Manipulating the transmission matrix of scattering media for nonlinear imaging beyond the memory effect

The measurement of the Transmission Matrix (TM) of a scattering medium is of great interest for imaging. It can be acquired directly by interferometry using an internal reference wavefront. Unfortunately, internal reference fields are scattered by the medium which results in a speckle that makes the TM measurement heterogeneous across the output field of view. We demonstrate how to correct for this effect using the intrinsic properties of the TM. For thin scattering media, we exploit the memory effect of the medium and the reference speckle to create a corrected TM. For highly scattering media where the memory effect is negligible, we use complementary reference speckles to compose a new TM, not compromised by the speckled reference anymore. Using this correction, we demonstrate large field of view second harmonic generation imaging through thick biological media

Reflection Matrix Method for Controlling Light After Reflection From a Diffuse Scattering Surface

This research focuses on reflective inverse diffusion, which was a proof-of-concept experiment that used phase modulation to shape the wavefront of a laser causing it to refocus after reflection from a rough surface. By refocusing the light, reflective inverse diffusion has the potential to eliminate the complex radiometric model of indirect photography by creating a virtual light source at the first diffuse reflector that satisfies the line-of-sight requirement of dual photography. However, the initial reflective inverse diffusion experiments provided no mathematical background and were conducted under the premise that the process operated similarly to transmissive inverse diffusion. In this research, diffraction modeling of the reflective inverse diffusion experiments led to the development of Fourier transform-based simulations. Simulations and experimentation were used to develop reflection matrix methods that determine the proper phase modulation to refocus light after reflection to any location in the observation plane. These techniques provide a new method for controlled illumination of an occluded scene that can be used in conjunction with dual photography. This document provides the mathematical background for reflective inverse diffusion, the reflection matrix methods for phase modulation, and describes the simulations and experiments conducted

Reflective Inverse Diffusion

Phase front modulation was previously used to refocus light after transmission through scattering media. This process has been adapted here to work in reflection. A liquid crystal spatial light modulator is used to conjugate the phase scattering properties of diffuse reflectors to produce a converging phase front just after reflection. The resultant focused spot had intensity enhancement values between 13 and 122 depending on the type of reflector. The intensity enhancement of more specular materials was greater in the specular region, while diffuse reflector materials achieved a greater enhancement in non-specular regions, facilitating non-mechanical steering of the focused spot. Scalar wave optics modeling corroborates the experimental results

Measuring the Reflection Matrix of a Rough Surface

Phase modulation methods for imaging around corners with reflectively scattered light required illumination of the occluded scene with a light source either in the scene or with direct line of sight to the scene. The RM (reflection matrix) allows control and refocusing of light after reflection, which could provide a means of illuminating an occluded scene without access or line of sight. Two optical arrangements, one focal-plane, the other an imaging system, were used to measure the RM of five different rough-surface reflectors. Intensity enhancement values of up to 24 were achieved. Surface roughness, correlation length, and slope were examined for their effect on enhancement. Diffraction-based simulations were used to corroborate experimental results

Speculative Attacks or Economic Fundamentals: Evidence from the Asian Currency Crisis

This paper argues that what led to the Asian financial crisis was a fatal combination of several self-reinforcing factors including external sector weaknesses, fragility in domestic financial markets due to inadequately administered financial liberalisation, loss of confidence, and short-term capital flows, maturing within less than a year and denominated in unhedged dollars. Some of these factors were country-specific while others were common to the entire region. Asia\u27s financial crisis will almost certainly lead to important changes in the international financial system, as countries try to find an appropriate balance between the benefits from gaining access to intentional capital flows and the potential for instability and ohter risks that also seem to be much greater in a world of large and highly mobile capital movements. The paper discusses important lessons from the crisis

Portugal Telecom takeover defense

This Masters Thesis aims to be used in class for teaching purposes. The primary objective of the case is to understand the way critical anti-takeover measures can be used in a hostile takeover environment. The case portrays not only all defensive tools used by PT against Sonaecom (conditioned by the Portuguese Securities Code) but also all major interactions with relevant stakeholders. Communication is positioned as a central tool to stakeholders’ interaction. As a consequence of that, students are supposed to comprehend how PT used this instrument to implement its defensive strategy. This Masters Thesis was written essentially from PT’s perspective but it also includes all available Sonacom’s public statements and data. It joins several different points of view (from PT’s CEO and top management to PT’s employees and labor unions). Therefore, it is not supposed to mach exactly to one’s perspective of Sonecom’s takeover attempt but instead to be an aggregate view of the case. The mains idea is to promote debate in order to create an interactive class environment among session’s attendants. Main areas developed in this Thesis are Mergers and Acquisitions, Corporate Communication and Corporate Governance. The case's recommended questions are in line with these areas. Suggested answers for these questions are balanced with some academic documentation that can be complemented with some in-class notes according to the session's objectives

Semiclassical Scattering in a Circular Semiconductor Microstructure

The conductance of a microscopic junction shows fluctuations caused by quantum interference of waves that follow different paths between the leads. We give a semiclassical formula for these fluctuations. The theory utilizes trajectories which travel between the centers of the lead apertures; it also incorporates diffraction at these apertures. We extend the theory to include ‘‘ghost paths,’’ which scatter diffractively off the lead mouths. Semiclassical S-matrix elements are computed for a circular junction over a range of Fermi wave numbers, and the large-scale structure of these matrix elements shows good agreement with quantum results. Finally, we propose a hypothesis about the effect of the quantum coherence length on the S matrix and on the semiclassical sum. © 1996 The American Physical Society

Longitudinal changes in brain structure and integrity during acute HIV

Cognitive impairment persists in the form of HIV-associated neurocognitive disorder (HAND) among chronically infected individuals despite successful viral suppression. Widespread access to combination antiretroviral therapy (cART) has allowed infected individuals to initiate treatment at an earlier time point and effectively reduce the risk of HIV-related mortality and morbidity. The current study examines whether cART, when initiated within days to weeks following infection, can longitudinally preserve brain health. Quantitative magnetic resonance image (MRI) methodologies were used to analyze T1-weighted structural images and diffusion tensor imaging (DTI) metrics. Specifically, region of interest and voxel-wise volume and tensor-based spatial statistics (TBSS) approaches were performed to evaluate differences in brain volumes and white matter microstructure. We examined 31 acute HIV (AHI) participants who had paired month 0 (baseline) and month 24 (two-year follow-up) scans. Participants were comparatively analyzed in both a longitudinal manner to themselves, and a cross-sectional manner against 25 healthy control (CO) participants. As an indication of inflammation, CD8 t-lymphocyte counts was examined as a clinical covariate. The 31 AHI participants had a median (IQR) age of 26 (23-30) years at the time of enrollment and a median (IQR) baseline CD4 count of 576 (370-868) cells/μL. All immediately initiated cART. The 25 healthy controls had a median (IQR) age of 31 (26-37) years. Differences of brain integrity in the AHI group followed longitudinally were observed. Baseline CD8 count was significantly associated with increased mean diffusivity (MD) in the longitudinally infected, present in the genu and the splenium of the corpus callosum, the corona radiata, and the superior longitudinal fasciculus (all p<0.05). Structural analysis revealed enlarged corpus callosum (p<0.01) volumes, as well as enlarged caudate and thalamus subcortical gray matter volumes in the longitudinally infected AHI participants (both p<0.05). Differences of brain integrity in the AHI group after 24 months of treatment were observed compared to healthy controls. Specifically, fractional anisotropy (FA) was reduced in AHI at 24 months compared to controls in models adjusting for age in the corpus callosum, the corona radiata, and the left superior longitudinal fasciculus (all p<0.05). Structural analyses revealed enlarged putamen and the caudate volumes (both p<0.05). We conclude that differences in both brain integrity and structural volumes can be seen in AHI with successful viral suppression when compared to healthy controls. Future work will include longitudinal imaging data from healthy controls who are followed over two-year follow-up to ensure that observed changes are disease specific. We will also comparatively investigate longitudinally treated chronic HIV-infected participants (‘positive controls’) to examine if the differences are similar to those seen when therapy is initiated in the chronic stage of infection. We will examine inflammatory plasma and CSF biomarkers to inform potential mechanisms and, separately, cognitive testing to inform clinical significance

Rollins Alumni Record, Summer 2004

Issue Title: A Rollins Valedictory! The Bornstein Era 1990-200

Diffusion Tensor Imaging in Pediatric Brain Tumor Patients

In this dissertation, we outline our efforts to introduce an advanced MRI imaging technique called Diffusion Tensor Imaging (DTI) to the pediatric brain tumor population. We discuss the theory and application of DTI as it was performed in a series of translational investigations at St. Jude Children’s Research Hospital. We present evidence of how the introduction of this technique impacted diagnosis, and treatment. And finally, we demonstrate how DTI was used to investigate cognitive morbidities associated with cancer treatment and how this research provided insight into the underlying pathophysiology involved in the development of these treatment sequela. This research has generated important insights into the fundamental causes of neuroanatomical and cognitive deficits associated with cancer and cancer therapy. The use of DTI has permitted us to identify potential targets for improved radiological and surgical techniques as well as targets for pharmacological and behavioral interventions that might improve cognitive function in cancer survivors. The discoveries here afford us an opportunity to reduce the negative effects of cancer therapy on patients treated in the future while maintaining successful survival rates