T_2ρ and T_1ρ adiabatic relaxations and contrasts

Transverse relaxation in the rotating frame (T_2ρ) is the dominant relaxation mechanism during a train of adiabatic full passage (AFP) radiofrequency (RF) pulses with no interpulse time intervals placed after the 90° excitation pulse. The magnetization components remain transverse to the time-dependent effective field and undergo relaxation with the time constant T_2ρ. Longitudinal relaxation in the rotating frame (T _1ρ) is the dominant relaxation mechanism during a train of AFP RF pulses placed prior to an excitation pulse. Here, magnetization is aligned along the time-dependent effective field during adiabatic rotation undergoes relaxation with the time constant T_1ρ. A detailed description of rotating frame relaxations due to dipolar interactions and exchange during adiabatic pulses is presented herein. The exchange-induced and dipolar interaction contributions depend on the modulation functions of the adiabatic pulses used. The intrinsic rotating frame relaxation rate constant is sensitive to fluctuations at the effective frequencey (ω_eff) in the rotating frame, and this is modulated differently during the two types of AFP pulses. This may lead to the possibility to assess T_ρ and T _2ρ relaxation influenced by dipolar relaxation pathways and exchange in human brain tissue and provide a means to generate T _1ρ and T_2ρ contrasts in MRI.