Transcranial magnetic stimulation and brain atrophy: a computer-based human brain model study - PubMed (original) (raw)

Transcranial magnetic stimulation and brain atrophy: a computer-based human brain model study

Tim Wagner et al. Exp Brain Res. 2008 Apr.

Abstract

This paper is aimed at exploring the effect of cortical brain atrophy on the currents induced by transcranial magnetic stimulation (TMS). We compared the currents induced by various TMS conditions on several different MRI derived finite element head models of brain atrophy, incorporating both decreasing cortical volume and widened sulci. The current densities induced in the cortex were dependent upon the degree and type of cortical atrophy and were altered in magnitude, location, and orientation when compared to healthy head models. Predictive models of the degree of current density attenuation as a function of the scalp-to-cortex distance were analyzed, concluding that those which ignore the electromagnetic field-tissue interactions lead to inaccurate conclusions. Ultimately, the precise site and population of neural elements stimulated by TMS in an atrophic brain cannot be predicted based on healthy head models which ignore the effects of the altered cortex on the stimulating currents. Clinical applications of TMS should be carefully considered in light of these findings.

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Figures

Fig. 1. Geometries

a Healthy head model and the model coordinate system: This image depicts the Wnite element mesh borders of the skin (flesh color), skull (yellow), CSF (light blue), gray matter (dark blue), and white matter (red). The coordinate system is shown in the foreground image and was used for all the models. b Increasing symmetric atrophy models: the models are displayed from the healthy head model to the 85% atrophy model. On the right side, the healthy head model (upper) and the 85% atrophy model (lower) are shown to highlight the increasing thickness of the CSF and the decreasing cortical thickness. The skin mesh is shown in the flesh color where the tissue thicknesses are highlighted in the transverse slices; scalp (flesh colored), skull (yellow), CSF (blue), brain (bright green). Notice the increasing CSF thickness and the decreased cortical size between the two models. c. Widened sulci model: the base sulci model is shown with the widened sulcal regions highlighted and sample MRI slices. d Evaluation line locations: the lines were located with the center line normal to the figure-of-eight coil center and the other lines 1 cm ventral, dorsal, anterior, and posterior to the center line (note that the figure is not drawn to scale but with the lines and the coil drawn to highlight their placement)

Fig. 2. Current density magnitudes

the current density magnitudes are plotted for the dorsal lateral prefrontal cortex (DLPC) and the motor strip (MS) coil position the healthy head model (HH) to the 85% atrophy model. Note that the normalized current density magnitude is relative to the maximum current density magnitude for the healthy head model for each coil position (i.e., the DLPC scale ranges from 0 to 2.82 A/m2 and the MS scale ranges from 0 to 3.57 A/m2)

Fig. 3. Current density behavior with distance

a Current density magnitude evaluated along the center evaluation line in the healthy head model with the coil in the dorsal lateral prefrontal cortex coil location. Note that the current density magnitude varies with the conductivity of the tissues. b Exponential models for maximum current density: exponential models for maximum cortical current density as a function of distance for position dependent and independent models. Circles and squares represent simulated current density values from the motor strip and the dorsal lateral prefrontal cortex coil positions respectively. The dashed line represents the mean exponential trend for the position independent model. The light green and light blue areas represent a 95% confidence region for the position dependent models

Fig. 4. Widened sulci current density variations

a Current density magnitudes: the current density magnitudes are shown for the base and 90% widened sulci models focused on the region near the central sulcus and in the same locations for the analogous models without the widened sulci in the cortex (i.e., healthy head and 90% atrophy models). Note that the current density magnitudes in the region of the figure-of-eight coil center’s, indicated by the _X_’s, are consistent for the models with the same % of atrophy. However, the current density magnitudes increase as one gets closer to the widened central sulcus; the black, blue, and red circles indicate analogous points in the models with consistent % atrophies at points 10, 5, and 2.5 mm, respectively. b Current density vector distribution: the left most image shows the vector density distribution on the surface of the cortex for the healthy head and base widened sulci model highlighting the behavior in the central sulcus region. The right most image shows the vector behavior on the surface of the CSF for the healthy head and base widened sulci model. Note that the current density scale is normalized to the maximum current densities for the widened sulci models (i.e., the models on the left are normalized to the maximum cortical current density in the base widened sulci models, 3.06 A/m2, and for the 90% widened sulci model, 2.42 A/m2)

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Transcranial magnetic stimulation and brain atrophy: a computer-based human brain model study - PubMed (original) (raw)