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 (Formula presented.) r.Z = r.Z_ref - r.Z_CEST and the reassembled exchange-dependen magnetization transfer ratio r.MTR_Rex = 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.MTR_Rex measured under 2 B_1,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 T₁. 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 MTR_{Rex_}LD. Results: For a broad choice of (Formula presented.) ranges and numbers, Δr.Z and r.MTR_Rex exhibited comparable quantification features with conventional LD and MTR_Rex_LD, respectively, when B_1,sat ≤ 1 μT. The subtraction of 2 REST values under distinct B_1,sat values showed linear relationships with exchange rate and obtained immunity to field inhomogeneity and variation in MT and water T₁. For both rat and human studies, REST images exhibited similar contrast distribution to MTR_Rex_LD, with superiority in contrast-to-noise ratio and acquisition efficiency. Compared with MTR_Rex_LD, 2-B_1,sat subtraction REST images displayed better resistance to B₁ 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,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.