Artefacts in multi-echo T₂ imaging for high-precision gel dosimetry: II. Analysis of B₁-field inhomogeneity

In BANG gel dosimetry, the spin-spin relaxation rate, R₂ = 1/T₂, is related to radiation dose that has been delivered to a gel phantom. R₂ is calculated by fitting the pixel intensities of a set of differently T₂-weighted base images. The accuracy that is aimed for in this quantitative MR application is about 5% relative to the maximum dose. In a conventional imaging MR scanner, however, several imaging artefacts may perturb the final dose map. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose maps are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B₁-field inhomogeneities as a source of dose inaccuracy. First, the influence of B₁-field inhomogeneities on slice profiles is investigated using a thin- slice phantom. Secondly, a FLASH sequence is used to map the B₁- 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 B₁-field inhomogeneities on the measured R₂. This work offers a method to correct R₂ maps for B₁-field inhomogeneities.