Thermal variation in gradient response: measurement and modeling.

Purpose: Many aspects and imperfections of gradient dynamics in MRI have been successfully captured by linear time-invariant (LTI) models. Changes in gradient behavior due to heating, however, violate time invariance. The goal of this work is to study such changes at the level of transfer functions and model them by thermal extension of the LTI framework.

Simultaneous feedback control for joint field and motion correction in brain MRI.

T2*-weighted gradient-echo sequences count among the most widely used techniques in neuroimaging and offer rich magnitude and phase contrast. The susceptibility effects underlying this contrast scale with B, making T2*-weighted imaging particularly interesting at high field. High field also benefits baseline sensitivity and thus facilitates high-resolution studies.

High-resolution short-T MRI using a high-performance gradient.

Purpose: To achieve high resolution in imaging of short-T materials and tissues by using a high-performance human-sized gradient insert with strength up to 200 mT/m and 100% duty cycle.

Methods: Dedicated short-T methodology and hardware are used, such as the pointwise encoding time reduction with radial acquisition (PETRA) technique with modulated excitation pulses, optimized radio-frequency hardware, and a high-performance gradient insert. A theoretical analysis of actual spatial resolution and SNR is provided to support the choice of scan parameters and interpretation of the results.

A Reconfigurable Platform for Magnetic Resonance Data Acquisition and Processing.

Developments in magnetic resonance imaging (MRI) in the last decades show a trend towards a growing number of array coils and an increasing use of a wide variety of sensors. Associated cabling and safety issues have been addressed by moving data acquisition closer to the coil. However, with the increasing number of radio-frequency (RF) channels and trend towards higher acquisition duty-cycles, the data amount is growing, which poses challenges for throughput and data handling.

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.

Gradient Response Harvesting for Continuous System Characterization During MR Sequences.

MRI gradient systems are required to generate magnetic field gradient waveforms with very high fidelity. This is commonly implemented by gradient system calibration and pre-emphasis. However, a number of mechanisms, particularly thermal changes, cause variation in the gradient response over time, which cannot be addressed by calibration approaches.

A high-performance gradient insert for rapid and short-T imaging at full duty cycle.

Purpose: The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting-edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short-T techniques, the hardware design focused on the head and the extremities as target anatomies.

Methods: A boundary element method with minimization of power dissipation and stored magnetic energy was used to design anatomy-targeted gradient coils with maximally relaxed geometry constraints.

Multi-Rate Acquisition for Dead Time Reduction in Magnetic Resonance Receivers: Application to Imaging With Zero Echo Time.

For magnetic resonance imaging of tissues with very short transverse relaxation times, radio-frequency excitation must be immediately followed by data acquisition with fast spatial encoding. In zero-echo-time (ZTE) imaging, excitation is performed while the readout gradient is already on, causing data loss due to an initial dead time. One major dead time contribution is the settling time of the filters involved in signal down-conversion.

Filling the dead-time gap in zero echo time MRI: Principles compared.

Purpose: MRI of tissues with short coherence lifetimes T or T2* can be performed efficiently using zero echo time (ZTE) techniques such as algebraic ZTE, pointwise encoding time reduction with radial acquisition (PETRA), and water- and fat-suppressed proton projection MRI (WASPI). They share the principal challenge of recovering data in central k-space missed due to an initial radiofrequency dead time. The purpose of this study was to compare the three techniques directly, with a particular focus on their behavior in the presence of ultra-short-lived spins.

Prospective motion correction with NMR markers using only native sequence elements.

Purpose: To develop a method of tracking active NMR markers that requires no alterations of common imaging sequences and can be used for prospective motion correction (PMC) in brain MRI.

Methods: Localization of NMR markers is achieved by acquiring short signal snippets in rapid succession and evaluating them jointly. To spatially encode the markers, snippets are timed such that signal phase is accrued during sequence intervals with suitably diverse gradient actuation.

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.

Feedback field control improves the precision of T * quantification at 7 T.

T * mapping offers access to a number of important structural and physiological tissue parameters. It is robust against RF field variations and overall signal scaling. However, T * measurement is highly sensitive to magnetic field errors, including perturbations caused by breathing motion at high baseline field.

Correction of parallel transmission using concurrent RF and gradient field monitoring.

Objectives: The accuracy and precision of the parallel RF excitations are highly dependent on the spatial and temporal fidelity of the magnetic fields involved in spin excitation. The consistency between the nominal and effective fields is typically limited by the imperfections of the employed hardware existing both in the gradient system and the RF chain. In this work, we experimentally presented highly improved spatially tailored parallel excitations by turning the native hardware accuracy challenge into a measurement and control problem using an advanced field camera technology to fully correct parallel RF transmission experiment.

Physiology recording with magnetic field probes for fMRI denoising.

Physiological noise originating in cardiovascular and respiratory processes is a substantial confound in BOLD fMRI. When unaccounted for it reduces the temporal SNR and causes error in inferred brain activity and connectivity. Physiology correction typically relies on auxiliary measurements with peripheral devices such as ECG, pulse oximeters, and breathing belts.

Analysis and correction of field fluctuations in fMRI data using field monitoring.

This work investigates the role of magnetic field fluctuations as a confound in fMRI. In standard fMRI experiments with single-shot EPI acquisition at 3 Tesla the uniform and gradient components of the magnetic field were recorded with NMR field sensors. By principal component analysis it is found that differences of field evolution between the EPI readouts are explainable by few components relating to slow and within-shot field dynamics of hardware and physiological origin.

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.

Symmetrically biased T/R switches for NMR and MRI with microsecond dead time.

For direct NMR detection and imaging of compounds with very short coherence life times the dead time between radio-frequency (RF) pulse and reception of the free induction decay (FID) is a major limiting factor. It is typically dominated by the transient and recovery times of currently available transmit-receive (T/R) switches and amplification chains. A novel PIN diode-based T/R switch topology is introduced allowing for fast switching by high bias transient currents but nevertheless producing a very low video leakage signal and insertion loss (0.

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.

Utility of real-time field control in T2 *-Weighted head MRI at 7T.

Purpose: Real-time field control can serve to reduce respiratory field perturbations during T2 * imaging at high fields. This work investigates the effectiveness of this approach in relation to key variables such as patient physique, breathing patterns, slice location, and the choice of sequence.

Methods: To cover variation in physical constitution and breathing behavior, volunteers with a wide range of body-mass-indices were asked to breathe either normally or deeply during T2 *-weighted image acquisition at 7T.

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.

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.