Purpose Specific absorption rate (SAR) amplification around active implantable medical devices during diagnostic MRI procedures poses a potential risk for patient safety. In this work we present a parallel transmit (pTx) strategy that can be used to safely scan patients with deep brain stimulation (DBS) implants. Methods We performed EM simulations at 3 T using a uniform phantom and a multi-tissue realistic head model with a generic DBS implant. Our strategy is based on utilizing implant-friendly modes which are defined as the modes of an array that reduce the local SAR around the DBS lead tip. These modes are used in a spokes pulse design algorithm in order to produce highly uniform magnitude least-squares flip angle excitations. Results Local SAR (1g) at the lead tip is reduced below 0.1 W/kg in comparison to 31.2W/kg which is obtained by a simple quadrature birdcage excitation without any sort of SAR mitigation. For the multi-tissue realistic head model, peak 10g local SAR and global SAR are obtained as 4.52 W/kg and 0.48 W/kg respectively. A uniform axial flip angle is obtained (NRMSE<3%). Conclusion pTx arrays can be used to generate implant-friendly modes and to reduce SAR around DBS implants while constraining peak local SAR and global SAR and maximizing flip angle homogeneity.
Abstract. This paper deals with the weak formulation of a free (moving) boundary problem arising in theoretical glaciology. Considering shallow ice sheet ow w e present the mathematical analysis and the numerical resolution of the second order, nonlinear, degenerate parabolic equation modelling, in the isothermal case, the ice sheet non-newtoniandynamics. An obstacle problem is then deduced and analysed. The existence of a free boundary generated by the support of the solution is proved and its location and evolution are qualitatively described by using a comparison principle and an energy method. Then the solutions are numerically computed with a method of characteristics and a duality algorithm to cope with the resulting variational inequalities. The weak framework we introduce and its analysis (both qualitative a n d n umerical) are not restricted to the simple physics of the ice sheet model we consider nor to the model dimension. They can be applied succesfully to more realistic and sophisticated models related to other geophysical settings.
i Al athematical l nstitute, 24-29 St Ciles; Oxford OX13LB, England 2Faculty cif E conomics, Universidad Aulonoma de M adrid, Cantoblanco, M adrid, Spa in ABSTRACT. A simplified model of a two-dimensional ice shee t is desc ribed . It includes basa l ice sliding dependen t on the bas al wa ter prcssure, which itself is described by a simple theory of basal drainage. '''le show th a t thi s simpl e but s~phi s ti cated m od el predicts surges of the ice mass in realistic circumsta nces, and we describe these su rges by solving th e problem numerically. "Ve also are ~bl e to . descnbe some par~s of.rhe surge a nalytically. The numerica l solution of the model IS a d elicate matter, a nd hIghlIghts pItfa ll s to be avoided if m ore complicated m odels are to be solved successfull y.
Purpose Local specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a “dark mode”) to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T. Methods We model electromagnetic fields in a head/torso model to calculate SAR and excitation B1+ patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity. Results For B1+ shimming in the spine, the addition of dipole elements did not significantly alter the B1+ spatial pattern but reduced local SAR by 36%. Conclusion The dipole elements provide a sufficiently complimentary B1+ and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR.
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