Glioma cell density in a rat gene therapy model gauged by water relaxation rate along a fictitious magnetic field

Longitudinal and transverse rotating‐frame relaxation time constants, T1ρ and T2ρ, have previously been successfully applied to detect gene therapy responses and acute stroke in animal models. Those experiments were performed with continuous‐wave irradiation or with frequency‐modulated pulses operating in an adiabatic regime. The technique called Relaxation Along a Fictitious Field (RAFF) is a recent extension of frequency‐modulated rotating‐frame relaxation methods. In RAFF, spin locking takes place along a fictitious magnetic field, and the decay rate is a function of both T1ρ and T2ρ processes. In this work, the time constant characterizing water relaxation with RAFF (TRAFF) was evaluated for its utility as a marker of response to gene therapy in a rat glioma model. To investigate the sensitivity to early treatment response, we measured several rotating‐frame and free‐precession relaxation time constants and the water apparent diffusion coefficients, and these were compared with histological cell counts in 8 days of treated and control groups of animals. TRAFF was the only parameter exhibiting significant association with cell density in three different tumor regions (border, intermediate, and core tissues). These results indicate that TRAFF may provide a marker to identify tumors responding to treatment. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.