FAST T-1 IMAGING OF DUAL GEL SAMPLES FOR DIFFUSION MEASUREMENTS IN NMR DOSIMETRY GELS

Diffusion of iron is one of the major problems limiting the usefulness of NMR gel dosimetry, This was studied in dual gel samples using a 4. 7T micro-imaging MR scanner and a fast T-1 imaging sequence which allo wed the acquisition of a 64 x 128 x 8 data sets (phase encoding x freq uency encoding x number of inversion times) in less than 15 min, The p rocedure enabled us to obtain relative relaxation times for any region of interest within the sample, After the two differently doped gels w ere brought into contact in the dual gel samples (diameter 12mm), the diffusion could be observed on subsequent images as a function of time , An inverse square root function was used to fit the change of 1/T-1 across the junction between the two gel phases, A diffusion constant o f 0.014 +/- 0.003 cm(2)/h was determined for Fe3+ in a typical dosimet ry gel (1.5% agarose, 50mM H2SO4). This could be lowered by adding a c helating agent such as xylenol orange to the gel, It was also found th at diffusion was slower in gelatine gels, however these gels tended no t to set properly when H2SO4 was added as required for NMR dosimetry, From the present results we propose that a gel consisting of 1.5% agar ose (for stability), 3% gelatine and 0.1mM xylenol orange (to combat d iffusion and allow a visual evaluation) is a suitable base for NMR dos imetry gels, The use of a fast T-1 imaging sequence reduces acquisitio n times and therefore the potential impact of diffusion, (C) 1997 Else vier Science Inc.