MULTIEXPONENTIAL T-2 RELAXATION IN FRICKE AGAROSE GELS - IMPLICATIONSFOR NMR DOSIMETRY

NMR relaxation times T-1 and T-2 of agarose and Fricke agarose gels ha ve been measured in the range 17-51 MHz. The analysis of the spin echo curves indicates a multiexponential behaviour, characterized by three components, at all the examined frequencies. The relative T-2 values, ranging from a few to a hundred milliseconds, can be attributed to di fferent species of water molecules present in the gel. Two of these co mponents are characterized by relaxation rates R(2)(a) and R(2)(b), mo re sensitive than R(1) to gamma irradiation, the sensitivity S being S (R(1)) = 0.066 s(-1) Gy(-1), S(R(2)(a)) = 0.088 s(-1) Gy(-1), S(R(2)(b )) = 0.17 s(-1) Gy(-1). The three T-2 values decrease as a function of frequency, but no gain in dose sensitivity is obtained by changing th e working frequency in the examined range. The relaxivity of agarose g els containing ferrous or ferric ions has also been measured and found to be different from those of the corresponding solutions in the abse nce of agarose. Thus it was possible to estimate the irradiation yield from three independent parameters, R(1), R(2)(a) and R(2)(b). No effe ct of the dose rate or of the source energy has been observed for any of these parameters.