Mechanism of magnetization transfer during on‐resonance water saturation. A new approach to detect mobile proteins, peptides, and lipids

Abstract

The mechanism of magnetization transfer (MT) between water and components of the proton spectrum was studied ex vivo in a perfused cell system and in vivo in the rat brain (n ‫؍‬ 5). Water was selectively labeled and spectral buildup consequential to transfer of longitudinal magnetization was followed as a function of time. At short mixing time (T m ), nitrogen-bound solventexchangeable protons were observed, predominantly assigned to amide groups of proteins and peptides. At longer T m , intramolecular nuclear Overhauser enhancement (NOE) was observed in the aliphatic proton region, leading to a mobile-macromolecule-weighted spectrum that resembles typical protein spectra described in the literature. This effect on the proton spectrum is distinct from that of classical off-resonance MT, which has been shown to be due to the immobile solid-like proton pool. When studying a solution of major brain metabolites under physiological concentrations and conditions (pH), no transfer effects were observed, in line with expectations based on reduced NOE effects in rapidly tumbling molecules and the fast proton exchange rates of amino, amine, SH, and OH groups. In vivo magnetization transfer (MT) effects from irradiated protons associated with immobile macromolecules and membranes can occur to water (1-3) and to metabolites (4 -13). The mechanism of MT during off-resonance irradiation (i.e., not on water) has been well described for both instances. The semisolid lattice protons are efficiently saturated via intramolecular dipolar transfer over the complete spin system (spin diffusion), which is followed by intermolecular transfer to interacting smaller molecules.This intermolecular saturation transfer can in principle occur both via proton exchange and dipolar cross relaxation to short-lived bound molecules (Fig. 1), the descriptions of which are theoretically equivalent. However, the sequence of events after transfer of saturation to metabolites is not yet well understood. For instance, for creatine in excised rat muscle, the off-resonance saturation effect was found to be pH-independent in H 2 O and to remain unchanged in D 2 O, interpreted as excluding a contribution of exchangeable NH protons to the transfer process (4,13). However, on-resonance water saturation was found to increase the saturation transfer effect to metabolites (4,12,13). In addition, selective inversion of the water magnetization caused a transient reduction in the creatine resonance in vivo, but not in vitro, the mechanism of which is also unknown.In the present article we investigate the effect of water perturbation on ex vivo and in vivo metabolite spectra using a selective water-exchange-filter experiment (WEXfilter) (14,15) in which the path of MT of RF-labeled water protons can be followed as a function of time. Using a detection scheme that does not suppress exchangeable protons, it is possible to follow the interaction between water, exchangeable protons, and other molecular sites on a time scale of a few milliseconds to a second. The present...

In vivo chemical exchange saturation transfer imaging allows early detection of a therapeutic response in glioblastoma

“…This was supported by an NMR study reporting that the amide protons of mobile protein/peptide side chains (Gln, Asn) and backbone amides resonate at 6.8 ppm and 8.2-8.4 ppm (2.1 ppm and 3.5-3.7 ppm downfield of water resonance) range, respectively (28). Zhou et al demonstrated in two models of orthotopic murine glioma that APT imaging could clearly differentiate viable glioma (hyperintense) from radiation necrosis (hypointense to isointense), and that the APT signal of the tumor decreased substantially after radiation (19).…”

Amide proton transfer imaging of human brain tumors at 3T

“…Selective irradiation is possible because there is a composite amide proton resonance at approximately 3.5 ppm downfield from the water resonance (21)(22)(23), and in the brain, amide protons exchange with water protons at a rate (k) of about 30 times per second (14). Thus, the frequency difference (⌬) with the water resonance is generally sufficient to provide conditions of slow exchange on the NMR time scale (⌬Ͼ Ͼk).…”

Simultaneous in vivo positron emission tomography and magnetic resonance imaging

“…To ensure that a subject is being imaged in the same physiologic state, and to correlate changes over time in the PET and MRI signals in response to an intervention, thus often requires that the data be acquired simultaneously. To give just one example, one might want to monitor dynamic changes in tumor physiology with MRI [e.g., cellularity by diffusion measurements (24), macromolecular environment (25,26), vasculature by contrast enhancement (27)] while imaging the delivery of a radiolabeled therapeutic agent to, or assessing the biochemistry of, the tumor. Simultaneous acquisition of PET and MRI data by using an integrated imaging device is, therefore, necessary to answer many important biomedical questions in dynamic, living systems.…”