High-resolution optical functional mapping of the human somatosensory cortex

Abstract

Non-invasive optical imaging of brain function has been promoted in a number of fields in which functional magnetic resonance imaging (fMRI) is limited due to constraints induced by the scanning environment. Beyond physiological and psychological research, bedside monitoring and neurorehabilitation may be relevant clinical applications that are yet little explored. A major obstacle to advocate the tool in clinical research is insufficient spatial resolution. Based on a multi-distance high-density optical imaging setup, we here demonstrate a dramatic increase in sensitivity of the method. We show that optical imaging allows for the differentiation between activations of single finger representations in the primary somatosensory cortex (SI). Methodologically our findings confirm results in a pioneering study by Zeff et al. (2007) and extend them to the homuncular organization of SI. After performing a motor task, eight subjects underwent vibrotactile stimulation of the little finger and the thumb. We used a high-density diffuse-optical sensing array in conjunction with optical tomographic reconstruction. Optical imaging disclosed three discrete activation foci one for motor and two discrete foci for vibrotactile stimulation of the first and fifth finger, respectively. The results were co-registered to the individual anatomical brain anatomy (MRI) which confirmed the localization in the expected cortical gyri in four subjects. This advance in spatial resolution opens new perspectives to apply optical imaging in the research on plasticity notably in patients undergoing neurorehabilitation.

A quantitative spatial comparison of high-density diffuse optical tomography and fMRI cortical mapping

“…Previous comparisons between fMRI and DOT have examined the co-localization between the two modalities in visual (Zhang et al, 2005) and motor Koch et al, 2010) cortices. However, these studies did not perform quantitative voxel-by-voxel comparisons.…”

“…However, these studies did not perform quantitative voxel-by-voxel comparisons. The motor papers used qualitative means to infer image agreement by projecting the fMRI data to individual DOI measurements at the scalp surface , or used a generic head model for the forward light modeling rather than using the subject-specific anatomy to generate a realistic photonpropagation model (Koch et al, 2010) and then projected the data from the 'model head' onto the subject specific MRI anatomy. In contrast this visual study compared fMRI activations to DOT reconstructions in a subject-specific brain volume.…”

“…Another strategy uses high-density DOT grids with overlapping continuous wave measurements at multiple SD-pair separations (Bluestone et al, 2001;Boas et al, 2004a;Joseph et al, 2006;Zeff et al, 2007). Relative to DOI, the newer HD-DOT methods achieve higher resolution and improved localization accuracy (Gibson and Dehghani, 2009;Gibson et al, 2005;Habermehl et al, 2011;Koch et al, 2010;White and Culver, 2010b;Zeff et al, 2007). While HD-DOT's ability to decipher detail has been established in studies of retinotopy in visual cortex (White and Culver, 2010a;Zeff et al, 2007) and finger-topy in the motor cortex (Custo et al, 2009;Koch et al, 2010;White et al, 2009), the image quality of HD-DOT at the voxel level has not been compared directly to fMRI, the current gold standard in hemodynamic-based functional neuroimaging.…”

Non-invasive optical imaging of stroke

“…We deem depth resolution necessary to clearly increase sensitivity to the bolus passage in cerebral tissue. This may be afforded by multi-distance approaches (as is currently being tested by our group along the principles of [93]), by frequency-domain [94] and ideally by time-resolved approaches [66]. Options to use ICG fluorescence to enhance sensitivity or assess dye extravasation await further evaluation in a clinical setting [95].…”

Quantitative evaluation of atlas-based high-density diffuse optical tomography for imaging of the human visual cortex

“…While most hemodynamic-based neuroimaging research studies in adult subjects are typically performed using functional magnetic resonance imaging (fMRI), its relative high cost, fixed scanner locations and physical constraints during imaging, limit fMRI's translation as a bedside clinical tool [10]. DOT has shown a strong potential in clinical application specifically for neonate and long-term bedridden patients [11] and NIR studies of the human brain have demonstrated its ability to recover abnormalities as haemorrhage detection from stroke patients [12] with accurate recovery of stimulated activations [13][14][15] and functional networks [5,16,17].…”