FAST MULTISLICE B1 MAPPING: MRI

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Introduction: Inhomogeneity of the applied B1 field compromises the accuracy of MR-measurements. The inhomogeneity depends on the RF-coil, the load, and characteristics of the observed object [1]. A map of the B1 field makes it possible to account for and correct problems associated with the RF-inhomogeneity, such as image artifacts or inaccurate quantization. Methods: In our method, a stimulated echo sequence (STE) is modified with an additional non-selective pulse. Due to the simple relationship between the observed signal and the flip angle, theta, of the extra pulse, we can robustly estimate B1. Multislice B1-maps were measured on a 1.5-T GE Signa system using the standard circularly polarized head coil. A 28-cm diameter sphere containing doped water was placed in the center of the RF-coil. Four sets of 7 slices were acquired with the nominal theta set to 0, 100, 200, and 300 degrees. Total imaging time was 2:20 min. Results: Multislice quantitative B1 maps were acquired from phantoms and normal volunteers. The maps confirm the exptected B1 variations of the head coil. Conclusions: Rapid quantitative volumetric estimates of RF profiles can be calculated from acquisitions with a modified stimulated-echo sequence.