Abstract
PurposefMRI is increasingly used for presurgical language mapping, but lack of standard methodology has made it difficult to combine/compare data across institutions or determine the relative efficacy of different approaches. Here, we describe a quantitative analytic framework for determining language laterality in clinical fMRI that addresses these concerns.MethodsWe retrospectively analyzed fMRI data from 59 patients who underwent presurgical language mapping at our institution with identical imaging and behavioral protocols. First, we compared the efficacy of different regional masks in capturing language activations. Then, we systematically explored how laterality indices (LIs) computed from these masks vary as a function of task and activation threshold. Finally, we determined the percentile threshold that maximized the correlation between the results of our LI approach and the laterality assessments from the original clinical radiology reports.ResultsFirst, we found that a regional mask derived from a meta-analysis of the fMRI literature better captured language task activations than masks based on anatomically defined language areas. Then, we showed that an LI approach based on this functional mask and percentile thresholding of subject activation can quantify the relative ability of different language tasks to lateralize language function at the population level. Finally, we determined that the 92nd percentile of subject-level activation provides the optimal LI threshold with which to reproduce the original clinical reports.ConclusionA quantitative framework for determining language laterality that uses a functionally-derived language mask and percentile thresholding of subject activation can combine/compare results across tasks and patients and reproduce clinical assessments of language laterality.
Highlights
Many institutions have replaced traditional Wada testing with clinical fMRI for presurgical language mapping [1]
For the full functional mask, and each subcomponent, the plots highlight how laterality indices (LIs) varied by threshold and task — for the leftdominant subject, verb generation (VG) produced larger LIs at higher thresholds when compared to the other tasks, while for the right-dominant subject, mental rhyming (MR) produced larger LIs at higher thresholds
We found that verb generation produced the most robustly lateralized activation, a result we confirmed through a separate group-level general linear model (GLM)-based analysis of the data that compared the relative strength of activation between tasks across the population
Summary
Many institutions have replaced traditional Wada testing with clinical fMRI for presurgical language mapping [1]. With increasing recognition that language networks are more complex than suggested by the classical model of anterior-expressive (Broca’s) and posterior-receptive (Wernicke’s) areas, even the set of candidate regions to consider in localizing language is unclear [3,4,5] Addressing these issues, the American Society of Functional Neuroradiology has recommended that the field adopt standard tasks and approaches [6]; these standards are currently largely based on consensus opinion rather than the systematic and quantitative comparison of approaches and data across tasks, patients, and institutions. As Bradshaw and colleagues detailed in a recent comprehensive review, quantitative methods for language lateralization with fMRI will need to address the key elements of thresholding, regions of interest, and tasks [7] We develop such a quantitative approach and use it to compare the relative efficacy of a limited set of language mapping tasks in a population of 59 patients who underwent presurgical fMRI language mapping at our institution
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