k_T‐points: Short three‐dimensional tailored RF pulses for flip‐angle homogenization over an extended volume

Abstract

With Transmit SENSE, we demonstrate the feasibility of uniformly exciting a volume such as the human brain at 7T through the use of an original minimalist transmit k-space coverage, referred to as ''k T -points.'' Radio-frequency energy is deposited only at a limited number of k-space locations in the vicinity of the center to counteract transmit sensitivity inhomogeneities. The resulting nonselective pulses are short and need little energy compared to adiabatic or other B þ 1 -robust pulses available in the literature, making them good candidates for shortrepetition time 3D sequences at high field. Experimental verification was performed on three human volunteers at 7T by means of an 8-channel transmit array system. On average, whereas the standard circularly polarized excitation resulted in a 33%-flip angle spread (standard deviation over mean) throughout the brain, and a static radio-frequency shim showed flip angle variations of 17% and up, application of k T -point-based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub-millisecond durations. Magn Reson Med 67:72-80,

Accelerating Parallel Transmit Array B1 Mapping in High Field MRI With Slice Undersampling and Interpolation by Kriging

“…The dipoles were previously described in [35]. The power budget at the input of each channel is limited to 650 W. More details about the operation of this transmit-array for in vivo experiments are found in [34]. The validation of interpolation algorithms and SU-XFL sequence follows different steps.…”

“…A set of phases was previously worked out, in such a way that, sending the same power on every input of the coil with phase for channel , gives a field pattern close to the well-known circularly polarized (CP) mode of a birdcage coil [34]. This pseudo CP-mode, should not show a location with zero-field (field level below the noise level) in the brain.…”

First and second order derivatives for optimizing parallel RF excitation waveforms

“…(68) and (69) and the effort to obtain theÃ-andB-values in Eqs. (52) and (53). Also, we shifted 2NSC operations for the calculation of theC-andD-values to the second order derivatives, since these intermediate values can be avoided, when only first order derivatives are needed.…”

“…At this point, we'd like to thank Nicolas Boulant for communicating the example parameters, giving additional explanations and partially checking our results. The two variants are a 30 degree excitation with 5 k T -points, and a 180 degree inversion with 7 k T -points [53]. The problem is to determine the RF voltages for each k T -point and channel such that the magnetization is rotated by one of these flip angles.…”

3D T 2-weighted imaging at 7T using dynamic kT-points on single-transmit MRI systems

“…In a previous study [10], improved-quality whole-brain T 2 -weighted imaging was achieved by replacing the original hard pulses of a 3D TSE sequence with k T -point [11] pulses. For both single and parallel transmit systems, a single k T -point pulse was designed in the small tip-angle regime with a uniform excitation profile as a target.…”