Ultrafast 3D Bloch-Siegert B -mapping using variational modeling.

Abstract

PurposeHighly accelerated B1+‐mapping based on the Bloch–Siegert shift to allow 3D acquisitions even within a brief period of a single breath‐hold.Theory and MethodsThe B1+ dependent Bloch–Siegert phase shift is measured within a highly subsampled 3D‐volume and reconstructed using a two‐step variational approach, exploiting the different spatial distribution of morphology and B1+‐field. By appropriate variable substitution the basic non‐convex optimization problem is transformed in a sequential solution of two convex optimization problems with a total generalized variation (TGV) regularization for the morphology part and a smoothness constraint for the B1+‐field. The method is evaluated on 3D in vivo data with retro‐ and prospective subsampling. The reconstructed B1+‐maps are compared to a zero‐padded low resolution reconstruction and a fully sampled reference.ResultsThe reconstructed B1+‐field maps are in high accordance to the reference for all measurements with a mean error below 1% and a maximum of about 4% for acceleration factors up to 100. The minimal error for different sampling patterns was achieved by sampling a dense region in k‐space center with acquisition times of around 10–12 s for 3D‐acquistions.ConclusionsThe proposed variational approach enables highly accelerated 3D acquisitions of Bloch–Siegert data and thus full liver coverage in a single breath hold.