B1 and T1 mapping of the breast with a reference tissue method

Abstract

Purpose: To develop a method for mapping the B1 field using a reference signal from a tissue with known T1.

Methods: Flip angle correction factors were calculated in a region with a known "gold standard" T1; by comparing T1 values from a variable flip angle (VFA) sequence to the "gold standard" and correcting the value of the Ernst angle. The resulting partial B1 map was interpolated for all other regions. In the breast, fat is an ideal reference tissue because its T1 is spatially homogeneous and interpatient variability is low. This method was tested with scans of phantoms and patients (n = 4) on a 3T magnet. The performance of the method was evaluated by comparing the results of VFA T1 mapping with and without B1 correction to inversion recovery (IR) T1 maps.

Results: Phantom data determined that a linear inverse distance weighted interpolation accurately recovered the full B1 map. Use of interpolated maps to correct the VFA data in vivo, reduced the average difference in the T1 of parenchyma between VFA and IR results from 58% to 8%.

Conclusion: This proof-of-principle study showed that it is possible to recover a full and accurate map of the B1 field in the breast by using a reference tissue (fat) with an accurately measured T1.

Keywords: B1 map; T1 mapping; quantitative breast imaging; reference tissue method.