Optimum flip-angles for exciting NMR with uncertain T1 values.

T1 is often ill-determined. This means that an Ernst angle excitation often cannot be precisely defined for the simple pulse and acquire experiment. Here, published 31P T1 values of metabolites in human muscle, liver, heart, and brain are archived, some new data on heart and brain added, and overall confidence intervals determined. Strategies for setting the flip-angle based on the confidence intervals are examined, and an optimum flip-angle derived which minimizes the signal loss relative to what could have been realized if T1 were precisely known. With such optimized pulses, signal loss can be limited to < or = 14% for up to a 10-fold variation in T1, with TR < or = T1. The effect that an uncertainty in T1 by a factor of two has on the saturation corrected signal is limited to < or = 20% in the optimum flip-angle experiment. Adiabatic B1-independent rotation phase-cycled (BIRP) excitation pulses are ideal as optimum flip-angle pulses as they can be prescribed without calibration.