Optimizing saturation-recovery measurements of the longitudinal relaxation rate under time constraints

The saturation-recovery method using two and three recovery times is studied for conditions in which the sum of recovery times is 1.5T(1) to 3T(1), where T(1) is the longitudinal relaxation time. These conditions can reduce scan time considerably for long T(1) species and make longitudinal relaxation rate R(1) (R(1) = 1/T(1)) mapping for body fluids clinically feasible. Monte Carlo computer simulation is carried out to determine the ideal set of recovery times under various constraints of the sum of recovery times. The ideal set is found to be approximately invariant to the signal-to-noise ratio. For the three-point method, two of the recovery times should be set the same or approximately the same and should be shorter than the third one. Only marginal improvements in accuracy and precision can be achieved by the three-point method over the two-point method under a common constraint of the sum of recovery times. Three-dimensional, high resolution, whole-brain saturation-recovery scans on volunteers with a fast-spin-echo technique (XETA) and completed in a scan time of 10 min generated R(1) measurements of cerebrospinal fluid (T(1) approximately 4 s) in agreement with the computer simulation and literature results, which demonstrates the clinical feasibility of applying the two-point saturation-recovery method for R(1) mapping for long relaxation components.