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Vadim S. Zotev¹, Andrei N. Matlashov¹, Petr L. Volegov¹, Igor M. Savukov¹, Michelle A. Espy¹, John C. Mosher¹, John J. Gomez¹, Robert H. Kraus, Jr. ¹

¹Los Alamos National Laboratory, Los Alamos, New Mexico, USA

Magnetic resonance imaging at ultra-low fields (ULF MRI) uses SQUIDs (superconducting quantum interference devices) to measure spin precession at a microtesla-range magnetic field after sample magnetization is enhanced by a stronger pre-polarizing field. Here, the first ULF images of the human head acquired at 46 microtesla measurement field with pre-polarization at 30 mT are reported. The imaging was performed with 3 mm x 3 mm x 6 mm resolution using the 7-channel SQUID system designed for both ULF MRI and magnetoencephalography (MEG). This result demonstrates feasibility and potential of human brain imaging at microtesla magnetic fields.

Modhurin Banerjee Snyder¹, Krishna Kurpad¹, Charles Paulson¹, Daniel van der Weide¹, Thomas M. Grist¹

¹University of Wisconsin,Madison, Madison, USA

A new mixing and filtering technique for magnetic signal detection based on the principles of  magnetic force microscopy (MFM) is described. MFM adapts sensitivity of MFM to force-detect RF-magnetic signals at distances necessary for in-vivo imaging. Instead of using a coil antenna with resonant LC network, an untuned coil antenna is coupled magnetically to an ultra-sensitive,  MFM cantilever with an integral coil. Forces on the cantilever tip from the untuned coil antenna cause deflection of the cantilever, whose instantaneous position is measured by reflection of a laser onto a detector. The resonant cantilever acts as an electromechanical filter and sensitive signal transducer. The system is frequency agile, allowing broadband operation and though thermal-noise limited, the system provides several potential advantages over conventional NMR detector coils; the parameters affecting the sensitivity of the antenna and their effects on signal-to-noise ratio are calculated.

Dennis W. Hwang¹, Susie Y. Huang², Lian-Pin Hwang³, Yung-Ya Lin¹

¹UCLA, Los Angeles, California , USA; ²Harvard Medical School, Boston, Massachusetts, USA; ³National Taiwan University, Taipei, Taiwan

A conceptually new approach to enhance MRI contrast by manipulating the intrinsic spin dynamics in the presence of nonlinear feedback interactions has recently been demonstrated. In this work, we fabricate an active RF feedback device to amplify and control the radiation damping feedback field. To validate the efficacy of active RF feedback, tumor detection and characterization in in vivo mice injected with human lung cancers was investigated. It is shown that active RF feedback circuit enables improved differentiation of neighboring normal and abnormal tissues at low fields using conventional probes/receiver coils.

Xiaoliang Zhang¹, ²

¹University of Minnesota, Minneapolis, USA; ²University of California San Francisco, San Francisco, USA

In this work, we investigate an unusual NMR experiment method which uses no RF coils. The method is based on the dielectric resonance (or sample self-resonance) property of a NMR sample with finite boundaries. The NMR images and proton spectra of a cylindrical water sample acquired on a 7T scanner are presented. NMR with no RF coil may provide a more sensitive and simplified way to perform NMR because an entire subsystem in the NMR signal receiving chain, RF coil, is eliminated.

Pekka Sipilä¹, ², Silke Lechner¹, ², Dirk Lange¹, Sebastian Greding¹, Gerhard Wachutka², Florian Wiesinger¹

¹GE Global Research, Munich, Germany; ²Technical University of Munich, Munich, Germany

Magnetic Field Monitoring (MFM) system based on NMR probes promises an effective method to correct B0 and gradient imperfections in MRI. This is of great interest, because the practical applicability of several advanced pulse sequences is hindered due to these non-idealities. However, receive-only NMR probes are relatively cumbersome to use, as the probes have to be aligned with the excitation plane of the imaged object itself.  In this paper, a more practicable MFM setup is described in form of transmit-receive NMR probes. Having a second exciter channel available, MFM monitoring can be flexibly performed almost independent of the pulse sequence chosen.

David Ian Hoult¹, Derek Foreman¹, Glen Kolansky¹

¹National Research Council Institute for Biodiagnostics, Winnipeg, Canada

Primarily for use with transmit/receive high-field phased arrays, the concept of distributing multiple, non-magnetic Cartesian feedback transceivers around the back of the imaging magnet is presented. Close proximity to the array coils ensures minimal cable RF power loss, while the feedback corrects distortion and allows inexpensive, non-magnetic transistor RF power amplifiers to be used. The feedback also ensures that induced currents in coupled coils (transmit and receive) are reduced by a factor of 100, and stable performance has been verified with 2 coupled coils attached to two instruments. Control is via a USB-based link to an external computer.

Juan Wei¹, Gary X. Shen¹

¹The University of HongKong, Hong Kong, People's Republic of China

The link budgets of analog and digital wireless transmission for MR signals have been made. Although analog transmission may be has higher SNR and sensitivity in theory, the digital one is more feasible with better stability and noise immunity. Also with the rapid growth of wireless LAN (WLAN) for the data rates from 2Mbps up to 108Mbps and precipitous drops in prices of WLAN products, an application of digital wireless transmission for MR signals based on 802.11b has been realized.

Gary X. Shen¹, Juan Wei, Jing Yuan, Yong Pang

¹The University of Hong Kong, Hong Kong, Hong Kong

A single analog optical link system for multiple channel RF transmission is designed and implemented. All the RF channel signals can be down converted to the different low frequencies using frequency division multiplexing (FDM) method, and combined to a single optical link for transmission. The primary bench tests show that the system has very good dynamic range, linearity and SNR. This design can significantly reduce the system complexity and cut the cost of using multiple independent optical transmit links and optical receivers for multi-channel array MRI applications.

Peter Ullmann¹, Gerhard Eber², Thomas Eckert², Paul Freitag¹, Gernot Götzelmann¹, Michael Heidenreich¹, Ulrich Heinen¹, Sven Junge¹, Pietro Lendi³, Balz Odermatt³, Hans Post¹, Jens Rommel², Arthur Schwilch³, Willy Uhrig², Ralf Velten¹, Uwe Wark¹, Ewald Weber⁴, Wolfgang Ruhm¹

¹Bruker BioSpin MRI GmbH, Ettlingen, Germany; ²Bruker BioSpin GmbH, Rheinstetten, Germany; ³Bruker BioSpin AG, Fällanden, Switzerland; ⁴The University of Queensland, Brisbane, Australia

In this work a multi-channel transmit extension for a broadband RF electronics with the potential of driving large numbers of transmit channels was developed. This novel electronics setup was implemented on a 9.4 T animal scanner with 8 transmit and receive channels and its functionality was successfully verified in experiments of B₁-shimming and Parallel Excitation.

Matthew G. Erickson¹, Krishna N. Kurpad², James H. Holmes², Sean B. Fain

¹University of Wisconsin, Madison, Wisconsin, USA; ²U of Wisconsin, Madison, Wisconsin, USA

A TEM transmission line coil with all port drive was reported in the October 2007 issue of MRM.  In this work, we describe the integration of this deisign with a state of the art, multi-channel all digital T/R chain.  The receiver in the chain is a Mercury Visage direct digital receiver which uses a fold-back Nyquist digitization scheme in conjunction with analog anti-aliasing filters.  The transmitter is an experimental prototype based on ultra high speed D/A converters controlled by field programmable gate array (FPGA) chips.  Phase, frequency, receiver gain, and transmitter amplitud are all software controlled.  The need for quadrature hybrids and/or power dividers is neatly eliminated.  The system is portable, and maybe used at any filed strength between 0.2T and 11T with no special modifications.