Multiparametric imaging with heterogeneous radiofrequency fields

Abstract

Magnetic resonance imaging (MRI) has become an unrivalled medical diagnostic technique able to map tissue anatomy and physiology non-invasively. MRI measurements are meticulously engineered to control experimental conditions across the sample. However, residual radiofrequency (RF) field inhomogeneities are often unavoidable, leading to artefacts that degrade the diagnostic and scientific value of the images. Here we show that, paradoxically, these artefacts can be eliminated by deliberately interweaving freely varying heterogeneous RF fields into a magnetic resonance fingerprinting data-acquisition process. Observations made based on simulations are experimentally confirmed at 7 Tesla (T), and the clinical implications of this new paradigm are illustrated with in vivo measurements near an orthopaedic implant at 3T. These results show that it is possible to perform quantitative multiparametric imaging with heterogeneous RF fields, and to liberate MRI from the traditional struggle for control over the RF field uniformity.

Visualizing electromagnetic fields in metals by MRI

“…(5) yields S ac /S ab = 161, rendering the contribution from the ab faces all but invisible in those images. It would be interesting to see future studies involving EM simulations, 14,25,26 B1 maps, 27 etc., to gain additional insight about EM field distribution around bulk metals and help understand the remaining gap between the observed and derived intensity ratios, and to see to what extent the effective voxel size emerges.…”

How to choose the right MR sequence for your research question at 7 T and above?

“…If the faster and B0 calibrations already available in the literature (Brunner and Pruessmann, 2016;Eggenschwiler et al, 2014;Moortele and Ugurbil, 2009;Setsompop et al, 2008) are made part of the initial scan preparations and rf-shimming, pulse design and SAR calculations (using methods as VOP (Eichfelder and Gebhardt, 2011;Lee et al, 2012)) get integrated into the sequence preparation, this would then bring us closer to the theoretical SNR increase associated with the higher magnetic fields. In this context, it is also interesting to note that pseudo-randomly varying the -field in a finger-printing sequence (by varying the transmit phase relationships across transmit channels), makes it possible to efficiently obtain MR parametric maps unaffected by inhomogeneity in without ever having to rely on any level of homogeneity of the transmitfield (Cloos et al, 2016).…”

Fast quantitative MRI as a nonlinear tomography problem

“…Subsequently, the tissue parameters quantification is carried out for each location separately. The second step (parameter estimation) is thus obtained from a series of magnetization images by fitting relatively simplistic signal models [4] or by searching over a dictionary of complex signal fingerprints [5,6]. …”

“…The RF excitation and gradient acquisition schemes need to be designed properly to ensure incoherence between the signal and the undersampling artifacts which are interpreted as zero-mean noise-like perturbations. Interleaved spiral [5] and radial [6] readout gradients are therefore preferred. These type of sequences are however, prone to gradient system imperfections such as eddy currents and thus require an additional sophisticated calibration of the hardware [7].…”