Near infrared topographic imaging is an effective instrument to image brain-cortex activity. The light scattering in tissue prevents us from improving the spatial resolution of the reconstructed image; hence it is important to evaluate the effect of scattering on the spatial resolution of the image. In this study, the light propagation in the adult head model was predicted by Monte Carlo simulation to investigate the effect of fiber arrangement on the spatial resolution
of NIR topographic imaging. The image of absorbers in the topographic images obtained from the double-density arrangement of source-detector pairs was compared with that from the conventional single-density arrangement. The double density arrangement improved the spatial resolution and accuracy of the position of the absorbers in the topographic image.
In medical instrumentation, optical topography (OT) refers to the use of near-infrared spectroscopy for measuring brain function in systems. Arrays of optical fibers are attached to the scalps of subjects; infrared light is passed through the fibers, and changes in the reflections depict blood-volume changes in the cortex. In this study, the spatial resolution and locational accuracy of topographical images obtained by three arrangements of optical fibers was analyzed through simulation. Three arrangements, a "lattice arrangement" (LA), "double-density arrangement" (DA), and "quadruple density arrangement" (QA) were investigated. The density of spatial-sapling points is higher in the DA and QA than in the LA, i.e. the distance between sampling points for these arrangements were 21, 15 and 11 mm, respectively. The efficacy of these arrangements was evaluated. An adult head-structure phantom was prepared. The absorption coefficient in the phantom was varied to simulate brain activation in the cortex, and the resulting absorbance change (ΔOD) was thus obtained. The 'activated' area in the overall measurement area was fixed and the ΔOD at each of the sampling point in each arrangement was obtained. The resulting distributions of ΔOD were spatially interpolated to obtain topographical images. The spatial resolution and locational accuracy was obtained for each of the images; the results indicated that the DA is the most efficacious of the three arrangements. An experimental DA-OT system was then built. Topographical images of motor-function activation obtained by this system and a commercial LA-based system were compared; the DA-OT system provided the higher spatial resolution.
The light propagation in a simple layered ellipse model and more complex neonatal head model are calculated by the finite-difference method. The finite-difference method has advantage of simple algorithm and fast calculation time however has been successful under restricted condition for a heterogeneous medium. The light propagation in the both models is predicted by Monte Carlo simulation to validate the results of the finite-difference method. The detected intensity and partial optical path length calculated by the finite-difference method agree with those by Monte Carlo simulation. The boundary of the grey and white matter in the neonatal head model is more complex than the simple ellipse model. However, the tendency of spatial sensitivity profiles in the neonatal head model is scarcely affected by the effect of heterogeneity of the brain tissue.
Authors proposed optical topography to visualize the blood- volume change in the cortex associated with brain activation. The diffusion equation was used to obtain the sensitivity distribution of the blood-volume change. We show that the sensitivity distribution agrees with that obtained by the Monte-Carlo simulation. We then developed a phantom that simulates the light scattering property and brain activation in the cortex. Topographic images of the absorber in the phantom are obtained, and high location accuracy but spatial resolution of the topographic image were found. In addition, we derived a methodology to arrange optodes at high density in order to improve the spatial resolution of the topographic image.
Optical tomography (OT) is a method for visualizing brain functions noninvasively. In an OT measurement system, near- infrared light, to which living tissue is highly permeable, is irradiated from the scale of the subject, and the scattered light reflected from the cerebral cortex is detected elsewhere on the scalp. The spatio-temporal blood volume change in the cortex is visualized based on the signal detected using two-dimensionally arranged optodes. The measurement imposes few constraints on the subject, either physically or mentally, thus the subject is in a natural and relaxed condition during measurement. Here we describe our OT system, then report on an experiment to evaluate the system using a phantom. We found that OT can accurately locate the activated region in the cortex. Also, as an example of a clinical application of OT, we used our system to measure the language function, demonstrating the system's ability to measure the activity of Broca's area.
A recently developed 24-channel optical topography (OT) system, that uses intensity-modulated near-infrared spectroscopy (NIRS) at wavelength of 780 nm and 830 nm can be used to visualize spatiotemporal changes of blood oxygenation states in human brain caused by cortical activity. We have used this system to estimate the hemodynamic changes during a language function task. To stimulate language function in the brain, we had a subject perform a writing task, which is more demanding than a speaking task. The subject, who was relaxed and sitting in a chair, was shown a card with a picture of an object for 3 seconds. During the 3 seconds, the subject wrote down the name of the object. The task was repeated 30 times, so the total stimulation period was 90 seconds. As a control task, the subject was shown figures with no meaning and drew each figure. The control task was performed as pre-stimulation for 60 seconds and as post-stimulation for 70 seconds, respectively. The subject rested for 30 seconds between post-stimulation and pre-stimulation. We observed a significant increase in the blood volume, which is proportional to the total change in the oxy-hemoglobin and deoxy-hemoglobin concentrations in the spatiotemporal topographic images in the area corresponding to Broca's area.
Optical topography (OT) was proposed in 1995 as a new imaging method for observing brain activity. It can bring out meaningful information such as spatio-temporal blood volume and oxygenation changes in cortices, and its facility can allow non-invasive measurement of human brain function under various conditions without restriction on the subjects. Researchers are beginning to use OT to investigate brain functions, and for clinical applications. In this paper, we introduce the evolution of the OT system and present a few results of brain functional measurement and clinical uses. The development of OT was advanced on 4 steps. The first generation system had only one probe with dual wavelengths. Therefore in order to reconstruct topographic images of cortical activity the probe had to be manually moved for 10 measurement positions. The second system was able to measure 12 positions, which were sequentially sampled by using a multiplexer and an optical switching device, in 6 seconds. The third system was developed to evaluate simultaneous measurement of 8 positions. At present, we finished the fourth OT system having 24 measurement channels with dual wavelengths. Using this system, we are performing dynamical observations of hemodynamic changes during brain activation and epileptic seizures.