CEST - Chemical Exchange Saturation Transfer
Chemical Exchange Saturation Transfer
(CEST) is a technique where one resonance, in slow exchange
with a second resonance, is saturated with a selective low power pulse followed by a hard non-selective 90° pulse. The intensity of the second resonance is then diminished due to the transfer of saturation from the first resonance as the result of chemical exchange
. The figure below demonstrates this for a 25 mM solution of salicylic acid in H2O/D2O buffered at pH 7.
The left-hand panel of the figure is a stacked plot of extracted spectra collected in a pseudo 2D acquisition as a function of saturation frequency. The saturation frequency was varied from an initial value of 20 ppm to a final value of -20 ppm in steps of 0.2 ppm. The spectra are plotted such that only the water resonance is on scale. One can see that the intensity of the water resonance dips when a saturation frequency of ~14 ppm is applied, corresponding to the resonance frequency of the –COOH and –OH protons of the salicylic acid (which appear to be in fast or intermediate exchange
with one another). The water resonance of course also dips to zero when a saturation frequency of ~4.7 ppm is used, corresponding to a simple presaturation of the water
. The right-hand panel of the figure is a plot of the integral of the water resonance as a function of saturation frequency, showing again a dip at ~14 ppm.
CEST is used in MRI
to provide image contrast where a chemical exchange agent is introduced and images are collected with and without saturation of the exchange agent. The difference provides an image enhanced by the presence of the chemical exchange agent.
Thank you to Dr. Mojmir Suchy
of Prof. Adam Shuhendler’s
group at the University of Ottawa
for arousing my interest in the use of CEST for MRI and preparing the sample used in this post.
Source: University of Ottawa NMR Facility Blog