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Paramagnetic chemical exchange saturation transfer (paraCEST) agents are well-suited for imaging tissue pH because the basis of CEST,c hemical exchange,i si nherently sensitive to pH. Several previous pH-sensitive paraCEST agents were based on an exchanging Ln 3+-bound water molecule as the CEST antenna but this design often added additional line-broadening to the bulk water signal due to T 2 exchange.W er eport herein ap H-sensitive paraCEST agent that lacks an inner-sphere water molecule but contains one Lnbound ÀOH group for CEST activation. The Yb 3+ complex, Yb(1), displayed as ingle,h ighly shifted CEST peak originating from the exchangeable Yb-OH proton, the frequency of which changed over the biologically relevant pH range.CEST images of phantoms ranging in pH from 6to8demonstrate the potential of this agent for imaging pH. Initial rodent imaging studies showed that Gd(1)remains in the vascular system much longer than anticipated but is cleared slowly via renal filtration.

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