Showing papers on "Point spread function published in 1996"

A regularization approach to joint blur identification and image restoration

Abstract: The primary difficulty with blind image restoration, or joint blur identification and image restoration, is insufficient information. This calls for proper incorporation of a priori knowledge about the image and the point-spread function (PSF). A well-known space-adaptive regularization method for image restoration is extended to address this problem. This new method effectively utilizes, among others, the piecewise smoothness of both the image and the PSF. It attempts to minimize a cost function consisting of a restoration error measure and two regularization terms (one for the image and the other for the blur) subject to other hard constraints. A scale problem inherent to the cost function is identified, which, if not properly treated, may hinder the minimization/blind restoration process. Alternating minimization is proposed to solve this problem so that algorithmic efficiency as well as simplicity is significantly increased. Two implementations of alternating minimization based on steepest descent and conjugate gradient methods are presented. Good performance is observed with numerically and photographically blurred images, even though no stringent assumptions about the structure of the underlying blur operator is made.

Target detection in optically scattering media by polarization-difference imaging

Abstract: Polarization-difference imaging (PDI) was recently presented by us as a method of imaging through scattering media [Opt. Lett. 20, 608 (1995)]. Here, PDI is compared with conventional, polarizationblind imaging systems under a variety of conditions not previously studied. Through visual and numerical comparison of polarization-difference and polarization-sum images of metallic targets suspended in scattering media, target features initially visible in both types of images are shown to disappear in polarization-sum images as the scatterer concentration is increased, whereas these features remain visible in polarization-difference images. Target features producing an observed degree of linear polarization of less than 1% are visible in polarization-difference images. The ability of PDI to suppress partially polarized background variations selectively is demonstrated, and discrimination of target features on the basis of polarization information is discussed. Our results show that, when compared with conventional imaging, PDI yields a factor of 2-3 increase in the distance at which certain target features can be detected.

Expanding the Realm of Microlensing Surveys with Difference Image Photometry

Abstract: We present a new technique for monitoring microlensing activity even in highly crowded fields, and use this technique to place limits on low-mass MACHOs in the haloes of M31 and the Galaxy. Unlike present Galactic microlensing surveys, we employ a technique in which a large fraction of the stellar sample is compressed into a single CCD field, rather than spread out in a way requiring many different telescope pointings. We implement the suggestion by Crotts (1992) that crowded fields can be monitored by searching for changes in flux of variable objects by subtracting images of the same field, taken in time sequence, positionally registered, photometrically normalized, then subtracted from one another (or a sequence average). The present work tackles the most difficult part of this task, the adjustment of the point spread function among images in the sequence so that seeing variations play an insignificant role in determining the residual after subtraction. The interesting signal following this process consists of positive and negative point sources due to variable sources. The measurement of changes in flux determined in this way we dub "difference image photometry" (also called "pixel lensing" [Gould 1996]). - The matching of the image point spread function (PSF) is accomplished by a division of PSFs in Fourier space to produce a convolution kernel, in a manner explored for other reasons by Phillips & Davis (1995). In practice, we find the application of this method is difficult in a typical telescope and wide field imaging camera due to a subtle interplay between the spatial variation of the PSF associated with the optical design and the inevitable time variability of the telescope focus. Such effects lead to complexities...(abstract continues)

Expanding the Realm of Microlensing Surveys with Difference Image Photometry

Abstract: We present a new technique for monitoring microlensing activity even in highly crowded fields, and use this technique to place limits on low-mass MACHOs in the haloes of M31 and the Galaxy. Unlike present Galactic microlensing surveys, we employ a technique in which a large fraction of the stellar sample is compressed into a single CCD field, rather than spread out in a way requiring many different telescope pointings. We implement the suggestion by Crotts (1992) that crowded fields can be monitored by searching for changes in flux of variable objects by subtracting images of the same field, taken in time sequence, positionally registered, photometrically normalized, then subtracted from one another (or a sequence average). The present work tackles the most difficult part of this task, the adjustment of the point spread function among images in the sequence so that seeing variations play an insignificant role in determining the residual after subtraction. The interesting signal following this process consists of positive and negative point sources due to variable sources. The measurement of changes in flux determined in this way we dub "difference image photometry" (also called "pixel lensing" [Gould 1996]). - The matching of the image point spread function (PSF) is accomplished by a division of PSFs in Fourier space to produce a convolution kernel, in a manner explored for other reasons by Phillips & Davis (1995). In practice, we find the application of this method is difficult in a typical telescope and wide field imaging camera due to a subtle interplay between the spatial variation of the PSF associated with the optical design and the inevitable time variability of the telescope focus. Such effects lead to complexities...(abstract continues)

System for creating a high resolution image from a sequence of lower resolution motion images

Abstract: A robust system, adaptive to motion estimation accuracy, for creating a high resolution image from a sequence of lower resolution motion images produces a mapping transformation for each low resolution image to map pixels in each low resolution image into locations in the high resolution image. A combined point spread function (PSF) is computed for each pixel in each lower resolution image employing the mapping transformations provided that they describe accurate motion vectors. The high resolution image is generated from the lower resolution images employing the combined PSF's by projection onto convex sets (POCS), where sets and associated projections are defined only for those pixels whose motion vector estimates are accurate.

Aberration control in quantitative imaging of botanical specimens by multidimensional fluorescence microscopy

Abstract: Three dimensional (3-D) fluorescence microscopes, including conventional instruments with digital deblurring, confocal systems and two-photon excitation, all exhibit monochromatic and chromatic aberrations. A simple Gaussian model of the aberrated point spread function and optical field for each instrument illustrates spatial distortions and blurring and some unique attenuation effects in confocal and two-photon microscopy. These properties depend on the manner in which illumination and detection combine to give the overall microscope performance and highlight the importance of both optics and sample aberrations; the specimen must be considered as an optical component of the integrated imaging system. Axial focus distortion, from refractive index boundaries at the sample, can be accurately modelled by a geometric ray tracing programme, considering weighted components across the entire objective lens working NA. The modal z-focus error, rather than the average of all weighted rays, agrees closely with empirical measurements of axial focus position from test samples. This agreement is particularly close for confocal measurements when NA4 weighting is used in the model calculations, but the situation is more complex for samples with non-planar refractive boundaries. Calibration of axial attenuation in a botanical sample, arising from the combination of optical sectioning with specimen-induced spatial distortions and blurring, is possible using an in situ fluorescence sea within a permeabilized preparation. Parametric descriptions of attenuation can be obtained through the guard cell complex of Commelina communis leaf epidermis. Improved images of the 3-D morphology of stomatal guard cells are then obtained by digital correction of attenuation and spatial distortion. Calibrations can be routinely used to correct experimental data by integration with a structured image file format.

Dispersion, aberration and deconvolution in multi-wavelength fluorescence images

Abstract: The wavelength dependence of the incoherent point spread function in a wide-field microscope was investigated experimentally. Dispersion in the sample and optics can lead to significant changes in the point spread function as wavelength is varied over the range commonly used in fluorescence microscopy. For a given sample, optical conditions can generally be optimized to produce a point spread function largely free of spherical aberration at a given wavelength. Unfortunately, deviations in wavelength from this value will result in spherically aberrated point spread functions. Therefore, when multiple fluorophores are used to localize different components in the same sample, the image of the distribution of at least one of the fluorophores will be spherically aberrated. This aberration causes a loss of intensity and resolution, thereby complicating the localization and analysis of multiple components in a multi-wavelength image. We show that optimal resolution can be restored to a spherically aberrated image by constrained, iterative deconvolution, as long as the spherical aberration in the point spread function used for deconvolution matches the aberration in the image reasonably well. The success of this method is essentially independent of the initial degree of spherical aberration in the image. Deconvolution of many biological images can be achieved by collecting a small library of spherically aberrated and unaberrated point spread functions, and then choosing a point spread function appropriate for deconvolving each image. The co-localization and relative intensities of multiple components can then be accurately studied in a multi-wavelength image.

Confocal microscopy by aperture correlation.

Abstract: Most confocal microscopes do not produce images in real time with nonlaser light sources. The tandem scanning confocal microscope does produce such images but, because the pinhole apertures of the Nipkov disk must be placed far apart to reduce cross talk between neighboring pinholes, only 1% or less of the light available for imaging is used. We show that, by using aperture correlation techniques and relaxing the requirement to obtain a pure confocal image directly, one can obtain real-time confocal images with a dramatically increased (25% or even 50%) light budget.

The telescope point spread function

Abstract: New observations are used to accurately define the stellar point spread function produced by atmospheric turbulence at the focus of large telescopes and to compare its profile to those computed from wavefronts characterized by various structure functions Excellent agreement is found with the PSF expected from Kolmogorov statistics, except for the presence of an extended aureole of light which appears to results from a combination of instrumental and atmospheric light scattering Simple yet accurate analytic fits are developed to represent the PSF profile over a range of 15 magnitudes in surface brightness The relation between the Strehl ratio S of the PSF and the value of the parameter D/r0 is re-discussed, both for a natural wavefront and for a wavefront whose variance DeltaN is reduced by an adaptive optics systems A simple expression for S(D/r0, DeltaN) is proposed and shown to yield essentially correct Strehl ratios for any value of the parameters

Identification of blur parameters from motion-blurred images

Abstract: A difficult problem in imaging systems is degradation of images caused by motion. This problem is common when the imaging system is in moving vehicles such as tanks or planes and even when the camera is held by human hands. For correct restoration of the degraded image we need to know the point spread function (PSF) of the blurring system. In this paper we propose a method to identify important parameters with which to characterize the PSF of the blur, given only the blurred image itself. A first step of this method has been suggested in a former paper where only the blur extent parameter was considered. The identification method here is based on the concept that image characteristics along the direction of motion are different than the characteristics in other directions. Depending on the PSF shape, the homogeneity and the smoothness of the blurred image in the motion direction are higher than in other directions. Furthermore, in the motion direction correlation exists between the pixels forming the blur of the original unblurred objects. The method proposed here identifies the direction and the extent of the PSF of the blur and evaluates its shape which depends on the type of motion during the exposure. Correct identification of the PSF parameters permits fast high resolution restoration of the blurred image.

Improved SPECT quantitation using fully three-dimensional iterative spatially variant scatter response compensation

Abstract: The quality and quantitative accuracy of iteratively reconstructed SPECT images improves when better point spread function (PSF) models of the gamma camera are used during reconstruction. Here, inclusion in the PSF model of photon crosstalk between different slices caused by limited gamma camera resolution and scatter is examined. A three-dimensional (3-D) projector back-projector (proback) has been developed which models both the distance dependent detector point spread function and the object shape-dependent scatter point spread function of single photon emission computed tomography (SPECT). A table occupying only a few megabytes of memory is sufficient to represent this scatter model. The contents of this table are obtained by evaluating an analytical expression for object shape-dependent scatter. The proposed approach avoids the huge memory requirements of storing the full transition matrix needed for 3-D reconstruction including object shape-dependent scatter. In addition, the method avoids the need for lengthy Monte Carlo simulations to generate such a matrix. In order to assess the quantitative accuracy of the method, reconstructions of a water filled cylinder containing regions of different activity levels and of simulated 3-D brain projection data have been evaluated for technetium-99m. It is shown that fully 3-D reconstruction including complete detector response and object shape-dependent scatter modeling clearly outperforms simpler methods that lack a complete detector response and/or a complete scatter response model. Fully 3-D scatter correction yields the best quantitation of volumes of interest and the best contrast-to-noise curves.

Optical Coherent Tomography with Electrically Tunable Semiconductor Laser Using FMCW Techniques

Abstract: Imaging of highly scattering objects in scattering media can play an important part in assessing melanoma in human skin. The technique to be presented is based on frequency modulated continuous waves using a coherent tunable semiconductor laser irradiating a Michelson interferometer. The electrically tunable laser is characterized and the procedure to linearize the instantaneous frequency with time is described. The temporal point spread function of dilute milk is measured. Finally the performance of our imaging system is demonstrated on 2D-images of solid scattering phantoms and of an eye of a pig.

Resolution in three-photon fluorescence scanning microscopy.

Abstract: Resolution in potential three-photon fluorescence scanning microscopy is discussed in terms of the threedimensional optical transfer function. Images of layers and sharp edges are presented for a comparison of the resolution with that in two-photon fluorescence microscopic imaging. For the same fluorescence wavelength the resolution is almost the same in both cases. However, for a given illumination wavelength the resolution for imaging a thick object in the case of three-photon fluorescence imaging can be improved by as much as 40-50% relative to that in two-photon imaging.

Imaging through scattering media by the use of an analytical model of perturbation amplitudes in the time domain

Abstract: A method of generating images through highly scattering media is presented that involves comparing measurements of the time-dependent intensity of transmitted light with an analytical model describing the sensitivity of that intensity on localized changes in optical properties. A least-squares fitting procedure is employed to derive the amplitudes of the measurement perturbations caused by embedded absorbers and scatterers located along a line of sight between the source and detector. Images are presented of a highly scattering, solid plastic phantom with optical properties closely matched to those of human breast tissue at near-infrared wavelengths. The phantom is a 54-mm-thick slab, containing four small cylinders of contrasting scatter and absorption. Results show that embedded absorbers can be distinguished from embedded scatterers, and that the diffusion perturbation amplitude provides inherently greater spatial resolution than the absorption perturbation amplitude.

Analysis of the numerically implemented angular spectrum approach based on the evaluation of two‐dimensional acoustic fields. Part I. Errors due to the discrete Fourier transform and discretization

Abstract: The numerically implemented angular spectrum approach (ASA) is investigated based on the evaluation of the 2‐D fields radiated by striplike planar sources and 1‐D focusing linear‐phased arrays with different aperture sizes and on the comparison with those obtained by using the analytical solution derived with the point spread function method. Since the discrete Fourier transform (DFT) of a finite‐size source produces an angular spectrum with spatial frequency aliasing and discretization of the source in even‐numbered samples causes a half‐sample length phase shift error in the angular spectrum, the effects of the aliasing and the phase shift on calculation accuracy are dealt with. The results show that the frequency aliasing causes an overestimation of the near field and the phase shift distorts the results. A numerical algorithm for eliminating the aliasing is proposed and correction of the phase shift is confirmed to be necessary. The algorithm proposed can completely remove the errors and obtain exact ...

Industrial ultrasonic imaging and microscopy

Abstract: Ultrasonic imaging and scanned acoustic microscopy are terms used to describe similar imaging processes at different magnifications and frequencies. Both processes form images by acquiring spatially correlated measurements of the interaction of high-frequency sound waves with materials. With the exception of the interference measurement, called V(z), and the gigahertz frequencies used by the higher frequency scanning acoustic microscopes, it is difficult to establish operational differences between them. This is especially true since almost all commercial ultrasonic imaging systems use transducers producing focused beams and can display magnified high-resolution images. Ultrasonic C-scan imaging was developed largely by the ultrasonic nondestructive testing industry. The development was gradual and evolutionary. Over a 50-year period, better and better broadband transducers, electronics and scanners were developed for operation at progressively higher frequencies, now ranging from 1.0 to 100 MHz. Conversely, scanning acoustic microscopes made a relatively sudden appearance 20 years ago on the campus of Stanford University. The first scanning acoustic microscopes operated at gigahertz frequencies and used microwave electronics that produced acoustic tone bursts with many wavelengths per pulse. Three factors control resolution in an acoustic image: diameter of the acoustic beam or its point spread function (PSF); size and spacing of the pixels making up the image; signal-to-noise ratio (contrast) of the feature being resolved. The beam diameter, or PSF, is controlled by the frequency of the ultrasonic pulse and the focal convergence of the beam (or focal length to diameter ratio Z/d). In the coupling fluid, the Z/d ratio is determined by the transducer diameter and lens, but in the material, Z/d is established by the materials ultrasonic velocities. Pixels are the squares of colour or greyscale that make up computer displays of scanned images. Following Nyquist's criterion, the resolution of those images is twice the size and spacing of the pixels. It follows, therefore, that in order to support the resolution of an ultrasonic beam, the pixels must be no larger than half that beam diameter. Finally, the contrast of the feature being studied must be (at least) a clear shade of grey above the background produced by the image noise. The noise can be due to the material or the electronics. Written to support industrial ultrasonic inspection of materials, this discussion will emphasise the similarities between imaging and microscopy rather than the differences. The roles of the focusing lens, the pulse frequency, and the material being imaged, with respect to the final resolution of an acoustic image, will be considered in detail. It will be shown that additional improvements in resolution can be achieved with image processing. Finally, applications studies in metals, ceramics, composites, attachment methods, coatings, and electronic assemblies will be used to demonstrate specific roles for imaging/microscopy in nondestructive testing.

Scanning singular-value-decomposition method for restoration of images with space-variant blur

Abstract: We present a method that can efficiently restore large images, blurred possibly nonuniformly and contaminated with noise, by use of a scanning singular-value-decomposition (SVD) method. Such an approach bypasses the prohibitive storage and speed limitations of the SVD method, thus, to our knowledge for the first time, making possible the restoration of reasonably sized images. We make use of the linear and local nature of the point spread function (PSF) to scan the image and restore it in the same raster without incurring blocking effects that are due to the overlap in neighboring reconstruction areas. The increase in speed compared with the conventional SVD approach can be many orders of magnitude, depending on the ratio of the point-spread blur to the image size. For example, if the linear extent of the PSF is one-eighth that of the image, a speed-up factor greater than 106 is achieved. A similar but less accurate solution to the problem of spatially variant blur by use of scanning Fourier transforms, which allows an even faster solution, is also described.

Avalanche photodiode detection with object scanning and image restoration provides 2–4 fold resolution increase in two-photon fluorescence microscopy

Abstract: High-quantum-efficiency photodetection, millisecond pixel dwell time stage scanning and image restoration by maximum-likelihood estimation are synergetically combined and shown to improve the resolution of two-photon excitation microscopy 2–4 fold in all directions. Measurements of the two-photon excitation point-spread function (PSF) of a 1.4 aperture oil immersion lens are carried out by imaging fluorescence beads with a diameter of one seventh of the excitation wavelength (830 nm) and subsequent deconvolution with the bead object function. The proposed method of resolution increase is applied to beads as well as to rhodamine labelled actin fibres in mouse fibroblast cells. As the resolution improvement is not based on the non-linear effect of two-photon excitation, the results imply a comparable resolution increase in single-photon excitation confocal microscopy. In the fibroblasts, we established a three-fold improvement in axial resolution, namely from 840 nm before, to 280 nm after restoration (full-width at half-maximum). Actin fibres with axial distances of 850 nm, otherwise difficult to discern, are fully separated. In the lateral direction, images of fluorescence beads of about 110 nm diameter are restored to the real dimensions of the beads with an accuracy of better than one pixel (41 nm).

Optimized single site update algorithms for image deblurring

Abstract: We present optimized algorithms for image deblurring in the case of a separable point spread function (PSF). Our work is in the usual context of Bayesian estimation with Gibbs random fields (GRF). The derived algorithms fall into the class of single site update algorithms (SSUAs), which exhibit a high convergence rate per iteration and small memory requirements, while hard domain constraints such as positivity are easily introduced. On the other hand, standard forms of SSUAs rapidly become intractable when the size of the PSF is large. We show how PSF separability can benefit the SSUAs, in order to reduce the cost of each pixel update from O(2pq) to O(p+q) (p/spl times/q is the size of the PSF). We show that the resulting deterministic SSUA compares very favorably with global update algorithms (GUAs). The new separable form can also benefit other SSUAs, especially stochastic versions such as simulated annealing (SA) and Monte Carlo Markov chain (MCMC) algorithms.

Wavelet-based neural network with fuzzy-logic adaptivity for nuclear image restoration

Abstract: A novel wavelet-based neural network with fuzzy-logic adaptivity (WNNFA) is proposed for image restoration using a nuclear medicine gamma camera based on the measured system point spread function. The objective is to restore image degradation due to photon scattering and collimator photon penetration with the gamma camera and allow improved quantitative external measurements of radionuclides in vivo. The specific clinical model proposed is the imaging of bremsstrahlung radiation using /sup 32/P and /sup 90/Y. The theoretical basis for four-channel multiresolution wavelet decomposition of the nuclear image into different subimages is developed with the objective of isolating the signal from noise. A fuzzy rule is generated to train a membership function using least mean squares to obtain an optimal balance between image restoration and the stability of the neural network, while maintaining a linear response for the camera to radioactivity dose. A multichannel modified Hopfield neural network architecture is then proposed for multichannel image restoration using the dominant signal subimages.

Iterative restoration of fast-moving objects in dynamic image sequences

Abstract: If a point on an object passes over two or more photoreceptors during image acquisition, a blur will occur. Under these conditions, an object or scene is said to move fast relative to the camera's ability to capture the motion. In this work, we consider the iterative restoration of images blurred by distinct, fast moving objects in the frames of a (video) image sequence. Even in the simplest case of fast object motion, the degradation is spatially variant with respect to the image scene. Rather than segmenting the image into regions where the degradation can be considered space invariant, we allow the blur to vary at each pixel and perform iterative restoration. Our approach requires complete knowledge of the blur point spread function (PSF) to restore the scene. The blur of fast moving object in a single frame is under specified. With the appropriate assumptions, an estimate of the blur PSF can be specified to within a constant scaling factor using motion information provided by a displacement vector field (DVF). A robust iterative restoration approach is followed which allows for the incorporation of prior knowledge of the scene structure into the algorithm to facilitate the restoration of difficult scenes. A bilinear approximation to the continuous PSF derived from the motion estimate is proposed to obtain results for real and synthetic sequences. We found this approach suitable for restoring motion degradations in a wide range of digital video applications. The results of this work reinforced the well known flexibility of the iterative approach to restoration and its application as an off-line image sequence restoration method.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new coded-excitation ultrasound imaging system. II. Operator design

Abstract: For pt.I see ibid. vol.43, pp.131-140 (1996). We introduced in pt. I a new approach for ultrasonic imaging using coded-excitation. Our formulation allows for the implementation of the imaging operator as a transversal filter bank with each filter designed to reconstruct scatterer distribution along one image line. This formulation allows for high-speed data acquisition when the reconstruction filters are utilized in parallel configuration. Furthermore, under high signal-to-noise ratio (SNR) conditions, our approach allows for spatial resolution that exceeds the resolution set by the diffraction limit. Ideally, the resolution limit for our system is set by the grid spacing in the region of interest (ROI). When the SNR of the system is finite, however, sensitivity to noise becomes a factor in image reconstruction quality. A trade-off exists between spatial and contrast resolutions with all elements of this trade-off captured by the singular value decomposition (SVD) of the imaging operator, G. In this paper, we demonstrate the use of the function given in pt.I in defining an optimal pseudo-inverse operator (PIO). The optimal PIO provides the highest spatial resolution at which the mean-square error (MSE) is minimized. We show that a design procedure for such an optimal image reconstruction operator is feasible, and we present an algorithm for operator design. Computer simulations are used to highlight the main features of the SVD-based operator design procedure; the SVD of the imaging operator consists of array-dependent (analytic) and code-dependent (nonanalytic) modes. The analytic modes yield a sine-like lateral point spread function (LPSF) of the imaging system. For a given SNR of the imaging system, a maximum number of modes (analytic and nonanalytic) can be used in the design of the imaging operator to minimize the LPSF width (i.e., maximize resolution) while minimizing the MSE. Finally, a two-dimensional cyst simulation is provided to demonstrate the potential advantage of our approach.

Blind image restoration via recursive filtering using deterministic constraints

Abstract: Classical linear image restoration techniques assume that the linear shift invariant blur, also known as the point-spread function (PSF), is known prior to restoration. In many practical situations, however, the PSF is unknown and the problem of image restoration involves the simultaneous identification of the true image and PSF from the degraded observation. Such a process is referred to as blind deconvolution. This paper presents a novel blind deconvolution method for image restoration. The method is flexible for incorporating different constraints on the true image. An example of the method is given for situations in which the imaged scene consists of a finite support object against a uniformly grey background. The only information required are the nonnegativity of the true image and the support size of the original object. For situations in which the exact object support is unknown, a novel support-finding algorithm is proposed.

Apparatus for and method of simulating ocular optical system

Abstract: An apparatus for simulating an ocular optical system simulates a retinal image produced by a human eye through an optical lens. Optical system data are produced from an optical system including a cornea, a pupil, an intraocular lens, a retina, etc. Based on the optical system data, point spread functions each indicative of a distribution on an image plane of light emitted from a certain point are calculated by PSF (Point Spread Function) calculating means. Image data are subjected to convolutional integration with the point spread functions, determining retinal image data. The retinal image data are converted into display data, which are supplied to a display unit to display a retinal image thereon. The retinal image displayed on the display unit is an image that would be actually formed on the retina of the human eye, and provides an accurate objective indication of how the image is seen by the patient.

Host galaxies of intermediate redshift radio-loud and radio-quiet quasars

Abstract: In a search for host galaxies associated with quasars, the authors present results from deep CCD imaging using the European Southern Observatory 3.5-m New Technology Telescope. Altogether 21 targets, 12 radio-loud and nine radio-quiet quasars, were observed in R, and additional V and Gunn i images were collected for a subgroup of these. The quasar redshifts are between 0.4 and 0.8, a range largely unexplored in previous studies. At these redshifts the R band corresponds to a rest-frame wavelength between 4600 and 3600 Aring. Thus, the authors are primarily probing the light from young stellar components of the presumed host galaxies. To separate out the light originating from the host object the authors remove the quasar contribution by scaling the point spread function (PSF). The PSF is a combination of an empirical PSF model, derived from stars lying in the same field as the quasars (used for the core of the PSF), and an analytical model for the wings of the PSF. After PSF subtraction of the QSO image the authors identify extended residual objects in 17 targets (nine in radio-loud and eight in radio-quiet quasars), of which a few are only marginal detections. These objects are large and luminous and their colours are relatively blue. Their V-R colours are consistent with a stellar population typical of late-type spirals and irregular galaxies. The blue colours could be caused by recent star formation events or by scattering of the QSO light

Influence of Collimator Characteristics on Quantification in SPECT

Abstract: Accurate, absolute quantification of activity in SPECT depends on the counting sensitivity of the gamma camera collimator system and on the contribution of the tail of the point spread function (PSF) that contains both scattered radiation and septal penetration. Methods : These factors were studied for the radionuclides 99m Tc, 131 I and 123 I on different cameras and collimators. The ability of the geometric mean line source scatter function subtraction technique to correct for the effect of the tail of the PSF was investigated in a computer simulation study with 99m Tc and a physical phantom study with 123 I. Results : The sensitivities of 99m Tc collimators were relatively constant with distance in air whereas those of 123 I and 131 I decreased considerably. For these radionuclides the contribution of the PSF tail was also increased as compared to 99m Tc. The geometric mean line source scatter function subtraction technique was confirmed as being able to correct effectively for this factor. Conclusion : Collimator design affects both the fall off of sensitivity in air and the tail of the PSF. Attention to correction of these factors enables accurate quantification.

On the measurements of particles by imaging methods: Theoretical and Experimental Aspects

Abstract: Imaging techniques in particle sizing have always been important. Nevertheless, owing to the progress in CCD cameras, these techniques have found new potential which strongly depends on the capability to describe the image formation. Based on a combination of the generalized Lorenz-Mie theory and the diffraction theory, the image characteristics are described, taking into account the beam size, particle location, direction of observation and collecting optics characteristics. In the case of forward detection, a finite coherence is introduced and the results are compared with those obtained by a convolution approach, with the point spread function assumed to be Gaussian. Then some experimental results are displayed which underline the improvement achieved in particle size measurement.

High modulation transfer function CCD X-ray image sensor apparatus and method

Abstract: Charge coupled device X-ray sensor and camera capable of imaging with high modulation transfer function for high resolution. The high resolution is achieved by a method of simultaneously measuring the modulation transfer functions of x-ray and visible images while imaging the target or the scene. Then, by using the point spread function and measured MTFs at various spatial frequencies to calculating spatial frequency dependent correction table or correction parameters. This correction is applied to the raw image of the target inside a workstation using a software embodiment of the correction algorithm. The high precision, multi-spatial frequency patterns that are used in x-ray image correction are provided on the sensor, the scintillator screen and the fiber optic face plate of a sensor device to enable the user of the workstation to measure the modulation transfer functions (contrast transfer function) in the horizontal, vertical and in oblique orientations at several spatial frequencies.

Autocorrelation of integrated power doppler signals and its application

Abstract: The integrated power Doppler signal arising from blood flow is a random process. In this article, a general approach to model this random process is studied. Both the theoretical and experimental results show that the temporal autocorrelation function of the integrated power Doppler signals is directly related to properties of the insonified medium, such as the scattering strengths and velocities of all moving scatterers, and as well as the properties of the Doppler imaging system, such as the point spread function (psf) of the power Doppler images. Some initial experiments are performed to test the proposed model. Its potential application for flow measurement, such as perfusion evaluation, is also discussed.

Subsurface electrical impedance imaging using orthogonal linear electrode arrays

Abstract: The main limitation of electrical impedance tomography as an imaging modality is its inherently poor spatial resolution. Although more information about the impedance distribution within an object can be retrieved by the use of larger numbers of surface electrodes, the additional information tends to relate to regions of the object close to the electrodes rather than at depth. The paper describes the application of an image reconstruction approach originally developed for medical electrical impedance tomography to the alternative problem of mapping electrical impedance distributions in a subsurface volume. The use of this geometry allows the chosen imaged plane to be approximately uniformly sensed by a surface grid of electrodes leading to uniform resolution in the image and the possibility of exploiting the relatively high resolution information available at shallow depths. The advantage of the use of an electrode grid, treated as two orthogonal sets of parallel linear arrays, is demonstrated and the behaviour of the point-spread function in planes parallel to the surface plane examined. The application of a spatial frequency filter to counteract reconstruction blur is described. Simulated reconstructions of subsurface structures and an example biomedical image are presented.