KEYWORDS: Near infrared spectroscopy, Sensors, Capacitance, Noise cancelling, Tissues, Signal to noise ratio, Interference (communication), Electromagnetism, Electromagnetic interference, Signal attenuation
A modern application of NIRS moves towards implantable methods to overcome the limitation. In implantable NIRS, the sensor is implanted adjacent to the organ of interest. The implant's mechanical structure, shape, and total volume are crucial to ensuring usability and minimizing invasiveness. Since thinner and smaller implant encapsulation reduces the distance between the electronic circuit of the sensor and the tissue, the equivalent capacitance between the tissue and the implantable system (consisting of the sensor and controller) can increase dramatically. The CMV (Common-Mode Voltage) is a voltage on the patient's body due to electromagnetic and electrical coupling. CMV is an essential noise source for recording biological signals; however, implantable NIRS sensors can induce a more significant noise because of the higher capacitance effect. During the preamplifier, the CMV can appear and be transformed to differential voltage, contaminating the original signal and decreasing the signal-to-noise ratio. Electromagnetic Shielding and a high CMRR (Common-Mode Rejection Ratio) amplifier are conventional methods for preventing noise contamination with common-mode voltage. However, these methods are not robust enough to protect the signal of interest in the presence of high-amplitude CMV. We proposed the active CMV reduction technique to eliminate the effect of CMV and improve the SNR of the NIRS signal. It can measure and eradicate induced CMV by injecting a minimal amount of electric current into the patient non-invasively. This paper proposes an ANC (Active noise cancellation) electronic circuit that eliminates CMV.
Cortical spreading depression (SD), a pathological cortical negative DC potential, is associated with various brain abnormalities. SD is a significant transient and localized relocation of ions within the neurons and spreads slowly like a wave in the brain tissue. SD results from a high extracellular K+ concentration, increasing neuronal excitability and, consequently, brain oxygen consumption. In our previous studies, we developed an electroencephalography (EEG) system capable of recording the SD from the surface scalp of epileptic patients. We demonstrated that SD is associated with seizures in patients with medically intractable epilepsy. In this paper, in addition to EEG measurements, near-infrared spectroscopy (NIRS) was used to measure local brain oxygen consumption during SD and seizures. NIRS is a non-invasive method to measure the hemodynamics of the tissue, such as oxy and deoxyhemoglobin concentrations, representing the gray matter's local neuronal metabolisms. By applying two or more wavelengths in the near-infrared window and measuring the attenuation variations of the relative change in the concentration of deoxyhemoglobin (HHb) and oxyhemoglobin (HbO2), the local oxygen consumption can be estimated. Method: We recorded SD and NIRS simultaneously during epileptiform EEG activities from twelve epileptic patients. Main result: SD occurred in the scalp of epileptic patients and preceded seizures with a varying time lag (0-30 minutes). HHb concentration increased during the SD duration. While HbO2 concentration decreased during the SD duration. Both returned to normal values after the SD event.
Spreading depression (SD) is an ultra-slow (30 to 90 seconds) brain electrical activity caused by the high concentration of extracellular potassium ions (K+) and plays an essential role in the pathophysiology of epilepsy. However, the SD signal amplitude is higher than the conventional EEG signal (10 to 300 microvolt). Due to filter effects of the skull and in the presence of other non-neuronal slow shift potentials (like electrode low-frequency shifts and motion artifact) the recording of an SD signal with the non-invasive method can be a difficult task. Near-infrared spectroscopy (NIRS) is wavelength-dependent absorption spectroscopy. Light absorption is a function of the molecular properties of substances within the light path. Hemodynamic variations accompany the propagation of the SD. Thus, using near-infrared spectroscopy besides EEG can provide an additional biomarker to distinguish SD from non-neuronal EEG slow shifts. This study used NIRS/EEG, a dual-modal NIRS and ultra-low frequency (0.01Hz to 80Hz) EEG device to record five Wistar rats (anesthetized). One NIRS source, NIRS detector, and EEG electrode were positioned above the somatosensory neocortex on the depilated skin. The EEG reference electrode was close to the rat’s nasion. The distance between source and detector was 8mm. KCL solution (3 mole/L, 10μl) was injected into the rat neocortex to generate the SD wave, and NIRS/EEG device performed the simultaneous recording. The increase of HHb (deoxyhemoglobin) accompanied by the slow shift of EEG was detected during SD. The rise of THb (Total hemoglobin) was also detected during the induced SD.
This experiment proposes a multi-modal measurement using near-infrared spectroscopy (NIRS) and electroencephalography (EEG) using a novel NIRS/EEG device to measure the effect of subjective pain on Gamma-band (GBO) and hemodynamic changes. A customized NIRS/EEG probe was designed and implemented. The NIRS/EEG signals were recorded during the cold pressor test (CPT). The experiment began with two minutes baseline, followed by two minutes CPT and repeated three times for each subject. The GBO extracted from the EEG signal was detected during subjective pain (CPT). The increase of tissue total blood volume associated with the rise of GBO power was observed and reported.
A newborn infant has an extraordinarily vulnerable and immature central nervous system, which is undergoing rapid structural and functional development. As these infants are pre-verbal and their neurological systems are immature, assessing accurately and treating effectively procedure-related pain is a significant challenge. The nociceptive signals caused by the pain are accompanied by changes in regional blood oxygenation and neuronal activity in the infant’s brain. In this study, we developed a dual-mode Near-Infrared Spectroscopy (NIRS) and electroencephalography (EEG) monitor that can measure regional brain oxygenation and neuronal activity concurrently (safe and non-invasive). The neuronal activity is measured by an innovative low-noise EEG amplifier in both conventional and ultra-low frequency bandwidths. This multimodal recording allows us to investigate the coupling of neuronal activity and the neurovascular system as never before. NIRS and EEG electrodes are miniaturized and unified in one sensor. This modification facilitates the use of a NIRS/EEG device for recording from neonatal subjects. Ten infants, born between 27-35 weeks gestational age, are being recruited from the NICU at BCWH. They are monitored during a single, routine blood draw required for clinical care. In this experiment, we investigate the change of cerebral hemodynamic across 3 phases of blood collection, baseline, heel lance, recovery. Variation of blood flow accompanied with the slow shift of EEG has been detected during the pain stimulus phase. Additionally, the increase of gamma-band correlated to a rise in blood flow is also observed
Electroencephalography (EEG) and cerebral near-infrared spectroscopy (NIRS) are both well-known monitoring methods to quantify cerebral neurophysiology and hemodynamics states of the brain. A stable regulatory system operates to guarantee sufficient spatial and temporal distribution of energy substrates for ongoing neuronal activity. Most EEG signals are associated with the neural activity of an enormous number of neurons that are interconnected and firing concurrently. The conventional EEG bandwidth is 0.16Hz to 70Hz. In this study, the EEG recording bandwidth is extended in low frequency (0.016Hz to 70Hz) by using a novel EEG amplifier. We aimed to investigate the low-frequency EEG and brain tissue deoxygenation by using novel multi-modal measurements. We used combined NIRS and EEG measurements for estimating the electrophysiological activity and hemodynamic changes in the adult human forehead during a hypoxic breathing condition. For the experiment, an altitude simulation kit was used to restrict the concentration of oxygen in the air that was inhaled by the subjects. The hypoxic breathing conditions led to variations in CO2 concentration (pCO2). Prolong (low-frequency) EEG signal shift, accompanied by an increase of deoxygenated hemoglobin during simulated hypoxic breathing were observed in this experiment.
Near-Infrared Spectroscopy (NIRS) is a non-invasive technique, extensively used to monitor the hemodynamic variations in cerebral neuronal tissues. For cerebral NIRS, the back-scattering probe is more prevailing, in which an incident beam is diffused, and only a slight fraction of the source optical energy reaches the light detectors. Multiplexing in the time domain is the conventional method used to distinguish the optical density of each NIR source at the receiver site. Even though time-multiplexing is straightforward and convenient, the ambient light can significantly contaminate the NIR beams during the sampling-path from the source to the detector. In this work, we present a novel method based on frequency division multiplexing (FDM) to overcome the interference of ambient light even without an external optical filter. The method proposes to modulate the NIR source intensities by using specific carrier frequencies distinct from the dominant frequency components of ambient light intensity. By modulating the intensity of each NIR source, and applying them at their specific frequency channels, the receiver is capable of distinguishing the received optical signals based on their frequency channel. Because the frequency channels are adjusted at distinct dominant frequency components of the ambient intensity, the latter ambient noise can be filtered out instantly. The method has been implemented by using electronic circuit design and evaluated both by numerical simulation and experimental measurements. The signal to noise ratio (SNR) has been improved at least by 45dB.
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