Optimization of analyzer‐based imaging systems for minimal surface absorbed dose

Analyzer‐based imaging has improved tissue X‐ray imaging beyond what conventional radiography was able to achieve. The extent of the improvement is dependent on the crystal reflection used in the monochromator and analyzer combination, the imaging photon energy, the geometry of the sample and the imaging detector. These many factors determine the ability of the system to distinguish between various bone tissues or soft tissues with a specified statistical certainty between pixels in a counting detector before any image processing. The following discussion will detail changes in the required number of imaging photons and the resulting surface absorbed dose when the imaging variables are altered. The process whereby the optimal imaging parameters to deliver the minimum surface absorbed dose to a sample while obtaining a desired statistical certainty between sample materials for an arbitrary analyzer‐based imaging system will be described. Two‐component samples consisting of bone and soft tissue are discussed as an imaging test case. The two‐component approach will then be generalized for a multiple‐component sample.     

基于声透镜的三维光声成像技术

基于声透镜的光声成像系统中, 由样品的光声压分布等效样品的吸收分布, 进行光声像重建, 但之前的这种等效是一种近似, 理论上并不准确. 本文阐述了声透镜三维光声成像的基本原理, 揭示了声透镜像面上光声压信号的时间分布与样品轴向吸收分布之间的关系; 提出用积分法和希尔伯特变换提取光声信号瞬时值法, 解调样品吸收系数分布并重建光声像; 实验上, 对不同样品分别用积分法和希尔伯特变换法获取样品的吸收系数, 重建光声像的横向和轴向分辨率均约为1 mm, 实现了真正的三维快速光声成像.     

Influence of photon energy on the efficiency of photochemotherapy

It is found that when indotricarbocyanine dye in HeLa cells is exposed to photons with different energies the efficiency of cell damage is wavelength independent provided the photosensitizer absorbs the same number of photons per unit time. In vivo animal experiments with two strains of tumor show that when the wavelength of the irradiating light is increased (668, 740, and 780 nm) and the number of photons absorbed per unit time per unit volume of the tumors is held constant, the damage depth increases by a factor of 1.5 and 3, respectively. The observed changes are related both to differences in the in vivo tissue optical transmission with increasing wavelength and an increased local concentration of oxygen owing to photodissociation of oxyhemoglobin.     

Dual-modal photoacoustic/CT imaging system

Photoacoustic(PA) tomography(PAT) breaks the barrier for high-resolution optical imaging in a strong lightscattering medium, having a great potential for both clinical implementation and small animal studies. However,many organs and tissues lack enough PA contrast or even hinder the propagation of PA waves. Therefore, it is challenging to interpret pure PAT images, especially three-dimensional(3 D) PA images for deep tissues, without enough structural information. To overcome this limitation, in this study, we integrated PAT with X-ray computed tomography(CT) in a standalone system. PAT provides optical contrast and CT gives anatomical information. We performed agar, tissue phantom, and animal studies, and the results demonstrated that PAT/CT imaging systems can provide accurate spatial registration of important complementary contrasts.     

Role of entanglement in two-photon imaging

The use of entangled photons in an imaging system can exhibit effects that cannot be mimicked by any other two-photon source, whatever the strength of the correlations between the two photons. We consider a two-photon imaging system in which one photon is used to probe a remote (transmissive or scattering) object, while the other serves as a reference. We discuss the role of entanglement versus correlation in such a setting, and demonstrate that entanglement is a prerequisite for achieving distributed quantum imaging.     

多模态光声分子成像进展*

现代的各种医学影像术,如射线成像、CT、正电子发射(PET)、磁共振(MR)、超声(US)、荧光(FL)等都各具特色,并成功地应用于多种疾病的诊疗。但每种影像术都不能对生物组织做出完整的描述。由若干个成像技术组成的多模态成像技术,是获得组织更多信息的有效途径。光声(PA)成像是能提供组织的成分和功能信息的新成像技术。它不仅灵敏,可以对较深层的组织进行实时、快速、安全的成像,而且可以利用光声光热造影剂实施非侵入的光热靶向治疗。因此,与光声成像相结合的多模态分子成像是实现精准诊疗的重要技术途径。该文以手持US-PA探头的双模态成像系统,直径为1 mm的血管內窥镜US-PA成像系统,可同时用于术前和术中的US-PA-FL三模态成像系统,以及采用外磁场可操控的磁共振-光声光热分子造影剂、进行MR-PA成像引导的光热治疗技术为例,对多模态光声分子成像系统在医学诊断、手术和光热治疗方面的进展做简单介绍。     

Surface antiferromagnetic domain structures of NiO (0 0 1) studied using UV photoemission electron microscope

Direct observation of the antiferro (AF) magnetic domain structures of a NiO (0 0 1) surface is found to be possible using a spectroscopy photoelectron low-energy electron microscope (SPELEEM) and a commercial UV Hg excitation light source without using any polarizers. The principle is based on the magnetic linear dichroism (MLD) effect, where different domain contrasts are produced according to the relative angle between the antiferromagnetic axis and the linearly polarized light. The observed AF magnetic domain structures are strongly affected by both bulk AF magnetic domain structures and the stresses induced during the sample cleaving process. Moreover, the AF magnetic domain structures are found to be irreversible when the sample is heated to over its Néel temperature and then cooled. The possibility of imaging AF magnetic domain structures without using synchrotron radiation or a polarizer is attractive.     

Molecularly sensitive optical coherence tomography

Molecular contrast in optical coherence tomography (OCT) is demonstrated by use of coherent anti-Stokes Raman scattering (CARS) for molecular sensitivity. Femtosecond laser pulses are focused into a sample by use of a low-numerical-aperture lens to generate CARS photons, and the backreflected CARS signal is interferometrically measured. With the chemical selectivity provided by CARS and the advanced imaging capabilities of OCT, this technique may be useful for molecular contrast imaging in biological tissues. CARS can be generated and interferometrically measured over at least 600 microm of the depth of field of a low-numerical-aperture objective.     

Complete-angle projection diffuse optical tomography by use of early photons

We present the first, to our knowledge, experimental images of complex-shaped phantoms embedded in diffuse media by use of optical tomography. Imaging is based on a complete-angle projection tomographic technique that utilizes transmitted early photons. Results are contrasted with measurements obtained at later gates as well as pseudocontinuous-wave data. The scanning system developed employs noncontact illumination and detection technologies that allow for high spatial sampling of transmitted photons. Combining this system with complete-angle illumination is found to be an important strategy toward improved imaging performance, resulting in a better-posed inversion problem. The appropriateness of reconstruction algorithms similar to those employed in x-ray computed tomography are showcased, and suggestions for model improvements are provided.     

Incoherent coincidence imaging and its applicability in X-ray diffraction

Entangled-photon coincidence imaging is a method to nonlocally image an object by transmitting a pair of entangled photons through the object and a reference optical system, respectively. The image of the object can be extracted from the coincidence rate of these two photons. From a classical perspective, the image is proportional to the fourth-order correlation function of the wave field. Using classical statistical optics, we study a particular aspect of coincidence imaging with incoherent sources. As an application, we give a proposal to realize lensless Fourier-transform imaging, and discuss its applicability in x-ray diffraction.     

Nonlinear photoacoustic imaging dedicated to thermal-nonlinearity characterization

We proposed a nonlinear photoacoustic(PA) technique as a new imaging contrast mechanism for tissue thermalnonlinearity characterization. When a sine-modulated Gaussian temperature field is introduced by a laser beam, in view of the temperature dependence of the thermal diffusivity, the nonlinear PA effect occurs, which leads to the production of second-harmonic PA(SHPA) signals. By extracting the fundamental frequency PA and SHPA signal amplitudes of samples through the lock-in technique, a parameter that only reflects nonlinear thermal-diffusivity characteristics of the sample then can be obtained. The feasibility of the technique for thermal-nonlinearity characterization has been studied on phantom samples. In vitro biological tissues have been studied by this method to demonstrate its medical imaging capability,prefiguring great potential of this new method in medical imaging applications.     

Reversible reconstruction and crystallization of GeSe2 glass

Absorption of photons at low power levels below the band edge reversibly alters the molecular structure of GeSe₂ glass. Several stages of ordering, including the formation of microcrystallites, have been observed by Raman scattering. The absorbed photons systematically increase the chemical ordering of the glass by breaking like-atom bonds and replacing them with Ge---Se bonds.     

Lensless high-resolution photoacoustic imaging scanner for in vivo skin imaging

We previously launched a high-resolution photoacoustic (PA) imaging scanner based on a unique lensless design for in vivo skin imaging. The design, imaging algorithm and characteristics of the system are described in this paper. Neither an optical lens nor an acoustic lens is used in the system. In the imaging head, four sensor elements are arranged quadrilaterally, and by checking the phase differences for PA waves detected with these four sensors, a set of PA signals only originating from a chromophore located on the sensor center axis is extracted for constructing an image. A phantom study using a carbon fiber showed a depth-independent horizontal resolution of 84.0 ± 3.5 µm, and the scan direction-dependent variation of PA signals was about ± 20%. We then performed imaging of vasculature phantoms: patterns of red ink lines with widths of 100 or 200 μm formed in an acrylic block co-polymer. The patterns were visualized with high contrast, showing the capability for imaging arterioles and venues in the skin. Vasculatures in rat burn models and healthy human skin were also clearly visualized in vivo.     

A compressed sensing approach for enhancing infrared imaging resolution

This paper presents a novel approach for improving infrared imaging resolution by the use of Compressed Sensing (CS). Instead of sensing raw pixel data, the image sensor measures the compressed samples of the observed image through a coded aperture mask placed on the focal plane of the optical system, and then the image reconstruction can be conducted from these samples using an optimal algorithm. The resolution is determined by the size of the coded aperture mask other than that of the focal plane array (FPA). The attainable quality of the reconstructed image strongly depends on the choice of the coded aperture mode. Based on the framework of CS, we carefully design an optimum mask pattern and use a multiplexing scheme to achieve multiple samples. The gradient projection for sparse reconstruction (GPSR) algorithm is employed to recover the image. The mask radiation effect is discussed by theoretical analyses and numerical simulations. Experimental results are presented to show that the proposed method enhances infrared imaging resolution significantly and ensures imaging quality.     

Non-descanned versus descanned epifluorescence collection in two-photon microscopy: Experiments and Monte Carlo simulations

We report on the design, test and Monte Carlo simulations of a non-descanned (NDS) collection port that we compare to a descanned (DS) port implemented on the same confocal microscope to carry out two-photon excitation fluorescence (TPEF) imaging. Our optical concept provides compactness, a wide field of view to the NDS port and allows the usage of small-area photosensors. The collection efficiency of the NDS port was measured with respect to those of the DS port as function of the imaging depth within a tissue-like optical phantom, for two high numerical aperture objectives. A NDS-to-DS collection ratio as high as about 30 was found for an imaging depth of 500 μm, corresponding to four mean scattering paths of the collected photons within the turbid medium. Measurements were fully interpreted by Monte Carlo simulations of light scattering through the turbid medium and collection by the spatio-angular apertured DS and NDS ports. Comparison of XZ cross-sectional views of mice liver samples imaged with the two ports emphasized the advantage of our NDS device for imaging deeply inside biological samples using TPEF microscopy.     

Retrieval of multiple scattering contrast from x-ray analyzer-based imaging

We present a moment-based alternative approach to retrieve multiple scattering contrasts from x-ray analyzer-based imaging. By use of the properties of moments of convolutions, the multiple-image radiography approach is theoretically validated. Furthermore, higher order moments of the object scattering distribution, inaccessible in multiple-image radiography, are simultaneously provided by this alternative approach. It is experimentally demonstrated that the skew and kurtosis information related to the distribution of sub-pixel features within the object can be obtained from those complementary contrasts. Finally, the sensitivity of the retrieved multiple scattering contrasts is investigated experimentally. The finding that the sensitivity is inversely proportional to the square root of the detected photon number essentially indicates that the retrieval of moments with an order higher than two can be achieved without increasing exposure time or dose. The presented alternative approach provides an access to the exploitation of multiple scattering contrasts, which is expected to be useful in biomedical research, materials science, security screening, etc.     

Effects of thermal energy deposition on material ejection in poly(methyl methacrylate)

The effects of absorption of 7.9 and 5.0 eV photons by the polymer poly(methyl methacrylate) are studied using molecular dynamics simulations. By rapidly depositing a critical amount of thermal energy in the surface region (greater than 0.03 eV Å⁻³), a pressure wave is formed which causes spallation of the substrate. If there is only one photon absorbed per monomer unit of the polymer, the 7.9 eV photons can supply sufficient energy density to initiate ejection.     

"Two-Photon" coincidence imaging with a classical source

Coincidence imaging is a technique that extracts an image of a test system from the statistics of photons transmitted by a reference system when the two systems are illuminated by a source possessing appropriate correlations. It has recently been argued that quantum entangled sources are necessary for the implementation of this technique. We show that this technique does not require entanglement, and we provide an experimental demonstration of coincidence imaging using a classical source. We further find that any kind of coincidence imaging technique which uses a "bucket" detector in the test arm is incapable of imaging phase-only objects, whether a classical or quantum source is employed.     

On the theory of quantum efficiency in germanium

A system of functional integral equations describing multiple impact ionization in semiconductors due to hot electrons and holes generated by the absorption of photons is solved. Two relaxation processes, namely interband Auger transitions and phonon emission, are analyzed theoretically to explain the spectral dependence of the quantum efficiency of the internal photo-electric effect. The dependence of the quantum efficiency in Ge on the energy of photons absorbed is calculated and compared with experiment.     

Near-infrared quantum cutting in transparent nanostructured glass ceramics

Quantum cutting downconversion involving the emission of two near-infrared photons for each blue photon absorbed is realized in transparent glass ceramics with embedded Pr3+/Yb3+: beta-YF3 nanocrystals. On excitation of Pr3+ ions with a visible photon at 482 nm, Yb3+ ions emit two near-infrared photons at 976 nm through an efficient cooperative energy transfer from Pr3+ to Yb3+, with optimal quantum efficiency close to 200%. The development of the near-infrared quantum cutting transparent glass ceramic could open a route to enhance the energy efficiency of the silicon solar cell by converting one blue solar photon to two near-infrared ones.