An In-Bore Receiver for Magnetic Resonance Imaging.

In magnetic resonance imaging, the use of array detection and the number of detector elements have seen a steady increase over the past two decades. As a result, per-channel analog connection via long coaxial cable, as commonly used, poses an increasing challenge in terms of handling, safety, and coupling among cables. This situation is exacerbated when complementary recording of radiofrequency transmission or NMR-based magnetic field sensing further add to channel counts.

Single-shot spiral imaging at 7 T.

Purpose: The purpose of this work is to explore the feasibility and performance of single-shot spiral MRI at 7 T, using an expanded signal model for reconstruction.

Methods: Gradient-echo brain imaging is performed on a 7 T system using high-resolution single-shot spiral readouts and half-shot spirals that perform dual-image acquisition after a single excitation. Image reconstruction is based on an expanded signal model including the encoding effects of coil sensitivity, static off-resonance, and magnetic field dynamics.

Ultrafast Ligand Self-Exchanging Gadolinium Complexes in Ionic Liquids for NMR Field Probes.

Concurrent magnetic field monitoring in MRI with an array of NMR field probes allows for reducing image imperfections. High F concentrations together with short relaxation times are basic probe properties. We present the NMR properties of [Gd(NTf)] dissolved in an ionic liquid consisting of an imidazolium cation and the [NTf] anion.

Rapid anatomical brain imaging using spiral acquisition and an expanded signal model.

We report the deployment of spiral acquisition for high-resolution structural imaging at 7T. Long spiral readouts are rendered manageable by an expanded signal model including static off-resonance and B dynamics along with k-space trajectories and coil sensitivity maps. Image reconstruction is accomplished by inversion of the signal model using an extension of the iterative non-Cartesian SENSE algorithm.

Dynamic nuclear magnetic resonance field sensing with part-per-trillion resolution.

High-field magnets of up to tens of teslas in strength advance applications in physics, chemistry and the life sciences. However, progress in generating such high fields has not been matched by corresponding advances in magnetic field measurement. Based mostly on nuclear magnetic resonance, dynamic high-field magnetometry is currently limited to resolutions in the nanotesla range.

Gradient and shim pre-emphasis by inversion of a linear time-invariant system model.

Purpose: The goal of this contribution is to enhance the fidelity and switching speed of gradient and shim fields by advancing pre-emphasis toward broadband and full cross-term correction.

Theory And Methods: The proposed approach is based on viewing gradient and shim chains as linear, time-invariant (LTI) systems. Pre-emphasis is accomplished by inversion of a broadband digital system model.

Single-shot spiral imaging enabled by an expanded encoding model: Demonstration in diffusion MRI.

Purpose: The purpose of this work was to improve the quality of single-shot spiral MRI and demonstrate its application for diffusion-weighted imaging.

Methods: Image formation is based on an expanded encoding model that accounts for dynamic magnetic fields up to third order in space, nonuniform static B , and coil sensitivity encoding. The encoding model is determined by B mapping, sensitivity mapping, and concurrent field monitoring.

Continuous Magnetic Field Monitoring Using Rapid Re-Excitation of NMR Probe Sets.

MRI relies on static and spatially varying dynamic magnetic fields of high accuracy. NMR field probes permit the direct observation of spatiotemporal field dynamics for diverse purposes such as data correction, field control, sequence validation, and hardware characterization. However, due to probe signal decay and dephasing existing field cameras are limited in terms of readout duration and the extent of k -space that can be covered.

Concurrent recording of RF pulses and gradient fields - comprehensive field monitoring for MRI.

Reconstruction of MRI data is based on exact knowledge of all magnetic field dynamics, since the interplay of RF and gradient pulses generates the signal, defines the contrast and forms the basis of resolution in spatial and spectral dimensions. Deviations caused by various sources, such as system imperfections, delays, eddy currents, drifts or externally induced fields, can therefore critically limit the accuracy of MRI examinations. This is true especially at ultra-high fields, because many error terms scale with the main field strength, and higher available SNR renders even smaller errors relevant.

Image reconstruction using a gradient impulse response model for trajectory prediction.

Purpose: Gradient imperfections remain a challenge in MRI, especially for sequences relying on long imaging readouts. This work aims to explore image reconstruction based on k-space trajectories predicted by an impulse response model of the gradient system.

Theory And Methods: Gradient characterization was performed twice with 3 years interval on a commercial 3 Tesla (T) system.

Diffusion MRI with concurrent magnetic field monitoring.

Purpose: Diffusion MRI is compromised by unknown field perturbation during image encoding. The purpose of this study was to address this problem using the recently described approach of concurrent magnetic field monitoring.

Methods: Magnetic field dynamics were monitored during the echo planar imaging readout of a common diffusion-weighted MRI sequence using an integrated magnetic field camera setup.

A field camera for MR sequence monitoring and system analysis.

Purpose: MR image formation and interpretation relies on highly accurate dynamic magnetic fields of high fidelity. A range of mechanisms still limit magnetic field fidelity, including magnet drifts, eddy currents, and finite linearity and stability of power amplifiers used to drive gradient and shim coils. Addressing remaining errors by means of hardware, sequence, or signal processing optimizations, calls for immediate observation by magnetic field monitoring.

Analysis of temperature dependence of background phase errors in phase-contrast cardiovascular magnetic resonance.

Background: The accuracy of phase-contrast cardiovascular magnetic resonance (PC-CMR) can be compromised by background phase errors. It is the objective of the present work to provide an analysis of the temperature dependence of background phase errors in PC-CMR by means of gradient mount temperature sensing and magnetic field monitoring.

Methods: Background phase errors were measured for various temperatures of the gradient mount using magnetic field monitoring and validated in a static phantom.

Real-time motion correction using gradient tones and head-mounted NMR field probes.

Purpose: Sinusoidal gradient oscillations in the kilohertz range are proposed for position tracking of NMR probes and prospective motion correction for arbitrary imaging sequences without any alteration of sequence timing. The method is combined with concurrent field monitoring to robustly perform image reconstruction in the presence of potential dynamic field deviations.

Methods: Benchmarking experiments were done to assess the accuracy and precision of the method and to compare it with theoretical predictions based on the field probe's time-dependent signal-to-noise ratio.

Retrospective correction of physiological field fluctuations in high-field brain MRI using concurrent field monitoring.

Purpose: Magnetic field fluctuations caused by subject motion, such as breathing or limb motion, can degrade image quality in brain MRI, especially at high field strengths. The purpose of this study was to investigate the feasibility of retrospectively correcting for such physiological field perturbations based on concurrent field monitoring.

Theory And Methods: High-resolution T2*-weighted gradient-echo images of the brain were acquired at 7T with subjects performing different breathing and hand movement patterns.

Matched-filter acquisition for BOLD fMRI.

We introduce matched-filter fMRI, which improves BOLD (blood oxygen level dependent) sensitivity by variable-density image acquisition tailored to subsequent image smoothing. Image smoothing is an established post-processing technique used in the vast majority of fMRI studies. Here we show that the signal-to-noise ratio of the resulting smoothed data can be substantially increased by acquisition weighting with a weighting function that matches the k-space filter imposed by the smoothing operation.

Single-shot imaging with higher-dimensional encoding using magnetic field monitoring and concomitant field correction.

Purpose: PatLoc (Parallel Imaging Technique using Localized Gradients) accelerates imaging and introduces a resolution variation across the field-of-view. Higher-dimensional encoding employs more spatial encoding magnetic fields (SEMs) than the corresponding image dimensionality requires, e.g.

Real-time feedback for spatiotemporal field stabilization in MR systems.

Purpose: MR imaging and spectroscopy require a highly stable, uniform background field. The field stability is typically limited by hardware imperfections, external perturbations, or field fluctuations of physiological origin. The purpose of the present work is to address these issues by introducing spatiotemporal field stabilization based on real-time sensing and feedback control.

Field camera measurements of gradient and shim impulse responses using frequency sweeps.

Purpose: Applications of dynamic shimming require high field fidelity, and characterizing the shim field dynamics is therefore necessary. Modeling the system as linear and time-invariant, the purpose of this work was to measure the impulse response function with optimal sensitivity.

Theory And Methods: Frequency-swept pulses as inputs are analyzed theoretically, showing that the sweep speed is a key factor for the measurement sensitivity.

Feedback field control improves linewidths in in vivo magnetic resonance spectroscopy.

Purpose: Magnetic resonance spectroscopy (MRS) experiments rely on a homogeneous and stable magnetic field within the sample. Field homogeneity is typically optimized by static B0 shimming while reproducible effects from dynamic field variation are commonly diminished by means of gradient system calibration as well as calibration based on non-water suppressed reference data. However, residual encoding deficiencies from incomplete calibration and nonreproducible field perturbations deteriorate the quality of the obtained data.

Monitoring and compensating phase imperfections in cine balanced steady-state free precession.

Purpose: To analyze and correct for eddy current-induced phase imperfections in cardiac cine balanced steady-state free precession (bSSFP) imaging.

Methods: Eddy current-induced phase offsets were measured for different phase-encoding schemes using a higher order dynamic field camera. Based on these measurements, offset phases were corrected for in postprocessing and by run-time phase compensation applying radiofrequency phase increments and additional compensatory gradient areas.

Single shot trajectory design for region-specific imaging using linear and nonlinear magnetic encoding fields.

It has recently been demonstrated that nonlinear encoding fields result in a spatially varying resolution. This work develops an automated procedure to design single-shot trajectories that create a local resolution improvement in a region of interest. The technique is based on the design of optimized local k-space trajectories and can be applied to arbitrary hardware configurations that employ any number of linear and nonlinear encoding fields.