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Quantification and Reproducibility of Tracking Cortical Extent of Activation by Use of Functional MR Imaging and Magnetoencephalography

Timothy P.L. RobertsGo,a, Elizabeth A. Disbrowa, Heidi C. Robertsa and Howard A. Rowleya

a From the Biomagnetic Imaging Laboratory, Department of Radiology, University of California, San Francisco, San Francisco, CA.



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  		FIG 1. Functional MR imaging for a single participant, single session, as a function of increasing number of stimulated digits. This functional MR imaging study from a single volunteer shows the difference in activation pattern (red pixels) resulting from the stimulation of one through four fingers of the left hand. Overlays are shown on source echo-planar images (2000/69/1 [TR/TE/excitations]; flip angle, 60°); four sections are shown covering the expected primary somatosensory areas. Yellow pixels indicate areas of significant correlation with the stimulus presentation function (r > 0.3), which were deemed not primary contralateral somatosensory cortex by the radiologic reviewers (H.C.R. and H.A.R.). Although this example tends to show an increasing extent of cortical activation in line with expected somatotopic distribution in primary somatosensory cortex, results from other participants were less clearly demonstrative. Furthermore, increases in the number of activated pixels on progression from one to four stimulated fingers are non-monotonic (being especially diminished in response to three-finger stimulation in this example)



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  		FIG 2. Magnetoencephalography waveforms and source localization.

A, Representative magnetoencephalography waveforms for one, two, three, and four digits, for a single participant, single session. Waveforms are shown from the 37 sensor channels, collapsed onto a single horizontal time axis, ranging from 150 ms before stimulation to 150 ms after stimulation, capturing the initial evoked response. The y axis of these plots is a measure of evoked magnetic field amplitude, and all plots are shown to scale (50 fT/vertical division). All have recognizable peaks occurring approximately 30 to 40 ms after stimulus onset (arrow), with subsequent activity that is not the focus of this study. The amplitude of this peak seems to increase, somewhat monotonically, with an increasing number of stimulated digits.

B, Source localization (single equivalent dipole modeling) of this magnetoencephalography-detected peak onto high resolution MR images (SPGR; 38/6/1; flip angle, 35°), to form magnetic source images (MSI) for one-, two-, three-, and four-digit stimulation shows the source estimate (colored point) to lie in the postcentral gyrus, which is expected to contain primary somatosensory representations. The ring surrounding the modeled current source indicates the 95% confidence limit for model/data fit. The 95% confidence volumes (from 3D ellipsoids of confidence) for the example in the figure are 0.17, 0.06, 0.08, and 0.02 cm3, respectively, for one-, two-, three-, and four-digit stimulation.



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  		FIG 3. Variation of derived measures of the extent of cortical activation with increasing numbers of stimulated digits: intra- and interparticipant.

A, Area of activation derived by functional MR imaging shows considerable intraparticipant variability and no trend toward increasing areas of activation with increasing numbers of stimulated digits, for averaged data for each individual. Measures of cortical activation are shown as a function of increasing numbers of stimulated digits with intraparticipant variability represented by the dispersion of like-colored dots. Mean individual participant trends are shown as solid lines.

B, Evoked field amplitude as determined by magnetoencephalography. Despite overlap in the range of evoked fields elicited by each stimulation type across participants, the data from each individual are relatively clustered for each stimulation type; clear trends indicating increased activation with increased numbers of stimulated digits are seen for every individual participant. Measures of cortical activation are shown as a function of increasing numbers of stimulated digits with intraparticipant variability represented by the dispersion of like-colored dots. Mean individual participant trends are shown as solid lines.

C, There is no evident correlation between functional MR imaging and magnetoencephalographic measures.



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  		FIG 4. Interrater comparison for functional MR imaging. Despite differing sensitivities to activation identification (the slope of the correlation plot is approximately 0.6), the correlation between readers is strong (r ;eq 0.93), suggesting that observations and derived conclusions are not sensitive to arbitrary reader bias in definition of region of interest





Copyright © 2009 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X