In medical near-infrared spectroscopy (NIRS), movements of the subject often cause large step changes in the baselines of the measured light attenuation signals. This prevents comparison of hemoglobin concentration levels before and after movement. We present an accelerometer-based motion artifact removal (ABAMAR) algorithm for correcting such baseline motion artifacts (BMAs). ABAMAR can be easily adapted to various long-term monitoring applications of NIRS. We applied ABAMAR to NIRS data collected in 23 all-night sleep measurements and containing BMAs from involuntary movements during sleep. For reference, three NIRS researchers independently identified BMAs from the data. To determine whether the use of an accelerometer improves BMA detection accuracy, we compared ABAMAR to motion detection based on peaks in the moving standard deviation (SD) of NIRS data. The number of BMAs identified by ABAMAR was similar to the number detected by the humans, and 79% of the artifacts identified by ABAMAR were confirmed by at least two humans. While the moving SD of NIRS data could also be used for motion detection, on average 2 out of the 10 largest SD peaks in NIRS data each night occurred without the presence of movement. Thus, using an accelerometer improves BMA detection accuracy in NIRS.
Spontaneous cerebral hemodynamic oscillations below 100 mHz reflect the level of cerebral activity, modulate
hemodynamic responses to tasks and stimuli, and may aid in detecting various pathologies of the brain. Near-infrared
spectroscopy (NIRS) is ideally suited for both measuring spontaneous hemodynamic oscillations and monitoring sleep,
but little research has been performed to combine these two applications. We analyzed 30 all-night NIRS-electroencephalography (EEG) sleep recordings to investigate spontaneous hemodynamic activity relative to sleep stages
determined by polysomnography.
Signal power of hemodynamic oscillations in the low-frequency (LF, 40-150 mHz) and very-low-frequency (VLF, 3-40
mHz) bands decreased in slow-wave sleep (SWS) compared to light sleep (LS) and rapid-eye-movement (REM) sleep.
No statistically significant (p < 0.05) differences in oscillation power between LS and REM were observed. However,
the period of VLF oscillations around 8 mHz increased in REM sleep in line with earlier studies with other modalities.
These results increase our knowledge of the physiology of sleep, complement EEG data, and demonstrate the
applicability of NIRS to studying spontaneous hemodynamic fluctuations during sleep.
Near-infrared spectroscopy (NIRS) can be used to assess the cerebrovascular response to breath hold. We measured eight
healthy subjects during voluntary end-expiratory breath hold to study inter- and intraindividual variability of the deoxy-
(HbR) and oxyhemoglobin (HbO2) response curves for the scalp and cerebral cortex. Although cortical [HbO2] behaves qualitatively similarly in all subjects, there is large inter- and intraindividual variability, and in the case of [HbR] also
qualitative variability. However, the linearity of [HbO2] increase during the breath hold has encouraging measurement
repeatability, and it may even indicate an individual's CO2 tolerance. This result may help understand why breath hold
duration varies between subjects more than the total [HbO2] increase during breath hold.
Near-infrared spectroscopy (NIRS) is a method for noninvasive estimation of cerebral hemodynamic changes. Principal component analysis (PCA) and independent component analysis (ICA) can be used for decomposing a set of signals to underlying components. Our objective is to determine whether PCA or ICA is more efficient in identifying and removing scalp blood flow interference from multichannel NIRS signals. Concentration changes of oxygenated (HbO2) and deoxygenated (HbR) hemoglobin are measured on the forehead with multichannel NIRS during hyper- and hypocapnia. PCA and ICA are used separately to identify and remove signal contribution from extracerebral tissue, and the resulting estimates of cerebral responses are compared to the expected cerebral responses. Both methods were able to reduce extracerebral contribution to the signals, but PCA typically performs equal to or better than ICA. The improvement in 3-cm signal quality achieved with both methods is comparable to increasing the source-detector separation from 3 to 5 cm. Especially PCA appears to be well suited for use in NIRS applications where the cerebral activation is diffuse, such as monitoring of global cerebral oxygenation and hemodynamics. Performance differences between PCA and ICA could be attributed primarily to different criteria for identifying the surface effect.
Medical near-infrared spectroscopy (NIRS) can be used to estimate cerebral haemodynamic changes non-invasively.
Sleep apnea is a common sleep disorder where repetitive pauses in breathing decrease the quality of sleep and exposes
the individual to various health problems. We have measured oxygenated and deoxygenated haemoglobin concentration
changes during apneic events in sleep from the forehead of one subject using NIRS and used principal component
analysis to extract extracerebral and cortical haemodynamic changes from NIRS signals. Comparison of NIRS signals
with EEG, bioimpedance, and pulse oximetry data suggests that termination of apnea leads to decreases in cerebral blood
volume and flow that may be related to neurological arousal via neurovascular coupling.
The quality of phase and amplitude data from two medical optical tomography systems were compared. The two systems are a 32-channel time-domain system developed at University College London (UCL) and a 16-channel frequency-domain system developed at Helsinki University of Technology (HUT). Difference data measured from an inhomogeneous and a homogeneous phantom were compared with a finite-element method (diffusion equation) and images of scattering and absorption were reconstructed based on it. The measurements were performed at measurement times between 1 and 30 s per source. The mean rms errors in the data measured by the HUT system were 3.4% for amplitude and 0.51 deg for phase, while the corresponding values for the UCL data were 6.0% and 0.46 deg, respectively. The reproducibility of the data measured with the two systems was tested with a measurement time of 5 s per source. It was 0.4% in amplitude for the HUT system and 4% for the UCL system, and 0.08 deg in phase for both systems. The image quality of the reconstructions from the data measured with the two systems were compared with several quantitative criteria. In general a higher contrast was observed in the images calculated from the HUT data.
Visually evoked hemodynamic responses and potentials were simultaneously measured using a 16-channel optical imaging instrument and a 60-channel electroencephalography instrument during normo-, hypo- and hypercapnia from three subjects. Flashing and pattern-reversed checkerboard stimuli were used. The study protocol included two counterbalanced measurements during both normo- and hypocapnia and normo- and hypercapnia. Hypocapnia was produced by controlled hyperventilation and hypercapnia by breathing carbon dioxide enriched air. Near-infrared imaging was also used to monitor the concentration changes of oxy- and deoxyhaemoglobin due to hypo- and hypercapnia. Hemodynamic responses and evoked potentials were successfully detected for each subject above the visual cortex. The latencies of the hemodynamic responses during hypocapnia were shorter whereas during hypercapnia they were longer when compared to the latencies during normocapnia. Hypocapnia tended to decrease the latencies of visually evoked potentials compared to those during normocapnia while hypercapnia did not show any consistent effect to the potentials. The developed measurement setup and the study protocol provide the opportunity to investigate the neurovascular coupling and the links between the baseline level of blood flow, electrical activity and hemodynamic responses in the human brain.
Kalle Kotilahti, Ilkka Nissila, Riikka Makela, Tommi Noponen, Lauri Lipiainen, Nasia Gavrielides, Timo Kajava, Minna Huotilainen, Vineta Fellman, Pekka Merilainen, Toivo Katila
We have used near-infrared spectroscopy (NIRS) to study hemodynamic auditory evoked responses on 7 full-term neonates. Measurements were done simultaneously above both auditory cortices to study the distribution of speech and music processing between hemispheres using a 16-channel frequency-domain instrument. The stimulation consisted of 5-second samples of music and speech with a 25-second silent interval. In response to stimulation, a significant increase in the concentration of oxygenated hemoglobin ([HbO2]) was detected in 6 out of 7 subjects. The strongest responses in [HbO2] were seen near the measurement location above the ear on both hemispheres. The mean latency of the maximum responses was 9.42±1.51 s. On the left hemisphere (LH), the maximum amplitude of the average [HbO2] response to the music stimuli was 0.76± 0.38 μ M (mean±std.) and to the speech stimuli 1.00± 0.45 μ± μM. On the right hemisphere (RH), the maximum amplitude of the average [HbO2] response was 1.29± 0.85 μM to the music stimuli and 1.23± 0.93 μM to the speech stimuli. The results indicate that auditory information is processed on both auditory cortices, but LH is more concentrated to process speech than music information. No significant differences in the locations and the latencies of the maximum responses relative to the stimulus type were found.
We have developed a frequency-domain near-infrared device suitable for physiological studies in human. In this work, a four-channel configuration of the instrument is applied to monitor hemodynamic and oxygenation changes in the frontal cortex of volunteers during different ventilation tasks. We use four different source-receiver separations (2, 3, 4, and 5 cm) and three wavelengths (760, 808, and 830 nm) to test the sensitivity of these parameters to cardiovascular and metabolic changes. Low-frequency oscillations (~ 0.02 Hz) and variations in heart rate during different ventilation tasks are investigated as well. We also study physiological changes during natural sleep using the frequency-domain instrument simultaneously with a polysomnography system containing a pulse oximeter. Our results indicate that hemodynamic and oxygenation changes in the frontal cortex during natural sleep can be detected using near-infrared measurements.
We used a four-channel intensity-modulated near-infrared spectroscopy device to study the hemodynamic responses due to brain activation in adults and neonates. The stimuli included finger tapping, tickling of the heel, and auditory stimuli. The subjects included two adults and ten neonates of age between 0.5 and 4 days. A block paradigm was used in the studies, and responses were successfully obtained from both subject groups.
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