Reassembled saturation transfer (REST) MR images at 2 B₁ values for in vivo exchange-dependent imaging of amide and nuclear Overhauser enhancement

Purpose Design an efficient CEST scheme for exchange-dependent images with high contrast-to-noise ratio. Theory Reassembled saturation transfer (REST) signals were defined as Delta$$ \Delta $$r.Z = r.Z(ref) - r.Z(CEST) and the reassembled exchange-dependen magnetization transfer ratio r.MTRRex = r.1/Z(ref) - r.1/Z(CEST), utilizing the averages over loosely sampled reference frequency offsets as Z(ref) and over densely sampled target offsets as Z(CEST). Using r.MTRRex measured under 2 B-1,B-sat values, exchange rate could be estimated. Methods The REST approach was optimized and assessed quantitatively by simulations for various exchange rates, pool concentration, and water T-1. In vivo evaluation was performed on ischemic rat brains at 7 Tesla and human brains at 3 Tesla, in comparison with conventional asymmetrical analysis, Lorentzian difference (LD), an MTRRex_LD. Results For a broad choice of Delta omega ref$$ \Delta {\omega}_{ref} $$ ranges and numbers, Delta r.Z and r.MTRRex exhibited comparable quantification features with conventional LD and MTRRex_LD, respectively, when B-1,B-sat <= 1 mu T. The subtraction of 2 REST values under distinct B-1,B-sat values showed linear relationships with exchange rate and obtained immunity to field inhomogeneity and variation in MT and water T-1. For both rat and human studies, REST images exhibited similar contrast distribution to MTRRex_LD, with superiority in contrast-to-noise ratio and acquisition efficiency. Compared with MTRRex_LD, 2-B-1,B-sat subtraction REST images displayed better resistance to B-1 inhomogeneity, with more specific enhanced regions. They also showed higher signals for amide than for nuclear Overhauser enhancement effect in human brain, presumably reflecting the higher increment from faster-exchanging species as B-1,B-sat increased. Conclusion Featuring high contrast-to-noise ratio efficiency, REST could be a practical exchange-dependent approach readily applicable to either retrospective Z-spectra analysis or perspective 6-offset acquisition.