Transmittance Geometry Research Articles

Non-Invasive Hemoglobin Assessment with NIR Imaging of Blood Vessels in Transmittance Geometry: Monte Carlo and Experimental Evaluation

Non-invasive methods for determining blood hemoglobin (Hb) concentration are urgently needed to avoid the painful and time-consuming process of invasive venous blood sampling. Many such methods rely on assessing the average attenuation of light over a tissue area where hemoglobin is the dominant chromophore, without separating those areas corresponding to vessels and bloodless tissue. In this study, we investigate whether it is possible to determine hemoglobin levels in the blood by assessing the changes in light intensity when passing through large vessels in comparison to adjacent tissues, using this as a Hb level predictor. Using Monte Carlo light transport modeling, we evaluate the accuracy of determining hemoglobin levels via light intensity contrast and vessel widths estimated in the transmittance illumination geometry and estimate the influence of physiologically significant parameters such as vessel depth, dermis vascularization, and melanin content in the epidermis on the blood Hb prediction error. The results show that physiological variations in tissue parameters limit the mean absolute error of this method to ~15 g/L for blood Hb levels varying in the 60–160 g/L range, which finding is also supported by experimental data obtained for volunteers with different total blood Hb levels that have been determined invasively. We believe the application of new approaches to the non-invasive assessment of Hb levels will lead to the creation of reliable and accurate devices that are applicable in point-of-care and clinical practice.

Breakthrough light harvesting in time-domain diffuse optics with 100 mm2 silicon photomultiplier

Time-Domain Diffuse Optics (TDDO) technologies are rapidly evolving towards compact and cheap devices, thus progressively reducing the gap with continuous-wave equivalent technologies while providing the additional advantages of higher information content and better depth penetration and selectivity. One of the key limitations for TDDO is the active area of microelectronic time-resolved single-photon detectors as it is not possible to efficiently focus Lambertian light exiting from highly scattering samples into small areas. Therefore, signal-to-noise ratio bottlenecks can be surpassed only by targeting a breakthrough in this direction. In this work we present the validation in laboratory settings of the largest detector (100 mm2 active area with 92% cells fill-factor) ever reported to our knowledge in TDDO applications. To objectively validate this device, we made use of established performance assessment protocols in the field of diffuse optics, which also define standard conditions enabling realistic benchmarking of novel devices. The detector demonstrated the largest light harvesting capability ever reported, capability to measure homogeneous optical properties in line with state-of-the-art devices, and superior depth penetration inside heterogeneous media (in reflectance geometry) with respect to all the previously reported technologies (i.e., >4 cm). The active area breakthrough permits to envisage the first TDDO measurements on different fruits in transmittance geometry, thus increasing the penetration depth with respect to the traditional reflectance approach and making it easier for a future evolution towards real time quality check of fruits on the conveyor belt.

Time-resolved diffuse optical tomography system using an accelerated inverse problem solver

A computationally efficient time-resolved diffuse optical tomography (TR-DOT) prototype was demonstrated using an accelerated inverse problem solver to reconstruct high quality 3D images of highly scattering media such as tissues. The inverse problem solver utilizes seven well-defined points on each experimentally recorded histogram of the distribution time-of-flight (DToF). In this work, the accuracy of the recovered optical properties, and the computational load and time of TR-DOT prototype were investigated using cylindrical turbid phantoms. These phantoms were measured using transmittance geometry under different conditions in multiple experiments to evaluate the performance of this prototype. Overall, the results of evaluation are important in the realization of a real-time and highly accurate TR-DOT system for diffuse optical imaging applications.

Non-Contact Inclusion Detection in Food Through a Single-Photon Time-of-Flight Imager

We present a non-contact and non-invasive diffuse optical imaging system, based on an array of $32 \times 32$ single-photon avalanche diodes and time-to-digital converters, able to measure the time-of-flight of single photons, with 350-ps resolution. Measurements using continuous-wave (CW) and time-domain (TD) techniques demonstrate the possibility to identify the presence of absorbing inclusions (not visible by eye because of high scattering) within a diffusive media in transmittance geometry, with no contact with the sample. Thanks to the 1024 fully independent and concurrent time-resolved detection points, TD acquisitions achieve better resolution than CW ones, with the same measurement time. Provided that the investigated medium presents adequate thickness and transmissivity at the wavelengths of interest in order to allow sufficient light penetration, the system can be employed as a quick imaging device in food industries for non-destructive quality control

Time-resolved diffuse optical tomography with patterned-light illumination and detection

This investigation explores the feasibility of performing diffuse optical tomography based on time-domain wide-field illumination and detection strategies. Wide-field patterned excitation and detection schemes are investigated in transmittance geometry with time-gated detection channels. A Monte Carlo forward model is employed to compute the time-resolved Jacobians for rigorous light propagation modeling. We demonstrate both in silico and experimentally that reconstructions of absorption structures based on wide-field patterned-light strategies are feasible and outperform classical point excitation schemes for similar data set sizes. Moreover, we demonstrate that time-domain information is retained even though large spatial areas are illuminated. The enhanced time-domain data set allows for quantitative three-dimensional imaging in thick tissue based on relatively small data sets associated with much shorter acquisition times.

In Vivo Measurement of Vascular Modulation in Experimental Tumors Using a Fluorescent Contrast Agent

We compared the effectiveness of three optical techniques based on fluorescence imaging and spectroscopy with indocyanine green (ICG) contrast agent to evaluate in vivo the disruption of the active vasculature induced by a vascular targeting agent. The blood perfusion of the MDA-MB-435 tumor model transplanted in nude mice was estimated from the signal of the contrast agent measured immediately after its systemic injection in mice. Optical measurements were performed using a fluorescence imaging setup and a fiber-based time correlated single photon counting (TCSPC) apparatus. This latter apparatus was used to measure the tumor fluorescence in transmittance geometry and the change in the basal optical absorption induced by the contrast agent, thus providing an alternative estimation of the blood content in the tumor. Mice were divided into four groups. Three groups were treated with different doses of the vascular disrupting agent ZD6126, the fourth group (control group) received the drug vehicle only. Optical measurements were carried out 3 h after pharmacologic treatment. After 24 h, mice were killed, tumors were excised and the extent of necrosis was evaluated with standard histologic analysis. On fluorescence imaging ICG emission from tumors of mice treated with ZD6126 significantly was lower compared with the emission from control mice. The histologic sections also showed a significantly higher amount of necrosis in tumors of treated mice. Both these findings, which correlate with each other, indicate an effective vascular shutdown induced by the drug. However, ICG fluorescence measured with the TCSPC apparatus in transmittance geometry and the estimate of the change in optical absorption did not allow a statistically significant differentiation between treated and control groups.

Physisorption of Small Molecules on Single Crystal Alkali Halide Surfaces: CO and N2 Adsorbed on KCl(100)

Abstract The adsorption of CO and N2 on KCl(100) single crystal cleavage planes has been investigated by means of low energy electron diffraction (LEED) at primary currents in the nA range as well as polarization infrared spectroscopy (PIRS) in transmittance geometry. These isoelectronic adsorbates behave very similarly, and three different adsorption phases can be distinguished as a function of surface coverage for both of them. Initially a (1×1) structure is observed and assigned to a commensurate monolayer with one molecule per KCl ion pair. The CO infrared spectrum of this ‘phase I’ is characterized by a doublet absorption of the CO stretching vibration, which is discussed in the context of a correlation field splitting. The lack of superstructure diffraction peaks is attributed to a high degree of orientational disorder in this phase. From LEED adsorption isotherms the isosteric heat of adsorption of N2 has been determined to be 11±3 kJ mol−1. Upon increase in coverage by 50% ‘phase II’ is formed which exhibits a large number of additional diffraction spots. It is assigned to a bilayer in which the second layer is only half filled. Based on the LEED experiments a structure model is proposed in which the second layer is characterized by growth in rows along the [210] direction and a high degree of roughness in the perpendicular direction. This model can explain all experimentally observed main features. Finally upon further exposure formation of three-dimensional solid is observed, which proceeds via Stransky–Krastanov growth of crystalline 3D clusters with the structure of the low-temperature cubic α-phase. A simulation of the CO cluster infrared spectra within the dipole–dipole coupling approach can reproduce all major observed vibrational features.

Degenerate four-wave-mixing spectroscopy on epitaxially laterally overgrown GaN: Signals from below the fundamental absorption edge

We performed photoluminescence (PL) and degenerate four-wave-mixing (FWM) spectroscopy in epitaxially laterally overgrown GaN at 10 K. Optical transitions based on exciton complexes such as biexciton emission, exciton–exciton scattering, electron–hole plasma, and so on, were revealed by PL under a wide range of excitation densities. The FWM signals were observed from states below the fundamental excitonic absorption edge, showing that nonlinear photoswitching can be performed with a transmittance geometry. The origin of such nonlinearlity was discussed by correlating with feasible many-body effects between excitons, biexcitons, and free carriers.

Time-correlated single photon counting imager for simultaneous magnetic resonance and near-infrared mammography

We describe a near infrared (NIR) imager for mammography, designed to work simultaneously with a magnetic resonance (MR) scanner. The imager employs two pulsing laser diodes, with average power of 25 μW, at 780 and 830 nm. The two wavelengths are time multiplexed into 24 source fibers. The detection part consists of eight parallel time-correlated photon-counting channels with overall counting capacity of 106 photons/s. We use long optical fibers to avoid interference with the magnetic field. Specially designed coupling plates, for breast soft compression, bear both the MR radio-frequency coils and the optical source and detector fibers. Capillaries containing water and copper sulfate mark the position of the plates on the MR images for accurate coregistration of NIR and MR images. Instrument compatibility has been successfully tested with volunteers in the MR scanner. The use of gallium arsenide photomultiplier tubes has allowed penetration depths of 10 cm in the human breast. Imaging algorithms, based on the analytical modeling of photon propagation in inhomogeneous media, have been applied successfully to image 0.8-mm-diam absorbing and scattering cylindrical perturbations in transmittance geometry of breast-like phantoms.

Near-IR Spectroscopic Examination of Thin-Layer Chromatography Plates in the Diffuse Transmittance Mode

The nondestructive detection of analyte spots on high-performance thin-layer chromatography (HPTLC) plates is demonstrated with the use of near-IR spectrometry (NIRS). The resulting hyphenated technique is called Thin-Layer Chromatography-Near-Infrared Spectrometry (TLC-NIRS). A transmittance geometry is employed, and a set of 18 interference filters is used for wavelength selectivity. Plate-thickness nonuniformity and varying amounts of water vapor adsorption onto the silica-gel TLC plates were found to be the major complications. Variations in silica-gel thickness could be compensated for by obtaining reference spectral scans of the HPTLC plates before performing the chromatography. The effects of water-vapor adsorption were reduced through use of a partial least-squares calibration model. Detection limits near 1 μg were obtained.

The Near Infrared Determination of Moisture in Whole Dates

Whole dates (Phoenix dactylifera L.) were analyzed for moisture content using near infrared spectrophotometry in a direct transmittance geometry. In the calibration experiment using 72 samples, the correlation coefficient was 0.977 and the standard error of calibration (SEC) was 0.89%. When the calibration equation was used to predict the moisture in another set of 72 date samples, the standard error of performance (SEP) was 1.5%. When the method was used to sort these 72 dates into four industry-standard grades, 74% were correctly graded and 15% missed the grade by <1 SEC.

NEAR-INFRARED SPECTROPHOTOMETRY FOR MEASUREMENT OF SOLUBLE SOLIDS IN INTACT HONEYDEW MELONS

A near-infrared spectrophotometric method for estimating the soluble solids in honeydew melons is presented. The method is based on a body transmittance geometry in which the angle between the source incident beam and the detector is approximately 45°. The regression analysis of the spectral and chemical data utilizes a ratio of two second derivatives and resulted in a correlation coefficient of 0.85 and a standard error of calibration of 1.5. The numerator wavelength occurs in a carbohydrate absorption band, thus the method can be interpreted as a measurement of carbohydrates.