David O. Brunner

Travelling-wave nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is one of the most versatile experimental methods in chemistry, physics and biology, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant—magnetic resonance imaging (MRI)—is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction in one or multiple radio-frequency resonators that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides, superconducting quantum interference devices, atomic magnetometers, Hall probes or magnetoresistive elements have been explored. However, a common feature of all NMR implementations until now is that they rely on close coupling between the detector and the object under investigation. Here we show that NMR can also be excited and detected by long-range interaction, relying on travelling radio-frequency waves sent and received by an antenna. One benefit of this approach is more uniform coverage of samples that are larger than the wavelength of the NMR signal—an important current issue in MRI of humans at very high magnetic fields. By allowing a significant distance between the probe and the sample, travelling-wave interaction also introduces new possibilities in the design of NMR experiments and systems.

B 1+ Phase mapping at 7 T and its application for in vivo electrical conductivity mapping

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B  1+ phase and the homogeneous Helmholtz equation. This new MRI technique might open future opportunities for tumor and lesion characterization based on conductivity differences, while it may also find application in radio frequency safety assessment. Prerequisites for conductivity mapping using only the B  1+ phase (instead of the complex B  +1 field) are addressed. Furthermore it was found that the B  1+ phase can be derived directly from the measurable transceive phase arg(B  +1 B  −1 ) in the head. Validation for a human head excited by a 7 T‐birdcage coil using simulations and measurements showed that it is possible to measure in vivo conductivity patterns in the brain using B  1+ phase information only. Conductivity contrast between different brain tissues is clearly observed. The measured mean values for white matter, gray matter and cerebrospinal fluid differed 54%, 26%, and −13% respectively from literature values. The proposed method for B  1+ phase measurements is very suited for in vivo applications, as the measurement is short (less than a minute per imaged slice) and exposes the patient to low RF power, contrary to earlier proposed approaches. Magn Reson Med, 2012.

ZTE imaging in humans.

Zero echo time (ZTE) imaging is a robust and silent 3D radial technique suitable for direct MRI of tissues with very rapid transverse relaxation. Given its successful application on micro‐ and animal MRI systems, the purpose of this work is to enable and demonstrate ZTE imaging in humans using a whole‐body magnet.

B1(+) interferometry for the calibration of RF transmitter arrays.

Multiple‐channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B  1+ , of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B  1+ and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B  1+ fields of individual transmitter elements are measured indirectly by observing their interference with a SAR‐efficient baseline RF field. In this fashion even small RF fields can be observed in the B  1+ ‐sensitive large‐flip‐angle regime. Based on a set of such experiments B  1+ maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off‐resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8‐channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T1 or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B  1+ maps accurately reflect the linearity of RF superposition. Magn Reson Med, 2009.

Optimal design of multiple‐channel RF pulses under strict power and SAR constraints

Parallel radio frequency transmission has recently been explored as a means of tailoring the spatial response of MR excitation. In particular, parallel transmission is increasingly used to accelerate radio frequency pulses that rely on time‐varying gradient fields to achieve selectivity in multiple dimensions. The design of the underlying multiple‐channel radio frequency waveforms is mostly based on regularized least‐squares optimization in close analogy with image reconstruction in parallel imaging. However, this analogy has important limitations. Unlike image reconstruction, the design of radio frequency waveforms is subject to multiple strict constraints, which arise from technical power limits, as well as safety limits on local and global energy deposition in vivo. To optimize excitation profiles under such strict constraints, it is proposed to depart from the regularization strategy and rely on semidefinite programming instead. To render this approach fast, it is performed in a reduced search space, which is obtained by initial Lanczos iteration. The proposed algorithm is demonstrated to enable efficient pulse optimization within exactly the given constraints, including local specific absorption rate limits for multiple compartments. It is also shown that the proposed approach readily accommodates advanced forward models of the excitation process, including the effects of local off‐resonance and transverse relaxation. Magn Reson Med 63:1280–1291, 2010.

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.

Thermal Tissue Damage Model Analyzed for Different Whole‐Body SAR and Scan Durations for Standard MR Body Coils

This article investigates the safety of radiofrequency induced local thermal hotspots within a 1.5T body coil by assessing the transient local peak temperatures as a function of exposure level and local thermoregulation in four anatomical human models in different Z‐positions.

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.

Traveling-wave RF shimming and parallel MRI.

At sufficiently high Larmor frequencies, traveling electromagnetic waves along a magnet bore can be used for remote magnetic resonance excitation and detection, effectively using the bore as a waveguide. So far, this approach has relied only on the lowest waveguide modes and thus has not supported multiple‐channel operation for radiofrequency shimming and parallel imaging. In this work, this limitation is addressed by establishing a larger number of propagating modes and tapping their spatial field diversity with multiple waveguide ports. The number of available modes is increased by loading with dielectric inserts; the ports are implemented by stub and loop couplers at the end of a waveguide extension. The resulting traveling‐wave array, operated at 298 MHz in a 7T whole‐body magnet, is shown to enable radiofrequency shimming as well as parallel imaging with commonly used acceleration factors. The last part of the study concerns the amount of dielectric loading that is required. For the given Larmor frequency and bore dimensions, it is found that rather few water‐filled inserts, occupying ∼5% of the bore cross‐section, are sufficient for effective parallel imaging. Magn Reson Med, 2011.

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.