Figure 4. Constrained Cramér-Rao bounds as coefficients of variation.

Parameter values are listed as sets 1–5 in Table 1, and accurate constraints on T_2,S,T_2,F, andk_FS are assumed. This study was performed at σ_SPGR = 1 × 10⁻³ M₀, which corresponds to a clinically-feasible SNR for gradient echo sequences, and it should be noted that coefficients of variation scale with image SNR. The dotted lines on each plot are a reference goal for quantitative precision. There is a drastic improvement in free-parameter precision when the T₂s and exchange rate are constrained. Acquiring using multiple phase cycling patterns in the SSFP images does not benefit precision as it does when parameters are left unconstrained, but still removes a source of bias; i.e., the implicit assumption of B₀ homogeneity described previously (25).