Mask transparency in coded aperture imaging

Purpose: Improved localization of gamma-ray emissions from radiotracers is essential to the progress of nuclear medicine. Polaris is a portable, room-temperature operated gamma-ray imaging spectrometer composed of two 3×3 arrays of thick CdZnTe (CZT) detectors, which detect gammas between 30keV and 3MeV with energy resolution of <1% FWHM at 662keV. Compton imaging is used to map out source distributions in 4-pi space; however, is only effective above 300keV where Compton scatter is dominant. This work extends imaging to photoelectric energies (<300keV) using coded aperture imaging (CAI), which is essential for localization of Tc-99m (140keV). Methods: CAI, similar to the pinhole camera, relies on an attenuating mask, with open/closed elements, placed between the source and position-sensitive detectors. Partial attenuation of the source results in a “shadow” or count distribution that closely matches a portion of the mask pattern. Ideally, each source direction corresponds to a unique count distribution. Using backprojection reconstruction, the source direction is determined within the field of view. The knowledge of 3D position of interaction results in improved image quality. Results: Using a single array of detectors, a coded aperture mask, and multiple Co-57 (122keV) point sources, image reconstruction is performed in real-time, on an event-by-event basis, resulting in images with an angular resolution of ∼6 degrees. Although material nonuniformities contribute to image degradation, the superposition of images from individual detectors results in improved SNR. CAI was integrated with Compton imaging for a seamless transition between energy regimes. Conclusion: For the first time, CAI has been applied to thick, 3D position sensitive CZT detectors. Real-time, combined CAI and Compton imaging is performed using two 3×3 detector arrays, resulting in a source distribution in space. This system has been commercialized by H3D, Inc. and is being acquired for various applications worldwide, including proton therapy imaging R&D.

In nuclear medicine, images of planar scintigraphy and single photon emission computerized tomography (SPECT) obtained through gamma camera (GC) appear to be blurred. Alternatively, coded aperture imaging (CAI) can surpass the quality of GC images, but still it is not extensively used due to the decoding complexity of some images and the difficulty in controlling the noise. Summing up, the images obtained through GC are low quality and it is still difficult to implement CAI technique. Here we present a full aperture imaging (FAI) technique which overcomes the problems of CAI ordinary systems. The gamma radiation transmitted through a large single aperture is edge-encoded, taking advantage of the fact that nuclear radiation is spatially incoherent. The novel technique is tested by means of Monte Carlo method with simple and complex sources. Spatial resolution tests and parallax tests of GC versus FAI were made, and three-dimensional capacities of GC versus FAI were analyzed. Simulations have allowed comparison of both techniques under ideal, identical conditions. The results show that FAI technique has greater sensitivity (approximately 100 times) and greater spatial resolution (>2.6 times at 40 cm source-detector distance) than that of GC. FAI technique allows to obtain images with typical resolution of GC short source-detector distance but at longer source-detector distance. The FAI decoding algorithm simultaneously reconstructs four different projections, while GC produces only one projection per acquisition. Our results show it is possible to apply an extremely simple encoded imaging technique, and get three-dimensional radioactivity information. Thus GC-based systems could be substituted, given that FAI technique is simple and it produces four images which may feed stereoscopic systems, substituting in some cases, tomographic reconstructions.

Summary form only given. The Coded Aperture Imaging (CAI) technique, which was originally developed for x-ray and γ-ray astronomy, has been applied to investigate the spatial distribution of DD fusion in a 1.6 kJ (1-3×108 n/shot) Plasma Focus device. The coded mask pattern is based on a Singer cyclic difference set with 341 open pixels in 91×15 array (25% open fraction). The physical mask was fabricated by laser-machining 300 μm square holes in stainless steel foil. As the plasma pinch is a semi-extended line source, the mask pattern rows (×15 dimension) were spaced by a factor of 6, to obtain an almost square mask and reduce the effective open fraction to about 4%, which enhances the image signal-to-noise ratio. The CAI cameras were placed at 90° to plasma focus axis, with CR-39 detectors used to register tracks of protons from the D(d,p)T reaction. A 75 μm Kapton film stopped all charged particles, other than the ~3 MeV protons, from reaching the CR-39. A deconvolution algorithm was applied to the measured proton tracks coordinates to obtain images of the DD fusion source. The number of proton tracks registered per shot was typically (1-2)×105. Two beryllium fast-neutron detectors and a plastic scintillator were employed simultaneously to measure the time-integrated and time-resolved neutron yield and anisotropy for each shot. The results clearly show that the CAI technique yields images with greater detail and considerably improved signal-to-noise ratio by comparison with conventional pinhole imaging.

The coded aperture imaging technique is a useful method of x-ray imaging in observational astrophysics. However, the presence of imaging noise or so-called artifacts in a decoded image is a drawback of this method. We propose a coded aperture imaging method using multiple different random patterns for significantly reducing the image artifacts. This aperture mask contains multiple different patterns each of which generates a different artifact distribution in its decoded image. By summing all decoded images of the different patterns, the artifact distributions are cancelled out, and we obtain a remarkably accurate image. We demonstrate this concept with imaging experiments of a monochromatic 16-keV hard x-ray beam at the synchrotron photon facility SPring-8, using the combination of a complementary metal-oxide-semiconductor image sensor and an aperture mask that has four different random patterns composed of holes with a diameter of 27 μm and a separation of 39 μm. The entire imaging system is installed in a 25-cm-long compact size and achieves an angular resolution of <30 arc sec (full-width at half-maximum). In addition, we show by Monte Carlo simulation that the artifacts can be reduced more effectively if the number of different patterns increases to 8 or 16.

Coded aperture imaging (CAI) has evolved as a standard technique for imaging high energy photon sources and has found numerous applications. Coded aperture arrays (CAAs) are the most important devices in the applications of CAI. In recent years, many approaches were presented to design optimum or near-optimum CAAs. Uniformly redundant arrays (URAs) are the most successful CAAs for their cyclic autocorrelation consisting of a sequence of delta functions on a flat sidelobe which can easily be subtracted when the object has been reconstructed. Unfortunately, the existing methods can only be used to design URAs with limited number of array sizes and fixed autocorrelative sidelobe-to-peak ratio. In this paper, we presented a method to design more flexible URAs by means of a global optimization algorithm named DIRECT. By our approaches, we obtain various types of URAs including the filled URAs which can be constructed by existing methods and the sparse URAs which never be constructed and mentioned by existing papers as far as we know.

The coded aperture imaging (CAI) technique has been applied to investigate the spatial distribution of DD fusion in a deuterium filled 1.6 kJ plasma focus (PF) device operated in its neutron-optimized regime. The coded mask has been fabricated in Havar alloy with 20 × 20 pixels and 57 square holes (14% open area) using a Singer-set Cyclic Difference pattern. Five CAI cameras were employed simultaneously: one placed on-axis (0°) and four at 45° to the PF axis. CR-39 polymer nuclear track detectors were used to register proton tracks from D(d,p)T reactions. The detectors were covered by 75 μm Kapton film to stop all energetic charged particles (mostly deuterons) other than the 3 MeV fusion protons. A de-convolution algorithm was applied to the measured proton track coordinates to obtain images of the fusion source from the five directions. Typically (2–8) × 104 proton tracks were registered by each detector per shot. Also two beryllium fast-neutron detectors were employed simultaneously to measure the associated neutron yield and anisotropy. The CAI images show the fusion source is a cigar-shaped region around the pinch column with no indication of plasma column instabilities. The proton anisotropy is comparable to the measured neutron anisotropy.

We report terahertz coded-aperture imaging using photo-induced reconfigurable aperture arrays on a silicon wafer. The coded aperture was implemented using programmable illumination from a commercially available digital light processing projector. At 590 GHz, each of the array element apertures can be optically turned on and off with a modulation depth of 20 dB and a modulation rate of ~ 1.3 kHz. Prototype demonstrations of 4 ×4 coded-aperture imaging using Hadamard coding have been performed. Continuous THz imaging with 8 ×8 pixels has also been demonstrated, using a slowly moving metal strip as the target. In addition, this technique has been successfully applied to mapping THz beams by using a 6 ×6 aperture array at 590 GHz. The imaging results agree closely with theoretical calculations based on Gaussian beam propagation, demonstrating that this technique is promising for realizing real-time and low-cost terahertz cameras for many applications. The reported approach provides a simple but powerful means to visualize THz beams, which is highly desired in quasi-optical system alignment, quantum-cascade laser design and characterization, and THz antenna characterization.

Coded aperture imager with depth of interaction scintillators

In the processing diagnosis of the inertia confinement fusion (ICF), the ring coded aperture imaging technique is applied in order to gain high space and time resolutions simultaneously. The key of acquiring high spatial resolution is how to obtain a point spread function of an imaging system. The common method is the firsthand projection method which is an approximate one. The x-ray diffraction effect is neglected in the method, so the resolution of reconstructed image is decreased. We have derived the point spread function (PSF) of the ring coded aperture from scalar diffraction theory. And the Wiener filter is fabricated on the basis of the PSF. In National Key Laboratory of Laser Fusion, at China Academy of Engineering Physics, the imaging experiment on the diagnosis of ICF is completed using the ring coded aperture plate with inner diameter <i>d</i>₁ = 250&mu;m and outer diameter <i>d</i>₂ = 260&mu;m .The obtained coded image is processed by the Wiener filter which diffractive effect is considered. The processed results have shown that resolution and modulation contrast of acquired image are evidently better than the results obtained by the firsthand projection method.

Coded aperture correlation holography (COACH) combines incoherent digital holography with coded aperture imaging. COACH is also a method to record incoherent digital holograms of three-dimensional object scenes. Still, COACH can be used for several other incoherent and coherent optical applications. In this chapter, we survey the prime landmarks on the topic of COACH from two major perspectives: architectures and applications of the various systems. We explore the main configurations of hologram recorders in the COACH systems. For each design, we describe some of the recent implementations of these recorders in optical imaging. We conclude the chapter with general ideas on this technology.

A novel coded aperture imaging system working in optical spectrum region is described. Excellent simulation results are presented. The camera is also shown to be capable of multi-wavelength imaging with a single monochromatic detector array.

Using computer simulations, comparisons are made between a one-dimensional Fresnel zone aperture and a one dimensional uniformly redundant array aperture. In the former case, artifacts may produce severe distortions in the reconstructed image, and an iterative technique to remove these artifacts is described. In comparison to the Fresnel zone aperture, images reconstructed from uniformly redundant array apertures have few artifacts. A quantum noise analysis for the uniformly redundant array is presented to indicate the conditions under which a uniformly redundant array will produce a signal-to-noise advantage compared to an equivalent pinhole aperture.

Coded aperture imaging with uniformly redundant arrays (URAs) is the standard technique for imaging above the limit of grazing incident x-ray telescopes. It is an ideal technique for high energy astrophysics because it has high throughput, excellent performance on point sources, and the ability to measure simultaneously signal and background. However, many sources of interest in high energy astrophysics are time variable or require detailed energy spectra. Until now, to obtain a single time (or energy) sample, the photons from the particular time (or energy) interval must be formed into an encoded pattern, then processed to obtain an image for that sample. Therefore, massive computations are required to cover the entire time and energy parameter space. We present a new method of coded aperture analysis called URA tagging, which provides time and/or energy resolved histories of sources with known positions without using a correlation operation. It can easily reduce the computation time by orders of magnitude ompared to the next fastest method, the fast delta Hadamard transform. URA tagging can also correct for improperly encoded images or motion blurred images. Whereas previous methods for quantifying performance have not taken into account the finite resolution or the quantized sampling, URA tagging provides a SNR equation that includes all such effects. URA tagging analysis explains why delta decoding has a somewhat poorer SNR than balanced correlation; naively, one expects the better angular resolution to yield a better SNR. In addition, we show that complementary URAs (exchanged opaque and transparent elements) have different properties, and those with an even number of transparent elements should be preferred.

Coded aperture imaging (CAI) technology is a rapidly evolving indirect imaging method with extraordinary potential. In recent years, CAI based on chaotic optical waves have been shown to exhibit multidimensional, multispectral, and multimodal imaging capabilities with a signal to noise ratio approaching the range of lens based direct imagers. However, most of the earlier studies used only narrow band illumination. In this study, CAI based on chaotic optical waves is investigated for white light illumination. A numerical study was carried out using scalar diffraction formulation and correlation optics and the lateral and axial resolving power for different spectral width were compared. A binary diffractive quasi-random lens was fabricated using electron beam lithography and the lateral and axial point spread holograms are recorded for white light. Three-dimensional imaging was demonstrated using thick objects consisting of two planes. An integrated sequence of signal processing tools such as non-linear filter, low-pass filter, median filter and correlation filter were applied to reconstruct images with an improved signal to noise ratio. A denoising deep learning neural network (DLNN) was trained using synthetic noisy images generated by the convolution of recorded point spread functions with the virtual object functions under a wide range of aberrations and noises. The trained DLNN was found to reduce further the reconstruction noises.

The resolution limit of the interferenceless coded aperture correlation holography system is improved by inserting a phase-only SLM before the system entrance and by introducing a sparse point response to improve the signal-to-noise ratio.