NeuroMeasure: A Software Package for Quantification of Cortical Motor Maps Using Frameless Stereotaxic Transcranial Magnetic Stimulation

Abstract

The recent enhanced sophistication of non-invasive mapping of the human motor cortex using MRI-guided Transcranial Magnetic Stimulation (TMS) techniques, has not been matched by refinement of methods for generating maps from motor evoked potential (MEP) data, or in quantifying map features. This is despite continued interest in understanding cortical reorganization for natural adaptive processes such as skill learning, or in the case of motor recovery, such as after lesion affecting the corticospinal system. With the observation that TMS-MEP map calculation and quantification methods vary, and that no readily available commercial or free software exists, we sought to establish and make freely available a comprehensive software package that advances existing methods, and could be helpful to scientists and clinician-researchers. Therefore, we developed NeuroMeasure, an open source interactive software application for the analysis of TMS motor cortex mapping data collected from Nexstim® and BrainSight®, two commonly used neuronavigation platforms. NeuroMeasure features four key innovations designed to improve motor mapping analysis: de-dimensionalization of the mapping data, fitting a predictive model, reporting measurements to characterize the motor map, and comparing those measurements between datasets. This software provides a powerful and easy to use workflow for characterizing and comparing motor maps generated with neuronavigated TMS. The software can be downloaded on our github page: https://github.com/EdwardsLabNeuroSci/NeuroMeasure.

Aim: This paper aims to describe a software platform for quantifying and comparing maps of the human primary motor cortex, using neuronavigated transcranial magnetic stimulation, for the purpose of studying brain plasticity in health and disease.

Keywords: brain mapping; motor cortex; neuroimaging; software package; transcranial magnetic stimulation.

Figure 1

Individual motor evoked potential amplitudes sampled three times per location in a 0.5 × 0.5cm grid. Color represents peak-to-peak MEP amplitude in μV. Recorded from the FDI muscle with TMS. (a) First recording. (b) Second recording begun immediately after the conclusion of the first with the same parameters.

Figure 2

Flowchart of the NeuroMeasure workflow.

Figure 3

(A) A point cloud generated from sampling the segmentation of the head every 25th voxel, (B) an ellipsoid fit to the point cloud via least squares regression, (C) the ellipsoid shown without the point cloud surrounding it, (D,E) the elliptic surface position is described by an angular coordinate system that, when projected onto an image, creates the topographic display.

Figure 4

A representation of how the angular coordinate system is mapped onto the surface of a sphere. © 2009 Geek3 / GNU-FDL, commons.wikimedia.org/wiki/File:Sphere_wireframe_10deg_10r.svg.

Figure 5

A graphic display of the four position measurements reported in NeuroMeasure's position table. The cyan arrow represents posterior->anterior distance, red arrow represents right->left distance, green arrow represents inferior->superior distance and the purple arrow represents Euclidean distance. The 3D slice is shown from different view angles: (A) isometric, (B) coronal, (C) sagittal, (D) transverse.

Figure 6

An example of the comparison window launched in continuous comparison mode. The reference point coordinates (in their respective coordinate system) is displayed on top and all position measurements are reported as distance between combinations of two points.

Figure 7

A visual aid for computing ROC between a predictive model and testing data, (a) a predictive model computed from a training set, (b) the testing set used to evaluate the model's predictions.

Figure 8

Diagram of the four possible outcomes of a model's prediction tested against a ground truth.

Figure 9

An example of the comparison window launched in categorical comparison mode.