Chelator effect on ion diffusion in ferrous‐sulfate‐doped gelatin gel dosimeters as analyzed by MRI

Abstract

Ferrous-sulfate-doped gelatin gel dosimeters are useful tools for the measurement of three-dimensional absorbed radiation dose distributions. The diffusion of ferric ions through these gels causes degradation with time of the dose distribution image. It would be useful to reduce ferric ion diffusion without decreasing gel sensitivity. The amount of ferric ion diffusion is a function of the time delay after radiation, the gel temperature, and the gel concentration. These effects can be quantified by measuring the ferric ion diffusion coefficient. Determination of the diffusion coefficient by irradiating the lower section of a cylinder of gel, which was then imaged repeatedly over time with a clinical magnetic resonance imager, is described. Analysis of the edge spread function formed at each of several times after irradiation by drawing a profile over the imaged junction between the irradiated and unirradiated halves of the cylinder, gave estimates of the variance of the edge spread function. These variances were used to obtain an estimate of the ferric ion diffusion coefficient for the gel. A method of reducing ferric ion diffusion by adding a chelator and the cross linkage agent formaldehyde is suggested. The chelators investigated were 1,10 phenanthroline, xylenol orange, and bathophenanthroline disulfonic acid. These reduced diffusion to varying extents, and influenced the gel sensitivity. The diffusion coefficient in gels containing xylenol orange was found to be 0.44 mm2h-1. The gel sensitivity was 0.0093 s-1Gy-1. This compared with a diffusion coefficient of 0.82 mm2h-1 for the base line gel that did not contain formaldehyde or chelators. The sensitivity of this base line gel was 0.0129 s-1Gy-1. The addition of xylenol orange produced the most improved gel dosimeter of the gels studied. This gel had a decreased ferric ion diffusion coefficient and a decreased sensitivity. It was still sensitive enough to be useful.

Ferrous Xylenol Gel measurements for 6 and 10 MV photons in small field sizes

“…Also, that all PDD values, obtained with the FXG, are more near to the values belonging to the smallest IC, than for 0.125 cm 3 . One can also see that the difference between the dosimeters results increases when the field size diminishes.…”

“…The output factor measurements [7] were obtained in the PMMA phantoms with FXG detectors and with the 0.125 cm 3 IC for an absorbed dose of 1 Gy, SSD of 100 cm, a 1.5 cm build-up plate for 6 MV photons and a 2.5 cm build-up plate for 10 MV photons. Single FXG dosimeters were inserted in the phantom PI and centered in the square field sizes of 1 x 1 cm 2 ; 2 x 2 cm 2 ; 2.5 x 2.5 cm 2 ; 4 x 4 cm 2 ; 5 x 5 cm 2 ; 7 x 7 cm 2 ; 8 x 8 cm 2 and 10 x 10 cm 2 .…”

“…In the Fricke Xylenol Gel (FXG) chemical system the Fe 3+ -Xylenol complex concentration, measured spectrophotometrically, is directly proportional to the absorbed dose [1][2][3][4]. The FXG dosimeter characteristics make it suitable for radiotherapy as: broad linear dependence on the absorbed dose up to 30 Gy for 60 Co radiation [5,6], effective atomic number of 7.75 and density of 1.05 g/cm 3 near to 7.64 and 1.04 g/cm 3 , respectively, for soft tissue. The first chemical dosimeter, prepared by Fricke in 1921 (ferrous sulfate, sulphuric acid and water), has been improved adding or substituting the originals components.…”

Measurement of Gd-DTPA diffusion through PVA hydrogel using a novel magnetic resonance imaging method

“…Diffusivity through biopolymers is critical in many biomedical and biochemical applications such as bioreactor systems using immobilized cells and enzymes (Merchant et al, 1987;Blanch and Clark, 1996;Pilkington et al 1998), drug delivery systems (Wood and Calton, 1993;Peppas and Wright, 1996), radiation dosimetric gels (Rae et al, 1996;Kron et al, 1997;Pedersen et al, 1997), and pancreatic islet encapsulation (Watler et al, 1988;Kessler et al, 1992). Traditional chemical engineering methods used for measuring the diffusion coefficient of a solute through a polymer sample requires an analytical technique (Geankoplis, 1978;Willard et al, 1981) to determine the solute concentration (within or adjacent to the sample) as a function of time.…”

Experimental determination of the diffusion coefficient in two-dimensions in ferrous sulphate gels using the finite element method

“…f i = ffRl Pi dxdy (12) T u = [u u K U]T u = rdU I d~... dUn 1 and I' 2' ' n and l dt ' dt ' , dt J . In an analogy with the finite element method tor structural motion problems, the matrix K is called the stiffuess matrix, the matrix M is called the mass matrix and the vector f is called the load vector.…”