A review of optimization and quantification techniques for chemical exchange saturation transfer MRI toward sensitive in vivo imaging

Abstract

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging method that probes the chemical exchange between bulk water and exchangeable protons. CEST imaging indirectly detects dilute labile protons via bulk water signal changes following selective saturation of exchangeable protons, which offers substantial sensitivity enhancement and has sparked numerous biomedical applications. Over the past decade, CEST imaging techniques have rapidly evolved due to contributions from multiple domains, including the development of CEST mathematical models, innovative contrast agent designs, sensitive data acquisition schemes, efficient field inhomogeneity correction algorithms, and quantitative CEST (qCEST) analysis. The CEST system that underlies the apparent CEST-weighted effect, however, is complex. The experimentally measurable CEST effect depends not only on parameters such as CEST agent concentration, pH and temperature, but also on relaxation rate, magnetic field strength and more importantly, experimental parameters including repetition time, RF irradiation amplitude and scheme, and image readout. Thorough understanding of the underlying CEST system using qCEST analysis may augment the diagnostic capability of conventional imaging. In this review, we provide a concise explanation of CEST acquisition methods and processing algorithms, including their advantages and limitations, for optimization and quantification of CEST MRI experiments.

Clinical applications of chemical exchange saturation transfer (CEST) MRI

“…The acquisition of a full CEST spectrum provides opportunities for more accurate analysis methods. For example, a series of analytical methods can account for “RF spillover,” or unintended direct saturation of the water when the MR frequency of the proton on the agent or biomolecule is close to the MR frequency of water . An analysis method that accounts for apparent exchange‐dependent relaxation (AREX) can remove the background MT effect when analyzing CEST spectra, while also using a T 1 relaxation map to correct for the effects of T 1 relaxation on CEST amplitudes, to measure a pure APT signal amplitude .…”

Interrogating Metabolism in Brain Cancer

“…However, the specific absorption rate (SAR) increases quadratically with B 0 , which places a constraint on sequence design and applications. Please refer to (88,89) for a comprehensive review of optimization and quantification techniques involved in CEST imaging. Here we will focus on the potential applications of CEST to image brain cancer.…”

Ratiometric NOE(−1.6) contrast in brain tumors

“…Magnetization transfer (MT) and chemical exchange saturation transfer (CEST) are two sensitivity enhancement mechanisms to detect solute molecules, which employ the chemical exchange or dipole–dipole interaction between solute protons and water protons . In MT or CEST imaging, a frequency‐selective RF pulse is applied to saturate solute protons for a few seconds, and a subsequent change in water signal due to an accumulative exchange/coupling effect between solute protons and water protons is detected.…”