Array‐optimized composite pulse for excellent whole‐brain homogeneity in high‐field MRI

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. Magn Reson Med 57:470 -474, 2007.

New high dielectric constant materials for tailoring theB1+distribution at high magnetic fields

“…Using an approach derived from such developments, one solution to at least partially solving the inhomogeneous EM distribution for high-field MRI is to drive each element of a resonator separately with a variable magnitude and phase, the so-called transmit-array approach [2,[14][15][16][17][18]. Other approaches involve the design of specialized radiofrequency (RF) pulses [19,20], or the combination of these two approaches of RF pulse design and transmit arrays [21][22][23][24][25].…”

Improvements in high‐field localized MRS of the medial temporal lobe in humans using new deformable high‐dielectric materials

“…Alternatively, the RF pulses must become extremely long, meaning that the minimum TE is increased and also the chemical shift displacement artifact increases because of the weaker gradients used. Approaches to resolving the issues of transmit field (B þ 1 ) inhomogeneity include the design of multi-element transmit arrays using 'B 1 shimming' (27)(28)(29)(30)(31), the design of specialized RF pulses (32)(33)(34)(35)(36)(37)(38)(39), and a combination thereof. Currently, these constitute approaches which are not easily implemented in a routine clinical set-up, and have not been used for localized MRS. Alternatively, a simple approach has been proposed recently (40), namely the use of external high-dielectric materials placed around the head to increase the homogeneity of the B þ 1 field [based on earlier work using simple water pads (41)].…”

Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects

“…Though it is at present largely experimental technology on a growing number of research systems, this technology has been used effectively to improve homogeneity of transmit field distributions (29,42,51), reduce SAR (53), and shorten the duration of RF tailored pulses (54). Numerical field calculations have been used extensively in the exploration of possibilities for this technology (18,23,29,(55)(56)(57)(58)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69)(70). The simplest approach to using transmit arrays is to vary the magnitude and phase in various elements to achieve a homogeneous distribution of the B 1 + field magnitude, and then use this more homogeneous distribution in standard pulses throughout the sequence.…”