Abstract. Performance assessment of instruments devised for clinical applications is of key importance for validation and quality assurance. Two new protocols were developed and applied to facilitate the design and optimization of instruments for time-domain optical brain imaging within the European project nEUROPt. Here, we present the "Basic Instrumental Performance" protocol for direct measurement of relevant characteristics. Two tests are discussed in detail. First, the responsivity of the detection system is a measure of the overall efficiency to detect light emerging from tissue. For the related test, dedicated solid slab phantoms were developed and quantitatively spectrally characterized to provide sources of known radiance with nearly Lambertian angular characteristics. The responsivity of four time-domain optical brain imagers was found to be of the order of 0.1 m 2 sr. The relevance of the responsivity measure is demonstrated by simulations of diffuse reflectance as a function of source-detector separation and optical properties. Second, the temporal instrument response function (IRF) is a critically important factor in determining the performance of time-domain systems. Measurements of the IRF for various instruments were combined with simulations to illustrate the impact of the width and shape of the IRF on contrast for a deep absorption change mimicking brain activation. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
The nEUROPt protocol is one of two new protocols developed within the European project nEUROPt to characterize the performances of time-domain systems for optical imaging of the brain. It was applied in joint measurement campaigns to compare the various instruments and to assess the impact of technical improvements. This protocol addresses the characteristic of optical brain imaging to detect, localize, and quantify absorption changes in the brain. It was implemented with two types of inhomogeneous liquid phantoms based on Intralipid and India ink with well-defined optical properties. First, small black inclusions were used to mimic localized changes of the absorption coefficient. The position of the inclusions was varied in depth and lateral direction to investigate contrast and spatial resolution. Second, two-layered liquid phantoms with variable absorption coefficients were employed to study the quantification of layer-wide changes and, in particular, to determine depth selectivity, i.e., the ratio of sensitivities for deep and superficial absorption changes. We introduce the tests of the nEUROPt protocol and present examples of results obtained with different instruments and methods of data analysis. This protocol could be a useful step toward performance tests for future standards in diffuse optical imaging.
The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump-probe experiments. The vacuum ultraviolet ͑VUV͒ pump pulses with a pulse width of 350-450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive à state has been directly observed for the first time: D р20 fs. For molecular oxygen highly excited in the Schumann-Runge band, a decay time of 40Ϯ20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times D ϭ(1.9Ϯ0.1) and D ϭ(90Ϯ20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.
Abstract. We present results of a clinical study on bedside perfusion monitoring of the human brain by optical bolus tracking. We measure the kinetics of the contrast agent indocyanine green using time-domain near-IR spectroscopy (tdNIRS) in 10 patients suffering from acute unilateral ischemic stroke. In all patients, a delay of the bolus over the affected when compared to the unaffected hemisphere is found (mean: 1.5 s, range: 0.2 s to 5.2 s). A portable time-domain near-IR reflectometer is optimized and approved for clinical studies. Data analysis based on statistical moments of time-of-flight distributions of diffusely reflected photons enables high sensitivity to intracerebral changes in bolus kinetics. Since the second centralized moment, variance, is preferentially sensitive to deep absorption changes, it provides a suitable representation of the cerebral signals relevant for perfusion monitoring in stroke. We show that variance-based bolus tracking is also less susceptible to motion artifacts, which often occur in severely affected patients. We present data that clearly manifest the applicability of the tdNIRS approach to assess cerebral perfusion in acute stroke patients at the bedside. This may be of high relevance to its introduction as a monitoring tool on stroke units. C 2010 Society of Photo-Optical Instrumentation Engineers.
Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.
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