Development of a CCD array imaging system for measurement of dose distributions in doped agarose gels.

An imaging system for agarose gel sections has been investigated for applications in rapid two-dimensional radiation dosimetry. The imaging system, with white light illumination and CCD camera detection, was designed for measurement of the radiation-induced optical density changes in iron- and xylenol orange dye-doped agarose gels. The performance of the imaging system was compared with that of a laser scanning system for the gels and with the accepted dosimetry standard, the ionization chamber. In measurement of beam profiles of two therapeutic radiation fields, relative dose values from the CCD camera imaging system were on average within 3% ranging from 0.005% to 7.5%) of values recorded with a parallel plate ionization chamber. In comparison with the laser scanner, the CCD camera imaging system provided comparable spatial resolution and an increased rate of data acquisition, although a consistently reduced signal to noise ratio was observed. Suggestions for improving the camera imaging technique include noise reduction through camera cooling and further frame averaging.

Laser-scanned agarose gel sections for radiation field mapping.

PURPOSE: A dedicated laser scanning system has been applied to the measurement of optical density change in ferrous sulphate-xylenol orange dye-agarose gel mixtures for applications in radiation field mapping. METHODS AND MATERIALS: The gels were poured, irradiated, and optically scanned in Lucite casts providing a uniform gel section thickness of 1 cm. The gel sections were optically scanned both pre- and postirradiation to determine the optical density change in the gel and consequently the absorbed dose. Irradiations of the gel sections were performed with an orthovoltage unit and a linear accelerator. RESULTS: For linear dose response up to 10 Gy, an appropriate gel mixture was found to be 0.4 mM Fe2+, 0.2mM xylenol orange dye, 25 mM sulphuric acid, and 1% by weight agarose gel with a mixing temperature of 60 degrees C. In dosimetry of a 20 x 10 cm 6 MV wedged x-ray field, good agreement in terms of relative dose was found between the gel values and ionization chamber readings. However, in repeated experiments with the gel dosimeter involving calculation of absolute dose at various points in the wedged field, variations in absolute dose measurements of up to +/- 5% were observed. CONCLUSION: The dosimetry technique involving laser scanning of agarose gel sections has potential for further applications in radiotherapy dosimetry.

Optical transmission properties of homogenised milk used as a phantom material in visible wavelength imaging.

Using the collimated output beam from a Helium-Neon laser, operating at 632.8 nm, transmission intensities through various thicknesses and concentrations of homogenised milk have been measured for the purpose of determining the suitability of milk as a phantom for visible wavelength imaging. Also, theoretical predictions to the laser beam attenuation in milk based on diffusion theory have been calculated, giving substantial agreement with experiment for milk concentrations greater than 10%. The transmission properties of milk have been compared with a commonly used optical phantom, namely Intralipid, and the scattering coefficient mu(s) measured for each phantom. The scattering coefficients for undiluted milk and 20% solids Intralipid are 52 +/- 1, and 139 +/- 6 mm-1, respectively. With appropriate normalisation of the physical parameters, the effective physical properties of the phantom materials vis a vis light transmission become the same within limits imposed by experimental error. Comparison of measured transmission properties of homogenised milk with tissue confirm its suitability as a phantom vis a vis scattering properties determined by mu(s) and its mean cosine of scatter g, but that its absorption coefficient mu(a) is approximately two orders of magnitude less than for tissue. Appropriate attenuators, such as India Ink, have been suggested as additives to increase the value of mu(a).

Absorbance measurement with a helium-neon laser for chemical dosimetry.

A laser-based system for absorbance measurement has been developed for use in chemical dosimetry as an alternative to the spectrophotometer. The system incorporating a Helium-Neon laser operating at 543.5 nm is designed specifically for measurement of absorbed doses below 10 Gy with the ferrous sulphate-benzoic acid-xylenol orange (FBX) dosimeter. Absorbance measurement for the FBX dosimeter is normally carried out at 540 nm with a spectrophotometer. The performance of the laser-based system in measurement of absorbance is comparable to that of a standard spectrophotometer and it is concluded that the absorbance measurement technique is not the limiting factor in the sensitivity of the FBX dosimeter for absorbed doses below 1 Gy.