'Shim pulses' for NMR spectroscopy in inhomogeneous magneticfields

Journal Article · Wed May 19 04:00:00 EDT 2004 · Journal of American Chemical Society

OSTI ID:886823

Topgaard, Daniel; Martin, Rachel W; Sakellariou, Dimitris; Meriles, Carlos; Pines, Alexander

NMR spectroscopy conveys information about chemical structure through ppm-scale shifts of the resonance frequency depending on the chemical environment. In order to observe these small shifts, magnets with highly homogeneous magnetic field B{sub 0} are used. The high cost and large size of these magnets are a consequence of the requirement for high homogeneity. In this contribution we introduce a new method for recording high-resolution NMR spectra from samples in inhomogeneous B{sub 0}, opening up the possibility of exploiting magnets of lower homogeneity and cost. Instead of using the traditional B{sub 0} ''shim coils'', adiabatic radiofrequency (RF) pulse sequences and modulated B{sub 0} gradients generated by coils in the probe are used to produce ''shim pulses''. A great deal of work has been devoted to finding methods for retrieving chemical shift information even when B{sub 0} is inhomogeneous. One class of methods relies on zero- or multiple quantum coherences which evolve independently of B{sub 0}. These methods are inherently two-dimensional and the high-resolution information is obtained indirectly. In order to minimize experimental time it is desirable to acquire a high-resolution spectrum directly just as for traditional NMR in homogeneous fields. A further advantage with direct acquisition is that modification of already existing multidimensional NMR techniques is facilitated. A fundamentally different approach utilizes inhomogeneity of the RF magnetic field to periodically refocus the phase dispersion from the inhomogeneous B{sub 0}. With this technique a high-resolution spectrum can indeed be acquired in a single shot. The main drawback is the requirement for spatial matching between the RF and B{sub 0} inhomogeneities. Based on this latter technique we propose the use of ''shim pulses'', i.e. modulated, spatially constant, B{sub 0} gradient pulses together with spatially homogeneous adiabatic frequency sweeps to induce non-linear phase shifts in three dimensions. An intuitive understanding of the approach can be obtained from the following: An adiabatic full passage applied to transverse magnetization effectively rotates the magnetization in the transverse plane with an amount that depends on the frequency offset. In homogeneous B{sub 0} this gives rise to a ''phase roll'' across the NMR spectrum. If the adiabatic full passage is applied in the presence of a constant B{sub 0} gradient, a phase shift approximately linear in space will be the result. A second adiabatic passage reverses this phase shift and the adiabatic double passage constitutes effectively a 360{sup o} pulse. However, if the amplitude of the B{sub 0} gradient is changing during the adiabatic passages, phase shifts, which are non-linear in space, can be achieved. With a proper choice of the RF and gradient modulation functions, the phase dispersion from the inhomogeneous B{sub 0} can be canceled. Application of a shim pulse between each detected point in the time-domain NMR signal yields an NMR spectrum free from the broadening caused by the B{sub 0} inhomogeneity.

• Research Organization: Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
• Sponsoring Organization: USDOE Director. Office of Science. Office of Basic EnergySciences; Department of Defense. US Army Contract DAMD 17-03-1-0476; Swedish Research Council Contract 2003-282
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 886823
• Report Number(s): LBNL--55112; BnR: KC0203010
• Journal Information: Journal of American Chemical Society, Journal Name: Journal of American Chemical Society Journal Issue: 51 Vol. 101
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
AMPLITUDES
CHEMICAL SHIFT
DIMENSIONS
MAGNETIC FIELDS
MAGNETIZATION
MAGNETS
MODIFICATIONS
MODULATION
NMR SPECTRA
OPENINGS
PHASE SHIFT
PROBES
RESONANCE
SPECTROSCOPY