Oliver G. Gruschke

Phased-array of microcoils allows MR microscopy of ex vivo human skin samples at 9.4 T

The aim of this study was to demonstrate the feasibility of a custom‐made phased‐array microcoil within a 400 MHz animal system for the morphological characterization of human skin tissue in correlation with histopathology.

Implementation of an in-field CMOS frequency division multiplexer for 9.4 T magnetic resonance applications

SUMMARY This paper introduces the implementation of an application-specific complementary metal oxide semiconductor frequency division multiplexer as a novel solution to interface magnetic resonance (MR) phased arrays of micro-detectors to an image-processing unit, thus reducing the complexity and space issues associated with MR detector arrays. The frequency multiplexer, in a compact 3 × 4 mm silicon die, is designed to operate at 400 MHz, which is the Larmor frequency of 1H protons in a 9.4-T MR imaging system. The system implements eight channels, where each channel consists of a low-noise amplifier, a frequency mixer, and a band-pass filter. The maximum gain of an individual channel after the band-pass filter stage is 38 dB. The suppression of the local oscillator ranges from 40 to −51 dB, and the maximum coupling between channels is −39 dB. The input dynamic range of an individual channel is 8 mV. Each channel consumes 54 mA from a 3.3-V source. The chip operates without errors within a high 9.4-T magnetic field. Copyright

Multilayer phased microcoil array for magnetic resonance imaging

We present for the first time wirebonded microcoils arranged in a planar phased-array configuration for large field of view (FOV) microscale magnetic resonance imaging (MRI) of 2D samples. The phased array consists of seven microcoils providing a sensitive area of 18.3 mm2. We demonstrate successful high-resolution imaging of a water phantom with 16 × 16 µm2 in-plane resolution and an signal-to-noise ratio (SNR) of 27 in 12 min 48 s acquisition time.

Hollow microcoils made possible with external support structures manufactured with a two-solvent process

We present a process to manufacture solenoidal microcoils with external support structures, which leaves the space within the coil windings free. The manufacturing procedure is based on a two solvent approach (water and acetone), for selectively etching polyvinyl alcohol and polymethyl methacrylate. Two sets of microcoils were manufactured with an inner diameter of 1.5 mm, an interwinding pitch of 100 μm and five or eight coil windings respectively. The coils were designed for application in magnetic resonance imaging and spectroscopy, and characterised in a 9.4 T MR scanner. An NMR spectrum of water and MR images in receive only and transceive mode were acquired as proof of concept.

The noise factor of receiver coil matching networks in MRI

In typical MRI applications the dominant noise sources in the received signal are the sample, the coil loop and the preamplifier. We hypothesize that in some cases (e.g. for very small receiver coils) the matching network noise has to be considered explicitly. Considering the difficulties of direct experimental determinations of the noise factor of matching networks with sufficient accuracy, it is helpful to estimate the noise factor by calculation. A useful formula of the coil matching network is obtained by separating commonly used coil matching network into different stages and calculating their noise factor analytically by a combination of the noise from these stages. A useful formula of the coil matching network is obtained. ADS simulations are performed to verify the theoretical predictions. Thereafter carefully-designed proof-of-concept phantom experiments are carried out to qualitatively confirm the predicted SNR behavior. The matching network noise behavior is further theoretically investigated for a variety of scenarios. It is found that in practice the coil matching network noise can be improved by adjusting the coil open port resonant frequency.

Design of a 3T preamplifier which stability is insensitive to coil loading

In MRI (magnetic resonance imaging), preamplifiers are needed to amplify signals obtained from MRI receiver coils. Under various loading conditions of the corresponding receiver coils, preamplifiers see different source impedance at their input and may become unstable. Therefore preamplifiers which stability is not sensitive to coil loading are desirable. In this article, a coil-loading-insensitive preamplifier for MRI is presented, derived from an unstable preamplifier. Different approaches to improve stability were used during this derivation. Since a very low noise factor is essential for MRI preamplifiers, noise contributions from passive components in the MRI preamplifier have to be considered during the stabilization process. As a result, the initially unstable preamplifier became stable with regard to coil loading, while other MRI requirements, as the extremely low noise factor, were still fulfilled. The newly designed preamplifier was manufactured, characterized and tested in the MRI spectrometer. Compared to a commercially available preamplifier, the newly designed preamplifier has similar imaging performance but other advantages like smaller size and better stability. Furthermore, presented stabilization approaches can be generalized to stabilize other unstable low-noise amplifiers.

CMOS 8-channel frequency division multiplexer for 9.4 T magnetic resonance imaging

We present a CMOS 8-channel frequency division multiplexer (FDM) to interface a phased array of micro coils for 9.4 T (400 MHz Larmor precession frequency ω0) for magnetic resonance imaging (MRI). The integrated multiplexer contributes towards a solution to achieve phased arrays with a massive number of coils without unnecessarily increasing system complexity, the size of hardware, and cost. The multiplexer is designed using commercially available 0.35 μm CMOS technology and consists of five major components: a low-noise amplifier (LNA), a frequency mixer, a voltage-controlled oscillator (VCO), a bandpass filter (BPF), and an adding operational amplifier. The maximum gain of a single channel is 79 dB, and the input referred noise is 1.4 nV/√Hz. The die area of the multiplexer is approximately 8 mm2, and requires 300 mA from a 3.3 V source.

Inductively coupled wirebonded microcoils for wireless on-chip NMR

We report for the first time single-chip MEMS-integration of a resonant LC-circuit for wireless magic angle coil spinning (MACS) NMR. The chip consists of a wirebonded 3D microcoil, an interdigitated capacitor and a sample container. The LC-circuit resonates at 700MHz for 1H spectroscopy in 16.4T magnetic field. The microcoil is inductively coupled with the probe-coil of the spectrometer. We prove the high sensitivity of the setup by detecting the NMR signal from 0.3nl H2O in a mixture with deuterium oxide (0.1% H2O: 99.9% D2O) with an SNR=8.5 in one scan.

A comparison of Lenz lenses and LC resonators for NMR signal enhancement

Abstract High signal‐to‐noise ratio (SNR) of the NMR signal has always been a key target that drives massive research effort in many fields. Among several parameters, a high filling factor of the MR coil has proven to boost the SNR. In case of small‐volume samples, a high filling factor and thus a high SNR can be achieved through miniaturizing the MR coil. However, under certain circumstances, this can be impractical. In this paper, we present an extensive theoretical and experimental investigation of the inductively coupled LC resonator and the magnetic Lenz lens as two candidate approaches that can enhance the SNR in such circumstances. The results demonstrate that the narrow‐band LC resonator is superior in terms of SNR, while the non‐tuned nature of the Lenz lens makes it preferable in broadband applications.

Improved method for MR microscopy of brain tissue cultured with the interface method combined with Lenz lenses

MR in microscopy can non-invasively image the morphology of living tissue, which is of particular interest in studying the mammalian brain. Many studies use live animals for basic research on brain functions, disease pathogenesis, and drug development. However, in vitro systems are on the rise, due to advantages such as the absence of a blood-brain barrier, predictable pharmacokinetics, and reduced ethical restrictions. Hence, they present an inexpensive and adequate technique to answer scientific questions and to perform drug screenings. Some publications report the use of acute brain slices for MR microscopy studies, but these only permit single measurements over several hours. Repetitive MR measurements in longitudinal studies demand an MR-compatible setup which allows cultivation for several days or weeks, and hence properly functioning in vitro systems. Organotypic hippocampal slice cultures (OHSC) are a well-established and robust in vitro system which still exhibits most histological hallmarks of the hippocampal network in vivo. An MR compatible incubation platform is introduced in which OHSC are cultivated according to the interface method following Stoppini et al. In this cultivation method a tissue slice is placed onto a membrane with nutrition medium underneath and a gas atmosphere above, where the air-tissue interface perpendicular to the B0 field induces strong artefacts. We introduce a handling protocol that suppresses these artefacts and increases signal quality significantly to acquire high resolution images of tissue slices. An additional challenge is the lack of available of MR microscopy equipment suitable for small animal scanners. A Lenz lens with an attached capacitor can dramatically increase the SNR in these cases, and wirelessly bring the detection system in close proximity to the sample without compromising the OHSC system through the introduction of wired detectors. The resultant signal gain is demonstrated by imaging a PFA-fixed brain slice with a 72 mm diameter volume coil without a Lenz lens, and with a broadband and a self-resonant Lenz lens. In our setting, the self-resonant Lenz lens increases the SNR 10-fold over using the volume coil only.