Somatosensory activation of two fingers can be discriminated with ultrahigh-density diffuse optical tomography

Abstract

Topographic non-invasive near infrared spectroscopy (NIRS) has become a well-established tool for functional brain imaging. Applying up to 100 optodes over the head of a subject, allows achieving a spatial resolution in the centimeter range. This resolution is poor compared to other functional imaging tools. However, recently it was shown that diffuse optical tomography (DOT) as an extension of NIRS based on high-density (HD) probe arrays and supplemented by an advanced image reconstruction procedure allows describing activation patterns with a spatial resolution in the millimeter range. Building on these findings, we hypothesize that HD-DOT may render very focal activations accessible which would be missed by the traditionally used sparse arrays. We examined activation patterns in the primary somatosensory cortex, since its somatotopic organization is very fine-grained. We performed a vibrotactile stimulation study of the first and fifth finger in eight human subjects, using a 900-channel continuous-wave DOT imaging system for achieving a higher resolution than conventional topographic NIRS. To compare the results to a well-established high-resolution imaging technique, the same paradigm was investigated in the same subjects by means of functional magnetic resonance imaging (fMRI). In this work, we tested the advantage of ultrahigh-density probe arrays and show that highly focal activations would be missed by classical next-nearest neighbor NIRS approach, but also by DOT, when using a sparse probe array. Distinct activation patterns for both fingers correlated well with the expected neuroanatomy in five of eight subjects. Additionally we show that activation for different fingers is projected to different tissue depths in the DOT image. Comparison to the fMRI data yielded similar activation foci in seven out of ten finger representations in these five subjects when comparing the lateral localization of DOT and fMRI results.

Fig. 1

a) Experimental setup: The fingers d1 and d5 of the right hand were stimulated by piezoelectric vibration devices. b) 30 optical fibers (inter-optode distance 7.5 mm) were placed around C3. The positions of the fiber grid corners were marked with vitamin E capsules c) Fiduciary mark approach for co-registration: subject’s MR scan in the real world space with the fiduciary marks on the head surface. d) Anatomic MR scan of the generic head atlas on which the FE mesh is based. e) Subject’s MR scan, warped into space and shape of the atlas. f) Atlas with a sub-mesh (blue) of the FE model, covering the area of interest. g) Within the sub-mesh, the locations of the optical fibers (red dots) on the boundary were defined individually for each subject.

Fig. 2

Results for vibrotactile stimulation of d1 (pink) and d5 (blue) of the right hand, mapped onto the individual brain anatomy of five subjects (s2–s6). Gray spheres indicate the edges of the grid. Gray dashed line: sulcus centralis. Colored boxes show the cut-off T-values for the different stimuli.

Fig. 3

Comparison of activation patterns for vibrotactile stimulation of the d1 (pink) and d5 (blue) of the right hand for two subjects (s2, s6) when simulating three different densities of probes. Top row: Topography approach using 12 NIRS- channels (light red ellipses between light source (red dots) and detectors (blue dots). Middle row: DOT approach with a medium-dense grid of co-located sources and detectors (bi-colored dots, minimum SD distance 15 mm) yields 225 optical data channels. Lower row: DOT approach with an ultrahigh-density grid and 900 channels. The increasing number of channels leads to a better lateral resolution and allows distinguishing between the two activations in both subjects (middle and right column).

Fig. 4

Reconstruction-based dislocation of the activation foci for d1 (pink) and d5 (blue) stimulation of the right hand in five subjects (s2–s6). a) Frontal view reveals that activation clusters for both fingers was determined in different tissue depths but still there is insufficient depth localization. b) Depth correction of the result volume localization: the volumes were translocated into the brain by 1.5 cm parallel to the head surface in x and z direction. c) Frontal view of a 1cm slice of the activated area for four subjects (s2–s5) after depth correction.

Fig. 5

Comparison of NIRS and fMRI activation. All results were mapped onto the individual anatomy. Left column: reconstructed activation maps from DOT experiment for vibrotactile stimulation of d1 (pink) and d5 (blue) of the right hand for five subjects (s2–s6). Colored boxes indicate the cut-off T-values. Right column: results for the fMRI experiment.