The Corsi block-tapping test (CBT) is an old neuropsychological test that, requiring the storage and the reproduction of spatial locations, assesses spatial working memory (WM). Despite its wide use in clinical practice, the specific contribution of prefrontal cortex (PFC) subregions during CBT execution has not been clarified yet. Considering the importance of spatial WM in daily life and the well-known role of ventrolateral-PFC/dorsolateral-PFC (VLPFC/DLPFC) in WM processes, the present study was aimed at investigating, by a 20-channel functional near-infrared spectroscopy (fNIRS) system (including four short-separation channels), the hemodynamic response of the VLPFC/DLPFC during a computerized version of the CBT. Thirty-nine university students were asked to perform CBT standard version (CBTs), block-suppression CBT (CBTb), and control task (CBTc). A VLPFC activation during CBTs and a DLPFC activation during CBTb were hypothesized. The results of the Bayesian analysis have not shown a delineated specific activation of VLPFC/DLPFC during either CBTs or CBTb. These results together with the related ones obtained by others using fMRI are not sufficient to definitively state the role of the PFC subregions during CBT execution. The adoption of high-density diffuse optical tomography would be helpful in further exploration of the PFC involvement in spatial WM tasks.
Working memory (WM) is fundamental for a number of cognitive processes, such as comprehension, reasoning and learning. WM allows the short-term maintenance and manipulation of the information selected by attentional processes. The goal of this study was to examine by time-resolved fNIRS neural correlates of the verbal and visual WM during forward and backward digit span (DF and DB, respectively) tasks, and symbol span (SS) task. A neural dissociation was hypothesised between the maintenance and manipulation processes. In particular, a dorsolateral/ventrolateral prefrontal cortex (DLPFC/VLPFC) recruitment was expected during the DB task, whilst a lateralised involvement of Brodmann Area (BA) 10 was expected during the execution of the DF task. Thirteen subjects were monitored by a multi-channel, dual-wavelength (690 and 829 nm) time-resolved fNIRS system during 3 minutes long DF and DB tasks and 4 minutes long SS task. The participants’ mean memory span was calculated for each task: DF: 6.46±1.05 digits; DB: 5.62±1.26 digits; SS: 4.69±1.32 symbols. No correlation was found between the span level and the heart rate data (measured by pulse oximeter). As expected, DB elicited a broad activated area, in the bilateral VLPFC and the right DLPFC, whereas a more localised activation was observed over the right hemisphere during either DF (BA 10) or SS (BA 10 and 44). The robust involvement of the DLPFC during DB, compared to DF, is compatible with previous findings and with the key role of the central executive subserving in manipulating processes.
This review celebrates the 30th anniversary of the first in vivo near-infrared (NIR) spectroscopy (NIRS) publication, which was authored by Professor Frans Jöbsis. At first, NIRS was utilized to experimentally and clinically investigate cerebral oxygenation. Later it was applied to study muscle oxidative metabolism. Since 1993, the discovery that the functional activation of the human cerebral cortex can be explored by NIRS has added a new dimension to the research. To obtain simultaneous multiple and localized information, a further major step forward was achieved by introducing NIR imaging (NIRI) and tomography. This review reports on the progress of the NIRS and NIRI instrumentation for brain and muscle clinical applications 30 years after the discovery of in vivo NIRS. The review summarizes the measurable parameters in relation to the different techniques, the main characteristics of the prototypes under development, and the present commercially available NIRS and NIRI instrumentation. Moreover, it discusses strengths and limitations and gives an outlook into the “bright” future.
The letter-fluency task-induced response over the prefrontal cortex is investigated bilaterally on eight subjects using a recently developed compact, eight-channel, time-resolved, near-IR system. The cross-subject mean values of prefrontal cortex oxygen saturation (SO2) were 68.8±3.2% (right) and 71.0±3.6% (left), and of total hemoglobin concentration (tHb) were 69.6±9.6 µM (right) and 69.5±9.9 µM (left). The typical cortical activation response to the cognitive task [characterized by an increase in oxyhemoglobin (O2Hb) with a concurrent decrease in deoxyhemoglobin (HHb)] at each measurement point is observed in only four subjects. In this subset, the amplitude of the O2Hb increase and HHb decrease is uniform over each prefrontal cortex area and comparable between the two hemispheres. These findings agree with previous studies using continuous wave functional near-IR spectroscopy and functional magnetic resonance imaging, therefore demonstrating the potential of a time-resolved spectroscopy approach. In addition, a significant increase in SO2 levels was observed in the right (1.1±0.5%) compared to left side of the prefrontal cortex (0.9±0.5%) (P=0.005). A different pattern of cortical activation (characterized by the lack of HHb decrease or even increased HHb) was observed in the remaining subjects.
A portable instrument for tissue oximetry based on time-resolved reflectance spectroscopy was developed. The performances of the system were tested on phantoms in terms of stability, reproducibility among channels, and accuracy in the determination of the optical properties. Preliminary in vivo measurements were performed on healthy volunteers to monitor spatial changes in calf (medial and lateral gastrocnemius) oxygen hemoglobin saturation and blood volume during dynamic plantar flexion exercise.
A portable instrument for tissue oximetry based on time-resolved reflectance spectroscopy was developed. The output pulses of 2 laser diodes (683 and 785 nm, 80 MHz pulse repetition rate, 1 mW average power, 100 ps FWHM) are delayed and coupled into a multimode graded-index fiber (50/125 μm and injected into the tissue. The reflectance photons are collected by 8 independent 1 mm fibers and detected by a 16-anode photomultiplier. A time-correlated single photon counting PC board is used for the parallel acquisition of the curves. Simultaneous estimate of the transport scattering and absorption coefficients is achieved by best fitting of time-resolved reflectance curves with a standard model of Diffusion Theory. The performances of the system were tested on phantoms in terms of stability, reproducibility among channels, and accuracy in the determination of the optical properties. Preliminary in vivo measurements were performed on healthy volunteers to monitor spatial changes in calf (medical and lateral gastrocnemius) oxygen hemoglobin saturation and blood volume during dynamic plantar flexion exercise.
The monitoring of a single muscle location does not reflect the heterogeneity of the muscle groups activation during exercise. In the past, measurements of oxygen consumption (VO2) at single muscle locations could be carried out non-invasively by near-infrared continuous wave spectroscopy (NIRCWS) at rest or during isometric contractions. In the present study, human regional quadriceps (vastus lateralis and rectus femoris) VO2 was investigated at rest and during maximal voluntary contractions using a 12- channel NIRCWS system with an acquisition time of 0.1 s.
Aim of this study was to investigate the oxygenation and the total hemoglobin concentration pattern in vastus lateralis and medial gastrocnemius muscle groups during a standardized treadmill exercise (n equals 6) by a new near infrared frequency-domain oximeter (ISS Oximeter model 96208). Vastus lateralis saturation and total hemoglobin concentration were 74 +/- 3% and 71 +/- 15 (mu) M at 0 mph and 72 +/- 5% and 79 +/- 16 (mu) M at 6 mph, respectively. Gastrocnemius saturation and total hemoglobin concentration were 74 +/- 2% and 107 +/- 18 (mu) M at 0 mph and 60 +/- 6% and 113 +/- 23 (mu) M at 6 mph, respectively. The saturation recovered gradually up to the baseline value when the speed was decreased.
We have measured transillumination spectra collected at various sites on the breasts of normal subjects. The absolute concentration of hemoglobin, lipids and water are derived using ratiometric methods. The lipids and deoxyhemoglobin signals overlap at 760 nm; the accurate separation of these thus requires explicit account to be taken of the diffusion of photons through the breast. The concentrations of Hb, lipids and water vary substantially from site to site, which will influence wavelength selection in imaging systems. Finite element calculations and solid phantom measurements demonstrate that this variability is not due to boundary effects. The spatial variation of (mu) a at wavelengths 720 nm, 820 nm, 930 nm and 970 nm is illustrated schematically. It is noted that (mu) a at 930 nm shows the least heterogeneity amongst the 8 normal subjects studied.
In the last four years near infrared spectroscopy (NIRS) has been used in cerebral functional activation studies to monitor changes in concentration of oxy-, deoxy- and total hemoglobin [(O2Hb), (HHb) and (tHb) respectively] in response to different stimuli. Previous studies were performed with a 1 - 2 Hz temporal resolution and a poor signal-to-noise (S/N) ratio. The aim of this study was to investigate the response of the motor cortex region during a finger opposition task in single subjects using a novel continuous wave NIRS instrument with enhanced temporal resolution and S/N ratio. Six subjects performed a sequential finger opposition task with the right hand (20 s duration; 2 Hz). The optodes were positioned over the left motor cortex region using an inter-optode distance of 3.5 cm. The high S/N ratio and 0.1 s sampling time allowed clear monitoring of (O2Hb) and (HHb) changes due to heart beat as well as to respiration. The contribution of the heart pulse to the total signal was less than 0.4%. As previously shown by others using pooled data, an increase of (O2Hb) during the activation accompanied by a decrease of (HHb) was found in most subjects for every activation cycle. Our approach provides a better insight into the underlying physiological mechanisms.
Near infrared spectroscopy (NIRS) was used to measure oxygen consumption (V ˙ O2) in the human forearm with two different methods.V ˙ O2 was measured in ten patients, first by inducing a forearm venous occlusion
and then an ischemia. The mean value ofV ˙ O2 was 3.3 6 1.1 mMO2 3 100 g−13min−1 during venous occlusion
and 2.960.9 mMO2 3 100 g−13min−1 during ischemia. The difference between the two means was 0.4 mMO2 3 100 g−13min−1. A good agreement between the two methods was demonstrated. The results showed that NIRS is a useful tool for the measurement of V ˙ O2 . The venous occlusion method was the method of choice because it could be easily repeated at the bedside without causing any discomfort to the patient.
The single scattering properties (extinction coefficient and scattering function at small forward scattering angles) of bovine and swine brain were obtained from transmissometric measurements on thin slices of tissues at l=633 nm. Large differences between the optical properties of white and gray matter were observed, whereas minor differences were found between bovine and swine samples. For gray matter, values between 10 and 30 mm−1 were obtained for the extinction coefficient. The scattering function was strongly forward peaked, indicating a highly anisotropic scattering. For white matter, values between 100 and 250 mm−1 were measured for the extinction coefficient. These values were so large that it was impossible to obtain a sample thin enough to measure the scattering function in single scattering conditions.
Absorption and reduced scattering coefficients at 715 and 825 nm as well as hemoglobin saturation and content of the forehead and the forearm were measured by a 110 MHz frequency-domain multisource instrument. The absolute data obtained by the frequency- domain spectrometer were compared with oxygenation changes measured by a continuous wave instrument during quadriceps ischemia and postural changes. These preliminary results indicate that portable frequency-domain instruments could be very helpful to investigate brain and muscle pathophysiology.
Vastus lateralis muscle oxygenation was investigated on volunteers as well as muscular dystrophy patients during a walking test, and on volunteers during a free running by a continuous wave near infrared instrument. The data were analyzed using an oxygenation index independent on pathlength changes. Walking did not significantly affect the oxygenation of volunteers and patients. A relative deoxygenation was found only during free running indicating an unbalance between oxygen supply and tissue oxygen extraction. Preliminary measurements of exercising muscle oxygen saturation were performed by a 110 MHz frequency-domain, multisource instrument.
We have compared in vivo spectra of the human forearm, brain, breast and blood-perfluorocarbon exchanged cat brain with reference spectra of deoxyhemoglobin, oxyhemoglobin, triglycerides, water and albumin to identify spectral features related to tissue biochemistry. From these investigations it is evident that bands due to fat, protein and water overlap the deoxyhemoglobin band at 760 nm. While these overlapping absorptions may be insignificant in whole blood, there is a potential for spectral interference in in vivo studies. Furthermore, unique features related the protein, lipids and water can be used to image breast tissue composition. The ability to resolve the overlapping absorption bands appears easily accomplished by using derivative mathematical approaches.
KEYWORDS: Near infrared spectroscopy, Blood, Near infrared, Tissue optics, Biopsy, Time resolved spectroscopy, Absorption, Fiber optics, Chromophores, Signal attenuation
The oxygenation of human muscle tissue can be investigated using near IR spectroscopy (NIRS). Oxy and deoxy hemoglobin changes can be quantified combining attenuation measurements with pathlength data obtained by time resolved spectroscopy. This study reports the application of NIRS to non- invasive measurements of quadriceps oxygenation on muscular dystrophy patients during treadmill exercise.
Pathlength can be evaluated by measuring the time taken from a picosecond (psec) near infrared (IR) laser pulse to cross tissue. Differential pathlength factor (DPF) is calculated by dividing the mean pathlength by the inter-fiber distance. Data on DPF variability on humans are scarce. We investigated the forehead and forearm DPF in resting conditions and dynamically during brain hypoxic hypoxia, muscle ischemia and voluntary isometric exercise. At 3 cm inter optode spacing DPF at 800 nm was 4.3 +/- 0.2 (n equals 14, mean +/- SD) on the forearm, and 6.5 +/- 0.5 (n equals 8) on the forehead. Brain, muscle, and breast DPF values were almost constant over the inter optode spacing 2.5 - 4 cm. DPF was roughly constant in the central region of forehead. DPF drastically decreased under the fronto- temporal junction for the presence of muscle in the optical field. DPF decreased 5 - 10% during forearm ischemia with and without maximal voluntary contraction and during brain hypoxic hypoxia.
By using both picosecond time-resolved spectroscopy and Monte Carlo simulations, we studied photon migration in a cylindrical phantom. Fan beam style source-receiver geometry was considered. The influence of inhomogeneities inside the phantom on impulse response of short incident light was studied considering a black body located at various positions inside the scattering medium. Applicability of time gating technique to the fan beam geometry for enhancement of image resolution was also examined.
Numerical and experimental results of a study carried out to investigate the capabilities of the time gating technique to enhance images of biological tissues are reported. In particular we have investigated the confocal scanning configuration. The experimental results were obtained by using a picosecond laser and a streak camera system. The numerical results were obtained by using Monte Carlo simulations. Both elementary and semianalytic Monte Carlo methods were used. The numerical results showed that by using the time gating technique the contrast can be significantly increased when a short gating time is used. The measured time resolved spread functions were in general good agreement with numerical results. However, the experimental results showed that with our experimental setup it is difficult to measure with sufficient accuracy the small energy received during a short gating time when the slab of tissue is larger than 1 cm.
The oxygenation of human forearm muscle tissue can be investigated using near infrared spectroscopy (NIRS). Oxy and deoxy hemoglobin changes can be quantified combining attenuation measurements with pathlength data obtained by time resolved spectroscopy. This study reports some applications of NIRS to non- invasive measurements of skeletal muscle oxygenation in untrained volunteers and patients.
KEYWORDS: Scattering, Time resolved spectroscopy, Tissues, Ischemia, Monte Carlo methods, Absorption, Light scattering, Near infrared, Spectroscopy, Imaging spectroscopy
Human forearm oxygenation and scattering changes during ischemia were investigated using picosecond near infrared laser spectroscopy. Path lengths were calculated for different geometries. At 760 nm, a plateau phaser was reached after 4 min occlusion. No changes were observed during the occlusion at 800 nm, the hemoglobin isosbetic wavelength. Time-resolved Monte Carlo simulations were performed also to mimic the propagation of a light pulse in a forearm phantom containing different quantity and size of polystyrene spheres as scattering media. The influence of both the scattering and absorption effects was studied.
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