Sensitivity to local dipole fields in the CRAZED experiment: An approach to bright spot MRI

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Sensitivity to local dipole fields in the CRAZED experiment: An approach to bright spot MRI

Cornelius Faber, Carolin Heil, Benjamin Zahneisen, David Z. Balla and Richard Bowtell
Journal of Magnetic Resonance
Article in Press, Corrected Proof

Local dipole fields such as those created by small iron-oxide particles are used to produce regions of low intensity (dark contrast) in many molecular magnetic resonance imaging applications. We have investigated, with computer simulations and experiments at 17.6 T, how the COSY revamped with asymmetric z-gradient echo detection (CRAZED) experiment that selects intermolecular double-quantum coherences can also be used to visualize such local dipole fields. Application of the coherence-selection gradient pulses parallel to the main magnetic field produced similar, dark contrast as conventional gradient echo imaging. Application of the gradient along the magic angle leads to total loss of signal intensity in homogeneous samples. In the presence of local dipole fields, the contrast was inverted and bright signals from the dipoles were observed over a very low background. Both simulations and experiments showed that the signal strongly decreased when a phase-cycle suppressing single-quantum coherences was employed. Therefore, we conclude that most of the signal comes from directly refocused magnetization or intermolecular single-quantum coherences. Finally, we demonstrate that bright contrast from local dipole fields can also be obtained, when the pair of coherence-selection gradient pulses is deliberately mismatched. Both methods allowed visualization of local dipole fields in phantoms in experimental times of about 3 min.

Indirect detection of nuclear magnetic resonance via geometrically induced long-range dipolar fields

by C.A. Meriles and Wei Dong
Department of Physics, CUNY—City College of New York, 138th Street and Convent Avenue, New
York, NY 10031, USA

In press on the Journal of Magnetic Resonance

We report the indirect detection of the magnetization of one spin species via the NMR signal of a second species. Our method relies on the control of long-range dipolar fields between two separate objects, in this case, a water droplet (sensor) immersed in a tube containing mineral oil (sample). Unlike prior experiments, no gradient pulses are used; rather, the setup geometry is exploited to select the part of the sample to be probed and modulate the spin alignment in the sensor. Our results are discussed in the context of Dipolar Field Microscopy, a proposed strategy in which the detector is a hyperpolarized tip.