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2 T1 Mapping/2-1 Inversion Recovery/01-introduction.md

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 ## Inversion Recovery _T_{sub}`1` Mapping
 
-Widely considered the gold standard for [_T_{sub}`1`](wiki:Spin–lattice_relaxation) mapping, the [inversion recovery](wiki:Inversion_recovery)technique estimates [_T_{sub}`1`](wiki:Spin–lattice_relaxation) values by fitting the signal recovery curve acquired at different delays after an inversion pulse (180°). In a typical [inversion recovery](wiki:Inversion_recovery) experiment ([](#irFig1)), the [magnetization](wiki:Magnetization) at thermal equilibrium is inverted using a 180° RF pulse. After the longitudinal [magnetization](wiki:Magnetization) recovers through [spin-lattice relaxation](wiki:Spin–lattice_relaxation) for predetermined delay (`inversion time`, TI), a 90° excitation pulse is applied, followed by a readout imaging sequence (typically a [spin-echo](wiki:Spin_echo) or [gradient-echo](wiki:MRI_pulse_sequence#Gradient_echo) readout) to create a snapshot of the longitudinal [magnetization](wiki:Magnetization) state at that TI.
+Widely considered the gold standard for [_T_{sub}`1`](wiki:Spin–lattice_relaxation) mapping, the [inversion recovery](wiki:Inversion_recovery)technique estimates [_T_{sub}`1`](wiki:Spin–lattice_relaxation) values by fitting the signal recovery curve acquired at different delays after an inversion pulse (180°). In a typical [inversion recovery](wiki:Inversion_recovery) experiment ([](#irFig1)), the [magnetization](wiki:Magnetization) at thermal equilibrium is inverted using a 180° RF pulse. After the longitudinal [magnetization](wiki:Magnetization) recovers through [spin-lattice relaxation](wiki:Spin–lattice_relaxation) for predetermined delay (inversion time, TI), a 90° excitation pulse is applied, followed by a readout imaging sequence (typically a [spin-echo](wiki:Spin_echo) or [gradient-echo](wiki:MRI_pulse_sequence#Gradient_echo) readout) to create a snapshot of the longitudinal [magnetization](wiki:Magnetization) state at that TI.
 
 [Inversion recovery](wiki:Inversion_recovery) was first developed for [NMR](wiki:Nuclear_magnetic_resonance) in the 1940s [@Hahn1949;@Drain1949], and the first [_T_{sub}`1`](wiki:Spin–lattice_relaxation) map was acquired using a saturation-recovery technique (90° as a preparation pulse instead of 180°) by [@Pykett1978]. Some distinct advantages of inversion recovery are its large dynamic range of signal change and an insensitivity to pulse sequence parameter imperfections [@Stikov2015]. Despite its proven robustness at measuring [_T_{sub}`1`](wiki:Spin–lattice_relaxation), inversion recovery is scarcely used in practice, because conventional implementations require repetition times (TRs) on the order of 2 to 5 [_T_{sub}`1`](wiki:Spin–lattice_relaxation) [@Steen1994], making it challenging to acquire whole-organ [_T_{sub}`1`](wiki:Spin–lattice_relaxation) maps in a clinically feasible time. Nonetheless, it is continuously used as a reference measurement during the development of new techniques, or when comparing different [_T_{sub}`1`](wiki:Spin–lattice_relaxation) mapping techniques, and several variations of the [inversion recovery](wiki:Inversion_recovery) technique have been developed, making it practical for some applications [@Messroghli2004;@Piechnik2010].