On-resonance longitudinal relaxation time in the rotating frame (T1rho) has been shown to provide unique information during the early minutes of acute stroke. In the present study, the contributions of the different relaxation mechanisms to on-resonance T1rho relaxation were assessed by determining relaxation rates (R1rho) in both protein phantoms and in rat brain at 2.35, 4.7, and 9.4 T. Similar to transverse relaxation rate (R2), R1rho increased substantially with increasing magnetic field strength (B0). The B0 dependence was more pronounced at weak spin-lock fields. In contrast to R1rho, longitudinal relaxation rate (R1) decreased as a function of increasing B0 field. The present data argue that dipole-dipole interaction forms only one pathway for T1rho relaxation and the contributions from other physicochemical factors need to be considered.

In the present study we derive a solution for two site fast exchange-induced relaxation in the presence of a fictitious magnetic field as generated by amplitude and frequency modulated RF pulses. This solution provides a means to analyze data obtained from relaxation experiments with the method called RAFFn (Relaxation Along a Fictitious Field of rank n), in which a fictitious field is created in a coordinate frame undergoing multi-fold rotation about n axes (rank n). The RAFF2 technique is relevant to MRI relaxation methods that provide good contrast enhancement for tumor detection. The relaxation equations for n = 2 are derived for the fast exchange regime using density matrix formalism. The method of derivation can be further extended to obtain solutions for n > 2. Published by Elsevier Inc.

Abstract. In order to analyze the relaxation characteristics in fresh human brain tissue and blood samples we studied the proton relaxation rate dispersion in the frequency interval from 0.01 to 10 MHz. With the Field cycling method we apply, the sample is first magnetized in a relatively high magnetic fields (0.5 Tesla), which then by electronic means rapidly (10-3 s) is reduced to lower values (0.0001 - 0.5 Tesla) where the excited proton spin may relax during a time interval of about 3*T1max. Then the magnetic field is again quickly raised up to higher level for the detection of NMR-signal. In order to analyse the relaxation characteristics we applied a model with three compartments of water exchange. For each compartment we estimate a characteristic frequency by fitting the dispersion curves to a sum of Lorentz distributions. The characteristic frequencies thus obyained for various human tissues are given in kHz: Whole blood 285; 7600 Blood plasma 37 ± 4 220 ± 10 1480 ± 40 Cereb...