Multiexponential relaxation in Fricke agarose gels: implications for NMR dosimetry

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Abstract
NMR relaxation times and of agarose and Fricke agarose gels have been measured in the range 17 - 51 MHz. The analysis of the spin echo curves indicates a multi-exponential behaviour, characterized by three components, at all the examined frequencies. The relative values, ranging from a few to a hundred milliseconds, can be attributed to different species of water molecules present in the gel. Two of these components are characterized by relaxation rates and , more sensitive than to gamma irradiation, the sensitivity S being . The three values decrease as a function of frequency, but no gain in dose sensitivity is obtained by changing the working frequency in the examined range. The relaxivity of agarose gels containing ferrous or ferric ions has also been measured and found to be different from those of the corresponding solutions in the absence of agarose. Thus it was possible to estimate the irradiation yield from three independent parameters, and . No effect of the dose rate or of the source energy has been observed for any of these parameters.