Other Saturation-Recovery T1 Mapping techniques

Several variations of the inversion recovery pulse sequence were developed to overcome challenges like those specified above. Amongst them, the Look-Locker technique Look & Locker, 1970 stands out as one of the most widely used in practice. Instead of a single 90° acquisition per TR, a periodic train of small excitation pulses θ are applied after the inversion pulse, {$\theta_{180}$ – 𝛕 – θ – 𝛕 – θ – ...}, where 𝛕 = TR/n and n is the number of sampling acquisitions. This pulse sequence samples the inversion time relaxation curve much more efficiently than conventional inversion recovery, but at a cost of lower SNR. However, because the magnetization state of each TI measurement depends on the previous series of θ excitation, it has higher sensitivity to B₁-inhomogeneities and imperfect spoiling compared to inversion recovery Gai et al., 2013Stikov et al., 2015. Nonetheless, Look-Locker is widely used for rapid T₁ mapping applications, and variants like MOLLI (Modified Look-Locker Inversion recovery) and ShMOLLI (Shortened MOLLI) are widely used for cardiac T₁ mapping Messroghli et al., 2004Piechnik et al., 2010.

Another inversion recovery variant that’s worth mentioning is saturation recovery, in which the inversion pulse is replaced with a saturation pulse: {$\theta_{90}$ – TI – $\theta_{90}$}. This technique was used to acquire the very first T₁ map Pykett & Mansfield, 1978. Unlike inversion recovery, this pulse sequence does not need a long TR to recover to its initial condition; every $\theta_{90}$ pulse resets the longitudinal magnetization to the same initial state. However, to properly sample the recovery curve, TIs still need to reach the order of ~T₁, the dynamic range of signal potential is cut in half ([0, $M_{0}$]), and the short TIs (which have the fastest acquisition times) have the lowest SNRs.