Showing papers on "Point spread function published in 1999"

Advanced Optical Imaging Theory

Abstract: Diffraction Theory.- Point Spread Function Analysis.- Transfer Function Analysis.- Imaging with an Ultrashort Pulsed Beam.- Imaging with a High Numerical-Aperture Objective.- Imaging with Aberration.

Blind image restoration by anisotropic regularization

Abstract: This paper presents anisotropic regularization techniques to exploit the piecewise smoothness of the image and the point spread function (PSF) in order to mitigate the severe lack of information encountered in blind restoration of shift-invariantly and shift-variantly blurred images. The new techniques, which are derived from anisotropic diffusion, adapt both the degree and direction of regularization to the spatial activities and orientations of the image and the PSF. This matches the piecewise smoothness of the image and the PSF which may be characterized by sharp transitions in magnitude and by the anisotropic nature of these transitions. For shift-variantly blurred images whose underlying PSFs may differ from one pixel to another, we parameterize the PSF and then apply the anisotropic regularization techniques. This is demonstrated for linear motion blur and out-of-focus blur. Alternating minimization is used to reduce the computational load and algorithmic complexity.

Extended depth of field and aberration control for inexpensive digital microscope systems.

Abstract: We present a new application and current results for extending depth of field using wave front coding. A cubic phase plate is used to code wave fronts in microscopy resulting in extended depths of field and inexpensive chromatic aberration control. A review of the theory behind cubic phase plate extended depth of field systems is given along with the challenges that are face when applying the theory to microscopy. Current results from the new extended depth of field microscope systems are shown

Weak Lensing Measurements: A Revisited Method and Application to HST Images

Abstract: The weak distortions produced by gravitational lensing in the images of background galaxies provide a method to measure directly the distribution of mass in the universe. However this technique requires high precision measurements of the lensing shear and cautious corrections for systematic effects. Kaiser, Squires, & Broadhurst (1995) proposed a method to calibrate the ellipticity-shear relation in the presence of Point Spread Function (PSF) anisotropies and camera distortions. We revisit the KSB method and show that both the PSF and the camera distortions can be corrected for using source moments, as opposed to ellipticities. We clarify the applicability of some of the approximations made in this method. We derive expressions for the corrections which only involve the galaxy moments. We derive an explicit relation between the shear and the average ellipticity. We discuss the shortcomings of the method, and test its validity using numerical simulations. As an application of the method, we repeat the analysis of the HST WFPC2 camera performed by Hoekstra et al. (1998). We confirm the presence of sizable (10%) PSF ellipticities at the edge of the WFPC2 chips. We also show that the PSF ellipticity varies by as much as 2% over time. We use these measurements to correct the shape of galaxies in the HST Survey Strip (``Groth'' Strip). By considering the dependence of the ellipticities on object size, we show that, after corrections, the residual systematic uncertainty for galaxies with radii greater than 0.15 arcsec, is about 0.4%, when averaged over each chip. We discuss how these results provide good prospects for measuring weak lensing by large-scale structure with deep HST surveys.

Third-Harmonic Generation Microscopy by Use of a Compact, Femtosecond Fiber Laser Source

Abstract: We demonstrate the first use, to our knowledge, of a compact, diode-pumped, femtosecond fiber laser for third-harmonic generation (THG) microscopy. We discuss the utility of this technique, as well as the technical issues involved in using this compact source, and demonstrate the first use, to our knowledge, of imaging by THG backlighting.

Three dimensional image restoration in fluorescence lifetime imaging microscopy

Abstract: A microscope set-up and numerical methods are described which enable the measurement and reconstruction of three-dimensional nanosecond fluorescence lifetime images in every voxel. The frequency domain fluorescence lifetime imaging microscope (FLIM) utilizes phase detection of high-frequency modulated light by homodyne mixing on a microchannel plate image intensifier. The output signal at the image intensifier's phosphor screen is integrated on a charge coupled device camera. A scanning stage is employed to obtain a series of phase-dependent intensity images at equally separated depths in a specimen. The Fourier transform of phase-dependent data gives three-dimensional (3D) images of the Fourier coefficients. These images are deblurred using an Iterative Constrained Tikhonov-Miller (ICTM) algorithm in conjunction with a measured point spread function. The 3D reconstruction of fluorescence lifetimes are calculated from the deblurred images of the Fourier coefficients. An improved spatial and temporal resolution of fluorescence lifetimes was obtained using this approach to the reconstruction of simulated 3D FLIM data. The technique was applied to restore 3D FLIM data of a live cell specimen expressing two green fluorescent protein fusion constructs having distinct fluorescence lifetimes which localized to separate cellular compartments.

Recursive ultrasound imaging

Abstract: Presents a new imaging method, applicable for both 2D and 3D imaging. It is based on Synthetic Transmit Aperture Focusing, but unlike previous approaches a new frame is created after every pulse emission. The elements from a linear transducer array emit pulses one after another. The same transducer element is used after N/sub xmt/ emissions. For each emission the signals from the individual elements are beam-formed in parallel for all directions in the image. A new frame is created by adding the new RF lines to the RF lines from the previous frame. The RF data recorded at the previous emission with the same element are subtracted. This yields a new image after each pulse emission and can give a frame rate of, for example, 5000 images/sec. The paper gives a derivation of the recursive imaging technique and compares simulations for fast B-mode imaging with measurements. A low value of N/sub xmt/ is necessary to decrease the motion artifacts and to make flow estimation possible. The simulations show that for N/sub xmt/=13 the level of grating lobes is less than -50 dB from the peak, which is sufficient for B-mode imaging and flow estimation. The measurements made with an off-line experimental system having 64 transmitting channels and 1 receiving channel, confirmed the simulation results. A linear array with a pitch of 208.5 /spl mu/m, central frequency f/sub otr/=7.5 MHz and bandwidth BW=70% was used. The signals from 64 elements were recorded, beam-formed and displayed as a sequence of B-mode frames, using the recursive algorithm. An excitation with a central frequency f/sub otr/=5 MHz (/spl lambda/=297 /spl mu/m in water) was used to obtain the point spread function of the system. The -6 dB width of the PSF is 1.056 mm at axial distance of 39 mm. For a sparse synthetic transmit array with N/sub xmt/=22 the expected grating lobes from the simulations are -53 dB down from the peak value at, positioned at /spl plusmn/28/spl deg/. The measured level was -51 dB at /spl plusmn/27/spl deg/ from the peak. Images obtained with the experimental system are compared to the simulation results for different sparse arrays. The application of the method for 3D real-time imaging and blood-velocity estimations is discussed.

Measurement of thickness and density of thin structures by computed tomography: a simulation study.

Abstract: The limited spatial resolution of clinical CT systems causes difficulties in the measurement of the density and thickness of thin structures such as the vertebral cortical shell. We simulated the imaging process by convolving experimentally determined point spread functions with rectangular and Gaussian profiles, for various fields of view or pixel sizes and reconstruction kernels. The simulations successfully explained the reported overestimation of thickness and underestimation of density when imaging thin structures. Both effects are larger for Gaussian profiles. For the rectangular profiles, experimental estimates of thickness and density will only be accurate when the true thickness is greater than about 1.5 times (for the bone reconstruction kernel) or 2.0 times (for the standard kernel) the full width at half maximum of the point spread function (PSF) of the imaging system. For Gaussian profiles imaged by a system with a Gaussian PSF, there are straightforward analytical expressions for the overestimation of thickness and underestimation of density: and these are useful approximations to the simulations of Gaussian profiles with experimental (pseudo-Gaussian) PSFs. We have demonstrated that thresholding of the vertebral image cannot provide accurate estimates of cortical thickness and density because the appropriate threshold level requires foreknowledge of the cortical thickness. To circumvent such difficulties we suggest that the average value of the peak CT numbers measured along the medial axis of the cortical shell be adopted as an index of cortical shell strength, since its value depends on both the density and the thickness of the shell.

Parametric blind deconvolution: a robust method for the simultaneous estimation of image and blur.

Abstract: Blind-deconvolution microscopy, the simultaneous estimation of the specimen function and the point-spread function (PSF) of the microscope, is an underdetermined problem with nonunique solutions that are usually avoided by enforcing constraints on the specimen function and the PSF. We derived a maximum-likelihood-based method for blind deconvolution in which we assume a mathematical model for the PSF that depends on a small number of parameters (e.g., less than 20). The algorithm then estimates the unknown parameters together with the specimen function. The mathematical model ensures that all the constraints of the PSF are satisfied, and the maximum-likelihood approach ensures that the specimen is nonnegative. The method successfully estimates the PSF and removes out-of-focus blur. The PSF estimation is robust to aberrations in the PSF and to noise in the image.

Data-driven multichannel superresolution with application to video sequences

Abstract: A method is proposed for superresolving multichannel data with application to video sequences. Based on a generalization of Papoulis’ sampling theorem, nonuniform samples of multiple channels are merged to generate high-resolution data. To overcome sampling ill posedness in the presence of noise, image frames are projected from standard orthonormal bases onto optimal Riesz bases defined by channel point spread functions (PSF’s). The method is therefore designed to perform under practical conditions of noise and other degradations. Unlike existing methods, where empirical models such as Gaussian, sinc, etc., are commonly used for characterizing channel PSF’s, the PSF’s are assumed unknown and possibly different and hence are blindly estimated from the observed data. The estimated PSF’s are then used to construct biorthogonal projection filters for the superresolution algorithm. This approach gives rise to a closed-form solution leading to a high-speed algorithm. The method has been tested and verified on PREDATOR video sequences (PREDATOR data are airborne video sequences of the Defense Advanced Research Projects Agency obtained by unmanned aircrafts).

An evaluation of two-photon excitation versus confocal and digital deconvolution fluorescence microscopy imaging in Xenopus morphogenesis.

Abstract: The ability to visualize cell motility occurring deep in the context of opaque tissues will allow many currently intractable issues in developmental biology and organogenesis to be addressed. In this study, we compare two-photon excitation with laser scanning confocal and conventional digital deconvolution fluorescence microscopy, using the same optical configuration, for their ability to resolve cell shape deep in Xenopus gastrula and neurula tissues. The two-photon microscope offers better depth penetration and less autofluorescence compared to confocal and conventional deconvolution imaging. Both two-photon excitation and confocal microscopy also provide improved rejection of “out-of-focus” noise and better lateral and axial resolution than conventional digital deconvolution microscopy. Deep Xenopus cells are best resolved by applying the digital deconvolution method on the two-photon images. We have also found that the two-photon has better depth penetration without any degradation in the image quality of interior sections compared to the other two techniques. Also, we have demonstrated that the quality of the image changes at different depths for various excitation powers. Microsc. Res. Tech. 47:172–181, 1999. © 1999 Wiley-Liss, Inc.

Theoretical limits of spatial resolution in elliptical-centric contrast-enhanced 3D-MRA.

Abstract: The point spread function (PSF) for contrast-enhanced threedimensional (3D) MR angiography using the elliptical centric view order is derived. This view order has been shown previously to provide high venous suppression thereby enabling long acquisition times capable of high spatial resolution. The dependence of the PSF on TR, field of view (FOV), scan time, and trigger time are shown explicitly. Theoretical predictions are corroborated with experimental results in phantoms and in vivo. The PSF width decreases as the square root of the product of TR and the two phase encoding FOV’s for fixed nominal voxel size. The PSF peak amplitude increases as the reciprocal of this product. Theory and experiment demonstrate that acquisition times over 40 sec provide superior resolution compared to shorter acquisitions, despite falling levels of contrast agent concentration. The analysis predicts that an isotropic spatial resolution of 1 mm before zero filling is possible in a FOV large enough to encompass the carotid and vertebral arteries bilaterally. Magn Reson Med 42:1106‐1116, 1999. r 1999 Wiley

Three-dimensional coherence imaging in the Fresnel domain

Abstract: We show that three-dimensional incoherent primary sources can be reconstructed from finite-aperture Fresnel-zone mutual intensity measurements by means of coordinate and Fourier transformation. The spatial bandpass and impulse response for three-dimensional imaging that result from use of this approach are derived. The transverse and longitudinal resolutions are evaluated as functions of aperture size and source distance. The longitudinal resolution of three-dimensional coherence imaging falls inversely with the square of the source distance in both the Fresnel and Fraunhofer zones. We experimentally measure the three-dimensional point-spread function by using a rotational shear interferometer.

Pile-up on X-ray CCD instruments

Abstract: This paper presents a statistical and analytical analysis of the pile-up phenomenon on X-ray CCD detectors, whereby two incoming X-rays are counted as one. The probability of measuring configurations involving 1, 2, 3 or 4 pixels is written down for a uniform incoming flux, thus allowing the computation of the flux loss (fraction of photons rejected because of pile-up) and of the pile-up fraction (fraction of events with wrong energy). For detectors with pixels which are small compared to the point spread function (PSF) of the telescope (such as XMM/EPIC-MOS or ASCA/SIS ) the formulae can be readily integrated over space to predict and account for pile-up on the total flux of point sources. It is shown that if only single events are selected the total pile-up fraction can never get very high for usual PSF shapes. The main effect is a loss of efficiency which is perfectly quantifiable. It is concluded that taking extreme care to avoid pile-up by adapting the instrument setting (for example restricting the useful area of the CCD) may not be so important after all, although it remains necessary if it is important to collect as many photons as possible from the source. An extension to the theory is proposed for detectors with pixels comparable to the PSF width (such as AXAF/ACIS ), and for extended sources.

Three-dimensional tomography using a cubic-phase plate extended depth-of-field system

Abstract: We use cubic-phase plate imaging to demonstrate an order-of-magnitude improvement in the transverse resolution of three-dimensional objects reconstructed by extended depth-of-field tomography. Our algorithm compensates for the range shear of the cubic-phase approach and uses camera rotation to center the reconstructed volume on a target object point.

Speckle-motion artifact under tissue shearing

Abstract: Research has shown that, for a rotating phantom, the speckle pattern may not replicate the phantom motion, rather it may show a large lateral translation component in addition to rotation. This translation effect was labeled speckle-motion artifact. An image formation model has been shown to explain the phenomenon, pointing to the curvature of the imaging system point spread function (PSF) at the origin of this effect. The present paper extends this analysis and proposes a model, which predicts that a lateral motion artifact also would occur with shear motion. In the model, the artifact is found to be proportional to the shear angle and dependent of shear orientation, being maximal for shear that runs parallel to the axial direction; as for rotation, the artifact increases with frequency and beamwidth. This would mean that, when viewing a parabolic flow in the far field or with a highly curved PSF, an apparent contraction/expansion pattern in the direction of the vessel wall would be superimposed to the real velocity profile. In elastography, when viewing an inclusion subjected to an axial strain, four motion artifact regions are expected near the inclusion. The model is developed using the Fourier domain representation of the speckles for tissue-motion compensated signals, also called Lagrangian speckle. It can explain the artifact in terms of a simple spectral translation of a parabolic phase profile; given this, it is shown the artifact would be proportional to the lateral derivative of the axial displacement field. The spectral representation of Lagrangian speckle, for shear, also provides a simple geometrical interpretation for speckle decorrelation in terms of the shear strength and orientation, and in terms of the beam characteristics, i.e., the axial and lateral bandwidth.

Effect of a finite-size pinhole on noise performance in single-, two-, and three-photon confocal fluorescence microscopy.

Abstract: It is known that signal level in single-, two- and three-photon confocal fluorescence microscopy increases with the size of the detector. Here we evaluate the signal-to-noise and the signal-to-background criteria for these microscopes. We investigate the effect of pinhole size on their ability to detect a weakly fluorescent point object in the presence of a uniformly fluorescence background. Numerical results based on a paraxial approximation theory show that optimization of these criteria gives an optimal value for pinhole size, which results in an improved imaging performance. The resulting improvement in noise performance, compared with the use of a large detector, is greater for three-photon than for two-photon confocal fluorescence microscopes.

Optical image scanner with internal measurement of point-spread function and compensation for optical aberrations

Abstract: An image scanner uses optical targets (120) within the scanner to characterize imperfections of a lens system (116) and to partially compensate for the imperfections using digital image processing. In one sample embodiment, a series of two-dimensional optical targets are placed outside the document scanning area. Each individual target is suitable for obtaining an estimate of the point-spread function for a small segment of the scan line. Each point-spread function is then used to compute a convolution kernel for the corresponding segment of the scan line. Alternatively, each point-spread function may be used in an iterative solution for a modified image. In an alternative embodiment, a two-dimensional known random pattern is provided for a target. Cross-correlation of a portion of the known random pattern with the scanned image of the same portion of known random pattern provides an estimate of the point-spread function for the portion of the scan line corresponding to the portion of the known random pattern. Providing a series of targets or continuous random target over the width of the scan line, within the scanner, enables determination of the point-spread function as a function of position for an assembled lens, at the humidity and temperature appropriate for the scan. As a result, a smaller, lower cost lens can be used and some the lens aberrations can be removed from the final scanned image.

Effective point-spread function for fast image modeling and processing in microscopic imaging through turbid media.

Abstract: An effective point-spread function (EPSF) for microscopic imaging through turbid media is proposed and calculated. The EPSF incorporates the property of a microscope system as well as the scattering property of a turbid medium. We prove that the image of a thin object embedded in a turbid medium can be expressed by the convolution of the EPSF with an object function. With the help of the convolution relation, image modeling for 5,000,000 incident photons can be approximately 15??times faster than the direct Monte Carlo simulation method for a one-dimensional object and can be at least 2??orders of magnitude faster for a two-dimensional object.

Performance of a volumetric CT scanner based upon a flat-panel imager

Abstract: To characterize the performance of a cone-beam computed tomography (CBCT) imaging system based upon an indirect- detection, amorphous silicon flat-panel imager (FPI). Tomographic images obtained using the FPI are presented, and the signal and noise characteristics of reconstructed images are quantified. Specifically, the spatial uniformity, CT linearity, contrast performance, noise characteristics, spatial resolution, and soft-tissue visualization are examined. Finally, the performance of the FPI-based CT system is discussed in relation to existing clinical technologies. A table-top measurements system was constructed to allow investigation of FPI performance in CBCT within a precisely controlled and reproducible geometry. The FPI incorporates a 512 X 512 active matrix array of a-Si:H thin-film transistors and photodiodes in combination with an overlying (133 mg/cm2 Gd2O2S:Tb) phosphor. The commercially available prototype FPI has a pixel pitch of 400 micrometer, a fill factor of approximately 80%, can be read at a maximum frame rate of 5 fps, and provides 16 bit digitization. Mounted upon an optical bench are the x-ray tube (in a rigid support frame), the object to be imaged (upon a precision rotation/translation table), and the FPI (mounted upon a precision translation table). The entire setup is directed under computer control, and volumetric imaging is accomplished by rotating the object incrementally over 360 degrees, delivering a radiographic x-ray pulse (e.g., 100 - 130 kVp, approximately 0.1 - 10 mAs), and acquiring a projection image at each increment. Prior to reconstruction, dark and flood- field corrections are applied to account for stationary nonuniformities in detector response and dark current. Tomographic images are reconstructed from the projections using the Feldkamp filtered back-projection algorithm for CBCT. The linearity of the CBCT system was compared to that of a commercial scanner (Philips SR-7000) using materials ranging in CT number from approximately 900 to 1100. The contrast sensitivity of the CBCT system and the conventional scanner was compared using these same materials. Images of a uniform water bath were acquired for characterization of the response uniformity and the dependence of noise on exposure. The spatial frequency response characteristics of the system were measured using a steel wire, from which the point spread function and modulation transfer function were determined. Finally, the soft-tissue contrast and spatial resolution of the CBCT system was demonstrated in volumetric images of a euthanized rat. The image quality was compared to images of the same subject acquired with an equivalent technique on the commercial scanner. A table-top CBCT scanner based upon an a- Si:H FPI has been constructed, and a system for CBCT image acquisition, processing, and reconstruction has been implemented. This system is capable of producing high-quality volumetric images. Reconstructions were generated from 300 radiographs (100 kVp; 1 mAs per projection) obtained at 1.2 degree increments through 360 degrees. Image acquisition and reconstruction required approximately 30 min and approximately 2 h 20 min (250 MHz UltraSparc), respectively. The system has demonstrated signal and noise performance comparable to that of commercial CT scanners. The imaging performance of the prototype supports the hypothesis that FPIs can be employed in computed tomography applications.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Crystal structure retrieval by maximum entropy analysis of atomic resolution incoherent images

Abstract: Summary In this paper, we discuss the application of the maximum entropy method to atomic resolution Z-contrast images acquired in a scanning transmission electron microscope. Zcontrast is an incoherent imaging technique, and can be described as a convolution between an object function (the real-space map of the columnar scattering cross-section to high angles) and a point spread function (the effective electron probe). As such, we show that the technique is ideally suited to maximum entropy analysis which can, given an electron probe distribution, retrieve the ‘most likely’ Z-contrast object function. Using both simulated and experimentally acquired data, we explore the capabilities of maximum entropy analysis when applied to atomic resolution Z-contrast images, drawing conclusions on both the range of applicability of the technique and the nature of the retrieved crystal structures. Ultimately, we show the way in which the combination of Z-contrast imaging with maximum entropy analysis can be used to yield important information on unexpected atomic structures.

Computerized adaptive imaging

Abstract: Apparatus for computational adaptive imaging comprises the following: an image information acquirer, which provides information relating to the refractive characteristics in a three-dimensional imaged volume; a ray tracer, which uses the information relating to the refractive characteristics to trace a multiplicity of rays from a multiplicity of locations in the three-dimensional imaged volume through the three-dimensional imaged volume, thereby providing a location dependent point spread function, and a deconvolver, which uses the location dependent point spread function, to provide an output image corrected for distortions due to variations in the refractive characteristics in the three-dimensional imaged volume.

Parallel-mode confocal microscope

Abstract: Conventional confocal microscopes are based on the dual scanning of a specimen by the images of a point source and of a point detector Hence, their imaging mode is serial, ie, the confocal image is obtained from the sequence of single-point or multiple-point partial images Parallel-mode confocal imaging is possible on the basis of broad- source image-plane holography We adapted this classical holography technique for real-time reflected-light microscopy The imaging speed of the parallel-mode confocal microscope is not limited by any part of the optical system, but only by the image detection and storage systems Both the image phase and the image amplitude are reconstructed from the interference signal We verify both theoretically and experimentally that the main imaging parameters of the microscope are comparable with those of a conventional confocal microscope The only exception is the imaging speed, which can be much higher

Optical transfer function in three dimensions for a large numerical aperture

Abstract: We calculate the three-dimensional optical transfer function for an ordinary microscope with a large numerical aperture by elementary mathematical methods. The support of this function has been found before but not its values. Numerical Fourier transformation gives the corresponding point spread function.

Ocular optical system simulating method and simulating apparatus

Abstract: There are provided an ocular optical system simulating method and a simulating apparatus which enable simulation of how things can be seen, together with fluctuation, deformation, blur, etc. , occurring when a lens system such as a progressive addition lens is worn. A rotation-based retinal image, defined as an image obtained by turning an eye-ball with respect to all object points within a field of vision and by connecting images caught at the fovea, is made. The image is made by first creating an original image having a specific angular field of vision and entering the eye having a specific rotation center point. Then, a deformed original image having deformation occurring when the original image is seen through the lens system is created by using ray tracing. A PSF on the retina of an eye-model from light from the object points of the original image in an optical system composed of the lens system and a spectacle model is determined. Next, the deformed original image and the PSF of each pixel of the original image are convoluted. The obtained rotation-based retinal image is edited further to result in a motion picture image of the rotation-based retinal image. The PSF is found by selecting sampling points on an object and the PSF other than those at the sampling points is found by using approximation methods including spline interpolation.