Coded Aperture

4 Key excerpts on "Coded Aperture"

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  eBook - ePub

  X-Ray Diffraction Imaging

  Technology and Applications

  • Joel Greenberg(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Furthermore, a variety of schemes to compressively and/or adaptively 10 sample and process the data have been developed and allow one to further improve the overall measurement efficiency in these systems. For example, Coded Aperture multiplexing can improve system sensitivity and SNR for compressible objects, even when photon noise (i.e., signal-dependent Poisson statistics) is dominant. 52 Beyond enhancing hyperspectral imaging in the conventional optical regime, Coded Apertures were also applied early on to X-ray astronomy, where one seeks to image a scene of stars (i.e., bright, point-like objects) on a dark background. Previous systems had employed a pinhole camera for forming images; to overcome the intrinsic tradeoff between throughput and resolution as the size of the single pinhole increases, however, Coded Apertures structured as Fresnel zone plates 53 or multi-pinhole arrays 54, 55 were employed. To recover sharp images of the stars with enhanced SNR (relative to the case of a single pinhole), one would then digitally correlate the recorded, multiplexed image with a complementary code. In the CA-XRDT scheme, there are two primary locations that one can place the Coded Aperture —between the source and object and between the object and detector. The former Coded Aperture allows one to limit the illuminated volume in a controlled way. Imaging a full three-dimensional volume with a source-side code therefore requires that one employ relative motion between the source, Coded Aperture, or object (as discussed in section on “cone beam CA-XRDT” below) to ensure that no locations are missed

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  eBook - ePub

  Tomographic Methods in Nuclear Medicine

  Physical Principles, Instruments, and Clinical Applications

  • Bhagwat D. Ahluwalia(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  In nuclear medicine the object of interest is the radioactive distribution within the patient. The image receptor is a film or, more often, a scintillation camera with the collimator removed. Decoding of the overlapping images cast by the aperture is usually done with the aid of a high-speed computer.

  Coded Aperture imaging can be interpreted as simply another approach to emission computed tomography. Tomography with Coded Apertures is sometimes referred to as longitudinal tomography, since the Coded Aperture approach reconstructs a set of planes parallel to the aperture plane.

  In principle, Coded Aperture data can be either multiplexed or unmultiplexed. The conventional approach to Coded Aperture imaging has been to cast overlapping images onto the detector to improve the photon collection efficiency. On the other hand, an approach such as seven-pinhole tomography19 that does not produce overlapping images is also a form of Coded Aperture imaging. Each pinhole records data on a distinct area of the scintillation crystal surface so that the data are “unmultiplexed”, or nonoverlapping, and no decoding algorithm is needed to obtain tomographic sections. High contrast images and short computing times provided by the impedance estimator algorithm developed by Kirch,20 as well as the low cost of this tomographic approach, account for the success of the seven-pinhole approach to tomographic imaging in nuclear medicine. This review concentrates on the clinical viability of multiplexed data collection using Coded Apertures.

  The primary motivation for replacing parallel multihole collimators or single-pinhole apertures with Coded Apertures in nuclear medicine is to improve the photon collection efficiency of the imaging system. Single-pinhole apertures and parallel multihole collimators used with scintillation cameras typically have collection efficiencies of 10–4

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  eBook - ePub

  Physical Principles of Astronomical Instrumentation

  • Peter A. R. Ade, Matthew J. Griffin, Carole E. Tucker(Authors)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  The principle is based on that of the pinhole camera, which produces an image of a point source at a well-defined position but with low SNR due to the fact that only a small fraction of the incident radiation is detected. Sensitivity can be improved by making the pinhole aperture larger (at the expense of some blurring) and/or having a random array of apertures. A coded mask is a plate with equal-sized square regions that are either transparent or opaque, which is placed in front of a position-sensitive detector with resolution equivalent to the mask element area. As shown in the one-dimensional schematic in Figure 11.22, a point source at a certain off-axis angle will create a laterally shifted shadowgram of the mask at the detector plane with the shift encoding the position and the signal strength encoding brightness. After exposure, the accumulated detector image from the whole field of view is decoded to form an image of the sky by determining the strength of all possible shifted mask patterns. Ideally, to ensure that every sky position is uniquely encoded, the autocorrelation function of the mask should be a delta function (as for the pinhole camera). Reconstruction of the sky image is achieved by correlating the image with a decoding pattern derived from the mask. The final angular resolution is equivalent to that defined by the mask element size but with much higher SNR given the fact that 30%–50% of the collecting aperture is utilised. Commonly used masks which provide a good approximation – a sharply peaked autocorrelation function with low sidelobes – are a random pattern (as used by the Swift-BAT instrument; see Section 11.11.2) or the Uniformly Redundant Array (as used by the INTEGRAL-IBIS instrument). FIGURE 11.22 Principle of coded mask imaging. 11.8.3 T he C ompton T elescope The physics of the Compton effect (Section 3.3.2) can be used to construct an imaging -ray telescope with a wide field of view and good angular resolution (a few degrees)

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  Displays

  Fundamentals & Applications, Second Edition

  • Rolf R. Hainich, Oliver Bimber(Authors)
  • 2016(Publication Date)
  • A K Peters/CRC Press

    (Publisher)

  Figure 8.21 ) and corresponding algorithms for static and programmable apertures are presented. It is explained how these patterns can be computed at interactive rates, by taking into account the image content and the limitations of the human visual system. Applications such as projector defocus compensation, high-quality projector de-pixelation, and increased temporal contrast of projected video sequences can be supported. Coded Apertures are a step toward next-generation auto-iris projector lenses.

  FIGURE 8.21 Two prototypes with a static broadband mask and an image-adaptive Coded Aperture realized with a programmable liquid crystal array (top). Focused and defocused image before and after deconvolution with different apertures (low-pass circular, broadband, adapted), and comparison of light loss when achieving the same depth of field with a circular aperture as with an adaptive Coded Aperture (bottom) [103 ].

  An alternative approach that is less dependent on the actual frequencies in the input image was introduced in [26 ]. Multiple overlapping projectors with varying focal depths illuminate arbitrary surfaces with complex geometry and reflectance properties. Pixel-precise focus values Φ

  i,x,y

  are automatically estimated at each camera pixel (x, y) for every projector. Therefore, a uniform grid of circular patterns is displayed by each projector and recorded by a camera. In order to capture the same picture (geometrically and color-wise) for each projection, these are pre-distorted and radiometrically compensated as described in Sections 8.3 and 8.4 .

  Once the relative focus values are known, an image from multiple projector contributions with minimal defocus can be composed in realtime. A weighted image composition represents a trade-off between intensity enhancement and focus refinement as:

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