Mantalena Sarafianou

Robust video broadcasting over 802.11a/g in time-correlated fading channels

In order to deliver video streams efficiently over WiFi to many thousands of consumer handheld devices, broadcast protocols must be employed. In this mode of operation the received video quality can deteriorate rapidly as a result of high application layer packet loss which occurs because MAC frame retransmission cannot be used. In this paper we develop a robust video delivery solution for broadcast transmission over 802.11a/g. Using a cross-layer WiFi simulator in combination with an accurate time-correlated fading channel, the received video quality is evaluated for broadcast H.264 video sequences. Application layer cross-packet forward error correction is then used together with error concealment at the video client. Furthermore, the application of an external packet interleaver is considered. Combining a block size of two hundred packets (which introduces a 4.8 second delay) and an application layer FEC code rate of 0.75 our results demonstrate that video can be successfully broadcast over WiFi to many thousands of handheld terminals at large-scale spectator events.

Microwave radar imaging of heterogeneous breast tissue integrating a priori information

Conventional radar-based image reconstruction techniques fail when they are applied to heterogeneous breast tissue, since the underlying in-breast relative permittivity is unknown or assumed to be constant. This results in a systematic error during the process of image formation. A recent trend in microwave biomedical imaging is to extract the relative permittivity from the object under test to improve the image reconstruction quality and thereby to enhance the diagnostic assessment. In this paper, we present a novel radar-based methodology for microwave breast cancer detection in heterogeneous breast tissue integrating a 3D map of relative permittivity as a priori information. This leads to a novel image reconstruction formulation where the delay-and-sum focusing takes place in time rather than range domain. Results are shown for a heterogeneous dense (class-4) and a scattered fibroglandular (class-2) numerical breast phantom using Bristols 31-element array configuration.

Compound Radar Approach for Breast Imaging

Multistatic radar apertures record scattering at a number of receivers when the target is illuminated by a single transmitter, providing more scattering information than its monostatic counterpart per transmission angle. This paper considers the well-known problem of detecting tumor targets within breast phantoms using multistatic radar. To accurately image potentially cancerous targets size within the breast, a significant number of multistatic channels are required in order to adequately calibrate-out unwanted skin reflections, increase the immunity to clutter, and increase the dynamic range of a breast radar imaging system. However, increasing the density of antennas within a physical array is inevitably limited by the geometry of the antenna elements designed to operate with biological tissues at microwave frequencies. A novel compound imaging approach is presented to overcome these physical constraints and improve the imaging capabilities of a multistatic radar imaging modality for breast scanning applications. The number of transmit-receive (TX-RX) paths available for imaging are increased by performing a number of breast scans with varying array positions. A skin calibration method is presented to reduce the influence of skin reflections from each channel. Calibrated signals are applied to receive a beamforming method, compounding the data from each scan to produce a microwave radar breast profile. The proposed imaging method is evaluated with experimental data obtained from constructed phantoms of varying complexity, skin contour asymmetries, and challenging tumor positions and sizes. For each imaging scenario outlined in this study, the proposed compound imaging technique improves skin calibration, clearly detects small targets, and substantially reduces the level of undesirable clutter within the profile.

Towards Enhancing Skin Reflection Removal and Image Focusing Using a 3-D Breast Surface Reconstruction Algorithm

Suppressing skin reflection is vital for successful tumor detection in radar breast imaging systems. In this communication, a novel skin reflection removal (SRR) algorithm is presented based on a previously-proposed breast surface reconstruction algorithm. This skin reflection removal algorithm is validated using numerical MRI-derived breast models. This communication also investigates how the same skin location information can be used to enhance the delay-and-sum algorithm.

A novel 3-D breast surface reconstruction algorithm for a multi-static radar-based breast imaging system

The estimation of the skin reflection is a crucial step towards successful breast tumour detection. This paper investigates the two-dimensional improved version of an existing general purpose technique for estimating target objects of arbitrary shape. The ability of the proposed method in estimating the breast skin location is tested over a wide range of realistic FDTD-based scenarios with results demonstrating accurate estimation of the breast skin location.

Evaluation of Two Approaches for Breast Surface Measurement Applied to a Radar-Based Imaging System

Locating the surface of an object, in this case the breast, is an important first step in many imaging situations; this surface information may be a necessary part of the reconstruction, it may be needed for the cancellation of the surface reflection, or (as herein) it may be needed as a preparatory step before imaging. This paper presents two complementary approaches developed for the purpose of surface localization. The proposed approaches are evaluated using data from both phantom measurements and volunteer scans.

MICROWAVE CONTRAST IMAGING OF BREAST TIS- SUE FROM LOCAL VELOCITY ESTIMATION

This paper proposes a new method to display microwave images of breast tissue, based on estimation of local microwave velocity from time of ∞ight measurements. Its computational demands are low compared with tomography. It has two major components: 1) the estimation of the travel time of microwaves across the tissue between a set of antennae using a wavelet decomposition, and 2) the estimation of the microwave velocity fleld from the set of travel times using a low dimensional set of radial basis functions to model local velocity. The technique is evaluated in 2-D on clinical MR-based numerical breast phantoms incorporated in Finite-Difierence Time-Domain simulations. The basis functions, used with a regularisation scheme to improve numerical stability, reduce the dimensionality of the inverse problem for computational e-ciency and also to improve the robustness to error in velocity estimation. The results support previously published flndings that the wavelet transform is suitable for robust measurement of time of ∞ight even in clinically signiflcant simulations, and shows that the velocity contrast images can be constructed so difierent regions of breast tissue type can be distinguished. In particular, the presence of a tumour is clearly detected, demonstrating the potential of this approach for breast screening. Keywords: Biomedical signal processing; Microwave imaging; Image reconstruction.