STrategically Acquired Gradient Echo (STAGE) imaging, part II: Correcting for RF inhomogeneities in estimating T1 and proton density

PURPOSE: To develop a method for mapping the B₁ transmit (B_1t) and B₁ receive (B_1r) fields from two gradient echo datasets each with a different flip angle and from these two images obtain accurate T₁ and proton density (PD) maps of the brain. METHODS: A strategically acquired gradient echo (STAGE) data set is collected using two flip angles each with multiple echoes. The B_1t field extraction was based on forcing cortical gray matter and white matter to have specific T₁ values and fitting the resulting B_1t field to a quadratic function. The B_1r field extraction was based on synthesizing isointense images despite there being two or three tissue types present in the brain. This method was tested on 10 healthy volunteers and 20 stroke patients from data acquired at 3.0Tesla. RESULTS: With the knowledge of the B_1t and B_1r fields, the uniformity of tissue T₁ and PD maps was considerably improved. T₁ values were measured for both the midbrain and basal ganglia and found to be in good agreement with the literature. DISCUSSION AND CONCLUSIONS: STAGE provides a practical way to assess the B_1t and the B_1r fields which can then be used to correct for spatial variations in the images.