Altering MRI rotating frame relaxations by changing the truncation level of Hyperbolic Secant pulse

In this work, we introduce a strategy entitled RETRO (RElaxation dependent on TRuncatiOn) to alter rotating frame relaxations in MRI by changing the truncation level of the amplitude and frequency modulation functions (AM and FM, respectively) of the Hyperbolic Secant (HS) radiofrequency pulses used for achieving adiabatic full inversion. For small truncation levels, as in typical HS pulses, B₁ is negligible at the beginning of the pulse. In this case, the relaxation process during the pulse is characterized solely by the longitudinal relaxation time constant T_1ρ,1(t) when magnetization M is initially aligned with the effective magnetic field B⁽¹⁾_eff(t), or by the transverse relaxation time constant T_2ρ,1(t) when M is initially placed on a plane perpendicular to B⁽¹⁾_eff(t). However, when the truncation level is non-zero, an instantaneous non-zero B₁ field is formed at the beginning of the HS pulse, leading to relaxation that comprises both T_1ρ,1(t) and T_2ρ,1(t) pathways, with relative contributions depending on the truncation level. Here, after introducing the basis of the RETRO strategy, we provide theoretical descriptions of homonuclear dipolar relaxations between like ½ spins in a weak collision regime, along with exchange-induced relaxations between two magnetic spin populations with different chemical shifts (Δω ≠ 0) during the pulses. In addition, we describe the features of in vivo RETRO contrasts obtained in the rodent brain with different pulse durations and truncation levels, and demonstrate that high truncation levels allow shortening pulse duration while maintaining RF amplitudes suitable for in vivo investigations. We conclude that, by enabling a wide range of HS pulse configurations to achieve full adiabatic inversion, the RETRO strategy generates flexible and robust MRI contrasts that can probe fast relaxing components typically not accessible in vivo with conventional adiabatic T_1,2ρ techniques.