In part 1 of this series we had a look at what magnetic field gradients (MFGs) are and learned that static gradients can be used to create a one-dimensional image of the sample. We also saw that the peaks broaden and consequently the chemical shift resolution is lost completely. It would be nice, for example to be able to do the position encoding using the gradients, but switch them off during the acquisition in order to preserve chemical shift resolution.
In fact, this is possible. Pulsed field gradients (PFG) are used in MRI scanners, where amazing images of the sample (usually the human body) are obtained. Magritek’s two founding fathers, Professor Bernhard Blümich and Professor Paul Callaghan, have both done pioneering work on NMR using pulsed magnetic field gradients, so there is a special interest in our company on this topic.
Figure 1: Schematic diagram of a simple pulse sequence without (a) and with (b) gradient pulse. The phase evolution of the spins at different locations along the gradient direction is shown in (c) and (d).
Figure 1 (a) shows a schematic of a simple pulse sequence. The acquired signal is a decaying exponential with the sample’s T2 as a time constant. The phase of the spins at different locations in the sample is shown in (c). Directly after the rf pulse all spins are in phase. After the delay, at the start of the acquisition, all spins are still in phase, since they are all exposed to the same field.
Let’s see what happens to the spin phases when we insert a gradient pulse between the rf pulse and acquisition, as indicated in (b). Directly after the rf pulse all spins are in phase. The gradient pulse exposes spins at different locations to different fields, so the phase evolution will depend on the location. At the end of the gradient pulse, the spin phases are encoded in a helix-like structure along the direction of the gradient, as indicated in (d). For the signal acquisition, the gradient is switched off, and consequently all spins are exposed to the same field. This means that the acquired signal will be very similar to what we see without the gradient pulse. But there is one fundamental difference. The total signal is the sum of the signal coming from all spins in the sample. In (c) all spins are in phase and add up coherently, but in (d) there is a phase wrap across the sample. Adding up the signal of all spins will result in signal attenuation and a phase shift. Both of these will depend on the length and amplitude, or in fact the area, of the gradient pulse, as well as the size and position of the sample in the gradient.
We can now run a series of experiments where the area of the gradient is stepped from one experiment to the next. This is very similar to the way a 2D COSY experiment is run and gives a two-dimensional data set. A subsequent two-dimensional Fourier transform yields a two-dimensional spectrum, where the chemical shift is along F2 and the position along the gradient direction is along F1. This is the simplest form of a Chemical Shift Image (CSI).
In fact, the Terranova-MRI teaching system uses the earth’s magnetic field, along with pulsed magnetic field gradients, to obtain MR images.
The conclusion of this post is that pulsed magnetic field gradients are a great tool for position encoding whilst preserving chemical shift resolution.
Please keep your eyes open for the next post in this series which will show how pulsed field gradients can be used to measure displacements.
The Spinsolve benchtop NMR spectrometer can now be purchased with additional hardware to enable PFGs. If you have any questions or want to discuss how gradients or other NMR methods can help with your application please Contact Us
References
- B. Blümich, NMR Imaging of Materials, Clarendon Press, Oxford 2000.
- P. T. Callaghan, Principles of Nuclear Magnetic Resonance Microscopy, Clarendon Press, Oxford 1991.
Click here to read Part 3: Encoding for Displacement using Pairs of Pulsed Gradients