The human cerebral cortex is composed of gyri of approximately 10–15 mm in width that have independent functions. To explore their activities with functional near-infrared spectroscopy (fNIRS), hemodynamic responses at single gyri should be measured. Most fNIRS devices only offer a channel arrangement with a larger interval size than the gyrus width, which can cause false negative errors in detecting cortical activation localized within 10–15 mm, and this has been an obstacle using fNIRS to explore cortical activities. Previously, we demonstrated doubling of the channel density using a triangular arrangement of dual-purpose optodes with a minimum number of optodes that was almost equivalent to that used in conventional arrangements. To implement this method as a wearable device for human measurement, we developed a dual-purpose optode to function both as the source and detector with the base unit triangularly mounted by three optodes, and the connectors joining plural base units with three-way joints. Optodes in this triangular arrangement illuminated and detected in sequence between adjacent optodes and performed high-density 15-mm measurements in channel intervals. Measurements of 30 channels on an adult human successfully detected hemodynamic responses to unilateral finger movements at the motor-related cortical regions according to their functions.
Many studies on CW spatially resolved spectroscopy (CW-SRS) have been conducted to noninvasively determine the optical properties, particularly the absorption and reduced scattering coefficients, μa and μs′, of biological tissues. To determine both μa and μs′, conventional CW-SRS employs measurements of the diffuse reflectances at short source-detector (SD) distances in the non-diffusion regime. In contrast, CW-SRS with long SD distances in the diffusion regime can determine only the effective attenuation coefficients, μeff = (3μaμs′)1/2 without separating μa and μs′. This study proposes a new method to separately determine μa and μs′ using CW-SRS with long SD distances, extending to conditions with high and low internal reflection at the boundary of homogeneous semi-infinite media. The proposed method used two ratios of the diffuse reflectances at two long SD distances, and μa and μs′ were determined by fitting the theoretical ratios to the measured values. Numerical simulations were conducted to validate the proposed method. As a light propagation model, the analytical solution of the time-dependent photon diffusion equation under the partial-current boundary condition (TD-DE-PCBC), which is verified for high internal reflection, was employed. Simulated measurements of the two ratios were compared with the calculated ratios (so-called look-up tables) using the TD-DE-PCBC to determine both μa and μs of the media. Simulation results demonstrate the validity of the proposed method. The effects of deviations in the SD distances and internal reflection coefficients were evaluated. Changes in the light propagation paths in the medium are discussed, and methods to realize the proposed method are suggested.
Significance: The establishment of a light propagation analysis-based scalp-cortex correlation (SCC) between the scalp location of the source–detector (SD) pair and brain regions is essential for measuring functional brain development in the first 2 years of life using functional near-infrared spectroscopy (fNIRS).
Aim: We aimed to reveal the optics-based SCC of 0-, 1-, and 2-year-olds (yo) and the suitable SD distance for this age period.
Approach: Light propagation analyses using age-appropriate head models were conducted on SD pairs at 10-10 fiducial points on the scalp to obtain optics-based SCC and its metrics: the number of corresponding brain regions (NCBR), selectivity and sensitivity of the most likely corresponding brain region (MLCBR), and consistency of the MLCBR across developmental ages. Moreover, we assessed the suitable SD distances for 0-, 1-, and 2-yo by simultaneously considering the selectivity and sensitivity of the MLCBR.
Results: Age-related changes in the SCC metrics were observed. For instance, the NCBR of 0-yo was larger than that of 1- and 2-yo. Conversely, the selectivity of 0-yo was lower than that of 1- and 2-yo. The sensitivity of 1-yo was higher than that of 0-yo at 15- to 30-mm SD distances and higher than that of 2-yo at 10-mm SD distance. Notably, the MLCBR of the fiducial points around the longitudinal fissure was inconsistent across age groups. An SD distance between 15 and 25 mm was found to be appropriate for satisfying both sensitivity and selectivity requirements. In addition, this work provides reference tables of optics-based SCC for 0-, 1-, and 2-yo.
Conclusions: Optics-based SCC will be informative in designing and explaining child developmental studies using fNIRS. The suitable SD distances were between 15 and 25 mm for the first 2 years of life.
Significance: Functional near-infrared spectroscopy (fNIRS) is a technique for detecting regional hemodynamic responses associated with neural activation in the cerebral cortex. The absorption changes due to hemodynamic changes in the scalp cause considerable signal contamination in the fNIRS measurement. A method for extracting hemodynamic changes in the cerebral tissue is required for reliable fNIRS measurement.
Aim: To exclusively detect cerebral functional hemodynamic changes, we developed an fNIRS technique using reflectance modulation of the scalp surface.
Approach: The theoretical feasibility of the proposed method was proven by a simulation calculation of light propagation. Its practical feasibility was evaluated by a phantom experiment and brain activation simulation mimicking human fNIRS experiments.
Results: The simulation calculation revealed that the partial path length of the scalp was changed by reflectance modulation of the scalp surface. The influence of absorption change in the superficial layer was successfully reduced by the proposed method, using only measurement data, in the phantom experiment. The proposed method was applicable to human experiments of standard designs, achieving statistical significance within an acceptable experimental time-frame.
Conclusions: Removal of the scalp hemodynamic effect by the proposed technique will increase the quality of fNIRS data, particularly in measurements in neonates and infants that typically would require a dense optode arrangement.
Diffuse optical tomography (DOT) images the distribution of the optical properties, such as the absorption and scattering coefficients, via the image reconstruction from the light intensities measured at the surface of the biological medium. The changes in the optical properties reflect the conditions of the tissues. Therefore, DOT image can provide the information which is not obtained from the other modalities and is useful for medical diagnoses. In this study, the application of the DOT to thyroid cancer diagnosis was investigated. The ultrasound technique is usually carried out for the thyroid cancer diagnosis. It is, however, difficult to distinguish follicular carcinoma from adenoma of thyroid. The optical properties may be helpful for the diagnosis. The image reconstruction algorithm employing the regularization minimizing lp-norm (0 < p < 2) of the reconstructed image was developed. The image was reconstructed from the timeresolved measurement data. The numerical simulations of the image reconstruction were tried. The numerical simulation demonstrated that the developed algorithm was able to image the changes in the optical properties in the medium. Additionally, the image reconstruction of the numerical neck phantom was simulated. The thyroid cancer region was reconstructed successfully. It was demonstrated that the developed algorithm had the possibility to image thyroid cancer.
Functional near infrared spectroscopy (fNIRS) can separately measure spatially differentiated brain functions by appropriately positioning irradiation and detection probes on the scalp, where brain region that could be assessed is limited to the adjacent region directly below the probe pair. A key challenge is determining the appropriate probe position for measuring the function of target brain region. Here, we propose an fNIRS probe positioning system using augmented reality technology. From a subject’s anatomical 3D magnetic resonance images, geometry of the head tissues including the appropriate position directly above the targeted brain region was obtained. The system captured an image of the subject’s head and several facial landmarks were extracted. Subsequently, the anatomical geometry was fitted into the captured image of the head to align with the landmark positions. Finally, the target probe positions were indicated icons on the captured head images. These were processed in real-time, while following the motion of the subject’s head. Therefore, the appropriate probe position was spatially determined by taking a video of the subject's head from various directions. The system was implemented on a generic tablet computer. Positioning accuracy of system in a mannequin head with a shape and color similar to that of a human face was assessed. Errors from the appropriate position were less than 10 mm, which is adequate for appropriate probe positioning in hemodynamic response measurement from the target gyrus, since brain gyri in human adults are approximately 10 mm in width.
In the brain function measurement by near infrared spectroscopy using a multi-distance probe configuration, the ratio of the partial optical path length in the scalp for the long spacing probe pair to that for the short spacing probe pair is important for the calculation to eliminate the scalp component in the signal. Light propagation in the subject specific head models of 45 volunteers was calculated to predict the dependence of the partial optical path length in the scalp and the weighting factor for the multi-distance probe configuration. The weighting factor ranges from 1.0 to 2.3 and increases with the scalp thickness when the scalp thickness is less than 15 mm.
The relationship between probe positions of near-infrared spectroscopy instruments and functional areas in the brain is very important for the brain function measurement. Light propagation in a standard brain was calculated to consider the broadening of the probing region caused by the tissue scattering in the NIRS measurements to determine the relationship between the probe positions and the functional areas. The NIRS signal tends to reflect the brain activation in different functional areas and the primary functional area is possibly different from that indicated by the simple projection of the measurement point.
Scalp hemodynamics contaminates the signals from functional near-infrared spectroscopy (fNIRS). Numerous methods have been proposed to reduce this contamination, but no golden standard has yet been established. Here we constructed a multi-layered solid phantom to experimentally validate such methods. This phantom comprises four layers corresponding to epidermides, dermis/skull (upper dynamic layer), cerebrospinal fluid and brain (lower dynamic layer) and the thicknesses of these layers were 0.3, 10, 1, and 50 mm, respectively. The epidermides and cerebrospinal fluid layers were made of polystyrene and an acrylic board, respectively. Both of these dynamic layers were made of epoxy resin. An infrared dye and titanium dioxide were mixed to match their absorption and reduced scattering coefficients (μa and μs’, respectively) with those of biological tissues. The bases of both upper and lower dynamic layers have a slot for laterally sliding a bar that holds an absorber piece. This bar was laterally moved using a programmable stepping motor. The optical properties of dynamic layers were estimated based on the transmittance and reflectance using the Monte Carlo look-up table method. The estimated coefficients for lower and upper dynamic layers approximately coincided with those for biological tissues. We confirmed that the preliminary fNIRS measurement using the fabricated phantom showed that the signals from the brain layer were recovered if those from the dermis layer were completely removed from their mixture, indicating that the phantom is useful for evaluating methods for reducing the contamination of the signals from the scalp.
Functional near-infrared spectroscopy (fNIRS) is suitable for measuring brain functions during neurorehabilitation
because of its portability and less motion restriction. However, it is not known whether neural reconstruction can be
observed through changes in cerebral hemodynamics. In this study, we modified an fNIRS system for measuring the
motor function of awake monkeys to study cerebral hemodynamics during neurorehabilitation. Computer simulation was
performed to determine the optimal fNIRS source–detector interval for monkey motor cortex. Accurate digital phantoms
were constructed based on anatomical magnetic resonance images. Light propagation based on the diffusion equation
was numerically calculated using the finite element method. The source–detector pair was placed on the scalp above the
primary motor cortex. Four different interval values (10, 15, 20, 25 mm) were examined. The results showed that the
detected intensity decreased and the partial optical path length in gray matter increased with an increase in the source-detector
interval. We found that 15 mm is the optimal interval for the fNIRS measurement of monkey motor cortex. The
preliminary measurement was performed on a healthy female macaque monkey using fNIRS equipment and custom-made
optodes and optode holder. The optodes were attached above bilateral primary motor cortices. Under the awaking
condition, 10 to 20 trials of alternated single-sided hand movements for several seconds with intervals of 10 to 30 s were
performed. Increases and decreases in oxy- and deoxyhemoglobin concentration were observed in a localized area in the
hemisphere contralateral to the moved forelimb.
The brain activation image obtained by diffuse optical tomography (DOT) is obtained by solving inverse problem using
the spatial sensitivity profile (SSP). The SSP can be obtained from the analysis of the light propagation using threedimensional
head models. The head model is based upon segmented magnetic resonance (MR) image and there are
several types of software based on binarization for segmentation of MR head images. We segmented superficial tissues
which effect the light propagation in human head from MR images acquired with FATSAT and FIESTA pulse sequences
by using region growing algorithm and morphological operation to facilitate the construction of the individual head
models for DOT. The pixel intensity distribution of these images has appropriate characteristics to extract the superficial
tissues by using algorithm based on binarization. The result of extraction was compared with the extraction from
T2-weighted image which is commonly used to extract superficial tissues. The result of extraction from FATSAT or
FIESTA image agree well with ground truth determined by manual segmentation.
In near-infrared topography, the topographic images of the brain activation considerably depend on the relative position
between the probe arrangement and brain activation. The variance of topographic image due to the relative position
between the brain activation and the probe arrangement is evaluated by the phantom experiment and simulation. We
examined five types of probe arrangements. The variation of the topographic images measured by 15-mm probe
arrangements is considerably improved in comparison with the image measured by the 30-mm probe arrangements. The
images are almost independent of the relative position in case where the diameter of the brain activation is greater than
30mm.
The poor spatial resolution and reproducibility of the images are disadvantages of near infrared topography. The authors
proposed the combination of the double-density probe arrangement and the image reconstruction algorithm using a
spatial sensitivity profile to improve the spatial resolution and the reproducibility. However, the proposed method was
evaluated only by the simplified adult head model. It is uncertain whether the proposed method is effective to the actual
head that has complicated structure. In this study, the proposed method is evaluated by the virtual head phantom the 3Dstructure
of which is based upon an MRI scan of an adult head. The absorption change the size of which is almost
equivalent to the width of the brain gyri was measured by the conventional method and the proposed method to evaluate
the spatial resolution of the topographic images obtained by each method. The positions of the probe arrangements are
slightly changed and the topographic images of the same brain activation measured by two probe positions are compared
to evaluate the reproducibility of the NIR topography. The results indicate that the combination of the double-density
probe arrangement and the image reconstruction algorithm using the spatial sensitivity profile can improve both the
spatial resolution and the reproducibility of the topographic image of brain activation in the virtual head phantom.
However, the uneven thickness of the superficial tissues affects the accuracy of the position of activation in the images.
The spatial resolution of current near-infrared topography is not enough for clinical application. In this study, the image reconstruction algorithm using prior knowledge about spatial sensitivity profile in the tissue and constraint of spatial frequency in image was proposed and was evaluated by simulation. The spatial resolution of topographic image obtained from the image reconstruction method is better than that obtained from the conventional mapping-interpolation method. The most appropriate cut-off frequency for the constraint for the image reconstruction method depends on the arrangement of fibres.
Multi-channel optical imaging system can obtain a topographical distribution of the activated region in the brain cortex by a simple mapping algorithm. Near-infrared light is strongly scattered in the head and the volume of tissue that contributes to the change in the optical signal detected with source-detector pair on the head surface is broadly distributed in the brain. This scattering effect results in poor resolution and contrast in the topographic image of the brain activity. We report theoretical investigations on the spatial resolution of the topographic imaging of the brain activity. The head model for the theoretical study consists of five layers that imitate the scalp, skull, subarachnoid space, gray matter and white matter. The light propagation in the head model is predicted by Monte Carlo simulation to obtain the spatial sensitivity profile for a source-detector pair. The source-detector pairs are one dimensionally arranged on the surface of the model and the distance between the adjoining source-detector pairs are varied from 4 mm to 32 mm. The change in detected intensity caused by the absorption change is obtained by Monte Carlo simulation. The position of absorption change is reconstructed by the conventional mapping algorithm and the reconstruction algorithm using the spatial sensitivity profiles. We discuss the effective interval between the source-detector pairs and the choice of reconstruction algorithms to improve the topographic images of brain activity.
A near infrared topographic system is an effective instrument for obtaining an image of brain activation. In the conventional mapping method, the signals detected with the source-detector pairs are simply mapped and interpolated to obtain the topographic image.
It is likely that an image reconstruction algorithm using a spatial
sensitivity profile will improve the spatial resolution of the topographic image. In this study, a one-dimensional distribution of the absorption change in the head model is calculated from the signals detected with various intervals of source-detector pairs by the conventional mapping method and an image reconstruction algorithm using the spatial sensitivity profile to evaluate the limit of spatial resolution of topographic imaging. Small intervals of the source-detector pairs improve the position of the absorption change in the topographic image calculated by both the conventional mapping method and the reconstruction algorithm. The size of the absorption change calculated from the intensity detected with a small interval of the source-detector pairs is sufficiently improved by the image reconstruction algorithm using the spatial sensitivity profile.
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