Approximate B1+ scaling of the SSFP steady state
Abstract
Purpose
It is shown that the steady state of rapid, TR-periodic steady-state free precession (SSFP) sequences at small to moderate flip angles exhibits a universal, approximate scaling law with respect to variations of B1+. Implications for the accuracy and precision of relaxometry experiments are discussed.
Methods
The approximate scaling law is derived from and numerically tested against known analytical solutions. To assess the attainable estimator precision in a typical relaxometry experiment, we calculate the Cramér–Rao bound (CRB) and perform Monte Carlo (MC) simulations.
Results
The approximate universal scaling holds well up to moderate flip angles. For pure steady state relaxometry, we observe a significant precision penalty for simultaneous estimation of R1 and B1+, whereas good R2 estimates can be obtained without even knowing the correct actual flip angle.
Conclusion
Simultaneous estimation of R1 and B1+ from a set of SSFP steady states alone is not advisable. Apart from separate B1+ measurements, the problem can be addressed by adding transient state information, but, depending on the situation, residual effects due to the scaling may still require some attention.