Abstract
Functional MRI (fMRI) enables evaluation of language cortical organization and plays a central role in surgical planning. Diffusion Tensor Imaging (DTI) or Tractography, allows evaluation of the white matter fibers involved in language. Unlike fMRI, DTI does not rely on the patient’s cooperation. In monolinguals, there is a significant correlation between the lateralization of language on fMRI and on DTI. Our objective is to delineate the arcuate fasciculus (AF) in right- and left-handed trilinguals and determine if the AF laterality on DTI is correlated to language lateralization on fMRI. 15 right and 15 left-handed trilingual volunteers underwent fMRI and DTI. Laterality Index was determined on fMRI (fMRI-LI). Mean Diffusivity, Fractional Anisotropy (FA), Number of Fibers, Fiber Length, Fiber Volume and Laterality Index (DTI-LI) of the AF were calculated on DTI. 28 of the 30 subjects presented a bilateral AF. Most subjects (52%) were found to have a bilateral language lateralization of the AF on DTI. Only 4 subjects had bilateral lateralization of language on fMRI. The right AF demonstrated lower diffusivity than the left AF in the total participants, the right-handed, and the left-handed subjects. FA, Volume and Length of the AF were not significantly different between the two hemispheres. No correlation was found between the DTI-LI of the AF and the fMRI-LI. A prominent role of the right AF and a bilateral structural organization of the AF was present in our multilingual population regardless of their handedness. While in prior studies DTI was able to determine language lateralization in monolingual subjects, this was not possible in trilingual highly educated subjects.
Highlights
Language lateralization and localization to the left frontal hemisphere has been described by Paul Broca in 18651
17 of the 30 participants had a normal brain MRI. 13 of the 30 subjects had minor brain abnormalities: 7 participants presented nonspecific signal abnormalities of the white matter, 5 had pineal cysts, and one subject demonstrated an arachnoid cyst. None of these abnormalities was significant enough to interfere with the results of the Diffusion Tractography Imaging (DTI) or the functional MRI (fMRI)
We hypothesized that most subjects would present leftward arcuate fasciculus (AF) asymmetry and that the lateralization of the AF on DTI would correlate with the lateralization of language by fMRI, at least in the right-handed subjects who in 92.5% demonstrate typical left hemisphere dominance of language[4]
Summary
Language lateralization and localization to the left frontal hemisphere has been described by Paul Broca in 18651. Even though leftward structural and morphological asymmetries are found in the language areas of the left cerebral hemisphere[2], right dominance of language and bilateral activation of language are present in the left-handed subjects[3] and, less frequently, in the right-handed s ubjects[4]. This variation in the hemispheric dominance of language illustrates why evaluation of language lateralization is important preoperatively. While fMRI studies activation and localization of the language areas in the cortex of the cerebral hemispheres, Diffusion Tractography Imaging (DTI) permits evaluation of the white matter fibers involved in language. Based on the available data in the literature in monolingual subjects, we hypothesized that most right-handed trilingual volunteers would present leftward asymmetry of the AF that would correlate with a left lateralization of language on fMRI
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