Relating MRI changes to motor deficit after ischemic stroke by segmentation of functional motor pathways

Background and Purpose-Infarct size on T2-weighted MRI correlates only mode stly with outcome, particularly for small strokes. This may be largely beca use of differences in the locations of infarcts and consequently in the fun ctional pathways that are damaged. To test this hypothesis quantitatively, we developed a "mask" of the corticospinal pathway to determine whether the extent of stroke intersection with the pathway would be more closely relat ed to clinical motor deficit and axonal injury in the descending motor path ways than total stroke lesion volume. Methods-Eighteen patients were studied greater than or equal to 1 month aft er first ischemic stroke that caused a motor deficit by use of brain T2-wei ghted imaging, MR spectroscopic (MRS) measurements of the neuronal marker c ompound N-acetyl aspartate in the posterior limb of the internal capsule, a nd motor impairment and disability measures. A corticospinal mask based on neuroanatomic landmarks was generated from a subset of the MRI data. The ma ximum proportion of the cross-sectional area of this mask occupied by strok e was determined for each patient after all brain images were transformed i nto a common stereotaxic brain space. Results-There was a significant linear relationship between the maximum pro portional cross-sectional area of the corticospinal mask occupied by stroke and motor deficit (r(2)=0.82, P<0.001), whereas the relationship between t he total stroke volume and motor deficit was better described by a cubic cu rve (r(2)=0.76, P<0.001), Inspection of the data plots showed that the tota l stroke volume discriminated poorly between smaller strokes with regard to the extent of associated motor deficit, whereas the maximum proportion of the mask cross-sectional area occupied by stroke appeared to be a more disc riminatory marker of motor deficit and also N-acetyl aspartate reduction, Conclusions-Segmentation of functional motor pathways on MRI allows estimat ion of the extent of damage specifically to that pathway by the stroke lesi on. The extent of stroke intersection with the motor pathways was more line arly related to the magnitude of motor deficit than total lesion volume and appeared to be a better discriminator between small strokes with regard to motor deficit. This emphasizes the importance of the anatomic relationship of the infarct to local structures in determining functional impairment. P rospective studies are necessary to assess whether this approach would allo w improved early estimation of prognosis after stroke.