Chemical Exchange Saturation Transfer

 

Introduction

A new technique known as Chemical Exchange Saturation Transfer (CEST) may provide a powerful new tool for MR molecular imaging.  CEST exploits the ability of Nuclear Magnetic Resonance (NMR) to resolve different signals arising from protons on different molecules. By selectively saturating a particular proton signal (associated with a particular molecule or CEST agent) that is in exchange with surrounding water molecules, the MRI signal from the surrounding bulk water molecules is also attenuated.  Images obtained with and without the RF saturating pulse reveal the location of the CEST agent.  The chemical exchange must be in the intermediate regime where exchange is fast enough to efficiently saturate the bulk water signal but slow enough that there is a chemical shift difference between the exchangeable proton and the water proton resonances.  The magnitude of the CEST effect therefore depends on both the exchange rate and the number of exchangeable protons:


where kCE is the single site exchange rate, n is the number of exchangeable protons/molecule, and T1 is the water spin-lattice relaxation time in the presence of the saturating pulse

 

In concept, CEST has three main advantages over traditional molecular imaging techniques: (1) the image contrast is controlled with radio-frequency (RF) pulses and can be turned on/off at will;  (2) The endogenous molecules of interest, in some cases, can be directly detected, eliminating the need for contrast agent to be delivered to, and to specifically react with, the molecule of interest;  (3) A variant of the CEST technique, known as PARACEST, may be much more sensitive than traditional molecular imaging techniques and should be able to detect nanomolar concentrations.  PARACEST typically relies on water exchange between the bulk water and water bound to paramagnetic Lanthanide complexes. Saturation of the Lanthanide ion bound water resonance leads to attenuation of the bulk water signal via water exchange.  The large paramagnetic chemical shift of the bound water molecules allows them to tolerate much faster exchange rates with the bulk water while still while still remaining in the intermediate exchange regime, thereby providing much more efficient saturation of the bulk water signal and much greater CEST sensitivity.

 

CEST Imaging of L-Arginine

Shown in Figure 1 are CEST images obtained of a tube of 100 mM L-arginine at pH=7 acquired with a saturating pulse applied at +2 ppm (Figure 1, left) and -2 ppm (Figure 1, middle) offset from the bulk water proton resonance.  The exchangeable amine proton of L-arginine has a chemical shift of +2 ppm relative to water and therefore saturation at +2 ppm results in attenuation of the bulk water signal.  The -2 ppm image is used as a control for other saturation and magnetization transfer effects not associated with the amine proton chemical exchange.

 

Figure 1: FLASH images of a tube of 100 mM L-arginine at pH=7 obtained with a RF saturating pulse applied +2 ppm (left) and  -2 ppm (middle) from the water proton resonance. The difference image is shown on the right.  A 65% attenuation of the bulk water proton signal is observed.

 

 L-arginine also has an exchangeable amide proton, with a resonance at + 3 ppm, that is in the intermediate exchange regime at pH’s below 5. In Figure 2, we show the pH and concentration dependence of the CEST effect for L-arginine. Changing the pH strongly effects the proton exchange rate and hence the observed CEST effect. The dominant CEST effect at low pH is attributed to the exchangeable amide proton, while the dominant CEST effect at netral pH is due to exchange of the amine proton.

                                                             

Figure 2: Dependence of the L-arginine CEST spectrum on pH (left) and L-arginine concentration (right). The data were acquired at 14 Tesla (600 Hz=1 ppm). CEST spectra for the concentration dependence plot were acquired at pH=7. A strong CEST effect is observed at +2 ppm for pH=6 and 7 and is attributed to the exchangeable amine proton. A strong CEST effect is also observed at +3 ppm for pH= 4 and 5 and is attributed to the exchangeable amide protons of L-arginine.

 

 

CEST Imaging of Creatine

Shown in Figure 3 is the pH and concentration dependence of the creatine CEST spectrum. A CEST effect is only observed for a saturation pulse frequency offset of +2 ppm, since creatine contains only an exchangeable amine proton.

 

Figure 3: Dependence of the creatine CEST spectrum on pH (left) and creatine concentration (right). The data were acquired at 14 Tesla (600 Hz=1 ppm). CEST spectra for the concentration dependence plot were acquired at pH=7. A strong CEST effect is observed at +2 ppm and is attributed to the exchangeable amine proton.

 

 

CEST Imaging of Albumin

 Shown in Figure 4 is the pH dependence of the CEST spectrum for albumin. A 30% CEST effect is observed at pH=5.5 for 1 mM albumin. The increased CEST sensitivity with respect to L-arginine and creatine is due to the significantly greater number of exchangeable amide protons.

 

Figure 4: Dependence of the albumin CEST spectrum on pH. The data were acquired at 14 Tesla (600 Hz=1 ppm) with an albumin concentration of 1 mM. A strong CEST effect is observed at +3 ppm at pH=8 and is attributed to exchangeable amide protons.