Reducing bias in DREAM flip angle mapping in human brain at 7T by multiple preparation flip angles

Abstract

DREAM (Dual Refocusing Echo Acquisition Mode) is an ultra-fast multi-slice B₁⁺-mapping technique based on the single-shot STEAM sequence. To study systematic errors at high actual flip angles (FA) and low SNR, DREAM B₁⁺ maps at 3.75×3.75×3.50 mm³ resolution were acquired at 7T in phantoms and human brain in vivo with nominal FAs between 20° and 90° for the two STEAM preparation pulses. Predicted B₁⁺ estimates were underestimated at actual FAs above 50° while noise was prominent below 20°. With a reliable interval of the actual FA between 20° and 50° identified, a B₁⁺ range of 33% - 200% of nominal FA is covered by varying the nominal preparation angle through 25°, 40°, and 60°. Individual B₁⁺ maps are thresholded according to the identified interval and combined into a single map. We demonstrate the benefit of the combined low-noise, low-bias B₁⁺ maps for dual flip angle T₁-mapping.