Measurement and correction of transmitter and receiver induced nonuniformities in vivo

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Abstract
Signal intensity nonuniformities in high field MR imaging limit the ability of MRI to provide quantitative information and can negatively impact diagnostic scan quality. In this paper, a simple method is described for correcting these effects based on in vivo measurement of the transmission field B1+ and reception sensitivity maps. These maps can be obtained in vivo with either gradient echo (GE) or spin echo (SE) imaging sequences, but the SE approach exhibits an advantage over the GE approach for correcting images over a range of flip angles. In a uniform phantom, this approach reduced the ratio of the signal SD to its mean from around 30% before correction to approximately 6% for the SE approach and 9% for the GE approach after correction. The application of the SE approach for correcting intensity nonuniformities is demonstrated in vivo with human brain images obtained using a conventional spin echo sequence at 3.0 T. Furthermore, it is also shown that this in vivo B1+ and reception sensitivity mapping can be performed using segmented echo planar imaging sequences providing acquisition times of less than 2 min. Although the correction presented here is demonstrated with a simultaneous transmit and receive volume coil, it can be extended to the case of separate transmission and reception coils, including surface and phase array coils. Magn Reson Med 53:408–417, 2005.