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.
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.