Transmit B1+ field inhomogeneity and T1 estimation errors in breast DCE-MRI at 3 tesla

Purpose: To quantify B1+ variation across the breasts and to evaluate the accuracy of precontrast T1 estimation with and without B1+ variation in breast MRI patients at 3 Tesla (T).

Materials and methods: B1+ and variable flip angle (VFA) T1 mapping were included in our dynamic contrast-enhanced (DCE) breast imaging protocol to study a total of 25 patients on a 3.0T GE MR 750 system. We computed precontrast T1 relaxation in fat, which we assumed to be consistent across a cohort of breast imaging subjects, with and without compensation for B1+ variation. The mean and standard deviation of B1+ and T1 values were calculated for statistical data analysis.

Results: Our measurements showed a consistent B1+ field difference between the left and right breasts. The left breast has an average 15.4% higher flip angle than the prescribed flip angle, and the right breast has an average 17.6% lower flip angle than the prescribed flip angle. This average 33% flip angle difference, which can be vendor and model specific, creates a 52% T1 estimation bias in fat between breasts using the VFA T1 mapping technique. The T1 variation is reduced to 7% by including B1+ correction.

Conclusion: We have shown that severe B1+ variation over the breasts can cause a substantial error in T1 estimation between the breasts, in VFA T1 maps at 3T, but that compensating for these variations can considerably improve accuracy of T1 measurements, which can directly benefit quantitative breast DCE-MRI at 3T.