NMR spectroscopy in inhomogeneous B₀ and B₁ fields with non-linear correlation

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• 作者：Topgaard ; Daniel ; Sakellariou ; Dimitris ; Pines ; Alexander
• 关键词：Inhomogeneous magnetic fields ; Chemical shift ; Numerical optimization ; Adiabatic pulses ; z-rotation
• 刊名：Journal of Magnetic Resonance
• 出版年：2005
• 出版时间：July, 2005
• 年：2005
• 卷：175
• 期：1
• 页码：1-10
• 全文大小：505 K

文摘

Resolved NMR spectra from samples in inhomogeneous B₀ and B₁ fields can be obtained with the so-called “ex situ” methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B₀ and B₁ fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry.