Quantitative comparison of planar coded aperture imaging reconstruction methods

Imaging distributions of radioactive sources plays a substantial role in nuclear medicine as well as in monitoring nuclear waste and its deposit. Coded Aperture Imaging (CAI) has been proposed as an alternative to parallel or pinhole collimators, but requires image reconstruction as an extra step. Multiple reconstruction methods with varying run time and computational complexity have been proposed. Yet, no quantitative comparison between the different reconstruction methods has been carried out so far. This paper focuses on a comparison based on three sets of hot-rod phantom images captured with an experimental γ-camera consisting of a Tungsten-based MURA mask with a 2 mm thick 256 × 256 pixelated CdTe semiconductor detector coupled to a Timepix© readout circuit. Analytical reconstruction methods, MURA Decoding, Wiener Filter and a convolutional Maximum Likelihood Expectation Maximization (MLEM) algorithm were compared to data-driven Convolutional Encoder-Decoder (CED) approaches. The comparison is based on the contrast-to-noise ratio as it has been previously used to assess reconstruction quality. For the given set-up, MURA Decoding, the most commonly used CAI reconstruction method, provides robust reconstructions despite the assumption of a linear model. For single image reconstruction, however, MLEM performed best of all analytical reconstruction methods, but took on average 45 times longer than MURA Decoding. The fastest reconstruction method is the Wiener Filter with a run time 4.3 times faster compared to MURA Decoding and a mediocre quality. The CED with a specifically tailored training set was able to succeed the most commonly used MURA decoding on average by a factor between 1.37 and 2.60 and an equal run time

Methodological approaches to planar and volumetric scintigraphic imaging of small volume targets with high spatial resolution and sensitivity

Single-photon emission computed tomography (SPECT) is a non-invasive imaging technique, which provides information reporting the functional states of tissues. SPECT imaging has been used as a diagnostic tool in several human disorders and can be used in animal models of diseases for physiopathological, genomic and drug discovery studies. However, most of the experimental models used in research involve rodents, which are at least one order of magnitude smaller in linear dimensions than man. Consequently, images of targets obtained with conventional gamma-cameras and collimators have poor spatial resolution and statistical quality. We review the methodological approaches developed in recent years in order to obtain images of small targets with good spatial resolution and sensitivity. Multipinhole, coded mask- and slit-based collimators are presented as alternative approaches to improve image quality. In combination with appropriate decoding algorithms, these collimators permit a significant reduction of the time needed to register the projections used to make 3-D representations of the volumetric distribution of target’s radiotracers. Simultaneously, they can be used to minimize artifacts and blurring arising when single pinhole collimators are used. Representation images are presented, which illustrate the use of these collimators. We also comment on the use of coded masks to attain tomographic resolution with a single projection, as discussed by some investigators since their introduction to obtain near-field images. We conclude this review by showing that the use of appropriate hardware and software tools adapted to conventional gamma-cameras can be of great help in obtaining relevant functional information in experiments using small animals.FAPESPFAPESP CInAPC

A Programmable Liquid Collimator for Both Coded Aperture Adaptive Imaging and Multiplexed Compton Scatter Tomography

A novel, fully reconfigurable collimator device for gamma-ray and X-ray imaging was built and tested as a coded aperture. The device consisted of 10x10, 5x5x5 mm3 chambers. Each chamber either was filled with an attenuating liquid, stopping photons, or evacuated of the attenuating liquid, allowing the photons to pass through. As the pattern of on and off chambers was manipulated, different, semi-independent views of the gamma-ray source were found. Noise in reconstructed images decreased in all tests. Image reconstruction was performed with correlation methods and Maximum Likelihood Expectation Maximization (ML-EM). Using 10 mask patterns, the signal-to-noise ratio (SNR) in images of a Co-57 point source increased by a factor of 4.3 using correlation methods and by a factor of at least 50 using ML-EM. SNR in images of a Cd-109 source with high background increased by a factor of 3.0 using correlation methods and by a factor of 1.8 using ML-EM. Two extended sources were imaged, and the images improved when more masks were used. The Multiplexed Compton Scatter Tomography (MCST) forward problem using a PHDs Co high purity germanium (HPGe) detector was tested and evaluated. Potential applications are discussed in detail

Quantification of Fast-Neutron Sources with Coded Aperture Imaging

Quantification of the mass of plutonium in facilities that process plutonium is important for both nuclear safeguards concerns and safety concerns, and multiple methods to nondestructively quantify plutonium sample characteristics have been proposed, particularly when the sample is located directly adjacent to or within the measurement device. In prior work, coded-aperture fast neutron imaging has been developed to demonstrate the imaging of neutron emitting radiation sources in a qualitative fashion, where the sources may be located meters to tens of meters away. Building upon prior work, this work develops the use of a Maximum Likelihood Expectation Maximization (MLEM) reconstruction technique to simultaneously reconstruct neutron sources measured from different detector positions. Moreover, a modified system response model is developed to accurately but quickly perform forward projections in order to accurately reconstruct and quantify neutron source characteristics including source intensity and location. The system response model incorporates mask transmission, a heterogeneous detector pixel array, scattering within the mask, and scattering within the detector, allowing for the expected detector data from a single source position to be generated in less than a second. The behavior of the MLEM reconstruction technique is discussed, and measurements of Cf-252 sources, acting as a surrogate Pu material, are reconstructed and analyzed. Using the methods developed here, a single 74 µCi Cf-252 point source placed at a distance of 200 cm is reconstructed within 2% of the known position and within 3% of known intensity at distances up to 300 cm. Measurements of more than one source and implications for Pu measurements in facilities are also discussed

Towards clinically useful coded apertures for planar nuclear medicine imaging.

Coded apertures have the potential to increase imaging e ciency in nuclear medicine without degrading resolution, but near- eld artifacts are present even under idealised aperture and imaging conditions. The purpose of this work is to reduce artifacts prior to reconstruction, and to work towards coded apertures that are clinically useful. A ray-tracing simulator was developed. Far- eld conditions produce nearperfect images, but the simulation of distributed sources under idealised near- eld conditions results in the presence of artifacts. Three core concepts are introduced in this thesis: a novel rotatable array of identical limited- eld-of-view coded apertures; the use of high-resolution aperture projections; and the deliberate and counter-intuitive use of thin, highly transparent aperture material. An array of identical limited- eld-of-view coded apertures, which can be rotated so as to implement an existing artifact-reduction technique, was simulated. The artifacts that exist for a single coded aperture under idealised conditions are removed. This novel technique remains e ective when realistic near- eld conditions are introduced into the simulation. However, realistic apertures increase artifacts due to nite pinhole widths and nite thickness of the aperture material. To address the pinhole width problem, high-resolution patterns, in which the smallest hole corresponds to a projection area of 1 1 detector pixels, o er the best trade-o between e ciency and resolution despite the partial volume e ect. The nite aperture thickness problem is addressed by another novel concept; viz. the deliberate reduction in material thickness, which results in a highly transparent coded aperture. Simulation shows that this counter-intuitive approach diminishes collimation e ects. The implementation of any or all of these three core concepts, however, reduces count statistics. An ultra-near- eld geometry, which would ordinarily result in severe artifacts, can theoretically be used to maintain count statistics, without altering either patient dose or image acquisition time. This was veri ed by simulation. ii ABSTRACT A prior-state-of-the-art 1 mm thick tungsten coded aperture, and a deliberately highly transparent 100 m tungsten foil coded aperture, were constructed for use with a dual head gamma camera. Phantom studies of Technetium-99m point, line, syringe and printed distributed sources were performed. The experimental acquisitions veri ed the simulation results, for both the prior-state-of-the-art coded aperture and the novel high-transparency coded aperture. The results and arguments presented in this thesis point to the potential for these three core developments to produce high-quality coded aperture images in a fraction of the time that is taken for a collimator acquisition. The limiting factor appears to be the poor count statistics that result from the low sensitivity of current gamma cameras; a situation which looks set to change given current research trends

A coded aperture with sub-mean free-path thickness for neutron implosion geometry imaging on inertial confinement fusion and inertial fusion energy experiments

Inertial confinement fusion and inertial fusion energy experiments diagnose the geometry of the fusion region through imaging of the neutrons released through fusion reactions. Pinhole arrays typically used for such imaging require thick substrates to obtain high contrast along with a small pinhole diameter to obtain high resolution capability, resulting in pinholes that have large aspect ratios. This leads to expensive pinhole arrays that have small solid angles and are difficult to align. Here, we propose a coded aperture with scatter and partial attenuation (CASPA) for fusion neutron imaging that relaxes the thick substrate requirement for good image contrast. These coded apertures are expected to scale to larger solid angles and are easier to align without sacrificing imaging resolution or throughput. We use Monte Carlo simulations (Geant4) to explore a coded aperture design to measure neutron implosion asymmetries on fusion experiments at the National Ignition Facility (NIF) and discuss the viability of this technique, matching the current nominal resolution of 10 µm. The results show that a 10 mm thick tungsten CASPA can image NIF implosions with neutron yields above 1014 with quality comparable to unprocessed data from a current NIF neutron imaging aperture. This CASPA substrate is 20 times thinner than the current aperture arrays for fusion neutron imaging and less than one mean free-path of 14.1 MeV neutrons through the substrate. Since the resolution, solid angle, and throughput are decoupled in coded aperture imaging, the resolution and solid angle achievable with future designs will be limited primarily by manufacturing capability

Reconstruction algorithms for multispectral diffraction imaging

Thesis (Ph.D.)--Boston UniversityIn conventional Computed Tomography (CT) systems, a single X-ray source spectrum is used to radiate an object and the total transmitted intensity is measured to construct the spatial linear attenuation coefficient (LAC) distribution. Such scalar information is adequate for visualization of interior physical structures, but additional dimensions would be useful to characterize the nature of the structures. By imaging using broadband radiation and collecting energy-sensitive measurement information, one can generate images of additional energy-dependent properties that can be used to characterize the nature of specific areas in the object of interest. In this thesis, we explore novel imaging modalities that use broadband sources and energy-sensitive detection to generate images of energy-dependent properties of a region, with the objective of providing high quality information for material component identification. We explore two classes of imaging problems: 1) excitation using broad spectrum sub-millimeter radiation in the Terahertz regime and measure- ment of the diffracted Terahertz (THz) field to construct the spatial distribution of complex refractive index at multiple frequencies; 2) excitation using broad spectrum X-ray sources and measurement of coherent scatter radiation to image the spatial distribution of coherent-scatter form factors. For these modalities, we extend approaches developed for multimodal imaging and propose new reconstruction algorithms that impose regularization structure such as common object boundaries across reconstructed regions at different frequencies. We also explore reconstruction techniques that incorporate prior knowledge in the form of spectral parametrization, sparse representations over redundant dictionaries and explore the advantage and disadvantages of these techniques in terms of image quality and potential for accurate material characterization. We use the proposed reconstruction techniques to explore alternative architectures with reduced scanning time and increased signal-to-noise ratio, including THz diffraction tomography, limited angle X-ray diffraction tomography and the use of coded aperture masks. Numerical experiments and Monte Carlo simulations were conducted to compare performances of the developed methods, and validate the studied architectures as viable options for imaging of energy-dependent properties