Spatial variation of radio frequency magnetic field exposure from clinical pulse sequences in 1.5T MRI

Sammanfattning: Cell biological exposure studies in magnetic resonance imaging (MRI) environment, where a complex mixture of strong magnetic fields are present, have attracted considerable interest in recent years. The outcome of such studies might depend strongly on the conditions, for example exposure parameters and spatial variations of exposure. The aim of this thesis has been to give a detailed description of how the radio frequency (RF) magnetic field varies with position and sequence choice within an MRI bore from a patient perspective and to highlight the need of better consistency in future research. Method: A straightforward theoretical description on the contribution to the RF magnetic field from a birdcage coil is given. A one dimensional coaxial loop antenna has been used as a probe to measure spatial variations of the RF magnetic field in a 1.5T MRI scanner. An exposure matrix containing RF magnetic field strength (H1-field) amplitudes in three dimensions was constructed and used to study several clinical protocols and sequences. A qualified correspondence measurement was also made on a 3T MRI scanner. Results: Around isocenter, for a common field-of-view (FOV), changes in exposure conditions were small; however, rapid changes of exposure conditions occurred upon approaching the end rings. The dominating H1-field component switched from lying in the xy-plane to pointing the z-direction and was roughly 3 times larger than in isocenter. Practical difficulties indicate even larger differences at positions not measurable with the equipment at hand. The strongest H1-field component was 32.6 A/m at position (x,y,z)=(-24,8,24) cm from the isocenter. Conclusions: Machine parameters such as repetition time, echo time and flip angle have little to do with actual exposure. Specic absorption rate (SAR) values correlated well with the square of measured root-mean-square (RMS) values of the magnetic field (B1,RMS) but not with peak values of the magnetic field (B1,peak), indicating that peak values are not unlikely to be part of compromising factors in previous contradictory exposure research on genotoxicity. Furthermore exposure conditions depend strongly on position and unfavorable situations may occur in the periphery of the birdcage coil. Potentially elevated risks for conducting surfaces, for example arms or external fixations, in the proximity of the end rings, are proposed. Aside from spatial variation consideration on which type of geometry exposed cell-biological samples are placed in should be held since eddy currents, hot-spots and proper SAR depend on geometry. Conditions may vary considerably between in-vitro, ex-vivo and in-vivo studies since geometries of test tubes, petri dishes and humans differ.