Additive angle method for fast large‐tip‐angle RF pulse design in parallel excitation

Abstract

Current methods for parallel excitation RF pulse design are based on the small-tip-angle approximation, which provides a computationally efficient means of pulse calculation. In general, pulses designed with those methods are inaccurate when scaled to produce large-tip angles, and methods for large-tipangle pulse design are more computationally demanding. This paper introduces a fast iterative method for large-tip-angle parallel pulse design that is formulated as a small number of Bloch equation simulations and fast small-tip-angle pulse designs, the results of which add to produce large-tip-angle pulses. Simulations and a phantom experiment demonstrate that the method is effective in designing multidimensional large-tip-angle pulses of high excitation accuracy, compared to pulses designed with small-tip-angle methods. Magn Reson Med 59:779-787, 2008.

An electromagnetic reverse method of coil sensitivity mapping for parallel MRI – Theoretical framework

“…In addition, the HF B þ 1 and B À 1 behave differently and therefore caution should be exercised when relating the reception sensitivity to the transmission sensitivity of the same coil using the reciprocity theorem. Currently we are working on extending the proposed concept for HF applications including parallel reconstruction and parallel transmission [29][30][31][32].…”

First and second order derivatives for optimizing parallel RF excitation waveforms

“…; Cg minimizing the value F mls , which is calculated through Eqs. (13), (14), (15), (9), (11), (12), (16), (17), (18), and (19). Frequently, additional restrictions are imposed on the variables, in order to comply with hardware limitations and patient safety regulations.…”

“…We focus on the magnitude least squares variant [31,32] which can be regarded as the analog of the function F mls in definition (19). This enables us to compare both the arbitrary flip angle method and small tip angle approximation with regard to their quality and computational burden.…”

“…For example, perturbation analysis methods [19][20][21] utilize a local approximation which is updated when the iterates move away from the point for which it has been established. Filter design methods [22] rely on the hard pulse approximation, but have not been extended to parallel RF waveform design yet.…”

Clinical applications of dual‐channel transmit MRI: A review

“…Several different mathematical methods have been presented for optimizing the pulse shapes of each individual RF pulse and the corresponding gradient waveforms. 63,[67][68][69] Finally, transmit SENSE can be used either to reduce the local SAR 70,71 by means of minimizing the mean RF power, or to minimize the peak power required for the RF pulses. 72 The natural extension of the advantages afforded by dualchannel transmit is to increase the channel count further.…”