Photon Counting System Research Articles

Dynamics of Two-Color Two-Photon Excited Fluorescence of p-Terphenyl: Determination and Analysis of the Molecular Parameters

We present the experimental and theoretical study of the two-photon excited polarized fluorescence of p-terphenyl dissolved in cyclohexane/paraffin. The fluorescence was produced within a two-color two-photon (2C2P) excitation scheme utilizing simultaneous absorption of two femtosecond laser pulses at 400 nm and at 800 nm with the total excitation energy of 4.649 eV. The fluorescence was detected by a time correlated single photon counting (TCSPC) system with two detectors. Using different combinations of the absorbed photon polarizations we extracted seven time-dependent molecular parameters from experiment that contain all information on the dynamics of the three-photon process under study. The analysis of the obtained molecular parameter values was based on the ab initio calculations of the vertical excitation energies and transition matrix elements in p-terphenyl and allowed for determination of the whole structure of the two-photon absorption tensor, fluorescence lifetime, and the rotational correlation time. The obtained results imply that the fluorescence in the conditions of our experiment was governed mostly by the d(z) component of the fluorescence transition dipole moment that is parallel to the molecular long axis Z. The tensor was found to be symmetric. The two-photon excitation in p-terphenyl occurs simultaneously via two channels, one of them resulting in the population of the totally symmetric excited state and the other in the population of the nontotally symmetric excited state. Moreover, the energetically allowed pure electron transitions are dipole forbidden and become allowed by vibronic coupling.

A 6 × 8 photon-counting array detector system for fast and sensitive analysis of protein microarrays

Fluorescence measurements on protein microarrays demand photodetectors with ultra-high sensitivity (down to the single photon level), low noise and parallel readout capability. In the frame of the NANOSPAD FP6 European project, a novel compact instrument capable of ultra-sensitive analysis of multiple proteins in a customized microarray has been designed and implemented. In order to obtain a very compact system, a monolithic 6 × 8 array of single photon avalanche diodes (SPAD) was developed. The SPAD detector technology combines the general advantages of silicon microelectronic devices (miniature size, ruggedness and reliability, low-voltage and low-power operation, insensitivity to magnetic fields, etc.) with very high sensitivity, due to the ability to detect single photons with high efficiency and low detector noise. For performing measurements of chemiluminescent microarrays, around this monolithic detector all the necessary electronic and opto-mechanical set-up and the glass slide coating procedure for protein immobilization were developed. A viable methodology for specific detection of any protein is demonstrated by the NANOSPAD microarray system, suitable for application in the rapidly growing field of proteomic research.

Deeply trapped electrons in imaging plates and their utilization for extending the dynamic range

The absorption spectra of deep centers in an imaging plate (IP) made of BaFBr0:85I0:15:Eu2+ have been studied in the ultraviolet region. Electrons trapped in deep centers are considered to be the cause of unerasable and reappearing latent images in IPs over-irradiated with X-rays. Deep centers showed a dominant peak at around 320nm, followed by two small peaks at around 345 and 380nm.By utilizing deeply trapped electrons, we have attempted to extend the dynamic range of an IP. The IP was irradiated by 150-kV X-rays with doses from 8.07mGy to 80.7Gy. Reading out the latent image by the stimulation of Eu2+ luminescence with a 633-nm He–Ne laser light from a conventional Fuji reader showed a linear relationship with irradiated dose up to 0.8Gy, but then becoming non-linear. After fully erasing with visible light, unerasable latent images were read out using 635-nm semi-conductor laser light combined with a photon-counting detection system. The dose–response curve so obtained gave a further two orders of magnitude extending the dynamic range up to 80.7Gy. Comprehensive results indicate that electrons supplied from deep centers to the F centers provided the extended dynamic range after the F centers became saturated. Based on these facts, a model of the excitation of deeply trapped electrons and PSL processes is proposed.

Development and applications of “Mössbauer cameras”

Two different types of “Mossbauer camera” are presented in this review article. The first one uses an imaging technique for γ-rays which are converted to visual light by a Photon-counting Image Acquisition System (PIAS) combined with a Fiber Optic plate with CsI (Tl) Scintillator (FOS), while the second one is a mapping technique for γ-rays which are first focused by a Multi Capillary X-ray lens (MCX), and subsequently electrons and γ-rays are detected as function of the sample position by a micro-channel plate (MCP) and a Si-PIN detector, respectively. A mapping image of a silicon solar cell is shown as an example obtained by the latter system. We also report on the present status in developing a new combined lens for the mapping using a MCX and a Fresnel Zone Plate (FZP) in order to realize a submicron-meter spatial resolution in a laboratory.

Photophysical processes of energy conversion in thylakoid membranes of Chlamydomonas reinhardtii mutants D1-R323H, D1-R323D, and D1-R323L

Chlamydomonas reinhardtii mutants D1-R323H, D1-R323D, and D1-R323L showed elevated chlorophyll fluorescence yields, which increased with decline of oxygen evolving capacity. The extra step K ascribed to the disturbance of electron transport at the donor side of PS II was observed in OJIP kinetics measured in mutants with a PEA fluorometer. Fluorescence decay kinetics were recorded and analyzed in a pseudo-wild type (pWt) and in mutants of C. reinhardtii with a Becker and Hickl single photon counting system in pico- to nanosecond time range. The kinetics curves were fitted by three exponentials. The first one (rapid, with lifetime about 300 ps) reflects energy migration from antenna complex to the reaction center (RC) of photosystem II (PS II); the second component (600–700 ps) has been assigned to an electron transfer from P680 to QA, while the third one (slow, 3 ns) assumingly originates from charge recombination in the radical pair [P680+• Pheo−•] and/or from antenna complexes energetically disconnected from RC II. Mutants showed reduced contribution of the first component, whereas the yield of the second component increased due to slowing down of the electron transport to QA. The mutant D1-R323L with completely inactive oxygen evolving complex did not reveal rapid component at all, while its kinetics was approximated by two slow components with lifetimes of about 2 and 3 ns. These may be due to two reasons: a) disconnection between antennae complexes and RC II, and b) recombination in a radical pair [P680+• Pheo−•] under restricted electron transport to QA. The data obtained suggest that disturbance of oxygen evolving function in mutants may induce an upshift of the midpoint redox potential of QA/QA− couple causing limitation of electron transport at the acceptor side of PS II.

Simultaneous fluorescence yield and lifetime tomography from time-resolved transmittances of small-animal-sized phantom

There has been recently a considerable interest in simultaneously reconstructing yield and lifetime distributions of fluorescent imaging agents inside a bulky tissue, since combined monitoring of these two parameters provides a potential means of in vivo interrogating quantitative and environmental information of specific molecules, as well as accessing interactions among them. It is widely accepted that an advantageous way of accomplishing the task in the context of small-animal imaging is to use a time-domain (TD) modality. In this paper, we present a full three-dimensional, featured-data algorithm for TD diffuse fluorescence tomography, which inverts the Laplace-transformed TD coupled photon diffusion equations and employs a pair of real-domain transform-factors to effectively separate the fluorescent yield and lifetime parameters. By use of a specifically designed 16x16 channel time-correlated single photon counting system and a normalized Born formulation for the inversion, the proposed scheme in a transmission mode is experimentally validated to achieve simultaneous reconstruction of the fluorescent yield and lifetime distributions with reasonable accuracy.

Reference optical phantoms for diffuse optical spectroscopy Part 1 – Error analysis of a time resolved transmittance characterization method

Development, production quality control and calibration of optical tissue-mimicking phantoms require a convenient and robust characterization method with known absolute accuracy. We present a solid phantom characterization technique based on time resolved transmittance measurement of light through a relatively small phantom sample. The small size of the sample enables characterization of every material batch produced in a routine phantoms production. Time resolved transmittance data are pre-processed to correct for dark noise, sample thickness and instrument response function. Pre-processed data are then compared to a forward model based on the radiative transfer equation solved through Monte Carlo simulations accurately taking into account the finite geometry of the sample. The computational burden of the Monte-Carlo technique was alleviated by building a lookup table of pre-computed results and using interpolation to obtain modeled transmittance traces at intermediate values of the optical properties. Near perfect fit residuals are obtained with a fit window using all data above 1% of the maximum value of the time resolved transmittance trace. Absolute accuracy of the method is estimated through a thorough error analysis which takes into account the following contributions: measurement noise, system repeatability, instrument response function stability, sample thickness variation refractive index inaccuracy, time correlated single photon counting system time based inaccuracy and forward model inaccuracy. Two sigma absolute error estimates of 0.01 cm(-1) (11.3%) and 0.67 cm(-1) (6.8%) are obtained for the absorption coefficient and reduced scattering coefficient respectively.

Contrast‐enhanced spectral mammography with a photon‐counting detector

Spectral imaging is a method in medical x-ray imaging to extract information about the object constituents by the material-specific energy dependence of x-ray attenuation. The authors have investigated a photon-counting spectral imaging system with two energy bins for contrast-enhanced mammography. System optimization and the potential benefit compared to conventional non-energy-resolved absorption imaging was studied. A framework for system characterization was set up that included quantum and anatomical noise and a theoretical model of the system was benchmarked to phantom measurements. Optimal combination of the energy-resolved images corresponded approximately to minimization of the anatomical noise, which is commonly referred to as energy subtraction. In that case, an ideal-observer detectability index could be improved close to 50% compared to absorption imaging in the phantom study. Optimization with respect to the signal-to-quantum-noise ratio, commonly referred to as energy weighting, yielded only a minute improvement. In a simulation of a clinically more realistic case, spectral imaging was predicted to perform approximately 30% better than absorption imaging for an average glandularity breast with an average level of anatomical noise. For dense breast tissue and a high level of anatomical noise, however, a rise in detectability by a factor of 6 was predicted. Another approximately 70%-90% improvement was found to be within reach for an optimized system. Contrast-enhanced spectral mammography is feasible and beneficial with the current system, and there is room for additional improvements. Inclusion of anatomical noise is essential for optimizing spectral imaging systems.

Discriminator threshold selection logic to improve signal to noise ratio in photon counting

The discriminator threshold plays a vital role in photon counting technique used with low level light detection in lidars and bio-medical instruments. A low discriminator threshold gives high counts due to secondary emitted electrons and ground level (electrical) noise, while high threshold skips the photon pulses having lower amplitude. In both the cases the signal to noise ratio of photon counting system deteriorates. In this article the impact of improper selection of discriminator threshold on photon counting has been described. Here we introduce a simple technique by which the system (micro pulse lidar) selects the discriminator threshold at the set PMT voltage and assigns this value to the multi-channel scaler. The technique is applicable in diverse fields employing photon counting system.

An experimental investigation on two-dimensional shape-based diffuse optical tomography

A two-dimensional (2D) shape-based approach of image reconstruction using a boundary element method is developed for diffuse optical tomography (DOT). The experimental validation uses a four-channel time-correlated single photon counting (TCSPC) system for detection and an intensity data-type for image reconstruction. The optical and geometric parameters are simultaneously recovered using a difference imaging scheme. Results demonstrate that the proposed DOT modality is a promising methodology of in vivo reconstruction of the optical structures of tissues.

Influence of annealing temperature on the performance of Ge film and photon-counting imaging system

Compared with the traditional image intensifier with phosphor screen readout, the photon-counting imaging detector with charge induction readout is more beneficial in several aspects (e.g., good imaging properties and time resolution) to astronomy, reconnaissance, bioluminescence, and materials research. However, the annealing temperature during the tube-making process can affect the properties of the Ge film, and consequently impair the performance of the detector. Therefore, the influence of annealing temperature on Ge film and on the detector is studied in order to determine the crucial parameters. The Ge films are prepared on ceramic and quartz glass by the use of an electron gun. They are analyzed by scanning electron microscope (SEM), high-resistance meter, and X-ray diffraction (XRD). The results show that the optimum substrate and annealing temperature are ceramic plate and 250 ?, respectively.

退火对Ge薄膜及光子计数成像系统性能的影响

退火对Ge薄膜及光子计数成像系统性能的影响

One-dimensional photon counting detector with vernier position sensitive anode

One-dimensional photon counting detector with vernier position sensitive anode is reported. The decode principle and design of vernier position sensitive anode are described in detail. An ultraviolet photon counting system was built based on one-dimensional photon counting detector with vernier position sensitive anode. One-dimensional coordinates of single-photon events can be simultaneously measured when the detection system works in photon counting mode. The spatial distribution of pulse counts, which corresponds to one-dimensional spatial distribution of incident light intensity, was obtained. The resolution of this system is better than 100 mμ according to the resolution test. The detector can detect one-dimensional spatial distribution of the intensity of very weak particle flow such as high-energy photons, electrons and ions flow, so it can be used for deep space exploration, spectral measurement, high-energy physics and bio-luminescence detection.

Metabolitinduzierte Biolumineszenz zur Tumorprädiktion

Solid tumors show an altered metabolism with respect to glycolysis in comparison to normal tissue. Recently, the determination of different glycolytic metabolites for tumor diagnosis and therapeutic decision-making became the focus of interest for various research groups. In particular an increased lactate concentration in tumor tissue appears to be a predictor of an adverse prognosis. Imaging of induced bioluminescence in rapidly frozen tumor biopsies is an established technique for the detection of selected substances. In this method the metabolites of interest are biochemically linked to luciferases. A microscopic photon counting system registers the light intensity and after calibration reflects the concentration distribution of metabolites. In contrast to other methods it is possible to detect different metabolites from one specific area of tissue. Preliminary results of a pilot study on oral cancer patients suggest a prognostic impact in terms of high lactate concentrations being associated with poor survival. This technique could increase the validity and significance of tumor grading and might be supportive decision guidance for tumor therapy in the future.

Scalable multiplexed detector system for high-rate telecom-band single-photon detection

We present an actively multiplexed photon-counting detection system at telecom wavelengths that overcomes the difficulties of photon-counting at high rates. We find that for gated detectors, the heretofore unconsidered deadtime associated with the detector gate is a critical parameter, that limits the overall scalability of the scheme to just a few detectors. We propose and implement a new scheme that overcomes this problem and restores full scalability that allows an order of magnitude improvement with systems with as few as 4 detectors. When using just two multiplexed detectors, our experimental results show a 5x improvement over a single detector and a greater than 2x improvement over multiplexed schemes that do not consider gate deadtime.

Parameters to consider in the construction of fiber-optic biosensors as alternative bioanalytical tools

A biosensor is a self-contained, bionic, integrated device that includes a biorecognition element that can respond in a concentration-dependent manner to a biochemical species. This article represents different possible methods of immobilizing biorecognition element used in fiber-optic-based biosensors. The method is chosen according to the analyte and the biorecognition element that will be used to sense it. In this review, two different approaches are represented: one using live organisms (based on bioluminescence) and one using immune reagents (based on chemiluminescence). Whole-cell fiber-optic biosensors have internal Bragg grating information with the potential to analyze samples without any additives in most cases. It includes a photon-counting system to measure a low-light response produced by markers of recognition elements. A flow-through, fiber-optic-based bacterial monitoring system for online monitoring of toxic pollutants in water has been developed. This monitoring system has proved to be a useful next step in the development of a simple online, realtime sensor for relevant human toxicants in flowing water.

Vacuum ultraviolet laser-induced breakdown spectroscopy analysis of polymers

Laser-induced breakdown spectroscopy (LIBS) in the vacuum ultraviolet range (VUV, λ < 200 nm) is employed for the detection of trace elements in polyethylene (PE) that are difficult to detect in the UV/VIS range. For effective laser ablation of PE, we use a F 2 laser (wavelength λ = 157 nm) with a laser pulse length of 20 ns, a pulse energy up to 50 mJ, and pulse repetition rate of 10 Hz. The optical radiation of the laser-induced plasma is measured by a VUV spectrometer with detection range down to λ = 115 nm. A gated photon-counting system is used to acquire time-resolved spectra. From LIBS measurements of certified polymer reference materials, we obtained a limit of detection (LOD) of 50 µg/g for sulphur and 215 µg/g for zinc, respectively. The VUV LIBS spectra of PE are dominated by strong emission lines of neutral and ionized carbon atoms. From time-resolved measurements of the carbon line intensities, we determine the temporal evolution of the electronic plasma temperature, T e. For this, we use Saha–Boltzmann plots with the electron density in the plasma, N e, derived from the broadening of the hydrogen H-α line. With the parameters T e and N e, we calculate the intensity ratio of the atomic sulphur and carbon lines at 180.7 nm and at 175.2 nm, respectively. The calculated intensity ratios are in good agreement with the experimentally measured results.

Time-correlated single photon counting system and light-collection system for studying fluorescence emitters under high-vacuum conditions: Use of immersion objective and ionic liquid

In this paper, we introduce a time-correlated single photon counting system and an efficient light-collection system for studying fluorescence emitters under high-vacuum conditions; the latter system has an immersion objective, and an ionic liquid is used as a refractive index matching medium. The ionic liquid is positioned in high vacuum. The time-correlated single photon counting system has modified photomultiplier tubes that act as photon detectors. The light-collection system is designed to be simple, compact, and easy to use. In order to verify the performance of these systems, the optical properties of colloidal semiconductor nanocrystals (CdSe/ZnS) and fluorescent dye molecules are studied as examples.

High-resolution mosaic imaging with multifocal, multiphoton photon-counting microscopy

High-resolution mosaic imaging is performed for the first time to our knowledge with a multifocal, multiphoton, photon-counting imaging system. We present a novel design consisting of a home-built femtosecond Yb-doped KGdWO(4) laser with an optical multiplexer, which is coupled with a commercial Olympus IX-71 microscope frame. Photon counting is performed using single-element detectors and an inexpensive electronic demultiplexer and counters.

Cramér–Rao lower bound of basis image noise in multiple-energy x-ray imaging

We present an analytical method to compute the basis image noise in the context of multi-energy x-ray imaging based on the Cramér–Rao lower bound (CRLB). The proposed formalism extends the original idea of Alvarez and Macovski (1976 Phys. Med. Biol. 21 733) to estimate the noise in the photo-effect and Compton-effect basis images in the case of dual-energy imaging. It includes an arbitrary number of independent, spectrally distinct attenuation measurements and also goes beyond the two-dimensional decomposition of the attenuation, including, e.g., a contrast agent as a third basis material. To illustrate our method, we consider three simple applications. The first application is to study the influence of the exact values for the energy thresholds on the basis image noise for a binned photon-counting system. The second application relates to the same detector system as the first and is an investigation of the dependence of the basis image noise on the energy resolution of the detector system. Finally, the third application provides an example for the case of an energy-integrating detector: the aim is to optimize the front-scintillator layer thickness of a dual-crystal detector for dual-energy imaging. The CRLB is used to minimize the noise of a photo-effect/Compton-effect basis material decomposition.