Figure 16.

Illustration of R _1ρ. Upper panel: scheme of a spin lock applied to two non‐exchanging spins with two different chemical shifts. If the spin lock power is large, both lines are spin locked in the x,y plane, for example, the effective field is in the x,y plane, and for increasing duration of the spin lock, the signals will decay according to T ₂ relaxation (shown on the right). In the case on the left, the spin lock power is smaller and on‐resonant only with respect to the signal at the blue chemical shift, while the signal at the red chemical shift still experiences the spin lock, it experiences significant off‐resonance effects. This manifests itself as an effective field, which is no longer in the x,y plane, therefore the signal with the red chemical shift is no longer spin locked in the x,y plane but along an effective field and the net magnetization of the red line during the spin lock has a significant z‐component. Thus, the signal with the red chemical shift will not only decay according to T ₂ but also T ₁. The contributions of the different relaxation phenomena can be calculated from simple trigonometric functions from the angle between the effective field and the z axes. Lower panel: we illustrated for the two different spin locks, a scenario with exchange. Now only one (broadened) line is observable in the spectrum and a spin lock is applied on‐resonant with this line. For a high power spin lock covering both, the GS chemical shift as well as the excited chemical shift, even with exchange happening during the spin lock, the net magnetization decay rate will correspond to R ₂ since the magnetization of an exchanging spin is spin locked in the x,y plane, irrespective of whether it is in the GS conformer or the ES conformer. In practice, experiments with different spin lock durations are carried out and the decaying intensity of the observable signal is fitted to an exponential decay to obtain R _1ρ. In case of a weaker spin lock, which does not cover the ES chemical shift without any off‐resonance effects, for the duration that a spin is in the ES conformation during the spin lock, the magnetization will decay with a different decay rate due to the different effective field, compared to the GS. This manifests itself in a faster decay rate, an R _EX contribution, for the observed signal for weaker spin locks when fitted to an exponential. In practice the measurements are repeated for different spin lock durations for different spin lock strengths.