Exchange-influenced T2p contrast in human brain images measured with adiabatic radio frequency pulses

Transverse relaxation in the rotating frame (T-2p) is the dominant relaxation mechanism during an adiabatic Carr-Purcell (CP) spin-echo pulse sequence when no delays are used between pulses in the CP train. The exchange-induced and dipolar interaction contributions (T-2p,T-ex and T-2p,T-dd) depend on the modulation functions of the adiabatic pulses used. In this work adiabatic pulses having different modulation functions were utilized to generate T-2 rho, contrast in images of the human occipital lobe at magnetic field of 4 T. T-2 rho time constants were measured using an adiabatic CP pulse sequence followed by an imaging readout. For these measurements, adiabatic full passage pulses of the hyperbolic secant HSn (n = 1 or 4) family having significantly different amplitude-and frequency-modulation functions were used with no time delays between pulses. A dynamic averaging (DA) mechanism (e.g., chemical exchange and diffusion in the locally different magnetic susceptibilities) alone was insufficient to fully describe differences in brain tissue water proton T-2 rho time constants. Measurements of the apparent relaxation time constants (T-2(dagger)) of brain tissue water as a function of the time between centers of pulses (tau(cp)) at 4 and 7 T permitted separation of the DA contribution from that of dipolar relaxation. The methods presented assess T-2 rho relaxation influenced by DA in tissue and provide a means to generate T-2 rho contrast in MRI. Magn Reson Med 53:823-829, 2005. (c) 2005 Wiley-Liss, Inc.