Lawrence Livermore National Laboratory is a large, multidisciplinary institution that conducts fundamental
and applied research in the physical sciences. Research programs at the Laboratory run the
gamut from theoretical investigations, to modeling and simulation, to validation through experiment.
Over the years, the Laboratory has developed a substantial research component in the areas of signal
and image processing to support these activities. This paper surveys some of the current research in
signal and image processing at the Laboratory. Of necessity, the paper does not delve deeply into any
one research area, but an extensive citation list is provided for further study of the topics presented.
The development of faster more reliable techniques to detect radioactive contraband in a portal type scenario
is an extremely important problem especially in this era of constant terrorist threats. Towards this goal the
development of a model-based, Bayesian sequential data processor for the detection problem is discussed. In the
sequential processor each datum (detector energy deposit and pulse arrival time) is used to update the posterior
probability distribution over the space of model parameters. The nature of the sequential processor approach
is that a detection is produced as soon as it is statistically justified by the data rather than waiting for a fixed
counting interval before any analysis is performed. In this paper the Bayesian model-based approach, physics
and signal processing models and decision functions are discussed along with the first results of our research.
Tomographic images of tissue phantoms and a sample of breast tissue have been produced from an acoustic synthetic array system for frequencies near 500 kHz. The images for sound speed and attenuation show millimeter resolution and demonstrate the feasibility of obtaining high-resolution tomographic images with frequencies that can deeply penetrate tissue. The image reconstruction method is based on the Born approximation to acoustic scattering and is a simplified version of a method previously used by Andre (Andre, et. al., Int. J. Imaging Systems and Technology, Vol 8, No. 1, 1997) for a circular acoustic array system. The images have comparable resolution to conventional ultrasound images at much higher frequencies (3-5 MHz) but with lower speckle noise. This shows the potential of low frequency, deeply penetrating, ultrasound for high-resolution quantitative imaging.
Extremely high quality data was acquired using an experimental ultrasound scanner developed at Lawrence Livermore National Laboratory using a 2D ring geometry with up to 720 transmitter/receiver transducer positions. This unique geometry allows reflection and transmission modes and transmission imaging and quantification of a 3D volume using 2D slice data. Standard image reconstruction methods were applied to the data including straight-ray filtered back projection, reflection tomography, and diffraction tomography. Newer approaches were also tested such as full wave, full wave adjoint method, bent-ray filtered backprojection, and full-aperture tomography. A variety of data sets were collected including a formalin-fixed human breast tissue sample, a commercial ultrasound complex breast phantom, and cylindrical objects with and without inclusions. The resulting reconstruction quality of the images ranges from poor to excellent. The method and results of this study are described including like-data reconstructions produced by different algorithms with side-by-side image comparisons. Comparisons to medical B-scan and x-ray CT scan images are also shown. Reconstruction methods with respect to image quality using resolution, noise, and quantitative accuracy, and computational efficiency metrics will also be discussed.
In contrast to standard reflection ultrasound (US), transmission US holds the promise of more thorough tissue characterization by generating quantitative acoustic parameters. We compare results from a conventional US scanner with data acquired using an experimental circular scanner operating at frequencies of 0.3 - 1.5 MHz. Data were obtained on phantoms and a normal, formalin-fixed, excised breast. Both reflection and transmission-based algorithms were used to generate images of reflectivity, sound speed and attenuation.. Images of the phantoms demonstrate the ability to detect sub-mm features and quantify acoustic properties such as sound speed and attenuation. The human breast specimen showed full field evaluation, improved penetration and tissue definition. Comparison with conventional US indicates the potential for better margin definition and acoustic characterization of masses, particularly in the complex scattering environments of human breast tissue. The use of morphology, in the context of reflectivity, sound speed and attenuation, for characterizing tissue, is discussed.
New ultrasound data, obtained with a circular experimental scanner, are compared with data obtained with standard X-ray CT. Ultrasound data obtained by scanning fixed breast tissue were used to generate images of sound speed and reflectivity. The ultrasound images exhibit approximately 1 mm resolution and about 20 dB of dynamic range. All data were obtained in a circular geometry. X-ray CT scans were used to generate X-ray images corresponding to the same 'slices' obtained with the ultrasound scanner. The good match of sensitivity, resolution and angular coverage between the ultrasound and X-ray data makes possible a direct comparison of the three types of images. We present the results of such a comparison for an excised breast fixed in formalin. The results are presented visually using various types of data fusion. A general correspondence between the sound speed, reflectivity and X-ray morphologies is found. The degree to which data fusion can help characterize tissue is assessed by examining the quantitative correlations between the ultrasound and X-ray images.
The scattering mechanism of diffraction tomography is described by the integral form of the Helmholtz equation. The goal of diffraction tomography is to invert this equation in order to reconstruct the object function from the measured scattered fields. During the forward propagation process, the spatial spectrum of the object under investigation is 'smeared,' by a convolution in the spectral domain, across the propagating and evanescent regions of the received field. Hence, care must be taken in performing the reconstruct, as the object's spectral information has been moved into regions where it may be considered to be noise rather than useful information. This will reduce the quality and resolution of the reconstruction. We show how the object's spectrum can be partitioned into resolvable and non-resolvable parts based upon the cutoff between the propagating and evanescent fields. Operating under the Born approximation, we develop a beam- forming on transmit approach to direct the energy into either the propagating or evanescent parts of the spectrum. In this manner, we may individually interrogate the propagating and evanescent regions of the object spectrum.
A two-dimensional hydrodynamic model incorporating turbulent mixing is
derived by averaging the three-dimensional hydrodynamics equations over slabs
of thickness L in the direction of propagation. The resulting equations are
identical with those for two-dimensional, incompressibl flow with additional
terms representing the average effect of velocity fluctuations within each
slab. These equations depend only parametrically on the coordinate in the
direction of propagation. The extra terms have a simple physical
interpretation but must be modeled in order to close the equations.
Correlation times of scintillations are computed from a numerical simulation of
plane-wave blooming using this model. A comparison with the correlation times
from simulations with only wind shear indicate that the effect of turbulent
mixing is relatively small for typical values of the Kolmogorov inner scale.
In this paper we show how the electric field spectrum for a high energy
laser beam propagating through a uniform atmosphere can be calculated using the
linear theory of small fluctuations. The beam is modeled analytically as an
infinite plane wave which propagates through a medium with constant absorption
and no transverse wind. Return-wave phase compensation is modeled as a filter
in the transverse Fourier domain. The linear theory describes the growth of
small fluctuations on the beam and accurately predicts the evolution of the
electric field spectrum until the magnitude of the fluctuations approach the
original beam irradiance. The accuracy is tested by comparing with the
spectrum calculated using ORACLE, a full wave optics thermal blooming code.
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