Date of Award
Master of Science (MS)
Optical neuroimaging stands as a promising potential surrogate for fMRI, while increasing imaging flexibility, as fMRIs require confinement in a loud restrictive chamber and lack compatibility with metal implants. Traditional high-density diffuse optical tomography (HD-DOT) systems can achieve comparable imaging quality on the cortical surface but require large opto-electronic consoles and bulky fiber optics which restrict head motion and require participants to remain stationary to avoid motion-induced noise. This has led to the development of portable fiberless HD-DOT systems, but these have multiple deficits including either lower resolution, smaller fields-of-view, strong image distortions, poor head conformability, or lower signal quality in comparison to traditional fiber systems. Wearable fiber-less systems often rely on PIN photodiodes as detectors which have a relatively low sensitivity, leading to the system noise being dominated by thermal noise from the transimpedance amplifier (TIA) that follows the photodiode. Fiber systems typically rely on avalanche photodiodes (APDs) which provide an intrinsic gain through the avalanche effect, reducing the effect of thermal amplifier noise, but require bias voltages in the hundreds of volts, which makes integration into a wearable system difficult. Recent improvements in silicon photomultipliers (SiPMs) in the near-infrared range have led to their consideration for use in wearable DOT systems, as their high inherent gain would circumvent the TIA noise bottleneck while requiring more moderate bias voltages. The use of silicon photomultiplier detection in a DOT system is investigated, with the aim of dramatically lowering noise floors compared to traditional photodiodes. The effectiveness of SiPMs are evaluated in a hybrid APD/SiPM system, showing potential to significantly expand the number of system channels, while managing complicating factors of SiPMs, including nonlinearity and temperature variability. A miniaturized module for a wearable system is designed and evaluated, with a measured NEP of 3.62 and 7.11fW/√Hz for 735 and 850nm wavelengths, lower than APD systems and more than an order of magnitude lower than comparable PIN photodiode systems. With dual SiPM detectors and LEDs, the module sets the path for the development of a wearable HD-DOT SiPM system.
Edward Richter, Jason Trobaugh
Available for download on Wednesday, June 19, 2024