Advanced coded-aperture imaging system for nuclear medicine.

Coded aperture imaging with multiple measurements

Abstract: In coded aperture imaging, only aperture arrays consisting of (0, 1) elements are physically realizable. If multiple coded images are obtained with different aperture masks and the resulting detector images are suitably combined, a larger variety of aperture arrays, such as multilevel, complex-valued, vector-valued, or complementary arrays becomes applicable. We present a general theory of coded aperture imaging with multiple measurements. An image reconstruction scheme from the coded images is described that results in a maximum signal-to-noise ratio. Also, the design of sets of aperture arrays is addressed and explicitly solved for several important cases. It is shown how known classes of correlation arrays can be beneficially applied to coded aperture imaging.

Complementary lattice arrays for coded aperture imaging

Abstract: In this work, we propose the concept of complementary lattice arrays in order to enable a broader range of designs for coded aperture imaging systems. We provide a general framework and methods that generate richer and more flexible designs compared to the existing techniques. Besides this, we review and interpret the state-of-the-art uniformly redundant array designs, broaden the related concepts, and propose new design methods.

Speeding Up Image Reconstruction Methods in Coded Mask γ Cameras Using Neural Networks: Application to the EM Algorithm

Abstract: When using γ-ray coded-mask cameras, one does not get a direct image as in classical optical cameras but the correlation of the mask response with the source. Therefore the data must be mathematically treated in order to reconstruct the original sky sources. Generally this reconstruction is based on linear methods, such as correlating the detector plane with a reconstruction array, or non-linear ones such as iterative or maximization methods (i.e. the EM algorithm). The latter have a better performance but they increase the computational complexity by taking a lot of time to reconstruct an image. Here we present a method for speeding up such kind of algorithms by making use of a neural network with a back-propagation learning rule.

Coded-aperture imaging system for reconstructing tomograms of human myocardium.

Abstract: To increase the detection efficiency and improve the spatial resolution, a coded-aperture imaging method is applied to nuclear medicine. The aperture consists of nine pinholes arranged in a square grid. Three kinds of coding are sequentially used to record the same number of projections including parallax and overlap. The overlapped images are partially separated, and good tomograms of a ring phantom and a human myocardium are reconstructed using a modified backprojection algorithm with variable damping factor.

Gamma Radiation Imaging System via Variable and Time-Multiplexed Pinhole Arrays.

Abstract: Biomedical planar imaging using gamma radiation is a very important screening tool for medical diagnostics. Since lens imaging is not available in gamma imaging, the current methods use lead collimator or pinhole techniques to perform imaging. However, due to ineffective utilization of the gamma radiation emitted from the patient’s body and the radioactive dose limit in patients, poor image signal to noise ratio (SNR) and long image capturing time are evident. Furthermore, the resolution is related to the pinhole diameter, thus there is a tradeoff between SNR and resolution. Our objectives are to reduce the radioactive dose given to the patient and to preserve or improve SNR, resolution and capturing time while incorporating three-dimensional capabilities in existing gamma imaging systems. The proposed imaging system is based on super-resolved time-multiplexing methods using both variable and moving pinhole arrays. Simulations were performed both in MATLAB and GEANT4, and gamma single photon emission computed tomography (SPECT) experiments were conducted to support theory and simulations. The proposed method is able to reduce the radioactive dose and image capturing time and to improve SNR and resolution. The results and method enhance the gamma imaging capabilities that exist in current systems, while providing three-dimensional data on the object.

Fresnel zone plate imaging of gamma rays; theory

Abstract: The use of a Fresnel zone plate as a coded aperture for imaging incoherent radiation such as gamma rays has been previously reported. The coded image is in many respects similar to a hologram and can be decoded or reconstructed with a coherent optical system. In this paper, the general theory of coded-aperture imaging is presented, first for an arbitrary code and then for an on-axis zone plate, an off-axis zone plate, and a one-dimensional zone plate (or linear chirp). With the on-axis plate, a matched imaging condition is suggested as a guide to optimizing image contrast. With the off-axis zone plate and the linear chirp, it is necessary to use a half-tone screen to spatially heterodyne the object spectrum into the passband of the aperture. In all three cases, expressions for the resolution, depth of field, field of view, and relative efficiency are derived. A simplified noise analysis is presented, and some practical system constraints are discussed.

Multiplex imaging with multiple-pinhole cameras

Abstract: When making photographs in x rays or γ rays with a multiple‐pinhole camera, the individual images of an extended object such as the sun may be allowed to overlap; then the situation is in many ways analogous to that in a multiplexing device such as a Fourier spectroscope. Some advantages and problems arising with such use of the camera are discussed, and expressions are derived to describe the relative efficacy of three exposure/postprocessing schemes using multiple‐pinhole cameras.

Coded Aperture Imaging with X-rays (Flashing Tomosynthesis)

Abstract: So far, three-dimensional X-ray imaging methods like tomography, etc. require exposure times of a few seconds or more. Hence, moving objects like the pulsating heart cannot be observed. This obstacle can be overcome by using an array of synchronously flashed X-ray sources. The source array acts as the coded aperture. The X-ray photograph is decoded optically, showing arbitrary layers of the object. We present four new versions of ‘flashing tomosynthesis’, as this approach is called. The obtainable image qualities and other practical features of these four new methods will be compared.

An advanced coded imaging without side lobes

Abstract: An advanced coded imaging is proposed which makes it possible to clear away the side lobes from reconstructed images by using a pair of coherent codes. Available coherent codes and their sizes are analyzed, and a computer simulation is presented.

Gamma-ray imaging with stochastic apertures.

Abstract: The spatial distribution of a radioactive fluid can be measured indirectly by observing the emerging gamma rays. A method is proposed and analyzed for gamma-ray imaging by stochastic time modulation and cross-correlation. Theoretical comparison is made to collimation and coded aperture techniques in gamma-ray image formation. Computed results are presented that illustrate the mean response and statistical error characteristics of this technique. Monte Carlo simulations are performed as a further verification. Because it relies upon a point-by-point reconstruction, rather than upon the integral properties of any particular aperture, the time modulation approach is seen to provide a theoretical basis for obtaining a smooth three-dimensional point response.