The impact of large structural brain changes in chronic stroke patients on the electric field caused by transcranial brain stimulation

Abstract

Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (TDCS) are two types of non-invasive transcranial brain stimulation (TBS). They are useful tools for stroke research and may be potential adjunct therapies for functional recovery. However, stroke often causes large cerebral lesions, which are commonly accompanied by a secondary enlargement of the ventricles and atrophy. These structural alterations substantially change the conductivity distribution inside the head, which may have potentially important consequences for both brain stimulation methods. We therefore aimed to characterize the impact of these changes on the spatial distribution of the electric field generated by both TBS methods. In addition to confirming the safety of TBS in the presence of large stroke-related structural changes, our aim was to clarify whether targeted stimulation is still possible. Realistic head models containing large cortical and subcortical stroke lesions in the right parietal cortex were created using MR images of two patients. For TMS, the electric field of a double coil was simulated using the finite-element method. Systematic variations of the coil position relative to the lesion were tested. For TDCS, the finite-element method was used to simulate a standard approach with two electrode pads, and the position of one electrode was systematically varied. For both TMS and TDCS, the lesion caused electric field “hot spots” in the cortex. However, these maxima were not substantially stronger than those seen in a healthy control. The electric field pattern induced by TMS was not substantially changed by the lesions. However, the average field strength generated by TDCS was substantially decreased. This effect occurred for both head models and even when both electrodes were distant to the lesion, caused by increased current shunting through the lesion and enlarged ventricles. Judging from the similar peak field strengths compared to the healthy control, both TBS methods are safe in patients with large brain lesions (in practice, however, additional factors such as potentially lowered thresholds for seizure-induction have to be considered). Focused stimulation by TMS seems to be possible, but standard tDCS protocols appear to be less efficient than they are in healthy subjects, strongly suggesting that tDCS studies in this population might benefit from individualized treatment planning based on realistic field calculations.

Transcranial Pulse Stimulation with Ultrasound in Alzheimer's Disease—A New Navigated Focal Brain Therapy

“…[32] In an AD mouse model, microglia activation with plaque reduction, clearing of Aβ into microglial lysosomes and improvements of spatial and recognition memory have been shown. [4] Therefore, TPS represents a promising novel brain stimulation technique with a mobile system adequate for human research and brain stimulation therapy. [18] Concerning study limitations, the patient pilot study was performed with an uncontrolled design and therefore further sham-controlled investigations are required to confirm the stimulation effects.…”

“…[1,2] However, in contrast to ultrasound tissue ablation, [3] for clinical brain stimulation no certified systems and clinical data exist and the current techniques have to be further developed. [4][5][6] Ultrasound can be reliably targeted and is the Ultrasound-based brain stimulation techniques may become a powerful new technique to modulate the human brain in a focal and targeted manner. In a comprehensive approach, we provide preclinical and clinical feasibility, safety, and efficacy data for TPS.…”

Transcranial Pulse Stimulation with Ultrasound in Alzheimer’s disease – A new navigated focal brain therapy

“…Our preclinical data and the quality of targeting ROI borders demonstrate the focal stimulation potential TPS bears. This is a particular advantage over electrophysiological brain stimulation techniques, where focality and deepness are generally difficult issues, especially in pathological brains (Minjoli et al 2017).…”

“…The new method is based on single ultrashort ultrasound pulses (transcranial pulse stimulation, TPS) that avoid secondary stimulation maxima or brain heating (Mueller et al 2017). In contrast to electrophysiological brain stimulation techniques that often suffer from conductivity effects (Minjoli et al 2017) and lack of deep stimulation capabilities (Spagnolo 2018), the target for ultrasound-based neuromodulation can be spatially distinct, highly focal, and is not restricted to superficial layers of the brain. For brain therapy, this enables a controlled modulation of a specific brain region with reduced probability of producing unwanted co-stimulations of other brain areas.…”

A flexible workflow for simulating transcranial electric stimulation in healthy and lesioned brains

“…Following the image segmentation, a surface-based meshing 54 approach is commonly used to create the head volume mesh. The advantage of this approach is a 55 maximum of control over the approximation of the boundaries of the sub-compartments of the head 56 model, which, on the other hand, must not intersect, restricting the topology of the included structures 57 and complicating the inclusion of irregular tissue such as lesioned tissue. ROAST circumvents this 58 restriction by applying an image-based meshing approach, which is free of any topological constraints 59 (12), with the drawback of less accurate feature edges, for example of the electrodes.…”