Pulse Check - Spectracular's NMR Blog

What is slice selection? Routinely used in MRI, slice-selection refers to the excitation and observation of signals from only a specific, well-defined ‘slice’ of the sample, instead of the whole sample. In MRI, this could be a ‘slice’ of the human brain: Magnetic field gradient causes the center frequency ( F c ) of each slice to vary by position. The range of frequencies ( ΔF ) contained in a slice depends on slice thickness ( ΔF ) and the strength of the gradient ( G ss ). Source: https://mriquestions.com/slice-selective-excitation.html   From an high-resolution NMR perspective, it most often refers to selectively observing different parts of the typical NMR tube, as described here in Glenn Facey’s blog .  As also described in Facey’s blog, slice-selection is achieved by applying a linear gradient simultaneously with the excitation pulse, which is usually a shaped RF pulse. A key difference between slice selection in imaging applications vs high-resolution NMR is that imagi...

Adiabatic Pulses Adiabatic pulses are a special category of pulses where both the amplitude and frequency (hence phase) of the pulses are modulated. They are designed to be insensitive to B1 inhomogeneities and miscalibrations, as long as the amplitude is above a certain threshold. They are ideal for uniform inversion or refocusing across large bandwidths. For more on adiabatic pulses, please refer to Chapter 5 in Ref. [1] and Chapter 12 in Ref. [2]. Some of these pulses have specific requirements that need to be met for them to behave ideally. In this section, we will discuss a few different types of adiabatic pulses and the ideal parameters needed for their optimal behavior. The Adiabatic Condition Adiabatic pulses need to satisfy the ‘adiabatic condition’ in order to achieve efficient inversion. The adiabatic condition is met when the effective B1 field is sufficiently larger than the rate of frequency sweep. That is, if the sweep is slow enough, the magnetization gets ‘dragged’ alo...