Three-dimensional dose measurement with a Leksell gamma knife by means of a dosimeter gel and MR imaging

This paper reports on a method of measuring in three dimensions the narrow absorbed dose distributions obtained from a Leksell gamma knife that is under evaluation. This irradiation unit comprises 201 hemispherically distributed Co-60 sources collimated so that a small volume located at the center of the unit is cross-fired by narrow beams. A cavity filled with dosimeter gel of ferrous sulphate solution and agarose was located at the center of a spherical Perspex phantom and stereotactically irradiated in the unit. The chemical changes in the gel introduced by the radiation were measured by means of T1 relaxation in an MR scanner. The value 1/T1 has been found to be proportional to the absorbed dose