Array-optimized composite pulse for excellent whole-brain homogeneity in high-field MRI. Journal   Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. Citation   Magn Reson Med. 57(3):470-4 Publication date   2007 Mar Authors   Collins CM Wang Z Mao W Fang J Liu W Smith MB Investigators   Christopher Michael Collins Jidong Fang Wenlei Liu Michael B. Smith Grant agencies   National Center for Research Resources National Institute for Biomedical Imaging and Bioengineering Grants   NCRR P41RR008079 NCRR P41RR16105 NIBIB R01EB000454 NIBIB R01EB000895 Abstract   A number of methods to improve excitation homogeneity in high-field MRI have been proposed, and some of these methods rely on separate control of radiofrequency (RF) coils in a transmit array. In this work we combine accurate RF field calculations and the Bloch equation to demonstrate that by using a sequence of pulses with individually optimized current distributions (i.e., an array-optimized composite pulse), one can achieve remarkably homogeneous distributions of available signal intensity over the entire brain volume. This homogeneity is greater than that achievable using the same transmit array to produce either a single optimized (or RF shimmed) pulse or a single RF shimmed field distribution in a standard 90x-90y composite pulse arrangement. Simulations indicate that with a very simple array-optimized composite pulse, excellent whole-brain excitation homogeneity can be achieved at up to 600 MHz.