Abstract
Measurements of dynamic near-infrared (NIR) light attenuation across the human head together with model-based image reconstruction algorithms allow the recovery of three-dimensional spatial brain activation maps. Previous studies using high-density diffuse optical tomography (HD-DOT) systems have reported improved image quality over sparse arrays. These HD-DOT systems incorporated multidistance overlapping continuous wave measurements that only recover differential intensity attenuation. We investigate the potential improvement in reconstructed image quality due to the additional incorporation of phase shift measurements, which reflect the time-of-flight of the measured NIR light, within the tomographic reconstruction from high-density measurements. To evaluate image reconstruction with and without the additional phase information, we simulated point spread functions across a whole-scalp field of view in 24 subject-specific anatomical models using an experimentally derived noise model. The addition of phase information improves the image quality by reducing localization error by up to 59% and effective resolution by up to 21% as compared to using the intensity attenuation measurements alone. Furthermore, we demonstrate that the phase data enable images to be resolved at deeper brain regions where intensity data fail, which is further supported by utilizing experimental data from a single subject measurement during a retinotopic experiment.
Highlights
Functional neuroimaging is a valuable medical tool that is employed in a wide area of medical applications, with several noninvasive or minimally invasive neuromonitoring techniques for examining functional brain activity available for clinical use
The effective resolution for continuous wave (CW) is 22.9 mm for NN2, 21.4 mm for NN3, and 11 mm for NN4, whereas for the FD recoveries is 17.7 mm, 16.2 mm and 9.8 mm respectively, demonstrating that for larger nearest neighbor (NN), the image recovery of the focal activation is improving for both CW and FD, and FD is consistently outperforming CW
Simulated data using 24 subject-specific models utilizing experimentally measured noise have been employed to provide a set of metrics for characterization of recovered image quality, as well as an in-vivo retinotopy experiment from a single subject
Summary
Functional neuroimaging is a valuable medical tool that is employed in a wide area of medical applications, with several noninvasive or minimally invasive neuromonitoring techniques for examining functional brain activity available for clinical use. Functional near-infrared spectroscopy (fNIRS)-based optical imaging is a noninvasive, relatively inexpensive technology applied in numerous applications where neuroimaging is crucial, such as functional brain mapping,[1] psychology studies,[2,3] intensive care unit patient monitoring,[4] mental disease monitoring,[5] and early dementia diagnosis.[6] Recent advances in system design have paved the way to promise wireless wearable systems, allowing neuroimaging to be performed in real-life contexts where it is otherwise not possible.[7]. Light propagation in biological tissue is dominated by scattering events in the near-infrared (NIR) window, where low absorption allows measurements on the surface of the head to sample the brain cortex.[8] Differential measurements at two NIR wavelengths, known as NIR spectroscopy (NIRS), can recover oxy (HbO2) and deoxy (HbR) hemoglobin concentration changes that convey information about functional brain vascular events. When more wavelengths are used, it is possible to recover concentration levels of water, lipids, and even measures of metabolism such as cytochrome c.9
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