Design of a Near Infrared Spectroscopy Microcontroller Based Data Logger for Cerebral Autoregulation Monitoring

In the biomedical field there is the need of providing physicians with quantitative information in a fast and reliable way for patients’ diagnosis and follow-up, preferring real-time, non-invasive, non-ionizing, portable and low cost techniques, when possible. Near Infrared Spectroscopy (NIRS) presents all of these advantages and is mostly used to monitor cerebral autoregulation and muscle oxidative metabolism: its applications continue to broaden as multi-channel and portable devices have permitted to deepen the exploration of physiological phenomena and obtain better temporal and spatial resolution with long duration acquisitions; in the future it is expected that NIRS devices will have a key role in brain computer interfaces and diagnostic purposes. This thesis focused on designing a portable and compact NIRS data logger that exploits a low power architecture to monitor cerebral changes in oxygenated and deoxygenated haemoglobin concentrations during different conditions and tasks. The hardware is composed by two probes, one with the IR emitting LEDs and one with the receiving photodiode, and their 3D printed container, realized in flexible resin in order to adapt to the forehead surface. The firmware was developed at first in the MATLAB environment, using the NIMAX USB-6002 DAQ to generate the impulse signals to control the LEDs’ lighting, and to acquire the voltage signal from the photodiode, which is used to solve the modified Lambert-Beer equation. Then the ATxmega128A1U microcontroller was programmed with the aim of starting the NIRS acquisition and storing voltage data on microSD card or sending them via USB. Both the first and the second application were tested on subjects performing apnea and hyperventilation tasks: the MATLAB project has been useful to confirm that the current given to the LEDs was enough to have a good SNR, and to verify the expected biological response to the related stimuli. Then the microcontroller system has been used later in the same conditions comparing the results with the one obtained previously. In both cases the changes in oxygenated and deoxygenated haemoglobin concentration were satisfying and in compliance with the results already present in the literature. The device was thought as battery-powered: at present, a prototype has been realized with the available evaluation board, but it may be easily converted into a wearable tool in the future, in order to improve its versatility.