Correction of Inhomogeneous RF Field Using Multiple SPGR Signals for High-Field Spin-Echo MRI

Abstract

To propose a simple and useful method for correcting nonuniformity of high-field (3 Tesla) T(1)-weighted spin-echo (SE) images based on a B1 field map estimated from gradient recalled echo (GRE) signals. To estimate B1 inhomogeneity, spoiled gradient recalled echo (SPGR) images were collected using a fixed repetition time of 70 ms, flip angles of 45 and 90 degrees, and echo times of 4.8 and 10.4 ms. Selection of flip angles was based on the observation that the relative intensity changes in SPGR signals were very similar among different tissues at larger flip angles than the Ernst angle. Accordingly, spatial irregularity that was observed on a signal ratio map of the SPGR images acquired with these 2 flip angles was ascribed to inhomogeneity of the B1 field. Dual echo time was used to eliminate T(2)(*) effects. The ratio map that was acquired was scaled to provide an intensity correction map for SE images. Both phantom and volunteer studies were performed using a 3T magnetic resonance scanner to validate the method. In the phantom study, the uniformity of the T(1)-weighted SE image improved by 23%. Images of human heads also showed practically sufficient improvement in the image uniformity. The present method improves the image uniformity of high-field T(1)-weighted SE images.