Magnetic resonance imaging (MRI) is arguably the most powerful and important imaging modality in clinical medicine. It is non-invasive and can provide anatomical details about static structures and dynamic processes from any region of the human body. Conventional low-molecular weight, extracellular, largely Gd3+-based contrast agents have had a significant impact in diagnostic medicine. As an alternative to T1 shortening agents for MR contrast enhancement, one could also introduce contrast by altering the proton density or total water signal detected by the MRI scanner. This can be accomplished using an old NMR technique called magnetization transfer (MT), first introduced by Forsén and Hoffman. Chemical Exchange Saturation Transfer (CEST) Agents are a class of agents that could potentially revolutionize the MRI field.
Magnetic resonance imaging (MRI) is arguably the most powerful and important imaging modality in clinical medicine. It is non-invasive and can provide anatomical details about static structures and dynamic processes from any region of the human body. Conventional low-molecular weight, extracellular, largely Gd3+-based contrast agents have had a significant impact in diagnostic medicine. These agents enter all extracellular space (vascular plus interstitial) and highlight only those tissue regions that temporally accumulate the paramagnetic complex. This accumulation of paramagnetic complex results in a shortening of the bulk water spin-lattice relaxation time (T1) and hence brightening of the image in that region. This mechanism of altering bulk water relaxation cannot be controlled externally and hence one cannot modulate (turn on/off) the relaxation effects of a T1 shortening agent; one can only compare image intensities pre- and post injection of the agent. Thus, one is dependent entirely upon getting enough agent targeted to a site of interest so that it may be detected by MRI. Various estimates of the lower detection limit of a typical low molecular weight Gd3+-based complex with a relaxivity of ~4 mM-1s-1 have been made and these vary somewhat but are typically in the range of 100-500 M. There has been considerable effort put forth to engineer Gd3+-based complexes with substantially higher relaxivities, but only modest success has been achieved at this point. Experts in this field agree that the only way to gain a substantial increase in relaxivity is to slow rotation of Gd3+ complexes by attachment to a larger structure such as biopolymer, a nanoparticle, or a naturally occurring polymer like albumin. Thus, to create new Gd3+-based systems for molecular imaging applications, one will need to make a high molecular weight polymer of some kind. This can introduce new complications and limitations that are not present with freely diffusible, low molecular weight complexes. If one however had the ability to modulate the signal provided by the agent at a prescribed rate, then one might be able to take advantage of known signal modulation algorithms to obtain a substantial gain in sensitivity even while using a low molecular weight complex.
As an alternative to T1 shortening agents for MR contrast enhancement, one could also introduce contrast by altering the proton density or total water signal detected by the MRI scanner. This can be accomplished using an old NMR technique called magnetization transfer (MT), first introduced by Forsén and Hoffman. This technique has been widely applied in chemistry and biology to determine unidirectional rate constants of simple reactions. To illustrate the basics of this technique, consider two exchanging water molecules, A and B, with different NMR chemical shifts (next page). Upon saturation of spin B by using a selective RF saturation pulse, the intensity of A will decrease to a new steady-state level due to chemical exchange of the two spins. At equilibrium, this new level is given by MA /MA0 = 1/(1 + k2T1A). This indicates that a MT effect will be observed if the T1 of bulk water (A in this model) is long compared to the lifetime (1/k2) of water at this site. This dictates that Gd3+ cannot be used in this experiment because the T1 of bulk water would be too short. However, Eu3+, Tm3+, Dy3+ and Yb3+ should work because these ions have smaller magnetic moments and consequently relax bulk water less efficiently. The model also indicates that the water at site B must have a different chemical shift than the water at site A and, as shown below, MT efficiency is related to this frequency difference ( ).
Ward et al. introduced the idea of using the MT effect to introduce contrast into an image and proposed a new class of MRI contrast agent based on chemical exchange saturation transfer (CEST) between intrinsic metabolites like amino acids, sugars, nucleotides or other heterocyclics having exchangeable OH or NH groups that exchange protons with bulk water. They demonstrated using simple diamagnetic molecules such as these that MRI contrast can be switched on/off by applying a saturating irradiation pulse at an exchangeable site a few ppm away from the bulk water resonance. One limitation of the simple diamagnetic agents described by Ward et al. is that the chemical shift of the exchanging OH or NH groups are rather close to the bulk water resonances, typically < 5 ppm, a serious limitation in any practical application of the CEST effect. We demonstrate below that this problem can be solved by using a paramagnetic CEST agent having one or more highly shifted NH or OH exchange sites.