Artefacts in multi-echo T-2 imaging for high-precision gel dosimetry: II. Analysis of B-1-field inhomogeneity

In BANG gel dosimetry, the spin-spin relaxation rate, R-2 = 1/T-2, is relat ed to radiation dose that has been delivered to a gel phantom. R-2 is calcu lated by fitting the pixel intensities of a set of differently T-2-weighted base images. The accuracy that is aimed for in this quantitative MR applic ation is about 5% relative to the maximum dose. In a conventional imaging M R scanner, however, several imaging artefacts may perturb the final dose ma p. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose map s are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B-1-field inhomogeneities as a source of dose inaccur acy. First, the influence of B-1-field inhomogeneities on slice profiles is investigated using a thin-slice phantom. Secondly, a FLASH sequence is use d to map the B-1-field by assessing the effective flip angle in each voxel of a homogeneous phantom. In addition. both experiments and computer simulations revealed the effects of B-1-field inhomogeneities on the measured R-2. This work offers a metho d to correct R-2 maps for B-1-field inhomogeneities.