A prototype, rapid, high precision fan beam optical computed tomography (OptCT) scanner for three-dimensional polymer gel dosimetry of complex radiotherapy protocols has been developed. The scanner employs a sixty degree fan beam of 543 nm laser light to interrogate irradiated gel samples up to 19 cm in cross-sectional diameter. Rapid data acquisition with minimal scattered light and minimal refraction and reflection artefacts is achieved with the unique radial design of the front and back apertures of the index matching medium tank, concentric arrangement of a CNC machined collimator and five photodiode detector arrays for light detection. Characteristics of the scanner include: an SNR of optical absorbance in light field projections of up to 100:1; SNR of optical absorbance in transmission projections of up to 92:1 for phantoms with absorbances ranging from 0.1 to 1.0 and a spatial resolution of 0.25 mm. OptCT dose maps extracted from normoxic NIPAM polymer gels irradiated with (a) a rectangular 3×3 cm2 field, (b) a 4-field wedged treatment and (c) a C-shaped IMRT plan are examined and compared to the Eclipse TPS. For these treatments, dose resolution is examined in steep and moderate dose gradient regions using dose difference and gamma function metrics. The effect of different OptCT reconstruction parameters on ring and streaking artefacts is also demonstrated.
Keywords: Computed tomography; Data acquisition; Dosimetry; Gels; Image scanners; Laser beam effects; Medical image noise; Optical polymers; Polymers; Radiation therapy.
© 2008 American Association of Physicists in Medicine.