Approximating the head-related transfer function using simple geometric models of the head and torso.

The head-related transfer function (HRTF) for distant sources is a complicated function of azimuth, elevation and frequency. This paper presents simple geometric models of the head and torso that provide insight into its low-frequency behavior, especially at low elevations. The head-and-torso models are obtained by adding both spherical and ellipsoidal models of the torso to a classical spherical-head model.

Elevation localization and head-related transfer function analysis at low frequencies.

Monaural spectral features due to pinna diffraction are the primary cues for elevation. Because these features appear above 3 kHz where the wavelength becomes comparable to pinna size, it is generally believed that accurate elevation estimation requires wideband sources. However, psychoacoustic tests show that subjects can estimate elevation for low-frequency sources.

High resolution 3D color flow mapping: applied to the assessment of breast vasculature.

In order to develop a three dimensional (3D) color flow map of low velocity flow through small vessels, we explore the use of a high resolution velocity estimation technique and a new technique for the differentiation of regions of flow and stationary tissue. Following the transmission of a wideband signal, a signal processing strategy that tracks the motion of small regions of blood is used to estimate velocity. We find that the use of wideband transmission, with coherent estimation over a long pulse train, provides the opportunity to map very low velocity flow, and to detect flow at beam-vessel angles near 90 degrees.

Linear filtering of images based on properties of vision.

The design of linear image filters based on properties of human visual perception has been shown to require the minimization of criterion functions in both the spatial and frequency domains. We extend this approach to continuous filters of infinite support. For lowpass filters, this leads to the concept of an ideal lowpass image filter that provides a response that is superior perceptually to that of the classical ideal lowpass filter.

The effect of frequency dependent scattering and attenuation on the estimation of blood velocity using ultrasound.

A comprehensive theoretical performance comparison of the wideband maximum-likelihood (WMLE) and cross-correlation strategies, previously proposed and evaluated for the estimation of blood velocity using ultrasound is presented. It is based on evaluation of the bias, local and global accuracy, and signal-to-noise ratio (SNR) performance. The results show that the intervening medium does not bias either wideband estimation, due to the effect of tracking the scattering target.