Magnetic field gradient waveform monitor (MFGM)

INTRODUCTION: Fast, efficient MRI measurements rely on magnetic field gradient system with high fidelities. Great hardware improvements, such as actively shielded gradient coil and delicate gradient waveform pre-emphasis, have been made to meet this goal. Modern MRI techniques, however, see a trend putting an increasing expectation on gradient system perfection [1]: single shot (EPI, RARE) and Non-Cartesian (radial, spiral) data acquisition, phase contrast imaging, diffusion weighted EPI, and the emerging techniques such as Ultrashort Echo Time (UTE) imaging [2]. Numerous methods have been developed to measure MRI gradient waveforms and k-space trajectories for correcting the remaining hardware imperfections. The most widely used method to characterize eddy currents behavior is a slice selection method by Duyn [3]. The most promising new strategy appears to be magnetic field monitoring (MFM) with RF microprobes [4]. The new concept, pure phase encode magnetic field gradient monitor (MFGM), was recently proposed by us [5,6] to solve four critical problems related to the above two methods: (i) limited gradient waveform duration (100 ms); (ii) limited k value (1 mm) / net gradient area (50 ms · mT/m); (iii) limited gradient strength (40 mT/m); (iv) susceptibility matched RF microprobes required. In the presented work an enhanced MFGM method is proposed to measure B0(t) field evolution associated with the gradient waveform. For the first time the four advantages outlined above are illustrated in a realistic application.