Advances in spiral fMRI: A high-resolution dataset.

We present data collected for the research article "Advances in Spiral fMRI: A High-resolution Study with Single-shot Acquisition" (Kasper et al. 2022). All data was acquired on a 7T ultra-high field MR system (Philips Achieva), equipped with a concurrent magnetic field monitoring setup based on 16 NMR probes. For task-based fMRI, a visual quarterfield stimulation paradigm was employed, alongside physiological monitoring via peripheral recordings. This data collection contains different datasets pertaining to different purposes: (1) Measured magnetic field dynamics (k0, spiral k-space trajectories, 2nd order spherical harmonics, concomitant fields) during ultra-high field fMRI sessions from six subjects, as well as concurrent temperature curves of the gradient coil, to explore MR system and subject-induced variability of field fluctuations and assess the impact of potential correction methods. (2) MR Raw Data, i.e., coil and concurrent encoding magnetic field (trajectory) data, of a single subject, as well as nominal spiral gradient waveforms, precomputed B0 and coil sensitivity maps, to enable testing of alternative image reconstruction approaches for spiral fMRI data. (3) Reconstructed image time series of the same subject alongside behavioral and physiological logfiles, to reproduce the fMRI preprocessing and analysis, as well as figures presented in the research article related to this article, using the published analysis code repository. All data is provided in standardized formats for the respective research area. In particular, ISMRMRD (HDF5) is used to store raw coil data and spiral trajectories, as well as measured field dynamics. Likewise, the NIfTI format is used for all imaging data (anatomical MRI and spiral fMRI, B0 and coil sensitivity maps).

Advances in spiral fMRI: A high-resolution study with single-shot acquisition.

Spiral fMRI has been put forward as a viable alternative to rectilinear echo-planar imaging, in particular due to its enhanced average k-space speed and thus high acquisition efficiency. This renders spirals attractive for contemporary fMRI applications that require high spatiotemporal resolution, such as laminar or columnar fMRI. However, in practice, spiral fMRI is typically hampered by its reduced robustness and ensuing blurring artifacts, which arise from imperfections in both static and dynamic magnetic fields. Recently, these limitations have been overcome by the concerted application of an expanded signal model that accounts for such field imperfections, and its inversion by iterative image reconstruction. In the challenging ultra-high field environment of 7 Tesla, where field inhomogeneity effects are aggravated, both multi-shot and single-shot 2D spiral imaging at sub-millimeter resolution was demonstrated with high depiction quality and anatomical congruency. In this work, we further these advances towards a time series application of spiral readouts, namely, single-shot spiral BOLD fMRI at 0.8 mm in-plane resolution. We demonstrate that high-resolution spiral fMRI at 7 T is not only feasible, but delivers both excellent image quality, BOLD sensitivity, and spatial specificity of the activation maps, with little artifactual blurring. Furthermore, we show the versatility of the approach with a combined in/out spiral readout at a more typical resolution (1.5 mm), where the high acquisition efficiency allows to acquire two images per shot for improved sensitivity by echo combination.

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. A generic way of addressing this trend is the transition to digital signal transmission, enabled by digitization and first-level digital processing close to detector coils and sensors in the magnet bore. The foremost challenge that comes with this approach is to achieve high dynamic range, linearity, and phase stability despite interference by strong static, audiofrequency, and radiofrequency fields. The present work reports implementation of a 16-channel in-bore receiver, performing signal digitization and processing with subsequent optical transmission over fiber. Along with descriptions of the system design and construction, performance evaluation is reported. The resulting device is fully MRI compatible providing practically equal performance and signal quality compared to state-of-the-art RF digitizers operating outside the magnet. Its use is demonstrated by examples of head imaging and magnetic field recording.

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. The latter are recorded concurrently with image acquisition, using NMR field probes. The resulting image resolution is assessed by point spread function analysis. RESULTS: Single-shot spiral imaging is achieved at a nominal resolution of 0.8 mm, using spiral-out readouts of 53-ms duration. High depiction fidelity is achieved without conspicuous blurring or distortion. Effective resolutions are assessed as 0.8, 0.94, and 0.98 mm in CSF, gray matter and white matter, respectively. High image quality is also achieved with half-shot acquisition yielding image pairs at 1.5-mm resolution. CONCLUSION: Use of an expanded signal model enables single-shot spiral imaging at 7 T with unprecedented image quality. Single-shot and half-shot spiral readouts deploy the sensitivity benefit of high field for rapid high-resolution imaging, particularly for functional MRI and arterial spin labeling.

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 19F concentrations together with short relaxation times are basic probe properties. We present the NMR properties of [Gd(NTf2)4]- dissolved in an ionic liquid consisting of an imidazolium cation and the [NTf2]- anion. These solutions achieve fluorine concentrations as high as 26 M and rapid relaxation times in the sub-ms time range. The second order self-exchange rate of coordinated [NTf2]- with [NTf2]- as determined with [Y(NTf2)4]- is 4.5 × 105·M-1·s-1. X-ray structure analyses confirm the coordination number of eight around Gd3+ and Y3+. These ionic-liquid solutions are thus excellent candidates for dynamically probing the NMR fields in MRI.

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 B0 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. Spiral readouts up to 25 ms are shown to permit whole-brain 2D imaging at 0.5 mm in-plane resolution in less than a minute. A range of options is explored, including proton-density and T2* contrast, acceleration by parallel imaging, different readout orientations, and the extraction of phase images. Results are shown to exhibit competitive image quality along with high geometric consistency.

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. In the multiple-channel case, it amounts to a matrix of broadband filters that perform concerted self- and cross-term correction. This approach is demonstrated with gradients and shims up to the third order in a 7 Tesla whole-body MR system. RESULTS: Pre-emphasis by LTI model inversion is first verified by studying settling speeds and response behavior without and with the correction. It is then demonstrated for rapid shim updating, achieving substantially enhanced fidelity of field dynamics and shim settling within approximately 1 ms. In single-shot echo-planar imaging (EPI) acquisitions in vivo, this benefit is shown to translate into enhanced geometric fidelity. CONCLUSIONS: The fidelity of gradient and shim dynamics can be greatly enhanced by pre-emphasis based on inverting a general LTI system model. Permitting shim settling on the millisecond scale, broadband multiple-channel pre-emphasis promises to render higher-order shimming fully versatile at the level of MRI sequence design. Magn Reson Med 78:1607-1622, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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 B0 , and coil sensitivity encoding. The encoding model is determined by B0 mapping, sensitivity mapping, and concurrent field monitoring. Reconstruction is performed by iterative inversion of the expanded signal equations. Diffusion-tensor imaging with single-shot spiral readouts is performed in a phantom and in vivo, using a clinical 3T instrument. Image quality is assessed in terms of artefact levels, image congruence, and the influence of the different encoding factors. RESULTS: Using the full encoding model, diffusion-weighted single-shot spiral imaging of high quality is accomplished both in vitro and in vivo. Accounting for actual field dynamics, including higher orders, is found to be critical to suppress blurring, aliasing, and distortion. Enhanced image congruence permitted data fusion and diffusion tensor analysis without coregistration. CONCLUSION: Use of an expanded signal model largely overcomes the traditional vulnerability of spiral imaging with long readouts. It renders single-shot spirals competitive with echo-planar readouts and thus deploys shorter echo times and superior readout efficiency for diffusion imaging and further prospective applications. Magn Reson Med 77:83-91, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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. Here we report a concerted approach involving tailored materials, magnetostatics and detection electronics to enhance the resolution of nuclear magnetic resonance sensing by three orders of magnitude. The relative sensitivity thus achieved amounts to 1 part per trillion (10-12). To exemplify this capability we demonstrate the direct detection and relaxometry of nuclear polarization and real-time recording of dynamic susceptibility effects related to human heart function. Enhanced high-field magnetometry will generally permit a fresh look at magnetic phenomena that scale with field strength. It also promises to facilitate the development and operation of high-field magnets.

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. The present work aims to overcome these limitations by the transition to short-lived NMR probes and rapid re-excitation. The proposed approach uses probes with T 2 so short that thermal relaxation dominates signal decay even in the presence of strongest gradients. They are integrated with transmit, receive and sequencing electronics that permit high-rate re-excitation with optional probe alternation as well as complementary RF pulse recording. The system is demonstrated by monitoring of sample MRI sequences with long readouts and large gradient moments. It is compared with the conventional long-lived probe concept and characterized in terms of net sensitivity and sources of systematic error. Continuous k -space trajectory mapping is demonstrated and validated by trajectory-based image reconstruction.

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. Higher baseline field also often requires higher acquisition bandwidths and faster signal encoding, increasing hardware demands and the severity of many types of hardware imperfection. To address field imperfections comprehensively, in this work we propose to expand the concept of magnetic field monitoring to also encompass the recording of RF fields. In this way, all dynamic magnetic fields relevant for spin evolution are covered, including low- to audio-frequency magnetic fields as produced by main magnets, gradients and shim systems, as well as RF pulses generated with single- and multiple-channel transmission systems. The proposed approach permits field measurements concurrently with actual MRI procedures on a strict common time base. The combined measurement is achieved with an array of miniaturized field probes that measure low- to audio-frequency fields via (19) F NMR and simultaneously pick up RF pulses in the MRI system's (1) H transmit band. Field recordings can form the basis of system calibration, retrospective correction of imaging data or closed-loop feedback correction, all of which hold potential to render MRI more robust and relax hardware requirements. The proposed approach is demonstrated for a range of imaging methods performed on a 7 T human MRI system, including accelerated multiple-channel RF pulses. Copyright © 2015 John Wiley & Sons, Ltd.

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. The measured gradient impulse response functions were used to predict actual k-space trajectories for single-shot echo-planar imaging (EPI), spiral and variable-speed EPI sequences. Image reconstruction based on the predicted trajectories was performed for phantom and in vivo data. Resulting images were compared with reconstructions based on concurrent field monitoring, separate trajectory measurements, and nominal trajectories. RESULTS: Image reconstruction using model-based trajectories yielded high-quality images, comparable to using separate trajectory measurements. Compared with using nominal trajectories, it strongly reduced ghosting, blurring, and geometric distortion. Equivalent image quality was obtained with the recent characterization and that performed 3 years prior. CONCLUSION: Model-based trajectory prediction enables high-quality image reconstruction for technically challenging sequences such as single-shot EPI and spiral imaging. It thus holds great promise for fast structural imaging and advanced neuroimaging techniques, including functional MRI, diffusion tensor imaging, and arterial spin labeling. The method can be based on a one-time system characterization as demonstrated by successful use of 3-year-old calibration data. Magn Reson Med 76:45-58, 2016. © 2015 Wiley Periodicals, Inc.

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. The present work presents a stand-alone monitoring system delivering insight into such field imperfections for MR sequence and system analysis. METHODS: A flexible NMR field probe-based stand-alone monitoring system, built on a software-defined-radio approach, is introduced and used to sense field dynamics up to third-order in space in a selection of situations with different time scales. RESULTS: Highly sensitive trajectories are measured and successfully used for image reconstruction. Further field perturbations due to mechanical oscillations and thermal field drifts following demanding gradient use and external interferences are studied. CONCLUSION: A flexible and versatile monitoring system is presented, delivering camera-like access to otherwise hardly accessible field dynamics with nanotesla resolution. Its stand-alone nature enables field analysis even during unknown MR system states.

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. The image encoding including encoding changes over the duration of entire scans were quantified and analyzed. Field perturbations were corrected by accounting for them in generalized image reconstruction. The impact on image quality along with geometrical congruence among different diffusion-weighted images was assessed both qualitatively and quantitatively. RESULTS: The most significant field perturbations were found to be related to higher-order eddy currents from diffusion-weighting gradients and B0 field drift as well as gradual changes of short-term eddy current behavior and mechanical oscillations during the scan. All artifacts relating to dynamic field perturbations were eliminated by incorporating the measured encoding in image reconstruction. CONCLUSION: Concurrent field monitoring combined with generalized reconstruction enhances depiction fidelity in diffusion imaging. In addition to artifact reduction, it improves geometric congruence and thus facilitates image combination for quantitative diffusion analysis.

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. An array of four field probes was used to perform real-time prospective motion correction in vivo. Images were reconstructed based on both predetermined and concurrently measured k-space trajectories. RESULTS: For observation windows of 4.8 ms, the precision of probe position determination was found to be 35 to 62 µm, and the maximal measurement error was 595 µm root-mean-square on a single axis. Sequence update per repetition time on this basis yielded images free of conspicuous artifacts despite substantial head motion. Predetermined and concurrently observed k-space trajectories yielded equivalent image quality. CONCLUSION: NMR field probes in conjunction with gradient tones permit the tracking and prospective correction of rigid-body motion. Relying on gradient oscillations in the kilohertz range, the method allows for concurrent motion detection and image encoding.

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. by applying two quadratic and two linear spatial encoding magnetic fields to reconstruct a 2D image. Images acquired with higher-dimensional single-shot trajectories can exhibit strong artifacts and geometric distortions. In this work, the source of these artifacts is analyzed and a reliable correction strategy is derived. METHODS: A dynamic field camera was built for encoding field calibration. Concomitant fields of linear and nonlinear spatial encoding magnetic fields were analyzed. A combined basis consisting of spherical harmonics and concomitant terms was proposed and used for encoding field calibration and image reconstruction. RESULTS: A good agreement between the analytical solution for the concomitant fields and the magnetic field simulations of the custom-built PatLoc SEM coil was observed. Substantial image quality improvements were obtained using a dynamic field camera for encoding field calibration combined with the proposed combined basis. CONCLUSION: The importance of trajectory calibration for single-shot higher-dimensional encoding is demonstrated using the combined basis including spherical harmonics and concomitant terms, which treats the concomitant fields as an integral part of the encoding.

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. METHODS: An array of NMR field probes is used to sense the field evolution in a whole-body MR system concurrently with regular system operation. The field observations serve as inputs to a proportional-integral controller that governs correction currents in gradient and higher-order shim coils such as to keep the field stable in a volume of interest. RESULTS: The feedback system was successfully set up, currently reaching a minimum latency of 20 ms. Its utility is first demonstrated by countering thermal field drift during an EPI protocol. It is then used to address respiratory field fluctuations in a T2 *-weighted brain exam, resulting in substantially improved image quality. CONCLUSION: Feedback field control is an effective means of eliminating dynamic field distortions in MR systems. Third-order spatial control at an update time of 100 ms has proven sufficient to largely eliminate thermal and breathing effects in brain imaging at 7 Tesla.

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. Field monitoring with a set of (19) F NMR probes distributed around the head was performed in two variants: concurrently with imaging or as a single field measurement per readout. The measured field fluctuations were then accounted for in the image reconstruction. RESULTS: Significant field fluctuations due to motion were observed in all subjects, resulting in severe artifacts in uncorrected images. The artifacts were largely removed by reconstruction based on field monitoring. Accounting for field perturbations up to the 1st spatial order was generally sufficient to recover good image quality. CONCLUSIONS: It has been demonstrated that artifacts due to physiologically induced dynamic field perturbations can be greatly reduced by retrospective image correction based on field monitoring. The necessity to perform such correction is greatest at high fields and for field-sensitive techniques such as T2*-weighted imaging.

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. The effect of thermal changes during k-space acquisition was simulated and confirmed with measurements in a stationary phantom. RESULTS: The temperature of the gradient mount was found to increase by 20-30 K during PC-CMR measurements of 6-12 min duration. Associated changes in background phase errors of up to 11% or 0.35 radian were measured at 10 cm from the magnet's iso-center as a result of first order offsets. Zeroth order phase errors exhibited little thermal dependence. CONCLUSIONS: It is concluded that changes in gradient mount temperature significantly modify background phase errors during PC-CMR with high gradient duty cycle. Since temperature increases significantly during the first minutes of scanning the results presented are also of relevance for single-slice or multi-slice PC-CMR scans. The findings prompt for further studies to investigate advanced correction methods taking into account gradient temperature and/or the use of concurrent field-monitoring to map gradient-induced fields throughout the scan.