PubMed · 8061222
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H Kiefer, F Jähnig. 1994. Neutral phospholipids do not buffer at neutral pH.. https://doi.org/10.1016/s0006-3495(94)80966-x
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The ferrous sulfate-doped gel dosimeters have been developed for three-dimensional magnetic resonance imaging of radiation dose distributions. When the gel dosimeter is irradiated, ferrous ions are converted to ferric ions and the nuclear magnetic spin relaxation of the dosimeter varies with dose. In this paper, a model is presented for the dose dependence of the spin-lattice relaxation rate R1 of the ferrous sulfate doped-gelatin dosimeter. The model is based on three basic physical quantities: the ferric ion yield and the ferrous and ferric ion relaxivities, r2+ and r3+, respectively. These relaxivities specify the ability of the ions to enhance the spin-lattice relaxation of water protons. The effects of gelatin and sulfuric acid concentration on the ferric ion yield and ion relaxivities are presented. The measured r2+ values agree with those predicted by a model in which the measured spin relaxation is considered the result of the fast exchange of water hydrating the ferrous ion with water in the bulk. The r3+ values are lower than predicted by the fast exchange model. The discrepancies in the measured and predicted r3+ values are shown to result from the complexing of ferric ions arising from pH variation caused by changes in gelatin or sulfuric acid concentrations. A modified version of the R1-dose response model accounting for ferric ion complexing is presented and tested spectrophotometrically.
An easy and rapid method for the measurement of the intrinsic spatial resolution of a gamma camera was developed. The measurement is based on the first and second statistical moments of regions of interest (ROIs) applied to bar phantom images. This leads to an estimate of the modulation transfer function (MTF) and the full-width-at-half-maximum (FWHM) of a line spread function (LSF). Bar phantom images were acquired using four large field-of-view (LFOV) gamma cameras (Scintronix, Picker, Searle, Siemens). The following factors important for routine measurements of gamma camera resolution with this method were tested: ROI placement and shape, phantom orientation, spatial sampling, and procedural consistency. A 0.2% coefficient of variation (CV) between repeat measurements of MTF was observed for a circular ROI. The CVs of less than 2% were observed for measured MTF values for bar orientations ranging from -10 degrees to +10 degrees with respect to the x and y axes of the camera acquisition matrix. A 256 x 256 matrix (1.6 mm pixel spacing) was judged sufficient for routine measurements, giving an estimate of the FWHM to within 0.1 mm of manufacturer-specified values (3% difference). Under simulated clinical conditions, the variation in measurements attributable to procedural effects yielded a CV of less than 2% in newer generation cameras. The moments method for determining MTF correlated well with a peak-valley method, with an average difference of 0.03 across the range of spatial frequencies tested (0.11-0.17 line pairs/mm, corresponding to 4.5-3.0 mm bars). When compared with the NEMA method for measuring intrinsic spatial resolution, the moments method was found to be within 4% of the expected FWHM.
In radiation dosimetry protocols, plastic is allowed as a phantom material for the determination of absorbed dose to water in electron beams. The electron fluence correction factor is needed in conversion of dose measured in plastic to dose in water. There are large discrepancies among recommended values as well as measured values of electron fluence correction factors when polystyrene is used as a phantom material. Using the Monte Carlo technique, we have calculated electron fluence correction factors for incident clinical beam energies between 5 and 50 MeV as a function of depth for clear polystyrene, white polystyrene and PMMA phantom materials and compared the results with those recommended in protocols as well as experimental values from published data. In the Monte Carlo calculations, clinical beams are simulated using the EGS4 user-code BEAM for a variety of medical accelerators. The study shows that our calculated fluence correction factor, phi pw, is a function of depth and incident beam energy Eo with little dependence on other aspects of beam quality. However the phi pw values at dmax are indirectly influenced by the beam quality since they vary with depth and dmax also varies with the beam quality. Calculated phi pw values at dmax are in a range of 1.005-1.045 for a clear polystyrene phantom, 1.005-1.038 for a white polystyrene phantom and 0.996-1.016 for a PMMA phantom. Our values of phi pw are about 1-2% higher than those determined according to the AAPM TG-25 protocol at dmax for clear or white polystyrene. Our calculated values of phi pw also explain some of the variations of measured data because of its depth dependence. A simple formula is derived which gives the electron fluence correction factor phi pw as a function of R50 at dmax or at the depth of 0.6R50-0.1 for any clinical electron beam with energy between 5 and 25 MeV for three plastics: clear polystyrene, white polystyrene and PMMA. The study also makes a careful distinction between phi pw and the corresponding IAEA Code of Practice quantity, hm.