Optimal design of multiple‐channel RF pulses under strict power and SAR constraints

Abstract

Parallel radio frequency transmission has recently been explored as a means of tailoring the spatial response of MR excitation. In particular, parallel transmission is increasingly used to accelerate radio frequency pulses that rely on time-varying gradient fields to achieve selectivity in multiple dimensions. The design of the underlying multiple-channel radio frequency waveforms is mostly based on regularized least-squares optimization in close analogy with image reconstruction in parallel imaging. However, this analogy has important limitations. Unlike image reconstruction, the design of radio frequency waveforms is subject to multiple strict constraints, which arise from technical power limits, as well as safety limits on local and global energy deposition in vivo. To optimize excitation profiles under such strict constraints, it is proposed to depart from the regularization strategy and rely on semidefinite programming instead. To render this approach fast, it is performed in a reduced search space, which is obtained by initial Lanczos iteration. The proposed algorithm is demonstrated to enable efficient pulse optimization within exactly the given constraints, including local specific absorption rate limits for multiple compartments. It is also shown that the proposed approach readily accommodates advanced forward models of the excitation process, including the effects of local off-resonance and transverse relaxation.

Local SAR management by RF Shimming: a simulation study with multiple human body models

“…Recently, a number of local SAR management techniques have been proposed. These can be classified as regularization-based approaches [34,[15][16][17] and approaches that enforce inequality constraints [14, 18,19].…”

“…Solving the problem with multiple SAR constraints via Lagrange multipliers drastically increases the number of unknowns such that approximating subspace solutions may be required (as used in [18] for Transmit SENSE pulses). Instead, a logarithmic barrier method is proposed in this work, offering a numerical effort which increases only linearly with the number of SAR constraints.…”

“…This can be beneficial in MR sequence design to increase the allowed range of flip-angles and repetition times for improved contrast and SNR or to reduce the total scan time. For local SAR management, the SAR model can in principle be considered directly in the RF pulse design by incorporating SAR constraints [15][16][17][18][19].…”

Fast design of local N‐gram‐specific absorption rate–optimized radiofrequency pulses for parallel transmit systems

“…This fact allows us to cast the problem as an unconstrained minimum norm problem as the Lagrange multiplier can be found by applying mCGLS to a (large) set of shifts, without slowing down the computation, which is a major concern in RF pulse design (29).…”

“…The mCGLS algorithm computes in a single run a set of solutions (each solution corresponds to a different regularization parameter). The choice of the optimal regularization parameter value, which is typically a major issue for RF pulse design (29), is thus efficiently solved.…”