Near-Infrared Spectroscopy (NIRS) is a non-invasive technique, extensively used to monitor the hemodynamic variations in cerebral neuronal tissues. For cerebral NIRS, the back-scattering probe is more prevailing, in which an incident beam is diffused, and only a slight fraction of the source optical energy reaches the light detectors. Multiplexing in the time domain is the conventional method used to distinguish the optical density of each NIR source at the receiver site. Even though time-multiplexing is straightforward and convenient, the ambient light can significantly contaminate the NIR beams during the sampling-path from the source to the detector. In this work, we present a novel method based on frequency division multiplexing (FDM) to overcome the interference of ambient light even without an external optical filter. The method proposes to modulate the NIR source intensities by using specific carrier frequencies distinct from the dominant frequency components of ambient light intensity. By modulating the intensity of each NIR source, and applying them at their specific frequency channels, the receiver is capable of distinguishing the received optical signals based on their frequency channel. Because the frequency channels are adjusted at distinct dominant frequency components of the ambient intensity, the latter ambient noise can be filtered out instantly. The method has been implemented by using electronic circuit design and evaluated both by numerical simulation and experimental measurements. The signal to noise ratio (SNR) has been improved at least by 45dB.
According to the WHO, 15,000 children under five years are dying every day from preventable causes with 80% of these children being born in low-income countries. Portable optical medical diagnostic devices can help physicians, nurses and untrained health workers to objectively identify children who are at a higher risk of dying. In the last 2 years, we collected the oxygenation values of the brachioradialis muscle, using a commercial Near Infrared Spectroscopy (NIRS) device, in 200 children under 5 years admitted in two hospitals in Uganda. Data revealed that the tissue oxygen saturation decrease during a vascular occlusion predicts children at higher risk better than other vital signs (SpO2, respiration rate, heart rate and temperature). Based on these results, we designed a low cost Continuous Wave Spatially Resolved NIRS device controlled by a smartphone in order to extend our study to a larger population and confirm our observation. The total cost of this device (excluding the smartphone) is less than $100. The preliminary tests suggest a significant potential of our low cost mobile NIRS device and oxygenation values closely matching those reported by the best device on the market.
Background: ‘Tisseel’ (Baxter Healthcare, Deerfield, IL) is a fibrin-based sealant that is commonly used during spine surgery to augment dural repairs. We wish to intra-operatively secure a near infrared spectroscopy (NIRS) sensor to the dura in order to monitor the tissue hemodynamics of the underlying spinal cord. To determine if ‘Tisseel’ sealant adversely attenuates NIR photon transmission. Methods: We investigated ‘Tisseel’ in both an in vitro and in vivo paradigm. For in vitro testing, we used a 1 mm pathlength cuvette containing either air or ‘Tisseel’ interposed between a NIR light source (760 and 850 nm) and a photodiode detector and compared transmittance. For in vivo testing, a continuous wave (760 and 850 nm) spatiallyresolved NIRS device was placed over the triceps muscle using either conventional skin apposition (overlying adhesive bandage) or bioadhesion with ‘Tisseel’. Raw optical data and tissue saturation index (TSI%) collected at rest were compared. Results: In-vitro NIR light absorption by ‘Tisseel’ was very high, with transmittance reduced by 95% compared to air. In-vivo muscle TSI% values were 80% with conventional attachment and 20% using fibrin glue. Conclusion: The optical properties of ‘Tisseel’ significantly attenuate NIR light during in-vitro transmittance and critically compromise photon transmission in-vivo.
Functional Near Infrared Spectroscopy (fNIRS) uses near infrared sources and detectors to measure changes in
absorption due to neurovascular dynamics in response to brain activation. The use of Silicon Photomultipliers (SiPMs) in
a fNIRS system has been estimated potentially able to increase the spatial resolution. Dedicated SiPM sensors have been
designed and fabricated by using an optimized process. Electrical and optical characterizations are presented. The design
and implementation of a portable fNIRS embedded system, hosting up to 64 IR-LED sources and 128 SiPM sensors, has
been carried out. The system has been based on a scalable architecture whose elementary leaf is a flexible board with 16
SiPMs and 4 couples of LEDs each operating at two wavelengths. An ARM based microcontroller has been joined with a
multiplexing interface, able to control power supply for the LEDs and collect data from the SiPMs in a time-sharing
fashion and with configurable temporal slots. The system will be validated by using a phantom made by materials of
different scattering and absorption indices layered to mimic a human head. A preliminary characterization of the optical
properties of the single material composing the phantom has been performed using the SiPM in the diffuse radial
reflectance measurement technique. The first obtained results confirm the high sensitivity of such kind of detector in the
detection of weak light signal even at large distance between the light source and the detector.
The Silicon Photomultiplier (SiPM) is a novel pixelated photon detector able to detect single photon arrival with good timing resolution and high gain. In this work we present a complete study of the performances of different SiPMs produced by STMicroelectronics. Their potentialities and limits were identified using experimental measurements and electrical simulations performed both on single pixel and SiPM having up to ∼4000 pixels. SiPM was tested in different experiments such as photoluminescence emission measurement, lifetime measurement of semiconductor materials and light diffusion in highly scattering material in the near infrared spectrum showing its aptitude to replace PMT in those applications.
KEYWORDS: Luminescence, Silicon photomultipliers, Sensors, Biosensors, Absorption, Single photon, Signal to noise ratio, Temperature metrology, Pulsed laser operation, Monochromators
A novel Si-based detector, having a low noise and a high sensitivity, up to a single photon detection, was used for a
biosensor application. It is a Silicon photomultiplier (SiPM), a device formed by avalanche diodes operating in Geiger
mode, in parallel connections. Arrays with different dimensions were electro-optically characterized (5×5; 10×10 and
20×20 pixels) in order to identify the best geometry to be used in terms of signal-to-noise ratio, for our purposes. The
SiPM array was used to study both traditional and innovative fluorophores. CY5 was chosen as “reference” marker. It
has an absorption peak at 649 nm and an emission peak at 670 nm. Ru(bpy)3[Cl]2 was identified as innovative fluorophore, since it has absorption and emission peaks at 455 and 630 nm, respectively. Measurements were carried out in both functional regimes: continuous and pulsed. Emission spectra in the range 550–750 nm were measured with both traditional photomultipliers tubes (PMT) and SiPM operating at room temperature in continuous mode. More
interestingly, fluorophore lifetimes were monitored showing that SiPM can measure lifetimes as short at 1 ns (CY5
lifetime), well below the lowest PMT limit (23 ns). Ru(bpy)3[Cl]2 lifetime characterization was performed with both
PMT and SiPM (being in the hundreds of ns range), as a function of the solvent and after deposition and drying on glass
substrates.
KEYWORDS: Silicon, Temperature metrology, Sensors, Optical testing, Quenching (fluorescence), Diodes, Data modeling, Single photon, Silicon photomultipliers, Surgery
We electrically and optically tested both single pixels and complete arrays of Silicon Photomultipliers, from 5×5 to
64x64, fabricated by STMicroelectronics. Single cell devices operation was studied as a function of the temperature from
-25°C to 65°C varying the voltage over breakdown, from 5% up to 20% of the breakdown voltage. Optical
characterization was performed using a laser at 659 nm and opportunely chosen filters to vary the optical power. We
determined the single pixel gain by using both the time resolved dark count signal and the current under controlled
illumination. Typical gain values above 1×105 and above were obtained for operation times of 10 ns, while higher gains
are obtained for longer integration times and lower photon flux.
Single photon Si detectors were fabricated by STMicroelectronics and fully characterized in standard operation
conditions and after irradiations. Both single cells and arrays, of dimensions ranging from 5x5 up to 64x64, were
electrically tested. The devices operation was studied as a function of the temperature from -25°C to 65°C varying the
voltage over breakdown, from 5% up to 20% of the breakdown voltage before and after irradiation using both light ions,
10 MeV B ions to doses in the range 3×107 - 5×1010 cm-2, and X-rays irradiations in the range 0.5 - 20 krad(Si). Optical
characterization was performed using a laser at 659 nm and opportunely chosen filters to vary the optical power. A
strong difference in the radiation damage effect is observed for the two different irradiation sources. Ion irradiation, or
better implantation, produces a damage preferentially sitting in the active device region, hence even at the lowest
irradiation dose the device functionality is compromised, while at the highest dose the device is completely blind. On the
other hand, X-rays produce damage in a low concentration, in fact it does not significantly affect the device dark current,
only an increase in the leakage current under breakdown is observed. Hence the device functionality is preserved to
doses up to 20 Krad(Si).
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