William J. Dallas

Deterministic Diffusers for Holography

We present here a new family of diffusers suitable for use in holography. We first exhibit a method for obtaining mathematical descriptions of diffusers that give nearly uniform amplitude at the hologram before insertion of an object. From these diffusers we select a few that redundantly map information into the hologram. Finally, we present a method of comparing the performance of different diffusers using a computer simulation technique that has a simple experimental analog.

Fourier space solution to the magnetostatic imaging problem

A method for constructing visualizations of current distributions with magnetic field measurements of biological current is presented. Two uses are discussed: SQUID tomography and longitudinal region-of-interest reconstruction.

A simple derivation of quasi-crystalline spectra

A quasi-crystalline solid is an aperiodic array of atoms which gives rise to Bragg-like diffraction peaks. Using the projection method for generating such structures, the authors provide a simple derivation of this paradoxical behaviour.

Computer-Generated Holograms

Computer-generated holograms (CGH’s) are diffractive-optical elements that offer the possibility of creating wave-optical display systems that are under complete computer control. Because CGH’s can control all aspects of the optical wave, three-dimensional imagery can be presented to an audience. After reviewing some mathematical preliminaries that are essential to the description of CGH’s, we move to the description of their construction and operation. We include descriptions of the effects that some common imperfections have on the image presented by a CGH. Although CGH’s have been successfully applied in applied in many areas including optical testing and security, we concentrate our attention on 3-D display. We will also examine some basic conditions on the use of CGH’s for video display of these images.

Fourier Imaging of Electrical Currents in the Human Brain from Their Magnetic Fields

Computer simulations of the reconstruction and imaging of neural currents from their external magnetic fields are presented. The method is based on the Fourier transformed Maxwell equations in combination with the Whittaker-Shannon sampling theorem. The simultaneous reconstruction of more than one current generator without any special physiological model assumptions of the current density distribution is discussed.

Linear estimation theory applied to the reconstruction of a 3-D vector current distribution

Linear estimation theory incorporating statistical a priori knowledge is applied to the inverse problem of reconstructing a static 3-D vector source field from another 3-D vector measurement field. The motivation for this development is to reconstruct 3-D electric current distributions from a set of magnetic measurements. Such a capability would be useful for the clinical determination of neural currents, for example. A simulation is presented to demonstrate the reconstruction of a class of simple nonbiological source objects, and to show the dependence of these reconstructions on the data taking configuration and the statistical a priori knowledge that is incorporated into the reconstruction process.

Dual detection full range frequency domain optical coherence tomography.

It has been shown that frequency domain optical coherence tomography (FD-OCT) systems achieve higher sensitivities compared to time domain optical coherence tomography (OCT) systems. However, the obscure object structure due to the mirror image generated by the Fourier transform is one of the remaining issues in the FD-OCT. We designed and developed what we believe to be a novel full range FD-OCT system that we refer to as the dual detection full range frequency domain optical coherence tomography (DD-FDOCT) that enables the instantaneous retrieval of quadrature components of the complex interferometric signal. The DD-FDOCT system enables full range imaging without loss of speed, and it may be less sensitive to phase error generated by involuntary movements of the subject compared to the other established full range OCT systems, because it uses two signals with a phase difference of pi/2 obtained simultaneously from two detection arms to remove mirror images at all depths.

Do patients with right ventricular outflow tract ventricular arrhythmias have a normal right ventricular wall motion? A quantitative analysis compared to normal subjects

Background/Aim: Patients with ventricular ectopy from the right ventricular (RV) outflow tract (RVOT) are often referred for RV angiography to exclude disorders such as arrhythmogenic RV cardiomyopathy/dysplasia (ARVC/D). This is usually based on a qualitative assessment of the wall motion. We present a method to quantify the wall motion and to apply this method to compare patients with RVOT ectopy to normal subjects. Methods: RV angiograms were analyzed from 19 normal subjects and 11 subjects with RVOT ventricular arrhythmias (RVOT arrhythmia subjects) who had no other clinical or other evidence for ARVC/D. By a newly developed computer-based method, RV contours were first traced from multiple frames spanning the entire cardiac cycle. The fractional change in area between contours was then calculated as a serial function of time and location to determine both total contour area change and timing of contour movement. Contour area strain, defined as the differential change in area between nearby regions, was also computed. Results: The contour area change was greatest in the tricuspid valve region and least in the RVOT and midanterior regions. The onset of contraction was earliest in the RVOT region and latest in the apical, inferior, inferoapical, and subtricuspid valve regions. The contour strain was largest in superior tricuspid valve and inferior wall and near zero within the lateral tricuspid valve region. There were significant pairwise differences in contraction area, timing, and strain in the various regions. There were no significant differences between normal subjects and RVOT arrhythmia subjects. Conclusions: The RV wall motion is nonuniform in contour area change, strain, and timing of motion. Patients with RVOT ventricular ectopy demonstrate wall motion parameters similar to those of normal subjects. This technique should be applicable in analyzing RV wall motion in patients suspected of having ARVC/D.

A LINEAR ESTIMATION APPROACH TO BIOMAGNETIC IMAGING

A reconstruction algorithm based on linear estimation theory in combination with Moore-Penrose pseudoinverse techniques is presented in order to image complex current distributions. Phantom experiments with current dipoles immersed in saline solution inside a glass phantom head and computer simulations demonstrate the applicability to biomagnetic imaging.

Artifact Suppression in Digital Chest Radiographs Enhanced With Adaptive Histogram Equalization

Adaptive Histogram Equalization (AHE) has been applied to high resolution digital chest radiographs to provide contrast enhancement. The method provides good contrast in uniform areas of the image, e.g. the lung field, but in so doing both overenhances noise and produces an artifact at boundaries between high density and low density regions. The artifact, which appears as a band of very low contrast data spanning such a boundary, has the effect of suppressing structural information. Although it is known that the problem of overenhancing noise is controlled by the algorithm known as Contrast Limited Adaptive Histogram Equalization (CLAHE), the boundary artifact is not removed by this technique. This paper concentrates on the boundary artifact. We present a method for processing a chest radiograph by means of background subtraction prior to applying the CLAHE algorithm which reduces contrast at high/low density boundaries and thus permits contrast enhancement free of both noise and boundary artifacts.