Parallel transmit pulse design for patients with deep brain stimulation implants

Abstract

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.

Construction and modeling of a reconfigurable MRI coil for lowering SAR in patients with deep brain stimulation implants

“…Theoretical studies that modelled straight wires [41, 42, 48, 49] and simulations of simplified DBS lead geometries [15, 50-52] have concluded that maximum SAR occurs at the electrode tip. Consequently, experimental studies assessing heating of DBS leads have mostly measured the temperature rise only at the location of the distal electrode contact [40, 46, 53, 54].…”

“…van den Bosch and Griffin have shown that for elongated metallic implants positioned parallel to the static magnetic field, the magnitude or phase of the image artifact in B 1 + field maps can be used to predict quantitative values of the RF currents induced on the implant, which in turn, correlate with tissue heating [26-28]. The visual artifact in B 1 + field maps have been also shown to correlate with the local SAR for planar [15] and simplified DBS lead models [18]. However, because the image artifact’s shape and location is dependent on the implant trajectory and its orientation with respect to the magnetic field, it is important to verify that the B 1 + field maps can be reliably used to predict the optimal rotation angle in realistic patient populations.…”

Safe guidewire visualization using the modes of a PTx transmit array MR system

“…It has been shown that RF‐induced current “modes” may exist on the guidewire and can be determined by 1 or more current sensors placed over the guidewire . Typically, 1 or more maximum current modes (MM) in which the PTx system produces a strong current on the conductor exist, along with additional null current modes (NM) in which RF excitation produces 0 measured current at the sensor’s location. Despite the NMs producing little or no RF current, they can still generate substantial transmit () field; hence, they can be harnessed for safe imaging of the anatomical structures with a guidewire in situ.…”

Parallel transmission to reduce absorbed power around deep brain stimulation devices in MRI: Impact of number and arrangement of transmit channels

“…Gudino et al reproduced the 4‐channel planar pTx coil setup of Etezadi‐Amoli et al, but this time they used a coil‐driving function derived from an electromagnetic simulation of the implant to find the complex‐valued shim weights yielding the minimum induced temperature at the tip of a guidewire . In addition to these experimental studies, Eryaman et al, Guerin et al, and McElcheran et al performed electromagnetic simulation of DBS patients in pTx coils, also indicating the potential of pTx to reduce the RF MRI safety concern in this patient cohort.…”