## 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 and 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, {θ180 – 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. 2013; Stikov 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. 2004; Piechnik 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: {θ90 – TI – θ90}. This technique was used to acquire the very first T1 map (Pykett and Mansfield 1978). Unlike inversion recovery, this pulse sequence does not need a long TR to recover to its initial condition; every θ90 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, M0]), and the short TIs (which have the fastest acquisition times) have the lowest SNRs.

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