Christoph Barmet

Spatiotemporal magnetic field monitoring for MR

MR experiments frequently rely on signal encoding by the application of magnetic fields that vary in both space and time. The accurate interpretation of the resulting signals often requires knowledge of the exact spatiotemporal field evolution during the experiment. To better fulfill this need, a new approach is presented that enables measuring the field evolution concurrently with any MR sequence. Miniature NMR probes are used to monitor the MR phase evolution around the object under investigation. Based on these data, a global phase model is calculated that can then be used as a basis for processing the actual image or spectroscopic data. The new method is demonstrated by MRI of a phantom, using spin‐warp, spiral, and EPI trajectories. Throughout, the monitoring results enabled highly accurate image reconstruction, even in the presence of massive gradient imperfections. Magn Reson Med 60:187–197, 2008.

NMR probes for measuring magnetic fields and field dynamics in MR systems

High‐resolution magnetic field probes based on pulsed liquid‐state NMR are presented. Static field measurements with an error of 10 nanotesla or less at 3 tesla are readily obtained in 100 ms. The further ability to measure dynamic magnetic fields results from using small (∼1 μL) droplets of MR‐active liquid surrounded by susceptibility‐matched materials. The consequent high field homogeneity allows free induction decay signals lasting 100 ms or more to be readily achieved. The small droplet dimensions allow the magnetic field to be measured even in the presence of large gradients. Highly sensitive detection yields sufficient SNR to follow the relevant field evolution without signal averaging and at bandwidths up to hundreds of kHz. Transient, nonreproducible effects and drifts are thus readily monitored. The typical application of k‐space trajectory mapping has been demonstrated. Potential further applications include characterization, tuning, and maintenance of gradient systems as well as the mapping of the static field distribution of MRI magnets. Connection of the probes to a standard MR spectrometer is similar to that used for imaging coils. Magn Reson Med 60:176–186, 2008.

Higher order reconstruction for MRI in the presence of spatiotemporal field perturbations

Despite continuous hardware advances, MRI is frequently subject to field perturbations that are of higher than first order in space and thus violate the traditional k‐space picture of spatial encoding. Sources of higher order perturbations include eddy currents, concomitant fields, thermal drifts, and imperfections of higher order shim systems. In conventional MRI with Fourier reconstruction, they give rise to geometric distortions, blurring, artifacts, and error in quantitative data. This work describes an alternative approach in which the entire field evolution, including higher order effects, is accounted for by viewing image reconstruction as a generic inverse problem. The relevant field evolutions are measured with a third‐order NMR field camera. Algebraic reconstruction is then formulated such as to jointly minimize artifacts and noise in the resulting image. It is solved by an iterative conjugate‐gradient algorithm that uses explicit matrix‐vector multiplication to accommodate arbitrary net encoding. The feasibility and benefits of this approach are demonstrated by examples of diffusion imaging. In a phantom study, it is shown that higher order reconstruction largely overcomes variable image distortions that diffusion gradients induce in EPI data. In vivo experiments then demonstrate that the resulting geometric consistency permits straightforward tensor analysis without coregistration. Magn Reson Med, 2011.

A transmit/receive system for magnetic field monitoring of in vivo MRI.

Magnetic field monitoring with NMR probes has recently been introduced as a means of measuring the actual spatiotemporal magnetic field evolution during individual MR scans. Receive‐only NMR probes as used thus far for this purpose impose significant practical limitations due to radiofrequency (RF) interference with the actual MR experiment. In this work these limitations are overcome with a transmit/receive (T/R) monitoring system based on RF‐shielded NMR probes. The proposed system is largely autonomous and protected against RF contamination. As a consequence the field probes can be positioned freely and permit monitoring imaging procedures of arbitrary geometry and angulation. The T/R approach is also exploited to simplify probe manufacturing and remove constraints on material choices. Probe miniaturization permits monitoring imaging scans with nominal resolutions on the order of 400 μm. The added capabilities of the new probes and system are demonstrated by first in vivo results, obtained with monitored gradient‐echo and spin‐echo echo‐planar imaging (EPI) scans. Magn Reson Med, 2009.

Gradient system characterization by impulse response measurements with a dynamic field camera.

This work demonstrates a fast, sensitive method of characterizing the dynamic performance of MR gradient systems. The accuracy of gradient time‐courses is often compromised by field imperfections of various causes, including eddy currents and mechanical oscillations. Characterizing these perturbations is instrumental for corrections by pre‐emphasis or post hoc signal processing. Herein, a gradient chain is treated as a linear time‐invariant system, whose impulse response function is determined by measuring field responses to known gradient inputs. Triangular inputs are used to probe the system and response measurements are performed with a dynamic field camera consisting of NMR probes. In experiments on a whole‐body MR system, it is shown that the proposed method yields impulse response functions of high temporal and spectral resolution. Besides basic properties such as bandwidth and delay, it also captures subtle features such as mechanically induced field oscillations. For validation, measured response functions were used to predict gradient field evolutions, which was achieved with an error below 0.2%. The field camera used records responses of various spatial orders simultaneously, rendering the method suitable also for studying cross‐responses and dynamic shim systems. It thus holds promise for a range of applications, including pre‐emphasis optimization, quality assurance, and image reconstruction. Magn Reson Med, 2013.

Real-time feedback for spatiotemporal field stabilization in MR systems

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.

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

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.

Analysis and correction of background velocity offsets in phase-contrast flow measurements using magnetic field monitoring.

The value of phase‐contrast magnetic resonance imaging for quantifying tissue motion and blood flow has been long recognized. However, the sensitivity of the method to system imperfections can lead to inaccuracies limiting its clinical acceptance. A key source of error relates to eddy current‐induced phase fluctuations, which can offset the measured object velocity significantly. A higher‐order dynamic field camera was used to study the spatiotemporal evolution of background phases in cine phase‐contrast measurements. It is demonstrated that eddy current‐induced offsets in phase‐difference data are present up to the second spatial order. Oscillatory temporal behaviors of offsets in the kHz range suggest mechanical resonances of the MR system to be non‐negligible in phase‐contrast imaging. By careful selection of the echo time, their impact can be significantly reduced. When applying field monitoring data for correcting eddy current and mechanically induced velocity offsets, errors decrease to less than 0.5% of the maximum velocity for various sequence settings proving the robustness of the correction approach. In vivo feasibility is demonstrated for aortic and pulmonary flow measurements in five healthy subjects. Using field monitoring data, mean error in stroke volume was reduced from 10% to below 3%. Magn Reson Med, 2012.

A field camera for MR sequence monitoring and system analysis

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.

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

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.