Strategies and tactics in NMR imaging relaxation time measurements. I. Minimizing relaxation time errors due to image noise--the ideal case.

The effect of NMR image noise on errors in calculated values of relaxation times is quantitatively assessed by use of relaxation time noise figures, which are derived on the basis of statistical principles as functions of pulse delay, repetition, and recovery intervals for several types of pulse sequences. Two strategies for determining relaxation times are considered: two point (ratio of intensities for two experiments) and multipoint (least-squares fit of intensities to pulse-sequence functions for n experiments). For given total measurement times, values of pulse interval times are found which give minimum relaxation time noise figures. A comparison of ratio methods shows that the best is a combination saturation-recovery, inversion-recovery (SR/IR) technique. For short measurement times (less than about 10T1) this optimized SR/IR ratio determination is also superior to the best multipoint method, a series of inversion-recovery experiments with equally incremented inversion-recovery times. An examination of the effect of signal averaging on the relaxation time noise shows that up to a measurement time characteristic of the particular method used (e.g., for times up to about 5T1 for the SR/IR ratio determination, 100T1 for the multipoint inversion-recovery method), increased measurement time is more effectively allotted to longer pulse intervals than to signal averaging. Numerical examples are tabulated which can help one to set optimum values for pulse intervals, given a rough estimate of the relaxation time to be determined.