(SQUIDS) BASED MICROTESLA NMR/MRI

Superconducting Quantum Interference Devices (SQUIDs) Based Microtesla NMR/MRI Instrumentation and Its Applications to Biosciences



Superconducting Quantum Interference Devices (SQUIDs) Based Microtesla NMR/MRI Instrumentation and Its Applications to Biosciences

1. Introduction

The general trend in conventional nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI)? has been to work at high fields? in order to increase the signal? and thus achieve better resolution with shorter measuring times. In fact? both the magnetization and its precession frequency increase linearly with the strength of the magnetic field? so that the signal produced by Faraday induction in pick-up coils increases quadratically with it. In fact? the polarization and the precession roles of the dc fields used in NMR can be discriminated. The two fields may differ in strength and/or orientation? and may be applied in two different regions (remote detection) (Lanczos? 1988? III.5? IV.22). When the time-dependent field produced by the precessing magnetization is detected by non-inductive sensors? the signal amplitude no longer depends on the precession frequency. For this reason? methods based on non-inductive detection show their competitiveness when the magnetization precesses in low magnetic fields. In this case the signal depends (linearly) only on the polarization field.

A recent renewed interest in low-field NMR (LF-NMR) measurements has been motivated by the use of Superconducting Quantum Interference Devices (SQUIDs) as sensitive and frequency-independent magnetic flux detectors ? ? and . Optical atomic magnetometers (OAMs) are alternative sensors based on the magneto-optical properties of atomic samples (Lanczos? 1988? III.5? IV.22). OAMs were first proposed decades ago and their performance has improved thanks to laser spectroscopy (Lanczos? 1988? III.5? IV.22). Nowadays? OAMs achieve sensitivity levels comparable to those of SQUIDs and are used in various fields of application ? ? ? and ? including NMR detection ? ? ? and .

The advantages of OAM sensors lie in the possibility of miniaturizing the sensor volume and ? while providing excellent time stability and reliability. OAMs do not require cryogenics? as they work at room temperature or higher (Callaghan? 1997? 4263-4270). This feature brings a further advantage to NMR as? besides dramatically reducing the cost of maintenance compared to SQUIDs? it helps to minimize the distance between sample and sensor? which is crucial for good sample-detector coupling (Ledbetter? 2008? 2286-2290).

From the point of view of sensitivity? SQUIDs operating at liquid He temperature reach a sensitivity in the few range. At liquid N2 temperature? this value increases to tens of . These values are improved by a factor 30 in the case of high-Q resonator SQUIDs operating at several hundred kHz (Callaghan? 1997? 4263-4270). For OAMs working in the so-called Spin-Exchange-Relaxation-Free (SERF) regime (which requires magnetic field compensation down to fractions of nT)? a sensitivity of has been demonstrated experimentally and ? while a fundamental limit of has been claimed and . For optical atomic set-ups working in a non-vanishing magnetic field the experimental limit is ? with a theoretical projection as low as . Sensitivity as good as with projection as low as have been reported ...