Abstract：
:In BANG gel dosimetry, the spin-spin relaxation rate, R2 = I/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted 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. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves 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 frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.
journal_name
Phys Med Bioljournal_title
Physics in medicine and biologyauthors
De Deene Y,De Wagter C,De Neve W,Achten Edoi
10.1088/0031-9155/45/7/307subject
Has Abstractpub_date
2000-07-01 00:00:00pages
1807-23issue
7eissn
0031-9155issn
1361-6560journal_volume
45pub_type
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