Local specific absorption rate (SAR), global SAR, transmitter power, and excitation accuracy trade‐offs in low flip‐angle parallel transmit pulse design

Abstract

Purpose We propose a constrained optimization approach for designing parallel transmit (pTx) pulses satisfying all regulatory and hardware limits. We study the trade-offs between excitation accuracy, local and global SAR, and maximum and average power for small flip-angle pTx (8 channels) spokes pulses in the torso at 3 T and in the head at 7 T. Methods We compare the trade-offs between the above-mentioned quantities using the L-curve method. We use a primal-dual algorithm and a compressed set of local SAR matrices to design RF pulses satisfying all regulatory (including local SAR) and hardware constraints. Results Local SAR can be substantially reduced (factor of 2 or more) by explicitly constraining it in the pulse design process compared to constraining global SAR or pulse power alone. This often comes at the price of increased pulse power. Conclusion Simultaneous control of power and SAR is needed for the design of pTx pulses that are safe and can be played on the scanner. Constraining a single quantity can create large increase in the others, which can then rise above safety or hardware limits. Simultaneous constraint of local SAR and power is fast enough to be applicable in a clinical setting.

Joint design of k T -points trajectories and RF pulses under explicit SAR and power constraints in the large flip angle regime

“…SAR and power constraints being dependent on the RF pulses only, previous tools incorporating explicitly those constraints in RF pulse design could still be used [10,46]. The cost-function and its derivatives with respect to all k-space and RF variables then could be calculated by using finite differences, which here were evaluated directly on the GPU and in one shot to avoid overhead communication time with the central processing unit.…”

First and second order derivatives for optimizing parallel RF excitation waveforms

“…Hence, the main difference between our derivatives and those of this optimal control method seems to be the missing averaging over the duration of a sample time interval captured in Eq. (35). Moreover, the optimal control method has been formulated for the Euclidean objective value F els only.…”

“…An important aspect of these examples is that RF amplitude bounds as well as nearly 500 specific absorbtion rate constraints have to be respected. Specialized algorithms for handling such constraints have been developed for the small tip angle approximation in [38][39][40][41]35]. An advantage of the interior-point method that it supports the handling of constraints.…”

“…In the literature, second order derivatives are rarely deployed for optimizing RF waveforms, with the exception of [12,17,23], where quasi Newton approximations for the Hessian are investigated. In [35] second order derivatives are used in quadratic small tip angle approximation with quadratic constraints. With our second order derivative formulas, the performance of numerical solution methods with exact Hessians can be compared to those using approximations (or no second order derivatives).…”

Realistic modeling of deep brain stimulation implants for electromagnetic MRI safety studies

“…A key component of the method is a topology correction algorithm that automatically removes self-intersections and curvature violations in the path estimated from the CT data (code available at http://ptx.martinos.org/index.php/Main_Page). We model the RF interactions between the DBS implant and the MRI coil using a simulation strategy based on the field solver HFSS (Kozlov and Turner, 2009; Guérin et al , 2014; Guérin et al , 2015), which allows efficient modeling of the smallest implant details (simulation time ~6 hours). We find that detailed modeling of the DBS implant path and internal components has a large impact on the SAR induced at the electrode-tissue interface tip by the MRI RF coil.…”