A new method entitled Relaxation Along a Fictitious Field (RAFF) was recently introduced for investigating relaxations in rotating frames of rank ≥2. RAFF generates a fictitious field (E) by applying frequency-swept pulses with sine and cosine amplitude and frequency modulation operating in a sub-adiabatic regime. In the present work, MRI contrast is created by varying the orientation of E, i.e. the angle between E and the z″ axis of the second rotating frame. When > 45°, the amplitude of the fictitious field E generated during RAFF is significantly larger than the RF field amplitude used for transmitting the sine/cosine pulses. Relaxation during RAFF was investigated using an invariant-trajectory approach and the Bloch-McConnell formalism. Dipole-dipole interactions between identical (like) spins and anisochronous exchange (e.g., exchange between spins with different chemical shifts) in the fast exchange regime were considered. Experimental verifications were performed in vivo in human and mouse brain. Theoretical and experimental results demonstrated that changes in induced a dispersion of the relaxation rate constants. The fastest relaxation was achieved at ≈ 56°, where the averaged contributions from transverse components during the pulse are maximal and the contribution from longitudinal components are minimal. RAFF relaxation dispersion was compared with the relaxation dispersion achieved with off-resonance spin lock T1ρ experiments. As compared with the off-resonance spin lock T1ρ method, a slower rotating frame relaxation rate was observed with RAFF, which under certain experimental conditions is desirable.
Bibliographical noteFunding Information:
The authors the following agencies for financial support: Instrumentarium Science Foundation (TL), Orion Corporation Research Foundation (TL), Finnish Cultural Foundation Northern Savo (TL), and NIH grants P30 NS057091, P41 RR008079, R01 NS061866, and R21 NS059813.
- Human brain
- Mouse brain
- Relaxation along fictitious field
- Relaxation dispersion
- Rotating frame