Fast T1 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 T1 imaging sequence which allowed the acquisition of a 64 × 128 × 8 data sets (phase encoding × frequency encoding × number of inversion times) in less than 15 min. The procedure enabled us to obtain relative relaxation times for any region of interest within the sample. After the two differently doped gels were 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 1T1 across the junction between the two gel phases. A diffusion constant of 0.014 ± 0.003 cm2/h was determined for Fe3+ in a typical dosimetry gel (1.5% agarose, 50mM H2SO4). This could be lowered by adding a chelating agent such as xylenol orange to the gel. It was also found that diffusion was slower in gelatine gels, however these gels tended not 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% agarose (for stability), 3% gelatine and 0.1mM xylenol orange (to combat diffusion and allow a visual evaluation) is a suitable base for NMR dosimetry gels. The use of a fast T1 imaging sequence reduces acquisition times and therefore the potential impact of diffusion.