Spatial-spectral modulating snapshot hyperspectral imager

Experimental results are presented for a computed tomography imaging spectrometer (CTIS) with imposed spatial-spectral modulation on the image scene. This modulation structure on the CTIS tomographic dispersion created substantial gains in spectral reconstruction resolution after standard iterative, nonlinear, inversion techniques were used. Modulation limits system ambiguities, so high-frequency spectral and low-frequency spatial scene data could be recovered. The results demonstrate how spatial modulation acts as a high-frequency spectral deconvolver for the snapshot hyperspectral imager technology.     

Automated determination of the center of rotation in tomography data

Tomographic reconstruction requires precise knowledge of the position of the center of rotation in the sinogram data; otherwise, artifacts are introduced into the reconstruction. In parallel-beam microtomography, where resolution in the 1 microm range is reached, the center of rotation is often only known with insufficient accuracy. We present three image metrics for the scoring of tomographic reconstructions and an iterative procedure for the determination of the position of the optimum center of rotation. The metrics are applied to model systems as well as to microtomography data from a synchrotron radiation source. The center of rotation is determined using the image metrics and compared with the results obtained by the center-of-mass method and by image registration. It is found that the image metrics make it possible to determine the axis position reliably at well below the resolution of one detector bin in an automated procedure.     

A new coded-excitation ultrasound imaging system. I. Basicprinciples

A new approach to ultrasound imaging with coded-excitation is presented. The imaging is performed by reconstruction of the scatterer strength on an assumed grid covering the region of interest (ROI). Our formulation is based on an assumed discretized signal model which represents the received sampled data vector as a superposition of impulse responses of all scatterers in the ROI. The reconstruction operator is derived from the pseudo-inverse of the linear operator (system matrix) that produces the received data vector. The singular value decomposition (SVD) method with appropriate regularization techniques is used for obtaining a robust realization of the pseudo-inverse. Under simplifying (but realistic) assumptions, the pseudo-inverse operator (PIO) can be implemented using a bank of transversal filters with each filter designed to extract echoes from a specified image line. This approach allows for the simultaneous acquisition of a large number of image lines. This could be useful in increasing frame rates for two-dimensional imaging systems or allowing for real-time implementation of three-dimensional imaging systems. When compared to the matched filtering approach to similar coded-excitation systems, our approach eliminates correlation artifacts that are known to plague such systems. Furthermore, the lateral resolution of the new system can exceed the diffraction limit imposed on conventional imaging systems utilizing delay-and-sum beamformers. The range resolution is compared to that of conventional pulse-echo systems with resolution enhancement (our PIO behaves as a pseudo-inverse Wiener filter in the range direction). Both simulation and experimental verification of these statements are given     

Feasibility Study for Improving the Image Characteristics in Digital Tomosynthesis (DTS) Using a Compressed-Sensing (Cs)-Based Pre-Deblurring Scheme

Digital tomosynthesis (DTS) has been widely used in both industrial nondestructive testing and medical x-ray imaging as a popular multiplanar imaging modality. However, although it provides some of the tomographic benefits of computed tomography (CT) at reduced dose and imaging time, the image characteristics are relatively poor due to blur artifacts originated from incomplete data sampling for a limited angular range and also aspects inherent to imaging system, including finite focal spot size of the x-ray source, detector resolution, etc. In this work, in order to overcome these difficulties, we propose an intuitive method in which a compressed-sensing (CS)-based deblurring scheme is applied to the projection images before common DTS reconstruction. We implemented the proposed deblurring algorithm and performed a systematic experiment to demonstrate its viability for improving the image characteristics in DTS. According to our results, the proposed method appears to be effective for the blurring problems in DTS and seems to be promising to our ongoing application to x-ray nondestructive testing.     

Effects of the time dependence of a bioluminescent source on the tomographic reconstruction

There are two goals in this simulation study: (1) to show that the time variation of the bioluminescence source can cause artifacts in the tomographic images such that quantification and localization becomes impossible; and (2) to show that the a priori knowledge of the light kinetics can be used to eliminate these artifacts. These goals are motivated by the fact that the half-life of luciferase has been reported as 30 min to 2 h in vivo. We perform two-dimensional simulations. We consider a 40 mm diameter circular region with an inclusion of 6 mm diameter located 10 mm away from the center. The measurement data is simulated using a finite-element-based forward solver. We model the noncontact measurements such that four-wavelength data is collected from four 90 degrees apart views. The results show that the ratio of the total imaging time to the half-life of the exponentially decaying bioluminescent source is the deciding factor in the reconstruction of the source. It is also demonstrated that a priori knowledge of the source kinetics is required to perform tomographic bioluminescence imaging of short half-life bioluminescent sources and the use of spatial a priori information alone is not adequate.     

Comparison of two- and three-dimensional reconstruction methods in optical tomography

We present a three-dimensional (3D) image reconstruction scheme for optical near-infrared imaging of highly scattering material. The algorithm reconstructs the spatial distribution of the optical parameters of a volume Omega from transillumination measurements on the boundary of Omega. We test the performance of the method for a cylindrical object with embedded absorbing perturbation for a number of different source and detector arrangements. Furthermore, we investigate the effect of a mismatched reconstruction, using a two-dimensional (2D) reconstruction model to image a single plane of the object from 3D tomographic data obtained in a single plane. The motivation for the application of 2D models is their advantage in speed and memory requirements. We found that the difference in the measurement data between 2D and 3D models depends greatly on the measurement type used, giving a much better agreement for mean time-of-flight data than for dc intensity data. Image artifacts that are due to data model mismatches can therefore be significantly reduced by use of mean time data.     

Truncation artifacts in tomographic reconstructions from projections

The artifacts in tomographic reconstructions from truncated sets of projections are analyzed. The shift-variant impulse response of the tomographic system for parallel-beam geometry is derived. A number of propositions are made describing the observed artifacts. A graphical scheme for the prediction of the location and shape of the truncation artifacts is presented and applied to reconstructions from simulated projections. The artifact analysis is applied to images obtained with the commonly used convolution backprojection reconstruction algorithms, and it is extended to reconstructions from fan-beam projections. The analysis is performed for the continuous imaging domain so as to separate the clipping artifacts clearly from those attributed to the digital implementation of the reconstruction algorithms.     

Phase grating design for a dual-band snapshot imaging spectrometer

Infrared spectral features have proved useful in the identification of threat objects. Dual-band focal-plane arrays (FPAs) have been developed in which each pixel consists of superimposed midwave and long-wave photodetectors [Dyer and Tidrow, Conference on Infrared Detectors and Focal Plane Arrays (SPIE, Bellingham, Wash., 1999), pp. 434-440]. Combining dual-band FPAs with imaging spectrometers capable of interband hyperspectral resolution greatly improves spatial target discrimination. The computed-tomography imaging spectrometer (CTIS) [Descour and Dereniak, Appl. Opt. 34, 4817-4826 (1995)] has proved effective in producing hyperspectral images in a single spectral region. Coupling the CTIS with a dual-band detector can produce two hyperspectral data cubes simultaneously. We describe the design of two-dimensional, surface-relief, computer-generated hologram dispersers that permit image information in these two bands simultaneously.     

Wavelet-based image enhancement in x-ray imaging and tomography

We consider an application of the wavelet transform to image processing in x-ray imaging and three-dimensional (3-D) tomography aimed at industrial inspection. Our experimental setup works in two operational modes-digital radiography and 3-D cone-beam tomographic data acquisition. Although the x-ray images measured have a large dynamic range and good spatial resolution, their noise properties and contrast are often not optimal. To enhance the images, we suggest applying digital image processing by using wavelet-based algorithms and consider the wavelet-based multiscale edge representation in the framework of the Mallat and Zhong approach [IEEE Trans. Pattern Anal. Mach. Intell. 14, 710 (1992)]. A contrast-enhancement method by use of equalization of the multiscale edges is suggested. Several denoising algorithms based on modifying the modulus and the phase of the multiscale gradients and several contrast-enhancement techniques applying linear and nonlinear multiscale edge stretching are described and compared by use of experimental data. We propose the use of a filter bank of wavelet-based reconstruction filters for the filtered-backprojection reconstruction algorithm. Experimental results show a considerable increase in the performance of the whole x-ray imaging system for both radiographic and tomographic modes in the case of the application of the wavelet-based image-processing algorithms.     

Image reconstruction in spherical-wave intensity diffraction tomography

A reconstruction theory for intensity diffraction tomography (I-DT) has been proposed that permits reconstruction of a weakly scattering object without explicit knowledge of phase information. We investigate the I-DT reconstruction problem assuming an incident (paraxial) spherical wave and scanning geometries that employ fixed source-to-object distances. Novel reconstruction methods are derived by identifying and exploiting tomographic symmetries and the rotational invariance of the problem. An underlying theme is that symmetries in tomographic imaging systems can facilitate solutions for phase-retrieval problems. A preliminary numerical investigation of the developed reconstruction methods is presented.     

Effects of background fluorescence in fluorescence molecular tomography

Recent advances in optical imaging systems and systemically administered fluorescent probes have significantly improved the ways by which we can visualize proteomics in vivo. A key component in the design of fluorescent probes is a favorable biodistribution, i.e., localization only in the targeted diseased tissue, in order to achieve high contrast and good detection characteristics. In practice, however, there is always some level of background fluorescence present that could result in distorted or obscured visualization and quantification of measured signals. In this study we observe the effects of background fluorescence in tomographic imaging. We demonstrate that increasing levels of background fluorescence result in artifacts when using a linear perturbation algorithm, along with a significant loss of image fidelity and quantification accuracy. To correct for effects of background fluorescence, we have applied cubic polynomial fits to bulk raw measurements obtained from spatially homogeneous and heterogeneous phantoms. We show that subtraction of the average fluorescence response from the raw data before reconstruction can improve image quality and quantification accuracy as shown in relatively homogeneous or heterogeneous phantoms. Subtraction methods thus appear to be a promising route for adaptively correcting nonspecific background fluorochrome distribution.     

Usefulness of ultrasonic strain measurement-based shear modulus reconstruction for diagnosis and thermal treatment

We previously reported an ultrasonic strain measurement-based one-dimensional (1-D) shear modulus reconstruction technique using a regularization method for differential diagnosis of malignancies on human superficial tissues (e.g., breast tissues). Here, ultrasonic strain measurement-based 2-D and 3-D shear modulus reconstruction techniques are described, and the 1-D technique is reviewed and subsequently applied to various human in vivo tissues, including deeply situated tissues (e.g., liver). Because soft tissues are deformed in 3-D space by externally situated arbitrary mechanical sources, the accuracy of the low-dimensional (i.e., 1-D or 2-D) reconstructions is lower to that of 3-D reconstruction due to occurrence of erroneous reconstruction artifacts (i.e., the reconstructed modulus is different than reality). These artifacts are confirmed on simulated inhomogeneous cubic phantoms containing a spherical homogenous inclusion using numerically calculated deformation data. The superiority of quasi-real-time imaging of the shear modulus is then demonstrated by comparing it with conventional B-mode imaging and strain imaging from the standpoints of monitoring the effectiveness of minimally invasive thermal therapy as well as differential diagnosis. Because the 2-D and 3-D techniques require special ultrasonic (US) equipment, the 1-D technique using conventional US imaging equipment is used, even though erroneous artifacts will occur. Specifically, the 1-D technique is applied as a diagnostic tool for differentiating malignancies in human in vivo liver and breast tissue, and a monitoring technique for determining the effectiveness of interstitial electromagnetic wave (micro and rf) thermal therapy on human in vivo liver and calf in vitro liver. Even when using the 1-D technique, reconstructed shear moduli were confirmed to be a suitable measure for monitoring thermal treatment as well as differential diagnosis. These results are encouraging in that they will promote use of 2-D and 3-D reconstruction techniques.     

Alternatives to the use of the DFT in MRI and spectroscopic reconstructions

Standard and functional magnetic resonance imaging (MRI and f MRI) make of use the two-dimensional (2D) discrete Fourier transform (DFT). Many MR spectroscopic techniques use the 1D DFT. Experimental or time constraints frequently require that the DFT be applied to finite-length (truncated) data sequences. Truncation is essentially a windowing of the data and introduces artifacts and resolution loss in images or spectra. A number of alternative reconstruction algorithms have been proposed to counteract these problems. These algorithms attempt to model the known data and use the modeling information to implicitly or explicitly extrapolate the data to overcome the windowing. One modeling approach, the Transient Error Reconstruction Algorithm (TERA), uses an autoregressive moving average method to recover the missing data. In this article, we briefly discuss variants of the TERA algorithm and development of neural networks to take better account of the differing data properties of MR data sets. Our success with neural networks in fMRI reconstruction has led us to challenge some of the standard approaches to validating MR algorithms and develop our own. These new approaches include k-space phantom generation and automated computer observer (ROC analysis) to evaluate algorithms in terms of their clinical relevance. We have also developed an upgraded image quality measure based on Daly's Visual Differences Predictor. This models the ability of the human visual system to detect significant differences between images produced by different MRI reconstruction algorithms. We also present a protocol for generating Shepp-Logan phantoms which avoids the introduction of the high-frequency k-space data distortion present in the existing approach. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 558–564, 1997     

Quantization bit rate reduction in scanning tomographic acousticmicroscopy

The imaging capability of the scanning laser acoustic microscope has been enhanced by the addition of a quadrature detector for holographic imaging and rotational and frequency-varying hardware for tomographic reconstruction. Because of the expansion of data acquisition capability, the memory-intensive demands placed on the processing computer by these modifications require efficient methods of representing and storing the tomographic data while retaining the quality of image reconstruction. This paper reports the performance evaluation of low bit-rate data acquisition. Results show the feasibility of high-quality imaging at bit-rate savings at the level of 33%     

Recent advances in scanning tomographic acoustic microscopy

We present the recent developments of the Scanning Tomographic Acoustic Microscope (STAM). The STAM was proposed as a method to achieve 3D imaging capability for the Scanning Laser Acoustic Microscope (SLAM). With the addition of a quadrature receiver, the complex scattered wave field can now be detected, and consequently the STAM is capable of subsurface holographic and tomographic imaging. The resolution improvement of the STAM can be attributed directly to the detection of the phase information and the image reconstruction technique. The STAM is sensitive to phase errors in the tomographic projections. In particular, the quadrature phase error and the initial phase error in the complex projections are critical to the tomographic reconstruction process. For multiple-angle tomography, high-precision projection registration and alignment become necessary. By obtaining solutions to these implementation problems, we have succeeded in obtaining images superior to the original SLAM images. In addition, quantitative ultrasonic imaging is possible with the STAM, and a method is presented to image the velocity parameter of simple specimens. With these capabilities, the STAM may become a useful tool for high-resolution subsurface nondestructive evaluation.     

A heuristic technique for CTIS image reconstruction

An iterative method is presented for computed tomography imaging spectrometer (CTIS) image reconstruction in the presence of both photon noise in the image and postdetection Gaussian system noise. The new algorithm, which assumes the transfer matrix of the system has a particular structure, is evaluated experimentally with the result that it is significantly better, for larger problems, than both the multiplicative algebraic reconstruction technique (MART) and the mixed-expectation image-reconstruction technique (MERT) with respect to accuracy and computation time.     

Toward atomic-scale bright-field electron tomography for the study of fullerene-like nanostructures

We present the advancement of electron tomography for three-dimensional structure reconstruction of fullerene-like particles toward atomic-scale resolution. The three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is achieved by the combination of low voltage operation of the electron microscope with aberration-corrected phase contrast imaging. The method enables the study of defects and irregularities in the three-dimensional structure of individual fullerene-like particles on the scale of 2-3 A. Control over shape, size, and atomic architecture is a key issue in synthesis and design of functional nanoparticles. Transmission electron microscopy (TEM) is the primary technique to characterize materials down to the atomic level, albeit the images are two-dimensional projections of the studied objects. Recent advancements in aberration-corrected TEM have demonstrated single atom sensitivity for light elements at sub?ngstr?m resolution. Yet, the resolution of tomographic schemes for three-dimensional structure reconstruction has not surpassed 1 nm3, preventing it from becoming a powerful tool for characterization in the physical sciences on the atomic scale. Here we demonstrate that negative spherical aberration imaging at low acceleration voltage enables tomography down to the atomic scale at reduced radiation damage. First experimental data on the three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is presented. The method is applicable to the analysis of the atomic architecture of a wide range of nanostructures where strong electron channeling is absent, in particular to carbon fullerenes and inorganic fullerenes.     

A block iterative algorithm for tomographic reconstruction of ionospheric electron density

Ionospheric tomography is a technique whereby a vertical cross section through ionospheric electron density can be imaged. The vertical resolution of ionospheric tomography systems is inherently poor, but can be improved by using a priori information in the tomographic reconstruction algorithm. Care must be exercised in using a priori information, since if too much of it is used, the reconstruction algorithm may discard some of the information contained in the tomographic data in favor of satisfying some of the a priori assumptions. Orthogonal decomposition (OD) is an existing technique that uses a priori information to constrain the reconstruction to lie in a space of reasonable images without weighting the reconstruction toward any particular a priori image. In this way a priori information can be used in a manner that does not overwhelm information contained in the data. Gauss-Seidel (GS) is an iterative algorithm that is used to calculate solutions for large systems of linear equations. In this article, a block version of the GS algorithm will be used to calculate the solution of the least-squares problem that is created using OD. The complete algorithm presented here will be called the residual correction method (RCM), since it involves calculation of successively better approximations based on the residual error. RCM is a fast and numerically stable algorithm that extracts as much information from the data as possible. A numerical example demonstrating the properties of RCM will also be presented. © 1996 John Wiley & Sons, Inc.     

Eigenmode super-resolution imaging in arbitrary optical systems

We investigate optical super-resolution by means of eigenmode decomposition in arbitrary imaging systems. This technique is applicable for arbitrary objects but requires a knowledge of the eigenmodes of the imaging system. We outline a reconstruction technique that can be applied even to systems in which the eigenmodes are not orthogonal, and we present numerical simulations of eigenmode super-resolution in systems with resolution limited both by diffraction and by aberrations. Our results indicate that optical super-resolution by direct eigenmode decomposition provides a versatile method of sub-diffraction and distortion-free imaging in arbitrary optical systems.     

Reconstruction of multidirectional interferometric data using an isoparametric finite-element method

The spatial resolution of tomographic reconstructions is critical when the object field contains large- and small-scale features. Simply increasing the number of elements used in the reconstruction process throughout the domain is generally an unsatisfactory method to achieve higher resolution because additional multiview data are required. Here a new series-expansion reconstruction procedure, based on isoparametric finite-element concepts, is described. This procedure permits the shape and size of the reconstruction elements to be arbitrarily specified. The method is demonstrated by the use of an analytic function and is directly compared with results obtained from other series-expansion methods on a uniform grid. Given identical input data and reconstruction grids, the absolute error of reconstruction is improved by the use of the new method. The advantages of performing the reconstruction of a complex field on a nonuniform grid is also demonstrated.