We have developed a nuclear magnetic resonance (NMR) microcapillary probe for the detection of single magnetic microbeads. The geometry of the probe has been optimized so that the signal from the background water has a similar magnitude compared to the signal from the dephased water nearby a single magnetic bead within the probe detector coil.In addition, the RF probes were tested in a 7 T (300 MHz) pulsed NMR spectrometer with sample volumes ranging from 5 nL down to 1 nL. The 1 nL probe had a single shot signal-to-noise ratio (SNR) for pure water of 27 and a volume resolution that exhibits a 600-fold improvement over a conventional (5 mm tube) NMR probe with a sample volume of 18 υL. This allowed for the detection of a 1 υm magnetite/polystyrene bead (m = 2 × 10-14Α m2) with an estimated experimental SNR of 30. Simulations of the NMR spectra for the different coil geometries and positions of the bead within the coil were developed that include the β0 shift near a single bead, the inhomogeneity of the coils, the local coil sensitivity, the skin effect of the coil conductor, and quantitated estimates of the proximity effect between coil windings.
Citation: Journal of Magnetic Resonance
Pub Type: Journals
magnetic particle, single particle detection, microcoil NMR, MRI contrast agent, medical imaging