Abstract
Calculations are presented of the spatial distribution of radiofrequency field from two parallel circular coils separated by a distance equal to the coil diameter, and driven in opposition, the so-called “straddle coil.” This configuration can be used for spatial localization experiments by applying a sequence of “prepulses” that saturate the nuclear spins in all sample regions except the “sensitive volume” close to the median plane, where the radiofrequency fields from the two coils cancel. Compared with the widely used “surfacecoil” approach, this straddle-coil geometry has a more convenient shape for the sensitive volume, resembling a flat disc with good spatial resolution in the axial (χ) dimension. Fast radiofrequency switching allows pulses of opposite phase to be applied to the two coils in turn. Pulse sequences are proposed that are insensitive to radiofrequency offset over an appreciable range, ensuring that the localization is essentially independent of chemical shift. The location of the sensitive volume can be tracked across the sample in the χ dimension by changing the ratio of the radiofrequency currents in the two coils. These predictions are corroborated by experimental measurements on a phantom made up of two small glass tubes containing ethanol and trichloroethane.
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