Introduction

A major MRI safety concern for patients with deep brain stimulation (DBS) devices is the potential for localized heating that can arise if the leads couple substantially with the excitatory radiofrequency (RF) field. It has been shown that parallel RF transmission (pTx) technology¹ may be very useful to reduce such RF coupling and localized power deposition³. Clinically, the lead trajectory can be considerably different between patients as the target of stimulation and the surgical implementation² are customized on an individual basis. We have previously used numerical simulations to demonstrate that the lead trajectory is an important factor for patient-specific pTx optimization to suppress heating³. In the present work, we conduct a proof-of-concept validation experiment to demonstrate that RF-induced heating can be suppressed for a high-risk patient-specific lead trajectory, using a set of optimized pTx input parameters.

Methods

From numerical simulation data that estimated the localized heating effects for the real lead trajectories of nine DBS patients⁴, the trajectory with worst-case heating was selected for experimental work (Fig. 1). An insulated 61 cm copper wire with an exposed tip was placed in a uniform head phantom to approximate this lead trajectory (Fig. 2A), including a looped four-turn figure-eight pattern representative of how excess lead length was bundled at the skull by the neurosurgeon. The penetrating portion of the lead conformed closely to the specific patient model. The head phantom (Fig. 2B) was filled with a 4.5 L oil-in-gelatin mixture⁵ (oil = 3 %, NaCl = 4.5 g/L, and gelatin = 170 g/L) to mimic gray matter tissue and its corresponding electromagnetic properties. Subsequent RF heating experiments were conducted on a 3 T MRI system (MAGNETOM Prisma, Siemens, Germany). A single-channel RF excitation pulse from the MRI system was split into a custom 4-channel pTx system and transceiver coil to suppress heating using optimized RF shim settings. The amplitude (A) and phase (Φ) of the pulse on each channel was controlled by a commercial RF modulator (CGP-128-4C, CPC, Hauppauge, USA). Amplitude and phase settings for “suppression mode” were determined in simulation as in our previous work⁴, using an optimization algorithm to minimize local SAR in a cubic (0.5 cm3) region of minimization (ROM) surrounding the tip of the lead, while maximizing RF magnetic field uniformity over a large field of view. The suppression mode settings were A = 1.24, 0.5, 1.25, and 0.71, Φ= 62°, 347°, 290°, and 310°. For comparison, pTx MRI was also performed in “quadrature mode” with identical RF pulse amplitude for each channel and a phase shift of 90° between neighbouring channels. Imaging was performed with a turbo spin-echo sequence with high power deposition (acquisition matrix = 256 x 205, in-plane resolution = 0.5 mm x 0.5 mm, TR/ TE = 516 ms / 6.7 ms, flip angle = 150°, slice thickness = 2 mm, and acquisition time = 3.8 mins). During imaging, the temperature change at the lead tip was monitored with a fibre-optic temperature probe (OTG-MPK5, Opsens Inc., Canada).

Results

Images in the plane of the lead tip for pTx quadrature mode and suppression mode are shown in Fig. 3A and Fig. 3B, respectively. In the rectangular region (dashed lines) encompassing the ROM, suppression mode resulted in better image uniformity than quadrature mode with associated improvement in signal-to-noise ratio of 51 %. The time-dependent temperature changes produced by pTx MRI are shown for both modes in Fig. 4. During pTx quadrature mode, the temperature increased by 2.6 °C and did not reach steady state over a 4 min. time period, whereas suppression mode limited the temperature increase to <1 °C (62 % suppression) with good stability over approximately the last half of the image acquisition period.

Discussion

Promising proof-of-concept results were obtained, demonstrating that suppression mode pTx can limit temperature increases to <1 °C at the lead tip for a high-risk, clinically realistic DBS lead trajectory, with locally-improved image uniformity compared to quadrature mode pTx. However, residual coupling effects were still observable in suppression mode pTx MR images, and even better image uniformity would be useful over a larger field of view. Some improvement is likely by accounting for hardware and experiment discrepancies in relation to parameters used in the numerical simulation procedures to determine the RF shim settings for suppression mode.

Conclusion

Suppression mode pTx is capable of suppressing localized heating effects in implanted wires that are representative of high-risk realistic DBS lead trajectories. Further work will be required to improve image uniformity and to develop practical procedures for implementing pTx suppression modes when the electromagnetic properties of phantoms and tissues are not known in advance.

Acknowledgements

The authors thank Mitacs, Siemens Healthcare and the Canadian Foundation for Innovation for funding support.