Composite RF pulses for -insensitive volume excitation at 7Tesla

Abstract

A new class of composite RF pulses that perform well in the presence of specific ranges of B 0 and inhomogeneities has been designed for volume (non-selective) excitation in MRI. The pulses consist of numerous (~ 100) short (~ 10µs) block-shaped sub-pulses each with different phases and amplitudes derived from numerical optimization. Optimized pulses are designed to be effective over a specific range of frequency offsets and transmit field variations and are thus implementable regardless of field strength, transmit coil configuration, or the subject-specific spatial distribution of the static and RF fields. In the context of 7T human brain imaging, both simulations and phantom experiments indicate that optimized pulses result in similar on-resonance flip-angle uniformity as BIR-4 pulses but with the advantages of superior off-resonance stability and significantly reduced average power. The pulse design techniques presented here are thus well-suited for practical application in ultra-high field human MRI.

Slice-selective excitation with -insensitive composite pulses

“…This was accomplished with a pre-existing protocol [10] for numerical optimization via a discrete grid of relevant Δ B 0 and $B_1^{+}$ values. The Δ B 0 and $B_1^{+}$ ranges of this grid were chosen to be ±150 Hz and [0.25, 1.00], respectively, with the latter being in units of the nominal $B_1^{+}$ field strength ( $B_{1,\text{nom}}^{+}$).…”

“…The Δ B 0 and $B_1^{+}$ ranges of this grid were chosen to be ±150 Hz and [0.25, 1.00], respectively, with the latter being in units of the nominal $B_1^{+}$ field strength ( $B_{1,\text{nom}}^{+}$). These choices designate the field variations over which the resulting pulse is required to perform and were guided by the previous 7 T study [10]. Field ranges were discretized into 31 steps in the Δ B 0 direction (resulting in steps of 10 Hz) and 16 steps in the $B_1^{+}$ direction (resulting in steps equivalent to 5% increments of $B_{1,\text{nom}}^{+}$).…”

“…The challenge of mitigating such signal variations in order to better realize the potential of high-field imaging systems has sparked a renewed interest in the design of RF pulses that invoke a magnetization response insensitive to the $B_1^{+}$ field strength [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Due to non-selective frequency profiles and non-linear transverse magnetization phase in the through-slice direction, many such pulses are limited in application to whole-volume acquisitions.…”

“…The work presented here uses the same spatial selectivity strategy as that discussed above but in conjunction with composite pulses designed to produce uniform flip angles over specified ranges of B 0 and $B_1^{+}$ field strengths [10] and is thus akin to recent work by Matson et al [16] and Boulant et al [17]. In general, this class of composite pulses is characterized by non-selective composite pulses numerically optimized for insensitivity to field variations and subsequently endowed with spectral selectivity via the use of shaped sub-pulses, e.g., Gaussian (hereafter referred to as GAU) or SINC sub-pulses.…”

Design of Crisscrossed Double-Layer Birdcage Coil for Improving B1+Field Homogeneity for Small-Animal Magnetic Resonance Imaging at 300 MHz

“…HF MRI scanner have found use in a wide range of medical applications from neuroimaging to oncological investigations for the image quality they provide [3][4][5]. In this backdrop, researchers have designed and developed various geometries of the RF coil that provides the excitation pulse for generating the transmitting (B 1 + ) field and highly sensitive receiving fields (B 1 − ) for the MR signals emitted from the imaged subject [6,7].…”