Analysis of the Precision of Variable Flip Angle T1 Mapping with Emphasis on the Noise Propagated from RF Transmit Field Maps

In magnetic resonance imaging, precise measurements of longitudinal relaxation time (T-1 ) is crucial to acquire useful information that is applicable to numerous clinical and neuroscience applications. In this work, we investigated the precision of T-1 relaxation time as measured using the variable flip angle method with emphasis on the noise propagated from radiofrequency transmit field (B-1(+)) measurements. The analytical solution for T-1 precision was derived by standard error propagation methods incorporating the noise from the three input sources: two spoiled gradient echo (SPGR) images and a B-1(+) map. Repeated in vivo experiments were performed to estimate the total variance in T-1 maps and we compared these experimentally obtained values with the theoretical predictions to validate the established theoretical framework. Both the analytical and experimental results showed that variance in the B-1(+) map propagated comparable noise levels into the T-1 maps as either of the two SPGR images. Improving precision of the BT measurements significantly reduced the variance in the estimated T-1 map. The variance estimated from the repeatedly measured in vivo T-1 maps agreed well with the theoretically-calculated variance in T-1 estimates, thus validating the analytical framework for realistic in vivo experiments. We concluded that for T-1 mapping experiments, the error propagated from the B-1(+) map must be considered. Optimizing the SPGR signals while neglecting to improve the precision of the B-1(+) map may result in grossly overestimating the precision of the estimated T-1 values.