Single-Sided Stray-Field NMR Profiling Using Chirped Radiofrequency Pulses

Single-sided nuclear magnetic resonance (NMR) scanners find increased use in applications where non-destructive measurements are needed. These single-sided scanners are characterized by a weak magnetic field and a large stray magnetic field gradient. These characteristics make these scanners suitable for determining a sample’s proton density profile, or for mapping NMR properties such as T₁, T₂ or diffusivity as a function of distance. The strong stray-field gradient generated by these magnets dictates a need for relatively high transmission/reception bandwidths, even when thin slices are involved. Consequently, scanning a large volume demands multiple separate measurements, associated with long scan times, potential inaccuracies associated with mechanical misplacements and limitations in tackling certain in vivo or dynamic systems. This work explores the consequences of replacing the hard pulses in the usual multi-echo sequence used in this kind of scanner, with frequency-swept (chirped) pulses. It was found that, under identical echo times and number of echoes, peak power-limited cases like the ones usually involved in these setups endow chirped-pulse sequences with a higher sensitivity than their square-pulse counterparts. Furthermore, data can be extracted in this manner faster; it can also be measured from larger slabs following a single excitation, thereby avoiding the need for multiple mechanical motions of the scanner/sample. Still, at least with the system hereby assayed, hardware limitations prevented us from utilizing equally short echo times for square- as well as chirped-pulse implementations. Given the shorter echo delays that could be used in the square-pulse versions, optimal acquisitions ended up endowing the latter with the best overall sensitivity defined as signal intensity per unit acquisition time. Potential bypasses of this limitation are briefly discussed.