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Stray Field Magnetic Resonance Imaging (STRAFI)

Magnetic resonance imaging, MRI, has found widespread application in medicine where it is often the imaging modality of choice for a wide range of clinical disorders. Less well known is the fact that MRI is also widely applicable to problems in materials science. The inherent image contrast originating from molecular dynamics as reflected in the nuclear spin relaxation rates, and the fact that MRI is both non-invasive and non-destructive far out weigh the limited spatial resolution and sensitivity of the method. However, unlike water in the human body, the resonances from many systems of industrial interest are broad, due variously to a lack of molecular mobilty as is the case with solids, to pore surface relaxation of liquids confined in small pores, to heterogeneity of the magnetic susceptibility across internal sample interfaces and to paramagnetic impurities.

Worldwide, numerous broad line MRI techniques which can overcome these difficulties have been proposed. Stray field imaging, STRAFI, first suggested by Andre Samoilenko in Moscow, is one such technique. Almost uniquely, STRAFI manages to readily preserve the dynamic contrast which makes magnetic resonance so powerful in the first place while at the same time offering the very highest spatial resolution.

The method exploits the extremely large magnetic field gradient found outside the central region of all high field superconducting NMR magnets. In this gradient, even a short radio frequency pulse only excites nuclei in a narrow slice of the sample orthogonal to the gradient direction, thus providing spatial localisation of the NMR measurement. By stepping the sample through the gradient and repeating the measurement at each position, it is possible to build a profile of the sample in the gradient direction.

Dr Eli Ciampi, the Second Unilever Surrey Scholar (right) with Dr John Godward, responsible for the STRAFI facility and Prof. Ed Randall of QMW, London University, (left) around the STRAFI facility.

Research students using GARField at Surrey to study the drying of photo-initiated latex paint systems

Prof. Peter McDonald and his colleagues at Surrey together with collaborators coming from a widespread industrial and academic consortium have been instrumental in pioneering the understanding, methodology, instrumentation and applications of STRAFI from the early ideas of Samoilenko.

The Surrey laboratories now house a unique suite of facilities for stray field imaging and conventional magnetic resonance microscopy. One of the most recent developments is GARField, a low cost, low field, permanent magnet alternative to high field superconducting magnets for the stray field like profiling of planar samples.

Developed at Surrey by Dr Paul Glover, GARField is ideally suited to the study of industrial coatings systems and also to skin. Work on clinical MRI by Dr Simon Doran is described here.