Exchange-influenced T sub(2) rho contrast in human brain images measured with adiabatic radio frequency pulses

Transverse relaxation in the rotating frame (T sub(2) rho ) 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 sub(2 rho ,ex) and T sub(2 rho ,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 sub(2) rho contrast in images of the human occipital lobe at magnetic field of 4 T. T sub(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 sub(2) rho time constants. Measurements of the apparent relaxation time constants (T sub(2) rho ) of brain tissue water as a function of the time between centers of pulses ( tau sub(cp)) at 4 and 7 T permitted separation of the DA contribution from that of dipolar relaxation. The methods presented assess T sub(2) rho relaxation influenced by DA in tissue and provide a means to generate T sub(2) rho contrast in MRI.