Chemical exchange saturation transfer imaging is a technique to indirectly measure pools of amide protons of low concentration. The key here is the amide protons must be physically exchanging with the bulk water. This method applies a saturation pulse at the resonance frequency of the amide protons for a period of time (e.g., 3 seconds) and then a standard water image is acquired. The change in signal intensity of the water image relative to a water image acquired with no saturation is related to the concentration and exchange rate of the amide proton pool (among a couple of other things).
The protons of the water molecules resonate at a particular frequency (gyromagnetic ratio).
Protons on other molecules such as the large macromolecule shown above resonate at slightly different frquencies. The way to picture this is to think of a graph such as:
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One interesting property is that there is a physical exchange of protons between the water and the macromolecule (as shown in the yellow shaded regions in the figure below). This physical exchange happens at a particular frequency and is known as the exchange rate.