al 1998). At 633 nm, the dose response is 1/3 of the peak, thus reducing the dual wavelength dose sensitivity to 2/3 of the peak (Ramm et al 2015).…”

“…For single laser operation, scanner tuning mainly consists of focal distance adjustments for rotating mirror and PD detector, and alignment of the beam to give a scan plane perpendicular to the cylinder axis. Lasers were always oriented to give a p-polarised beam as with previous work (Ramm et al 2015). A dual laser configuration requires laser beam size and position matching.…”

“…Pre-irradiation reference scans have become a common part of quantitative optical imaging to provide a zero dose optical attenuation reference and to assist in reduction of reproducible artefacts (Doran et al 2001, Oldham et al 2001. In an attempt to simplify the scanning process by removal of the pre-irradiation scan step, a dual wavelength scanner has been proposed (Ramm and Rutten 2015). A second wavelength with reduced dose response may be used as the reference scan instead of a pre-irradiation scan.…”

“…The basis for the fluid-less optical geometry has been described previously (Ramm et al 2012). The feasibility of dual wavelength operation was initially investigated by calculation of light ray paths and reflection losses (Ramm et al 2015). Here, design and setup issues are detailed and imaging performance is assessed using dyed water solutions.…”