A Spatial Resolution Study of a New Optical Tomography-Based Polymer Gel Dosimetry System

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A spatial resolution investigation of the OCTOPUS™-IQ scanner in combination with the new BANG3-Pro2® polymer gel was performed by scanning a high-contrast needle phantom. The phantom contained five thin needles (0.3 mm diameter) embedded in gel positioned in different patterns: needles were inserted (a) at 45° angle from the center of the gel container, and (b) vertically along the gel axis. The non-irradiated needle phantoms were scanned at various slice spacings (0.25–1.0 mm) and for two different laser beam orientations. Optical density profiles and their full width at half maximum (FWHM) were evaluated for resolution limit. The modulation transfer function (MTF) corresponding to measured point spread function (PSF) data was calculated. With high resolution scanning mode and 0.25 mm pixel resolution, the measured PSFs at the center of the gel dosimeter have a FWHM of 0.95 mm. The MTF for the 0.25 mm reconstruction pixel size suggests that the resolution of the system is 0.5 mm or less. We also observed a progressive degradation of the vertical needle images with off-axis distance, attributable to the defocusing of the laser beam. No significant degradation was observed up to the maximum useful reconstructed image radius of 50 mm from the gel dosimeter center axis.

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