Reassembled saturation transfer (REST) MR images at 2 B1 values for in vivo exchange-dependent imaging of amide and nuclear Overhauser enhancement
Yanrong Chen, Xujian Dang, Wanting Hu, Yaozong Sun, Yan Bai, Xiaoli Wang, Xiaowei He, Meiyun Wang, Xiaolei Song
Abstract
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 ∆r.Z = r.Zref - r.ZCEST and r.MTRRex = r.1/Zref - r.1/ZCEST, utilizing the averages over loosely-sampled reference frequency offsets as Zref and over densely-sampled target offsets as ZCEST. Using r.MTRRex measured under two B1,sat’s, exchange rate (kex) 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 MTRRex_LD.
Results
For a broad choice of Δω_ref ranges and numbers, Δr.Z and r.MTRRex exhibited comparable quantification features with conventional LD and MTRRex_LD respectively, when B1,sat ≤1 μT. The subtraction of two REST values under distinct B1,sat’s showed linear relationships with kex 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 MTRRex_LD, with superiority in CNR and acquisition efficiency. Compared with MTRRex_LD, 2-B1,sat subtraction REST images displayed better resistance to B1 inhomogeneity, with more specific enhanced regions. They showed higher signals for amide than for NOE in human brain, presumably reflecting the higher increment from faster-exchanging species as B1,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.