Mobile NMR

Abstract

The widespread use of NMR in materials testing is hampered by the fact that the object needs to be carried to the NMR equipment and needs to fit inside the magnet[1]. Both limitations are removed by mobile unilateral NMR at the expense of a lower and inhomogeneous magnetic polarization field [2]. Originally, open sensors were developed by the well-logging industries [3]. There, transverse relaxation decays are measured from the fluids in the porous formation with a spectrometer positioned inside the borehole. For well-logging sensors, the field gradient is minimized in a sweet spot or adjusted to a small value so as to eliminate signal attenuation from translational diffusion for short echo times [4-6]. Parallel to the development of the first well-logging tools, the first unilateral sensors were developed mostly for measuring moisture in soil, bridge decks, building materials, and food [7-12]. Some of these instruments employed electromagnets weighing several hundred kilograms. As long as solid materials including rubber are investigated even large field gradients can be tolerated as diffusion is absent. This is the idea behind the NMR-MOUSE, which operates with permanent magnets at frequencies between 10 and 20 MHz with magnetic field gradients up to 20 T/m (Figure 1) [13-16]. The NMR-MOUSE weights less than 1 kg, but is limited to depths typically less than 15 mm.With the commercialization of well-logging instruments and the availability of the NMR-MOUSE about 10 years ago, the NMR methods for use in inhomogeneous magnetic fields were systematically developed [1724]. A variety of open magnet geometries are currently being explored for portable use [25-32]. For investigation of large objects, open magnets are fitted with surface coils that provide a magnetic radiofrequency (rf) field B [15,16,33]. The volume outside the magnet, where B exhibits perpendicular components to the polarization field Bo, is the sensitive volume of the sensor. A unilateral NMR sensor essentially selects the signal of a pixel from the object, the size of which is defined by the sensitive volume. A recent development complementary to unilateral NMR devices is lightweight and comparably inexpensive, cylindrical magnets in the Halbach geometry constructed from many small blocks of permanent magnets [34,35].Such magnets are suitable for studying pipe flow, geophysical drill cores [36], and plants at the site of the object.