Artefacts in multi-echo T-2 imaging for high-precision gel dosimetry: I. Analysis and compensation of eddy currents

In BANG gel dosimetry, the spin-spin relaxation rate, R-2 = 1/T-2, is relat ed to the radiation dose that has been delivered to the Eel phantom. R-2 is calculated by fitting the pixel intensities of a set of differently T-2-we ighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower. is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. Howev er, in a conventional MR scanner eddy currents will be provoked by switchin g the imaging gradients. As the eddy currents change in the course of the s equence, the net magnetization will be affected accordingly. Hence, eddy cu rrents may have a significant influence on the quantitative R-2 images them selves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a freque ncy shift method resulting in eddy current field maps. The related geometri cal displacements are obtained by use of a pyramidal phantom. The R-2 versu s dose relation is determined in the three main directions of the magnet, r evealing a dependence of the measured R-2 on slice orientation. The time co urse of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R-2 images. A compensation method for eddy current effects in multi-echo T-2 mapping is described.