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Friday, April 22, 2016

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. 

4 comments:

Anonymous said...

So I guess one could use this in order to see non-exchangeable protons below the water signal. If one presaturates the water line then these other signals are also killed,. But if the presaturation is done on one of the exchangeable protons then the main water peak will also get suppressed but the signals underneath it not. I am trying to think of a nice sample one could use to demonstrate this...

Glenn Facey said...

Anonymous,
Yes, you are correct however I suspect it may not be possible to suppress the water signal enough by saturating the other exchanging signal even at very long saturation times. The water signal would decrease to a plateau - not to zero. See this link.

http://u-of-o-nmr-facility.blogspot.ca/2011/08/saturation-transfer-and-exchange.html

Glenn

Anonymous said...

Thank you for the nice explanation and illustration of CEST. In a CEST experiment, do you vary the frequency of the hard pulse too or only the saturation pulse? what are typical saturation pulse durations?

Glenn Facey said...

Anonymous,
The frequency of the hard pulse is constant while the frequency of the saturation pulse is varied. Typical saturation times are on the order of seconds.

Glenn