Slice-selective excitation with -insensitive composite pulses

Abstract

Spatially selective excitation pulses have been designed to produce uniform flip angles in the presence of the RF and static field inhomogeneities typically encountered in MRI studies of the human brain at 7 T. Pulse designs are based upon non-selective, composite pulses numerically optimized for the desired performance over prescribed ranges of field inhomogeneities. The non-selective pulses are subsequently transformed into spatially selective pulses with the same field-insensitive properties through modification of the spectral composition of the individual sub-pulses which are then executed in conjunction with an oscillating gradient waveform. An in-depth analysis of the performance of these RF pulses is presented in terms of total pulse durations, slice profiles, linearity of in-slice magnetization phase, sensitivity to RF and static field variations, and signal loss due to T2∗ effects. Both simulations and measurements in phantoms and in the human brain are used to evaluate pulses with nominal flip angles of 45° and 90°. Target slice thickness in all cases is 2 mm. Results indicate that the described class of field-insensitive RF pulses is capable of improving flip-angle uniformity in 7 T human brain imaging. There appears to be a subset of pulses with durations ≲ 10 ms for which non-linearities in the magnetization phase are minimal and signal loss due to T2∗ decay is not prohibitive. Such pulses represent practical solutions for achieving uniform flip angles in the presence of the large field inhomogeneities common to high-field human imaging and help to better establish the performance limits of high-field imaging systems with single-channel transmission.

Toward imaging the body at 10.5 tesla

“…However, achieving the desired transmit performance when performing RF pulse sequences with high levels of power deposition (i.e., FSE) at 10.5T may not be accomplished using static or dynamically applied static RF shimming methods inside medium‐ and large‐sized targets. Several RF management strategies such as RF excitation using time‐interleaved acquisition of modes , multi‐spoke pulses , pTx RF pulse design , or direct signal control approaches may be used to overcome some of the challenges. Furthermore, incorporation of novel quantitative imaging methods that can perform in the presence of heterogeneous RF fields may alleviate the need of strict transmit field control .…”

7 Tesla MRI with a transmit/receive loopless antenna and B1-insensitive selective excitation

“…Spatially selective composite pulses were designed using a Matlab‐based (Mathworks Inc., Natick, MA) optimization algorithm . The transverse magnetization at the end of the RF pulse was numerically computed by solving the Bloch equations for a ±3‐mm range with 0.1‐mm increments and B 1 over 0–30 μT with 0.1 μT increments .…”

“…Here, we demonstrate scout and high‐resolution imaging with a 7T internal transmit/receive loopless antenna using composite, spatially selective, B 1 ‐insensitive, excitation pulses . RF safety is tested experimentally using a gel phantom.…”

A comparison and evaluation of reduced-FOV methods for multi-slice 7T human imaging

“…As demonstrated, a benefit of the selective composite pulse, beyond achieving improved B1 uniformity, is an insensitivity to change in the B1 profile across patients [23], eliminating the need to use B1 mapping methods commonly applied by multi-transmit approaches for RF shimming [42]. Adiabatic pulses are similarly relatively insensitive to B1 variation; however, they also introduce a quadratic phase distribution that results in signal loss when used for refocusing.…”

“…To achieve a reduction in B1 inhomogeneity caused by spatial variations in flip angles at high field strength, IVI was modified to use a composite RF pulse for slice selective excitation [23, 24, 28] and a refocusing pulse in the orthogonal direction with an adiabatic shape (Figure 1B). The composite consisted of eight Gaussian pulses each with 1.06 ms durations with 22 mT/m trapezoid gradients to achieve a 4 mm slice thickness.…”