Development of novel chemical exchange saturation transfer contrast techniques for MRI

Abstract

Magnetic resonance imaging (MRI) is a powerful medical imaging modality due to its excellent soft tissue contrast and use of non-ionising radiofrequency (RF) radiation. However, MRI is not without shortcomings as some tissue types, such as brain tumours, are not easily distinguished from surrounding brain tissue due to a lack of image contrast. This necessitates the utilisation of MRI contrast enhancement agents (CAs) which can generate MRI signal disparity between the tissue of interest and the surrounding tissue. These MRI CAs are usually chelates of Gadolinium(III), or Gd(III), and typically work by shortening the 'longitudinal' or T1 relaxation time of vicinal water protons. For example, greater amounts of these Gd(III) CAs accumulate in brain tumour tissue due to the breakdown of the blood-brain-barrier in tumours, resulting in the shortened T1 relaxation time of the tumour tissue water protons compared to the healthy tissue water protons. This ultimately results in the MRI contrast, that is the contrast between the "brighter" MRI pixels for the tumour tissue and the "darker" pixels for the surrounding healthy tissue. Although these relaxation-based MRI CAs yield excellent contrast enhancement, they are often limited in their specificity. Furthermore, the safety of these Gd(III) CAs have been put into question, with recent reports of their use being associated with the development of nephrogenic systemic fibrosis in patients with chronic or acute kidney diseases. This has led to a push for the development of safer alternative MRI contrast enhancement techniques. Chemical exchange saturation transfer (CEST) is one such alternative MRI contrast enhancement technique with potentially widespread applications due to its molecular imaging capabilities. The CEST technique involves RF saturation of the labile proton spins of a CEST CA, which are in constant chemical exchange with the vicinal detectable water protons, resulting in a reduced water signal intensity. The requirement for a CEST CA to have labile protons means that any molecules containing hydroxyl, amide, amine, or thiol protons, or coordinated water molecules may potentially act as CEST CAs, provided that the proton exchange rate (ksw) is slow enough. To date, only hydroxyl, amide, and amine functional groups have been exploited to generate CEST MRI contrast. CEST contrast from thiol functional groups has yet to be explored despite the important chemistry of thiol compounds in biological systems (e.g., thiol-disulfide exchange and oxidant scavenging). In this study, the thiol-based CEST effect from several well-known thiol-containing metabolites and pharmaceutical drugs was investigated. The thiol compounds in question are glutathione (GSH), cysteine (Cys), homocysteine (hCys), N-acetylcysteine (NAC), and D-penicillamine (Dpen). The investigation began with using slice-selective and ultrafast CEST 1H-NMR pulse sequences to quantify the thiol CEST effect, as well as the amine and amide CEST effects, of each compound at 37 °C and different pH values. It was found that all five compounds elicited a thiol CEST effect at 2 - 2.7 ppm upfield from the water resonance, with the greatest thiol CEST magnitude occurring at acidic pH values. This natural upfield CEST effect is particularly advantageous due to increased specificity since it does not have the potential to overlap with the CEST effect from hydroxyl, amide, or amine groups, which are all found downfield from the water resonance. Among the five thiol compounds, only NAC was found to elicit detectable thiol CEST effect (a 9.5% attenuation of the water signal when a saturation RF pulse of amplitude B1 = 5 µT and duration tsat = 5 s was applied at -2.7 ppm) at physiologically relevant conditions (i.e., near-neutral pH and 37 °C). The NAC thiol CEST effect was found to increase linearly with concentrations between 1 - 20 mM, similar to other CEST CAs reported in the literature, and was also found to be detectable in the presence of relayed nuclear Overhauser enhancements (rNOEs) and conventional semisolid magnetisation transfer (MT) effects in human serum albumin and agarose phantoms, respectively.

Date of Award	2020
Original language	English