pH Imaging Based on Chemical Exchange Saturation Transfer: Principles, Methods, Applications and Recent Progresses

Abstract

Chemical exchange saturation transfer (CEST) is a novel molecular magnetic resonance imaging (MRI) method that makes full use of the chemical exchange between water protons and exchangeable protons from solute molecules. Through chemical exchanges, the MRI signal of bulk water decreases when saturating at specific frequency of solute protons. Important biological information related to the chemical exchange processes could be extracted from the amplitude of water signal decrease. For example, the CEST signals have been used for in vivo pH imaging since the proton exchange rate is often pH-dependent. CEST signals originated from endogenous proteins/peptides and exogenous small molecule/metal chelate probes have been used for in vivo pH imaging. Using the ratiometric methods or the amine and amide concentration-independent detection (AACID) methods, in vivo pH maps have been acquired from kidneys, ischemic brain and tumors. In this paper, we thoroughly reviewed the progresses during the past twenty years in the field of in vivo pH imaging with CEST contrast. Principles, methods and applications were discussed, as well as development trends and future directions.

Keywords

• magnetic resonance imaging (MRI)
• chemical exchange saturation transfer (CEST)
• pH imaging
• ratiometric procedure
• contrast agents