Optimal radiofrequency and gradient spoiling for improved accuracy of T1 and B1 measurements using fast steady‐state techniques

Abstract

Variable flip angle T(1) mapping and actual flip-angle imaging B(1) mapping are widely used quantitative MRI methods employing radiofrequency spoiled gradient-echo pulse sequences. Incomplete elimination of the transverse magnetization in these sequences has been found to be a critical source of T(1) and B(1) measurement errors. In this study, comprehensive theoretical analysis of spoiling-related errors in variable flip angle and actual flip-angle imaging methods was performed using the combined isochromat summation and diffusion propagator model and validated by phantom experiments. The key theoretical conclusion is that correct interpretation of spoiling phenomena in fast gradient-echo sequences requires accurate consideration of the diffusion effect. A general strategy for improvement of T(1) and B(1) measurement accuracy was proposed based on the strong spoiling regimen, where diffusion-modulated spatial averaging of isochromats becomes a dominant factor determining magnetization evolution. Practical implementation of strongly spoiled variable flip angle and actual flip-angle imaging techniques requires sufficiently large spoiling gradient areas (A(G)) in combination with optimal radiofrequency phase increments (phi(0)). Optimal regimens providing <2% relative T(1) and B(1) measurement errors in a variety of tissues were theoretically derived for prospective in vivo variable flip angle (pulse repetition time = 15-20 ms, A(G) = 280-450 mT.ms/m, phi(0) = 169 degrees) and actual flip-angle imaging (pulse repetition time(1)/pulse repetition time(2) = 20/100 ms, A(G1)/A(G2) = 450/2250 mT.ms/m, phi(0) = 39 degrees) applications based on 25 mT/m maximal available gradient strength.