Point Spread Function Measurements Research Articles

PSF modelling for very wide-field CCD astronomy

One of the possible approaches to detecting optical counterparts of GRBs requires monitoring large parts of the sky. This idea has gained some instrumental support in recent years, such as with the "Pi of the Sky" project. The broad sky coverage of the "Pi of the Sky" apparatus results from using cameras with wide-angle lenses (20x20 deg field of view). Optics of this kind introduce significant deformations of the point spread function (PSF), increasing with the distance from the frame centre. A deformed PSF results in additional uncertainties in data analysis. Our aim was to create a model describing highly deformed PSF in optical astronomy, allowing uncertainties caused by image deformations to be reduced. Detailed laboratory measurements of PSF, pixel sensitivity, and pixel response functions were performed. These data were used to create an effective high quality polynomial model of the PSF. Finally, tuning the model and tests in applications to the real sky data were performed. We have developed a PSF model that accurately describes even very deformed stars in our wide-field experiment. The model is suitable for use in any other experiment with similar image deformation, with a simple tuning of its parameters. Applying this model to astrometric procedures results in a significant improvement over standard methods, while basic photometry precision performed with the model is comparable to the results of an optimised aperture algorithm. Additionally, the model was used to search for a weak signal -- namely a possible gamma ray burst optical precursor -- showing very promising results. Precise modelling of the PSF function significantly improves the astrometric precision and enhances the discovery potential of a wide-field system with lens optics.

Correction of partial volume effect in 18F-FDG PET brain studies using coregistered MR volumes: Voxel based analysis of tracer uptake in the white matter

A voxel-based algorithm to correct for partial volume effect in PET brain volumes is presented. This method (named LoReAn) is based on MRI based segmentation of anatomical regions and accurate measurements of the effective point spread function of the PET imaging process. The objective is to correct for the spill-out of activity from high-uptake anatomical structures (e.g. grey matter) into low-uptake anatomical structures (e.g. white matter) in order to quantify physiological uptake in the white matter. The new algorithm is presented and validated against the state of the art region-based geometric transfer matrix (GTM) method with synthetic and clinical data. Using synthetic data, both bias and coefficient of variation were improved in the white matter region using LoReAn compared to GTM. An increased number of anatomical regions doesn't affect the bias (<5%) and misregistration affects equally LoReAn and GTM algorithms. The LoReAn algorithm appears to be a simple and promising voxel-based algorithm for studying metabolism in white matter regions.

Reconstruction algorithm for improved ultrasound image quality

A new algorithm is proposed for reconstructing raw RF data into ultrasound images. Previous delay-and-sum beamforming reconstruction algorithms are essentially one-dimensional, because a sum is performed across all receiving elements. In contrast, the present approach is two-dimensional, potentially allowing any time point from any receiving element to contribute to any pixel location. Computer-intensive matrix inversions are performed once, in advance, to create a reconstruction matrix that can be reused indefinitely for a given probe and imaging geometry. Individual images are generated through a single matrix multiplication with the raw RF data, without any need for separate envelope detection or gridding steps. Raw RF data sets were acquired using a commercially available digital ultrasound engine for three imaging geometries: a 64-element array with a rectangular field-of- view (FOV), the same probe with a sector-shaped FOV, and a 128-element array with rectangular FOV. The acquired data were reconstructed using our proposed method and a delay- and-sum beamforming algorithm for comparison purposes. Point spread function (PSF) measurements from metal wires in a water bath showed that the proposed method was able to reduce the size of the PSF and its spatial integral by about 20 to 38%. Images from a commercially available quality-assurance phantom had greater spatial resolution and contrast when reconstructed with the proposed approach.

The beam-based calibration of an X-ray pinhole camera at SSRF

A pinhole camera for imaging X-ray synchrotron radiation from a dipole magnet is now in operation at the Shanghai Synchrotron Radiation Facility (SSRF) storage ring. The electron beam size is derived by unfolding the radiation image and the point spread function (PSF) with deconvolution techniques. The performance of the pinhole is determined by the accuracy of the PSF measurement. This article will focus on a beam-based calibration scheme to measure the PSF system by varying the beam images with different quadrupole settings and fitting them with the corresponding theoretical beam sizes. Applying this method at SSRF, the PSF value of the pinhole is revised from 37 to 44 μm. The deviation in beam size between the theoretical value and the measured value is minimized to 4% after calibration. This optimization allows us to observe the horizontal disturbance due to injection down to as small as 0.5 μm.

A Spatial Resolution Study of a New Optical Tomography-Based Polymer Gel Dosimetry System

A spatial resolution investigation of the OCTOPUSTM-IQ scanner in combination with the new BANG3-Pro2(r) polymer gel was performed by scanning a high-contrast needle phantom. The phantom contained five thin needles (0.3_mm diameter) embedded in gel positioned in different patterns: needles were inserted (a) at 45° angle from the center of the gel container, and (b) vertically along the gel axis. The non-irradiated needle phantoms were scanned at various slice spacings (0.25-1.0_mm) and for two different laser beam orientations. Optical density profiles and their full width at half maximum (FWHM) were evaluated for resolution limit. The modulation transfer function (MTF) corresponding to measured point spread function (PSF) data was calculated. With high resolution scanning mode and 0.25_mm pixel resolution, the measured PSFs at the center of the gel dosimeter have a FWHM of 0.95_mm. The MTF for the 0.25_mm reconstruction pixel size suggests that the resolution of the system is 0.5_mm or less. We also observed a progressive degradation of the vertical needle images with off-axis distance, attributable to the defocusing of the laser beam. No significant degradation was observed up to the maximum useful reconstructed image radius of 50_mm from the gel dosimeter center axis.

Deconvolved Spatial Light Interference Microscopy for Live Cell Imaging

Spatial light interference microscopy (SLIM) is a recently developed method for the label-free imaging of live cells, using the quantitative optical path length through the sample as an endogenous source of contrast. In conventional SLIM, spatial resolution is limited by diffraction and aberrations. This paper describes a novel constrained deconvolution method for improving resolution in SLIM. Constrained deconvolution is enabled by experimental measurement of the system point-spread function and the modeling of coherent image formation in SLIM. Results using simulated and experimental data demonstrate that the proposed method leads to significant improvements in the resolution and contrast of SLIM images. The proposed method should prove useful for high-resolution label-free studies of biological cells and subcellular processes.

A single-photon sensitive ebCMOS camera: The LUSIPHER prototype

Processing high-definition images with single-photon sensitivity acquired above 500 frames per second (fps) will certainly find ground-breaking applications in scientific and industrial domains such as nano-photonics. However, current technologies for low light imaging suffer limitations above the standard 30fps to keep providing both excellent spatial resolution and signal-over-noise. This paper presents the state of the art on a promising way to answer this challenge, the electron bombarded CMOS (ebCMOS) detector. A large-scale ultra fast single-photon tracker camera prototype produced with an industrial partner is described. The full characterization of the back-thinned CMOS sensor is presented and a method for Point Spread Function measurements is elaborated. Then the study of the ebCMOS performance is presented for two different multi-alkali cathodes, S20 and S25. Point Spread Function measurements carried out on an optical test bench are analysed to extract the PSF of the tube by deconvolution. The resolution of the tube is studied as a function of temperature, high voltage and incident wavelength. Results are discussed for both multi-alkali cathodes as well as a Maxwellian modelization of the radial initial energy of the photo-electrons.

Investigation of the resolution ability of an image inversion interferometer

The present paper presents the experimental measurement of the interferometric point spread function of an image inversion interferometer ( III). The measurement was realized by scanning a pointlike source in the object plane and integrating the intensity in the image plane for each position of the point source. The achieved results are compared to the theoretical expectations and the results, which we received in a previous work by using a different method. The improved resolution of two neighbouring points in comparison to the conventional image is demonstrated. In addition we propose an explanation concerning the divergences of the values to be expected in theory.

Method of obtaining wavefront slope data from through-focus point spread function measurements

A method is described for analyzing point source imagery collected with various amounts of defocus to obtain wavefront slope data at the exit pupil of an imaging system. Integration of this slope data yields the system wavefront aberration function. The method is based on a geometric optics interpretation of intensity point spread function measurements in the caustic region near focus. Algorithm performance is examined through Monte Carlo simulations. Application of the method to segmented-aperture systems is also explored. The proposed method is used to generate initial wavefront estimates for an iterative phase retrieval algorithm, significantly improving the capture range over a blind phase retrieval approach when segment tilt errors are large.

Generation of a high depth of focus with constant transversal spot size using a phase-only pupil filter

A novel approach is proposed to obtain an extended depth of focus (DoF) in white light with constant transversal spot size within the DoF. It combines a phase-only pupil filter based on multiplexed radial zones with alternating quartic phase functions. The design is first tested via numerical simulations of the point spread function (PSF) based on the scalar diffraction theory. The results for a fourfold gain of the depth of focus are experimentally verified with a phase-only spatial light modulator liquid crystal device combined with a 3D PSF measurement system. A close conformity between the experimental and simulation results proves the effectiveness of the proposed approach.

Optical Dot Gain Analysis by The Measured Point Spread Function of Paper

Optical Dot Gain Analysis by The Measured Point Spread Function of Paper

Evaluation of the higher order wavefront aberrations and the quality of vision after correction of aphakia by a retropupillary implantation of the Verisyze® iris‐claw intraocular lens

Abstract Purpose To investigate the influence of the secondary implantation site of the Verisyse® iris‐claw intraocular lens (IOL) on high order aberrations (HOAs) using wavefront analysis in aphakic patients. Methods Twenty five aphakic patients (25 eyes) who had been left aphakic after complicated phacoemulsification, leaving no capsular support, but good iris support and clear unwounded cornea implantated with the aphakic Verisyse®(AMO) IOL either implanted retropupilarely or over the iris. Wavefront aberrations and contrast sensitivity were measured using the Imagine Eyes IRX3 SH aberrometer. Results Twelve patients were implanted in the anterior chamber versus thirteen, who had the IOL clipped behind the iris. Surgery was uneventful in all patients. No postoperative complication was observed in the two groups. Best corrected visual acuity was significantly higher and HOAs were significantly lower in the retropupilarely implanted group. Simulated contrast sensitivity results calculated from Point Spread Function measurements were significantly better in the retropupilarly implanted group. There was no significant difference regarding HOAs and vision quality between the retropupillary implanted group and the panel of control âtients implanted with standard monofocal IOL in the capsular bag. Conclusion In addition to being atraumatic, the Verisyse® IOL implanted behind the iris may restore vision in the absence of capsular support in a more physiological way than when fixated over the iris. The overall quality of vision is better when the Verisyse® is implanted behind the iris and comparable to values obtained with standard monofocal IOLs.

Estimation and application of the solar adaptive optics corrected long-exposure point spread function

Telemetry data from the solar adaptive optics (AO) system at the 0.76m aperture Dunn Solar Telescope (DST) was used to estimate the long-exposure point spread function (PSF) delivered by the AO. The AO-corrected images were deconvolved ex post facto using the PSF estimate in order to improve the quantitative photometry of the images and the reliability of other derived physical quantities. Observations of the bright star Sirius obtained with the solar AO system produced direct measurements of the long-exposure PSF that were compared to the estimated PSFs. The match between estimates and direct measurements of the PSF is excellent, thus validating the implementation of the PSF estimation method for the DST AO system, which deploys a correlating Shack&#x2013;Hartmann wavefront sensor. The beneficial effect of postprocessing long-exposure solar images with the estimated PSFs is demonstrated for several observing situations, including stable and variable seeing conditions. The advantages and limitations of the application of the PSF estimation method to solar imagery and the quantitative measurements obtained from the observations are discussed in the broader context of solar AO observations.

Two-photon fluorescence correlation microscopy combined with measurements of point spread function; investigations made in human skin

Two-photon excitation fluorescence correlation spectroscopy (TPFCS) has been applied in connection to measurements of the point spread function (PSF) for quantitative analysis of sulphorhodamine B (SRB) in excised human skin. The PSF was measured using subresolution fluorescent beads embedded in the skin specimen. The PSF, measured as full width at half maximum (FWHM) was found to be 0.41 +/- 0.05 microm in the lateral direction, and 1.2 +/- 0.4 microm in the axial direction. The molecular diffusion of SRB inside the skin ranged between 0.5 and 15.0 x 10(-8) cm(2)/s. The diffusion coefficient is not dependent on depths down to 40 microm. The fluorophores were found to accumulate on the upper layers of the skin. This work is the first TPFCS study in human skin. The results show that TPFCS can be used for quantitative analyses of fluorescent compounds in human skin.

Three-dimensional characterization of optical coherence tomography point spread functions with a nanoparticle-embedded phantom

We present a novel (to our knowledge) approach for measurement of the three-dimensional point spread function (PSF) of optical coherence tomography (OCT) systems using a nanoparticle-embedded phantom (NEP), toward development of standardized test methods for biophotonic imaging. The NEP comprises highly reflective plasmonic nanoparticles, homogeneously distributed in a transparent silicone matrix. OCT image volumes were analyzed to characterize PSFs in axial and lateral directions at a variety of locations in the NEP. Results indicate submicrometer agreement with conventional approaches to measure dimensions of the PSF. The NEP offers a robust approach for validating and comparing imaging performance of OCT devices.

Spatially deconvolved optical coherence tomography

In this paper we present spatially mapped point-spread function (PSF) measurements of an optical coherence tomography (OCT) instrument and subsequent spatial deconvolution. The OCT B-scan image plane was divided into 2400 subimages, for which PSFs were determined from OCT measurements of a specially designed phantom. Each PSF was deconvolved from its corresponding subimage of the phantom using the Lucy-Richardson algorithm. Following deconvolution, all of the subimages were reassembled to form a final deconvolved image, from which the resolution improvement was quantitatively assessed. The lateral resolution was found to improve by 3.1 microm compared to an axial resolution enhancement of 4.5 microm. The spatial uniformity of both axial and lateral resolution was also observed to increase following deconvolution, demonstrating the advantage of deconvolving local PSFs from their associated subimages.

Noise and signal properties in PSF-based fully 3D PET image reconstruction: an experimental evaluation

The addition of accurate system modeling in PET image reconstruction results in images with distinct noise texture and characteristics. In particular, the incorporation of point spread functions (PSF) into the system model has been shown to visually reduce image noise, but the noise properties have not been thoroughly studied. This work offers a systematic evaluation of noise and signal properties in different combinations of reconstruction methods and parameters. We evaluate two fully 3D PET reconstruction algorithms: (1) OSEM with exact scanner line of response modeled (OSEM+LOR), (2) OSEM with line of response and a measured point spread function incorporated (OSEM+LOR+PSF), in combination with the effects of four post-reconstruction filtering parameters and 1–10 iterations, representing a range of clinically acceptable settings. We used a modified NEMA image quality (IQ) phantom, which was filled with 68Ge and consisted of six hot spheres of different sizes with a target/background ratio of 4:1. The phantom was scanned 50 times in 3D mode on a clinical system to provide independent noise realizations. Data were reconstructed with OSEM+LOR and OSEM+LOR+PSF using different reconstruction parameters, and our implementations of the algorithms match the vendor's product algorithms. With access to multiple realizations, background noise characteristics were quantified with four metrics. Image roughness and the standard deviation image measured the pixel-to-pixel variation; background variability and ensemble noise quantified the region-to-region variation. Image roughness is the image noise perceived when viewing an individual image. At matched iterations, the addition of PSF leads to images with less noise defined as image roughness (reduced by 35% for unfiltered data) and as the standard deviation image, while it has no effect on background variability or ensemble noise. In terms of signal to noise performance, PSF-based reconstruction has a 7% improvement in contrast recovery at matched ensemble noise levels and 20% improvement of quantitation SNR in unfiltered data. In addition, the relations between different metrics are studied. A linear correlation is observed between background variability and ensemble noise for all different combinations of reconstruction methods and parameters, suggesting that background variability is a reasonable surrogate for ensemble noise when multiple realizations of scans are not available.

Complex pupil retrieval with undersampled data

The ability to retrieve the complex-valued, generalized pupil function of an imaging system from undersampled measurements of the defocused system point spread function (PSF) is examined through numerical simulations. The ability to do so degrades as the detector pixel pitch increases when using a fixed number of PSF measurements. Two strategies for obtaining better results with undersampled data are demonstrated using additional PSF measurements with (i) random shifts due to system pointing fluctuations and (ii) intermediate amounts of defocus.

Evaluation of Noise Properties in PSF-Based PET Image Reconstruction.

The addition of accurate system modeling in PET image reconstruction results in images with distinct noise texture and characteristics. In particular, the incorporation of point spread functions (PSF) into the system model has been shown to visually reduce image noise, but the noise properties have not been thoroughly studied. This work offers a systematic evaluation of noise and signal properties in different combinations of reconstruction methods and parameters. We evaluate two fully-3D PET reconstruction algorithms: (1) OSEM with exact scanner line of response modeled (OSEM+LOR), (2) OSEM with line of response and a measured point spread function incorporated (OSEM+LOR+PSF), in combination with the effects of 4 post filtering parameters and 1-10 iterations. We used a modified NEMA IQ phantom, which was filled with 68Ge and consisted of 6 hot spheres of different sizes with a target/background ratio of 4:1. The phantom was scanned 50 times in 3D mode on a clinical system to provide independent noise realizations. Data were reconstructed with OSEM+LOR and OSEM+LOR+PSF using different reconstruction parameters. With access to multiple realizations, 4 metrics are adopted to quantify the noise characteristics in the reconstructed images. Image roughness and the standard deviation image are measures of the pixel-to-pixel variation, while NEMA and ensemble noises quantify the region-to-region variation. In addition to 4 noise metrics, we also evaluate signal to noise performance with accepted signal strength measures (recovery coefficient, SNR for quantitation), and study the relations between different metrics. From the analysis results, a linear correlation is observed between NEMA noise and ensemble noise for all different combinations of reconstruction methods and parameters, suggesting that NEMA style noise is a reasonable surrogate for ensemble noise when multiple realizations of scans are not available in practice. At the same number of iterations, the addition of PSF reduces image roughness for unfiltered images by roughly 35%, while the addition of PSF does not reduce NEMA style or ensemble noise. When noise is measured across realizations, the PSF based method offers slightly improved ( 7%) signal to noise performance across a range of reconstruction parameters.

Optical qualities of the Boston keratoprosthesis

Abstract Purpose To evaluate the optical characteristics of the Boston Keratoprosthesis (KPro) in relation to visual function. To determine glare sources and evaluate possible glare control. To explore the feasibility of fundus imaging. To examine the binocular consequences of implanting the device when the fellow eye has normal vision. Methods The computed and optical bench measured point spread function and corresponding modulation transfer function (MTF), as well as glare sources, were compared. A model eye construct was used to determine the relative impact of different sources of glare and its control. The effect of glare in eyes implanted with a KPro was measured with a Brightness Acuity Test (BAT) with and without a dark iris contact lens (dCL) for 8 eyes with Type I and 2 eyes with Type II KPro. Computed and measured visual fields were compared. The power of a lens that provides a wide field view of the fundus was calculated. Images with a fundus camera and lenses were obtained. Stereopsis was measured in 5 Type I KPro patients with an intact fellow eye. Results The computed and measured MTF for the KPro were found to be very close to the diffraction limit. Both the model eye and patients testing identified the hazy corneal graft as the main source of glare, which was controlled with a dCL. In 3 patients with an intact fellow eye, modest stereo acuity was gained with KPro implantation. Binocular visual field is not expanded when implanting a patient with normal fellow eye. Good image quality of the fundus is achieved in eyes with clear media. Conclusion Disability glare can be reduced with the use of a dCL. Implanting the KPro in a patient with an intact fellow eye only marginally improves visual function. Adequate fundus imaging can be obtained.