The electric field distributions in anatomical head models during transcranial Direct Current Stimulation for post-stroke rehabilitation

Purpose: The lack of knowledge of the electric field distribution inside the brain of a stroke patient receiving transcranial direct current stimulation (tDCS) increases the need to estimate it computationally, rather than using reports of various efficient clinical applications of this technique. Moreover, an evaluation of a novel approach which examines the role of secondary motor areas such as SMA in post stroke rehabilitation, in contrast with the classic electrode montages stimulation of the primary motor cortex, needs to be performed. Methods: This work presents the development of two different anatomical models (a female and a male one) with an ischemic stroke region of spherical volume 10cm3 or 50cm3. The stroke phase was considered as acute or chronic, resulting in different electrical properties of the stroke lesion. Two different electrode montages were used. One over the lesion area and the contralateral supra-orbital region and the other over the SMA and the supra-orbital region. Computational calculations gave quantitative information about the electric field distributions in the primary motor cortex (M1) and the SMA, the cerebellum and the hypothalamus. Results: In the acute phase of an ischemic stroke the electric field intensity distributions do not differ noticeably compared to those in the brain of a healthy person. The difference becomes clear for the chronic phase of an ischemic stroke and more specific for the chronic phase of a 50cm3 volume lesion, in both human models and electrode montages. Moreover, the maximum values of the induced electric field in the tissues in SMA area are almost equal for both electrode montages. The peak values of the electric field distribution in cerebellum and hypothalamus for both electrode montages in stroke patients are slightly different to those in the healthy subjects. A notable difference occurs in the spread of the electric field in the hypothalamus by the presence of a chronic stroke with 50cm3 volume with the use of the electrode montage over the M1 cortex. In addition, the spread of the electric field inside the brain of a young patient compared to an older one seems to be more focused. Conclusions: The presence and the phase of an ischemic stroke lesion, as well as the configuration of electrode montages affect the distribution and the maximum value of the electric field induced in tissues. Moreover, the patients that seem to benefit most from tDCS are those in the chronic phase of an ischemic stroke.