Spatiotemporal magnetic field monitoring for MR

Abstract

MR experiments frequently rely on signal encoding by the application of magnetic fields that vary in both space and time. The accurate interpretation of the resulting signals often requires knowledge of the exact spatiotemporal field evolution during the experiment. To better fulfill this need, a new approach is presented that enables measuring the field evolution concurrently with any MR sequence. Miniature NMR probes are used to monitor the MR phase evolution around the object under investigation. Based on these data, a global phase model is calculated that can then be used as a basis for processing the actual image or spectroscopic data. The new method is demonstrated by MRI of a phantom, using spin-warp, spiral, and EPI trajectories. Throughout, the monitoring results enabled highly accurate image reconstruction, even in the presence of massive gradient imperfections. Magn Reson Med 60:187-197, 2008.

Higher order reconstruction for MRI in the presence of spatiotemporal field perturbations

“…Time integration according to Eq. 1 then yields the phase model (37) [3] where B ref (r) denotes the net field at some reference point in time and the sum represents phase accumulation due to any field dynamics relative to B ref (r). h l denotes the l-th out of N l solid harmonics (36) and k l the corresponding expansion coefficient.…”

“…37. Subsequently the phase coefficients k l (t) were obtained for each scan by fitting the 3rd-order harmonic model to the probes' phase time courses.…”

A transmit/receive system for magnetic field monitoring of in vivo MRI

“…Four field probes were arranged around the eight-channel head array in an approximately tetrahedral fashion. In each setup the probe system was calibrated as described previously (19), involving measurements of the probes' positions and static frequency offsets. For each EPI segment a first-order phase model was calculated according to Ref.…”

“…To address this issue it has recently been proposed to monitor the actual field evolution concurrently with any MR sequence (19). In this approach miniature NMR probes (20 -25) are used to record the phase evolution of MR signals originating from a set of positions around the imaging region.…”

“…The field monitoring approach has thus far been demonstrated for phantom imaging with fast readouts of gradient echoes (19), using receive-only NMR probes. While proving the monitoring principle, the receive-only approach has important limitations.…”