Single Photon Counting System Research Articles

Analytical method for localizing a fluorescent inclusion in a turbid medium

We describe a novel method for localizing a fluorescent inclusion in a homogeneous turbid medium through the use of time-resolved techniques. Based on the calculation of the mean time of the fluorescence curves, the method does not require a priori knowledge of either the fluorescence lifetime or the mean time of the instrument response function since it adopts a differential processing approach. Theoretical expressions were validated and experiments for assessing the accuracy of localization were carried out on liquid optical phantoms with a small fluorescent inclusion. The illumination and detection optical fibers were immersed in the medium to achieve infinite medium geometry as required by the model used. The experimental setup consisted of a time-correlated single-photon counting system. Submillimeter accuracy was achieved for the localization of the inclusion.

Time-resolved diffuse optical tomography and its application to in vitro and in vivo imaging

This work reviews our research during the past several years on time-resolved (TR) near-infrared diffuse optical tomography (DOT). Following an introduction of the measuring modes, two proposed schemes of image reconstruction in TR-DOT are described: one utilizes the full TR data, and the other, referred to as the modified generalized pulse spectrum technique (GPST), uses the featured data extracted from the TR measurement. The performances of the two algorithms in quantitativeness and spatial resolution are comparatively investigated with 2-D simulated data. TR-DOT images are then presented for phantom experiments, which are obtained by using a 16-channel time-correlated single photon counting system, and the factors affecting the quantification of the reconstruction are discussed. Finally, in vitro and in vivo imaging examples are illustrated for validating the capibility of TR-DOT to provide not only the anatomical but also the physiological information of the objects.

Preliminary study of the advantages of X-ray energy selection in CT imaging

It is well known that a monochromatic X-ray source with an energy optimized for the organ thickness to be imaged could result in a better image quality in transmission radiology. In this paper we present the preliminary investigation for the implementation of this technique in computer tomography (CT) imaging. The detection system is based on a 1 mm thick silicon pixel detector bump bonded to a VLSI read-out, Medipix2. This detector ensures a good detection efficiency (46%) in the used energy range (60 kVp) with a good spatial resolution that arises from a 55 μm square pixel. The Medipix2 read-out electronics is not only a single photon counting system, but has also the capability of dual-energy threshold, that allows us to detect only photons that are in a chosen energy window. In this paper we present the results obtained in CT imaging of small samples, by selecting various energy windows within a standard X-ray tube spectrum so as to maximize the differentiation between significant attenuation coefficients. This study is preliminary for a future development of a dual-energy CT that could add functional information to the morphological information that is obtained in a CT examination.

Formation of Trititanate Nanotubes by Non‐Hydrothermal Methods: Optical Properties and Surface‐Exciton Dynamics Studied by Photoluminescence Spectroscopy

AbstractTrititanate (H2Ti3O7) nanotubes were prepared by a non‐hydrothermal digestion of TiO(OH)2 solution with 5 M NaOH, and their optical properties and exciton dynamics were studied by using steady‐state and time‐resolved photoluminescence (PL) spectroscopy. Based on TEM, AFM and XRD measurements, the tubular structure was identified to be the same as that of trititanate crystalline multi‐walled scroll nanotubes prepared hydrothermally, which has intershell d‐spacing of 0.79 nm with inner diameters of 5 nm and the lengths of several tens of nanometers. The UV‐absorption spectrum of trititanate nanotubes showed weak surface‐state absorption at 425 nm as well as the absorption maximum band around 350 nm with onset energy 3.03 eV corresponding to the indirect band gap energy. The steady‐state PL spectra of the trititanate nanotubes showed a band‐edge emission band around 360 nm as well as a broad surface emission band originating from widely dispersed surface‐state energy levels (2.84, 2.65, 2.41 2.21 2.03 eV), exhibiting higher relative intensity ratio of the band‐edge emission to the surface emission as compared to that of TiO2 nanoparticles. These results with PL excitation spectra indicate that the density of the surface states was found to be higher in the nanotubes than in TiO2 nanoparticles. The PL decay profiles were measured by using the time correlated single photon counting system (TCSPC) adopting fs‐Ti‐Sappire laser at 350 nm, and they were analyzed to fit a tri‐exponential equation. The decay times depend on monitoring emission wavelength, and at least four different decay time components (˜28 ps, ˜70 ps, ˜700 ps and ˜4 ns) were resolved, implying the existence of charge‐carriers trapping surface states with different energy levels. The two longer decay times are attributed to the deep‐trap surface states of trititanate nanotubes, which are much longer than those of TiO2 nanoparticles. These results suggest that the nonradiative route to the surface excitons recombination is more dominant in trititanate nanotubes than in titania nanoparticles because of strong coupling of excitons wave functions with lattice phonons.

蛍光寿命イメージングシステムの製作と生体試料および高分子試料への応用

We have constructed a fluorescence lifetime imaging (FLIM) system to study the photo-induced dynamics of living cells. The output from a femtosecond mode-locked Ti: sapphire laser is frequency-doubled and focused onto the sample with an objective lens of a confocal microscope. The fluorescence decay of the sample is measured at each pixel of a scanning image. Each fluorescence lifetime is evaluated quickly using time-gating electronics, and the fluorescence lifetime image can be obtained within several minutes. We have combined the FLIM system with a picosecond time-correlated single-photon counting system in which a microchannel-plate photomultiplier is used for detection, so that the value of the fluorescence lifetime at a position can be analyzed quantitatively. A high contrast light-scattering image of a polymer film is also found to be obtained with the FLIM system.

Performances of different digital mammography imaging systems: Evaluation and comparison

Digital mammography is entering routine clinical use and many commercial systems are now in use in the radiological units for screening and diagnostic mammography. At the same time, the research in the digital mammography field is very active in the development of more and more performing devices. In this paper we present the performance of single photon counting pixel detectors (MedipixI) tailored for digital mammographic applications. These devices are based on semiconductor (Si and GaAs) pixel detectors of different thickness, read-out by custom designed integrated circuits. To assess the imaging capability of such systems, the images of a mammographic phantom have been acquired in standard conditions for a clinical examination. The signal-to-noise ratio (SNR) of details simulating tumor masses has been evaluated. The same phantom has been also radiographed by three different commercial digital mammographic systems in the same reference conditions and a comparison in terms of SNR has been carried out. The spatial resolution of the single photon counting systems has also been evaluated by measuring the line spread function with the edge technique and then calculating the Modulation Transfer Function (MTF). The MTFs of the single photon counting systems have been compared with the MTFs of the commercial systems.

照射粉末食品の光ルミネセンス法による検知

Photostimulated luminescence (PSL) method provides a rapid and markedly sensitive technique to detect of any irradiated food that contain minerals. This method is also convenient as a first screening means in comparison with thermoluminescence (TL) method which requires separating minerals from the food materials. The present study reports the results of PSL measurements for both irradiated and non-irradiated powdered dry leaf vegetable obtained with the prototype of PSL system developed by our groups. This PSL system consists of pulsed Infra-Red (IR) source for photostimulation, single photon counting system for high sensitive detection of luminescence, and computer for data treatments. The PSL intensity of the leaf vegetable increased with increase of gamma radiation dose and show a linear relationship up to a dose of 1kGy. The PSL intensities after four months of storage under light shielding following gamma irradiation (0.49-3.1kGy) decreased to less than half of initial intensities. The PSL intensities of those decreased rapidly for 60 seconds and reached a steady level close to intensity of non-irradiated sample after 300 seconds following photostimulation. We used the information on the shape and slope of PSL intensity curve as a criteria to discriminate whether irradiated foods or not, and possible to detect the irradiation history of food irradiated with a 0.49kGy dose after four months of storage.

Excited-state proton-dissociation of N-alkylated anilinium ions in aqueous solution studied by picosecond fluorescence measurements

The proton-dissociation reactions from the excited state of protonated aniline (AN) and its derivatives in aqueous solution were investigated by using a femtosecond laser combined with a time-correlated single-photon counting system. The proton-dissociation rate of protonated AN at 298 K was obtained to be 1.4 × 1010 s−1, which is the fastest of those reported so far for protonated aromatic amines. The rate decreased markedly on alkylation of the amino group, being 6.2 × 109 s−1, 3.7 × 109 s−1, 7.6 × 108 s−1, and 4.4 × 107 s−1, for N-methylaniline (NMeA), N-ethylaniline (NEtA), N,N-dimethylaniline (DMA), and N,N-diethylaniline (DEA), respectively, despite the larger exothermic enthalpy changes for the proton dissociation of alkylated compounds. The activation energies for the proton-dissociation reactions of dialkylated anilines, DMA (19 kJ mol−1) and DEA (56 kJ mol−1), were much larger than those of the other anilines, AN (11 kJ mol−1), NMeA (8 kJ mol−1), and NEtA (11 kJ mol−1), suggesting that hydrophobic hydration around the alkyl amino group and steric hindrance due to the alkyl group cause an activation barrier for the movement of proton to water.

Emission properties of gossypol in solution

Steady-state emission properties of gossypol in acetonitrile at room temperature have been studied using a HPLC system equipped with a specially improved highly sensitive fluorescence detector ( Φ F⩾10 −7). The fluorescence lifetimes ( τ) have been determined using a time-correlated single photon counting system. At room temperature gossypol shows weak fluorescence with the overall quantum yield of Φ F=5×10 −5. On the basis of a comparison of the excitation emission spectra with the absorption spectrum and the radiative properties of gossypol, the origin of gossypol fluorescence from two tautomeric forms: aldehyde in S 1 (n, π ∗) (τ=300 ps) and S 2 ( π, π ∗) (τ=10 ps) and lactol in S 1 (n, π ∗) (τ=18 ps) state was suggested.

Internal conversion of o-aminoacetophenone in solution

The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a femtosecond laser–single photon counting system and time-resolved thermal lensing (TRTL) method. The fluorescence quantum yield (Φf) and lifetime (τf) of o-AAP depend strongly on the nature of the solvent. In nonpolar solvents, o-AAP gives very small Φf values (Φf = 2.4 × 10−4 in n-hexane) and remarkably short fluorescence lifetimes (τf = 9.4 ps in n-hexane), suggesting the presence of very fast nonradiative deactivation processes. The measurements of the quantum yield (Φisc) of intersystem crossing based on the energy transfer and TRTL methods clearly show that the fast radiationless processes in nonpolar solvents are due to internal conversion. In aprotic solvents, the rate (kic) of internal conversion for o-AAP decreases significantly with increasing solvent polarity (kic = 1.0 × 1011 s−1 in n-hexane, kic = 2.4 × 109 s−1 in acetonitrile). In protic solvents, the kic value tends to increase with an increase of hydrogen-bonding donor ability of the solvent. The internal conversion rate in aprotic solvents is scarcely affected by deuterium substitution of the NH2 group in o-AAP, while a large isotope effect is found for o-AAP in deuteriated protic solvents. It is concluded that the efficient S1 → S0 internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying 1(π,π*) and 1(n,π*) states (the proximity effect), and in protic solvents intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative process.

Measurements of the electric-field-induced change in fluorescence decay profile of a mixture of ethylcarbazole and dimethylterephthalate in a PMMA polymer film

Electric field effects on photoinduced intermolecular electron transfer between ethylcarbazole (ECz) and dimethylterephthalate (DMTP) doped in a PMMA film have been investigated by using a picosecond time-correlated single-photon counting system combined with a bipolar sample bias, which enables the direct measurements of the field-induced change in fluorescence decay profile. The difference of the lifetime in the presence and absence of applied electric fields was successfully measured. The obtained results show that the electron transfer rate is enhanced by an electric field in agreement with those by the steady-state fluorescence measurements.

Development of a high-quantum-efficiency single-photon counting system

A high-quantum-efficiency single-photon counting system has been developed. In this system, single photons were detected by a visible light photon counter operated at 6.9 K. The visible light photon counter is a solid state device that makes use of avalanches across a shallow impurity conduction band in silicon. Threefold tight shielding and viewports that worked as infrared blocking filters were used to eliminate the dark count caused by room-temperature radiation. Corrected quantum efficiencies as high as 88.2%±5% (at 694 nm) were observed, which we believe is the highest reported value for a single-photon detector. The dark count increased as the exponential of the quantum efficiency with changing temperature or bias voltage, and was 2.0×104 cps at the highest quantum efficiency.

Polymer–protein interactions in aqueous two phase systems: fluorescent studies of the partition behaviour of human serum albumin

This study describes the partitioning of fluorescent macromolecules in aqueous two-phase systems (ATPS) comprising phosphate salt and poly(ethylene glycol) of three different molecular masses (i.e. 1000, 1450 and 2000 Da). The impact of system assembly was studied with fluorescent macromolecules introduced in contact with either (i) first salt, then polymer or (ii) first polymer, then salt, or (iii) with both salt and polymer simultaneously. Native human serum albumin (HSA) and derivatives labelled with N-(iodoacetylaminoethyl)-5-naphthylamine-1-sulphonic acid (1,5-IAEDANS) were partitioned using selected ATPS. Partitioning behaviour was characterised by molecular rotational studies of recovered proteins based upon changes of depolarisation. Measurements were undertaken by steady-state fluorescence or time-decay fluorescence using a single-photon counting system. In addition, circular dichroism was used as a tool for the study of macromolecular secondary structure. Two discrete categories of stable molecular structure have been identified that exist irrespective of the phase environment. The findings form the basis for a discussion of polymer–protein interactions and the molecular micro-environment of proteins in ATPS.

Fast fluorescence decay of naphthalene induced by Ar ion irradiation

A single photon counting system was constructed for a time resolved fluorescence measurement with pulsed heavy ions from the AVF cyclotron in Takasaki Ion Accelerators for Advanced Radiation Application (TIARA). Fluorescence spectra and decay of naphthalene in poly(vinyl butylal) (PVB) film were observed from the irradiation with Ar ions. Though fluorescence spectra were assigned to the first singlet excited state of naphthalene, the decay with Ar ion irradiation were faster than that excited by UV light. This faster decay results from quenching by the transient species produced. The relative initial concentration of the excited state of naphthalene to that of the radicals increases with increasing energy deposited along the incident ion's trajectory.

Fluorescence lifetime imaging of experimental tumors in hematoporphyrin derivative-sensitized mice.

Tumor detection has been carried out in mice sensitized with hematoporphyrin derivative (HpD) by measuring the spatial distribution of the fluorescence lifetime of the exogenous compound. This result has been achieved using a time-gated video camera and a suitable mathematical processing that led to the so-called "lifetime images." Extensive experimental tests have been performed on mice bearing the MS-2 fibrosarcoma or the L1210 leukemia. Lifetime images of mice show that the fluorescence decay of HpD is appreciably slower in the tumor than in healthy tissues nearby, allowing a reliable detection of the neoplasia. The lengthening of the lifetime in tumors depends little on the drug dose, which in our experiments could be lowered down to 0.1 mg/kg body weight, still allowing a definite tumor detection. In order to ascertain the results achieved with the imaging apparatus, high-resolution spectroscopy, based on a time-correlated single photon counting system, has also been performed to measure the fluorescence lifetime of the drug inside the tumor and outside. The outcomes obtained with two techniques are in good agreement.

Time-resolved imaging on a realistic tissue phantom: μs′ and μa images versus time-integrated images

A method is proposed by which we construct images through turbid media, plotting directly either the transport-scattering coefficient μ(s) ' or the absorption coefficient μ(a). These optical parameters are obtained from the best fit of the time-resolved transmittance curves with a diffusion model. Measurements were performed with a time-correlated single-photon counting system on realistic tissue phantoms simulating a tumor mass within a breast. Images were obtained with an incident power of <1 mW and an acquisition time of 1 s/point. Comparison of μ(s) ' and μ(a) images with time-integrated images constructed from the same experimental data shows that the fitting method discriminates between scattering and absorption inhomogeneities and improves image quality for scattering but not for absorption inhomogeneities.

Picosecond Fluorescence of Merocyanines Derived from Photochromic Spironaphthoxazines

AbstractThe emission of the merocyanines derived from the photochromic 1, 3, 3‐trimethylspiro[indoline‐2,3′[3H]naphth[2,1‐b][1, 4]oxazine] 1, the 4, 5‐dimethyl‐substituted compound 2 and the 6′‐piperidino‐substituted derivative 3, in solution with acetonitrile or ethanol, was studied by means of a time‐correlated single photon counting system with ca 10 ps time resolution. The signal decays were described by a sum of two or three exponential components with τ1=10–60 ps, τ2= 0.5–1ns, τ3=1–2 ns, corresponding to the contribution of different fluorescing merocyanine isomers. The most intense emission, the shortest lived one, is characterized by extremely low quantum yields (&lt;10−6). The effect of the addition of LiClO4 to acetonitrile solutions of 2 is in agreement with the formation of ground‐state complexes with particular open isomers.

Picosecond Tryptophan Fluorescence of Human Blood Serum Orosomucoid

The state of three tryptophyl residues in human serum orosomucoid was estimated by prediction methods based on parameters characterizing their hydrophobicity either directly, or in terms of buried surfaces of the individual amino acid residues. It is shown that tryptophan 25 is the most buried, while Trp 160 is the most exposed to the solvent. Trp 122 is in this respect in an intermediate state. The fluorescence decay behaviour was determined using a picosecond single photon counting system. The multiwavelength data were analyzed using a global analysis as well as a distribution of lifetimes program. Both procedures yielded the existence of four wavelength independent lifetimes (0.22 ns, 1.0 ns, 2.5 ns, and 8.4 ns). A tentative assignment of the decay associated spectra of the four components to the three individual tryptophans is presented.

Fluorescence decay studies applying a CW femtosecond dye laser pumped ungated inverse time-correlated single photon counting system

An ungated inverse time-correlated single-photon counting system is described. The excitation source is a CW-pumped, linear prism-pair-stabilized, passive mode-locked femtosecond dye laser. The excitation pulse repetition rate is 120 MHz. The sample emission is detected with a microchannel plate photomultiplier tube. An iterative least-squares reconvolution method is applied to the data analysis. The fluorescence lifetimes, the reorientation times and the anisotropy factors of the dye solutions cresyl violet perchlorate and rhodamine 101 perchlorate in ethanol are determined.

An ultra-high counting rate 2D proportional counter for synchrotron radiation

Multiwire proportional counters are widely used in synchrotron radiation studies, for example in small angle diffraction experiments on biological systems. The active area and positional resolution of the detector can be conveniently matched to the dimensions of the synchrotron beam and to the optics of the beamline. Furthermore, the large dynamic range available from single photon counting systems is vital to exploit the true benefits of using synchrotron radiation for X-ray diffraction. The systems that are currently in routine use at the Daresbury SRS are based on the delay line method of position readout, a technique which inherently limits the count rate capability of the system to approximately 1 MHz. Large scale improvements in count rate performance require different readout systems which in general use many channels to improve throughput. A prototype high counting rate readout system has been constructed and commissioned. The system which is modular in nature and can therefore be tailored to particular applications, is described, and results are presented on its performance.