Barbara L. Beck

In vivo 1H magnetic resonance imaging and spectroscopy of the rat spinal cord using an inductively-coupled chronically implanted RF coil.

An inductively coupled, chronically implanted short‐solenoid coil was used to obtain in vivo localized 1H NMR spectra and diffusion‐weighted images from a rat spinal cord. A 5 × 8 mm two‐turn elliptically shaped solenoid coil was implanted in rats at the site of a T‐12 vertebral‐level laminectomy. Excitation was achieved solely by a 3 × 3 cm external surface coil, and signal detection was achieved by inductively coupling the external coil to the implanted coil. The image signal‐to‐noise ratio (SNR) obtained with the inductively‐coupled implanted coil was compared with that obtained using a linear or a quadrature external surface coil. The implanted coil provided a gain by over a factor of 3 in SNR. The implanted coil was used to measure localized 1H spectra in vivo at the T13/L1 spinal‐cord level within a 1.85 × 1.85 × 4.82 mm (16.5 μL) volume. With 256 averages, a ∼3‐s repetition delay and respiratory gating, a high‐quality spectrum was acquired in 13 min. In addition, water translational diffusion was measured in three orthogonal directions using a stimulated‐echo imaging sequence, with a short echo time (TE), to produce a quantitative map of diffusion in a rat spinal cord in vivo. Magn Reson Med 46:1216–1222, 2001.

Observation of significant signal voids in images of large biological samples at 11.1 T

Proton MRI of large biological samples were obtained on an 11.1 T / 40 cm instrument. Images were obtained of a fixed human brain and a large piece of fresh beef. The proton MR images demonstrate severe distortions within these conductive samples, indicative of shortened electrical wavelengths and wave behavior within the sample. These observations have significant implications with respect to the continuing evolution of MR to higher magnetic field strengths on large samples, particularly on humans. Magn Reson Med 51:1103–1107, 2004.

Intraorbital Optic Nerve and Experimental Optic Neuritis: Correlation of Fat Suppression Magnetic Resonance Imaging and Electron Microscopy

To provide magnetic resonance imaging (MRI)-ultrastructural correlations of demyelinating lesions of the intraorbital optic nerve, the authors performed gadolinium-enhanced/T2-weighted fat-suppressed MRI and transmission electron microscopy of the optic nerves of animals with experimental allergic encephalomyelitis. Gadolinium enhancement of the optic nerve adjacent to the globe was seen on fat-suppressed T1-weighted MRI as early as 3 days after antigenic sensitization, increased in severity involving longer segments of nerve at 10 to 14 days, and persisted at 30 days. Gadolinium enhancement preceded T2-weighted signal aberrations. Ultrastructural evaluation of the intraorbital nerve revealed: (1) expansion of the extracellular space and inflammatory infiltrate that correlated with the intensity of gadolinium enhancement; (2) the degree of demyelination correlated with T2-weighted signal aberrations; (3) as deduced from gadolinium enhancement and T2 signal aberrations, breakdown of the blood-brain barrier preceded widespread demyelination; (4) lesions appeared to start at the optic nerve insertion into the globe and then progress toward the orbital apex.

Phased array imaging on a 4.7T/33cm animal research system

Although phased array technology has been standard on clinical magnetic resonance imaging (MRI) systems for many years, it is just now becoming available on small animal MRI systems. Bruker Instruments Ltd. has recently provided multiple rf channels on a 4.7T/33cm Avance, but further development was necessary to complete the phased array implementation. This work discusses the development of this other hardware, specifically the rf array coil, low impedance preamplifiers, excitation coil, and image combination algorithm. Comparison of the array coil to a quadrature coil indicates superior signal-to-noise ratio from the array. In vivo images of a cat spine acquired simultaneously from the individual channels of the array and a sum of squares combination are shown.

A theoretical comparison of two optimization methods for radiofrequency drive schemes in high frequency MRI resonators

In this paper, numerical simulations are used in an attempt to find optimal source profiles for high frequency radiofrequency (RF) volume coils. Biologically loaded, shielded/unshielded circular and elliptical birdcage coils operating at 170 MHz, 300 MHz and 470 MHz are modelled using the FDTD method for both 2D and 3D cases. Taking advantage of the fact that some aspects of the electromagnetic system are linear, two approaches have been proposed for the determination of the drives for individual elements in the RF resonator. The first method is an iterative optimization technique with a kernel for the evaluation of RF fields inside an imaging plane of a human head model using pre-characterized sensitivity profiles of the individual rungs of a resonator; the second method is a regularization-based technique. In the second approach, a sensitivity matrix is explicitly constructed and a regularization procedure is employed to solve the ill-posed problem. Test simulations show that both methods can improve the B(1)-field homogeneity in both focused and non-focused scenarios. While the regularization-based method is more efficient, the first optimization method is more flexible as it can take into account other issues such as controlling SAR or reshaping the resonator structures. It is hoped that these schemes and their extensions will be useful for the determination of multi-element RF drives in a variety of applications.

Nuclear magnetic resonance energy harvesting for ultra-low power biomedical implants

A selective wirelessly-adjustable multiple-frequency probe (SWAMP) system allows for Nuclear Magnetic Resonance (NMR) imaging and spectroscopy with increased signal sensitivity through the coupling of the NMR surface coil with an implanted system capable of selectively resonating at multiple NMR frequencies. The ability to wirelessly recharge the biomedical implant in the NMR environment allows for long term in vivo operation without the need for surgery to replace the depleted secondary power source. This paper explores the capability of wirelessly recharging and powering the implanted electronics by leveraging the readily available energy from the transmitted RF pulses used for NMR measurements. The wireless power transfer between the NMR and the SWAMP systems are studied and relevant results are reported.