Nuts and bolts of chemical exchange saturation transfer MRI

Abstract

Chemical Exchange Saturation Transfer (CEST) has emerged as a novel MRI contrast mechanism that is well suited for molecular imaging studies. This new mechanism can be used to detect small amounts of contrast agent through saturation of rapidly exchanging protons on these agents, allowing a wide range of applications. CEST technology has a number of indispensable features, such as the possibility of simultaneous detection of multiple “colors” of agents and detecting changes in their environment (e.g. pH, metabolites, etc) through MR contrast. Currently a large number of new imaging schemes and techniques have been developed to improve the temporal resolution and specificity and to correct the influence of B0 and B1 inhomogeneities. In this review, the techniques developed over the last decade have been summarized with the different imaging strategies and post-processing methods discussed from a practical point of view including describing their relative merits for detecting CEST agents. The goal of the present work is to provide the reader with a fundamental understanding of the techniques developed, and to provide guidance to help refine future applications of this technology. This review is organized into three main sections: Basics of CEST Contrast, Implementation, Post-Processing, and also includes a brief Introduction section and Summary. The Basics of CEST Contrast section contains a description of the relevant background theory for saturation transfer and frequency labeled transfer, and a brief discussion of methods to determine exchange rates. The Implementation section contains a description of the practical considerations in conducting CEST MRI studies, including choice of magnetic field, pulse sequence, saturation pulse, imaging scheme, and strategies to separate MT and CEST. The Post-Processing section contains a description of the typical image processing employed for B0/B1 correction, Z-spectral interpolation, frequency selective detection, and improving CEST contrast maps.

Magnetic Resonance Imaging of Neurotransmitter-Related Molecules

“…ADC: apparent diffusion coefficient, CASL: continuous arterial spin labeling, TTC: 2,3,5-triphenyl tetrazolium chloride, Glu: glutamate, Lac: lactate. However, some issues such as a high specific absorption rate (SAR) 17 and susceptibility to magnetic field heterogeneity 18 should be resolved before applying this technique to routine clinical settings.…”

Technical and experimental features of Magnetic Resonance Spectroscopy of brain glycogen metabolism

“…This technique can thus detect low-concentration metabolites through the saturation of rapidly exchanging protons on these metabolites [68]. CEST MRI has been explored for assessing glycogen content in perfused liver [69] and to some extent in human skeletal muscle [70].…”

Functional magnetic resonance imaging techniques and their development for radiation therapy planning and monitoring in the head and neck cancers

“…In CEST MRI, different irradiation frequencies of a longer but weaker saturation RF pulse than spin-lock pulse for spin-lock imaging are applied to obtain the socalled Z-spectrum, the signal intensity ratios compared to the unsaturated signal intensity as a function of offset frequency from water. Asymmetric magnetization transfer ratio (MTRasym), the subtraction of the MTR at the opposite offset frequency of labile proton pool and that at the offset frequency, and other parameters can be derived from Z-spectrum analysis (119). Some animal studies have suggested the potentials of CEST MRI for RT applications, for example, the differentiation of brain tumor reoccurrence from radiation induced necrosis (120), and in vivo imaging of glucose uptake and metabolism in tumors (121).…”