Geometric distortion evaluation of magneticresonance images by a new large field of viewphantom for magnetic resonance based radiotherapypurposes

Background: the magnetic resonance imaging (mri)-based radiotherapyplanning method have been considered in recent years because of theadvantages of mri and the problems of planning with two images modality.the first step in mri-based radiotherapy is to evaluate magnetic resonance(mr) images geometric distortion. therefore, the present study aimed toevaluate system related geometric distortion by a new large field of viewphantom. materials and methods: a homemade phantom with perspexsheets and plastic pipes containing water was built for evaluating mr imagesdistortion. the phantom size is 48×48×37 cm3 and includes 325 water pipes.the study evaluated four different protocols from a 60 cm bore magnetom®symphony syngo 1.5 t (siemens). results: it was found that the amount ofdistortion for all protocols is under 2 mm for the radial distances less than 10cm (field of view (fov) = 20 cm), but distortion increased in radial distancesgreater than 10 cm, and reached about 5 cm for radial distances greater than25 cm. conclusion: geometric distortion of each mr scanner has been shownto be dependent on the radio frequency (rf) sequence pulse (spin echo orgradient echo) and image parameters (echo time (te), repetition time (tr),and receiver band-width)). the geometric distortion could be ignored for thefov<20 cm (for the head region), and must be evaluated and corrected formore fovs before the mr only radiotherapy.