T_1ρ MRI contrast in the human brain: Modulation of the longitudinal rotating frame relaxation shutter-speed during an adiabatic RF pulse

Longitudinal relaxation in the rotating frame (T_1ρ) is the dominant mechanism during a train of adiabatic full passage (AFP) RF pulses with no interpulse intervals, placed prior to an excitation pulse. Asymptotic apparent time constants (T_{1 ρ}^′) were measured for human occipital lobe ¹H₂O at 4 T using brief imaging readouts following such pulse trains. Two members of the hyperbolic secant (HSn) AFP pulse family (n = 1 or 4; i.e., arising from different amplitude- and frequency-modulation functions) were used. These produced two different non-monoexponential signal decays during the pulse trains. Thus, there are differing contrasts in asymptotic T_{1 ρ}^′ maps derived from these data. This behavior is quite different than that of ¹H₂O signals from an aqueous protein solution of roughly the same macromolecular volume fraction as tissue. The ROI-averaged decays from the two acquisitions can be simultaneously accommodated by a two-site-exchange model for an equilibrium isochronous process whose exchange condition is modulated during the pulse. The model employs a two-spin description of dipolar interaction fluctuations in each site. The intrinsic site R_1ρ(≡ T_{1 ρ}^{- 1}) value is sensitive to fluctuations at the effective Larmor frequency (ω_eff) in the rotating frame, and this is modulated differently during the two types of AFP pulses. Agreement with the data is quite good for site orientation correlation time constants characteristic of macromolecule-interacting water (site A) and bulk-like water (site B). Since R_1ρA is significantly modulated while R_1ρB is not, the intrinsic relaxographic shutter-speed for the process (≡|R_1ρA - R_1ρB|), and thus the exchange condition, is modulated. However, the mean residence time (67 ms) and intrinsic population fraction (0.2) values found for site A are each rather larger than might be expected, suggesting a disproportionate role for the water molecules known to be "buried" within the large and concentrated macromolecules of in vivo tissue.