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Dynamic slicewise shimming improves B0 field homogeneity by updating shim coil currents for every slice in a multi-slice acquisition, producing better field homogeneity over a volume than can be obtained by a single static global shim. The first aim of this work was to evaluate the performance of slice-wise field map based 2nd order dynamic shimming in a human high field 7 Tesla clinical scanner vis-à-vis image based 2nd order static global shimming. Another goal was to characterize eddy currents induced by 2nd and 3rd order shim switching. A final aim was to compare global and dynamic shimming through shim orders to elucidate the relative benefits of going to higher orders and to dynamic shim updating from a static shimming regime. An external hardware module was used to store and dynamically update slice-optimized shim values during multislice data acquisition. High bandwidth multislice gradient echo scans with B0 field mapping and low bandwidth single shot echo planar scans were performed on phantoms and humans using 2nd order dynamic and static global shims. For the measurement of 2nd and 3rd order shim induced eddy currents, step response temporal phase changes of individual shims were measured and fit to shim harmonics spatially and to multiexponential decay functions temporally. Finally, an order wise fieldmap based comparison was performed with 1st, 2nd and 3rd order global static shimming, 1st and 2nd order dynamic shimming, as well as combined 2nd or 3rd order global and 1st order dynamic shim. Dynamic shimming considerably improved B0 homogeneity compared to static global shimming, both in phantoms and in human subjects, reducing image distortion and signal drop-out. The unshielded 2nd and 3rd order shims generated strong B0, self- and cross-term eddy fields with multiple time constants ranging from milliseconds to seconds. Field homogeneity improved with increasing order of shim, with dynamic shimming performing better than global shimming. Hybrid global and dynamic shimming approach yielded field homogeneity better than global static shims but worse than dynamic shims.

Purpose Head motion continues to be a major source of artifacts and data quality degradation in MRI. The goal of this work was to develop and demonstrate a novel technique for prospective, 6 degrees of freedom (6DOF) rigid body motion estimation and real time motion correction using inductively coupled wireless nuclear magnetic resonance (NMR) probe markers. Methods Three wireless probes that are inductively coupled with the scanner’s RF setup serve as fiducials on the subject’s head. A 12 ms linear navigator module is interleaved with the imaging sequence for head position estimation, and scan geometry is updated in real time for motion compensation. Flip angle amplification in the markers allows the use of extremely small navigator flip angles (~1°). A novel algorithm is presented to identify marker positions in the absence of marker specific receive channels. The method is demonstrated for motion correction in 1 mm3 gradient recalled echo experiments in phantoms and humans. Results Significant improvement of image quality is demonstrated in phantoms and human volunteers under different motion conditions. Conclusion A novel real time 6 DOF head motion correction technique based on wireless NMR probes is demonstrated in high resolution imaging at 7 Tesla.

Purpose The non‐uniform fast Fourier transform (NUFFT) involves interpolation of non‐uniformly sampled Fourier data onto a Cartesian grid, an interpolation that is slowed by complex, non‐local data access patterns. A faster NUFFT would increase the clinical relevance of the plethora of advanced non‐Cartesian acquisition methods. Methods Here we customize the NUFFT procedure for a radial trajectory and GPU architecture to eliminate the bottlenecks encountered when allowing for arbitrary trajectories and hardware. We call the result TRON, for TR ajectory O ptimized N UFFT. We benchmark the speed and accuracy TRON on a Shepp‐Logan phantom and on whole‐body continuous golden‐angle radial MRI. Results TRON was 6–30× faster than the closest competitor, depending on test data set, and was the most accurate code tested. Conclusions Specialization of the NUFFT algorithm for a particular trajectory yielded significant speed gains. TRON can be easily extended to other trajectories, such as spiral and PROPELLER. TRON can be downloaded at http://github.com/davidssmith/TRON .

Purpose Continuously Moving Table (CMT) MRI is a high throughput technique that has multiple applications in whole-body imaging. In this work, CMT MRI based on a Golden Angle (GA, 111.246° azimuthal step) radial sampling is developed at 3 Tesla, with the goal of increased flexibility in image reconstruction using arbitrary profile groupings. Methods CMT MRI with GA and Linear Angle (LA) schemes were developed for whole-body imaging at 3 Tesla with a table speed of 20 mm/sec. Imaging was performed in phantoms and a human volunteer with extended z field of views of up to 1.8 meters. Four separate LA and a single GA scan were performed to enable slice reconstructions at four different thicknesses. Results GA CMT MRI produced high image quality in phantoms and humans and allowed complete flexibility for reconstruction of slices with arbitrary slice thickness and position from a single data set. LA CMT MRI was constrained by predetermined parameters, required multiple scans and suffered from stair step artifacts that were not present in GA images. Conclusion GA sampling provides a robust flexible approach to CMT MRI for whole-body examination with the ability to reconstruct slices at arbitrary positions and thicknesses from a single scan.

Purpose Artifacts caused by large magnetic susceptibility differences between metallic needles and tissue are a persistent problem in many interventional MRI applications. The signal void caused by the needle can hide procedure targets and prevent accurate image‐based monitoring. In this paper, a solution to this problem is presented in the form of an active shim insert inspired from degaussing coils used in naval vessels, that is designed to correct the field disturbance (ΔB0) caused by the needle. Methods The ΔB0 induced by a 10 gauge hollow single‐beveled titanium needle at 3T is modeled in different orientations. A set of 63 orthogonal coil pairs with unique tip paths are evaluated for shimming performance, and an optimal coil pair is chosen. Shimming performance and current demands are evaluated over a range of needle orientations. Results Robust correction of the titanium needle induced ΔB0 is predicted using a flat no‐loop coil combined with an orthogonal 1½ turn loop coil angled at the bevel angle for most orientations, with currents well below 1 amp per coil. Reductions in ΔB0 standard deviations with shimming ranged from ~49% to ~10% depending on needle orientation, with performance worsening as the needle is aligned more along B0. Conclusion Simulations predict that it is possible to minimize metallic probe induced ΔB0 and signal losses using externally supplied direct current shim coil inserts in arbitrary orientations for potential benefits in many interventional MRI applications.

PurposeThe eye and its accessory structures, the optic nerve and the extraocular muscles, form a complex dynamic system. In vivo magnetic resonance imaging (MRI) of this system in motion can have substantial benefits in understanding oculomotor functioning in health and disease, but has been restricted to date to imaging of static gazes only. The purpose of this work was to develop a technique to image the eye and its accessory visual structures in motion.MethodsDynamic imaging of the eye was developed on a 3-Tesla MRI scanner, based on a golden angle radial sequence that allows freely selectable frame-rate and temporal-span image reconstructions from the same acquired data set. Retrospective image reconstructions at a chosen frame rate of 57 ms per image yielded high-quality in vivo movies of various eye motion tasks performed in the scanner. Motion analysis was performed for a left–right version task where motion paths, lengths, and strains/globe angle of the medial and lateral extraocular muscles and the optic nerves were estimated.ResultsOffline image reconstructions resulted in dynamic images of bilateral visual structures of healthy adults in only ∼15-s imaging time. Qualitative and quantitative analyses of the motion enabled estimation of trajectories, lengths, and strains on the optic nerves and extraocular muscles at very high frame rates of ∼18 frames/s.ConclusionsThis work presents an MRI technique that enables high-frame-rate dynamic imaging of the eyes and orbital structures. The presented sequence has the potential to be used in furthering the understanding of oculomotor mechanics in vivo, both in health and disease.

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