Ultrasonic Imaging

In order to enhance the resolution of the ultrasonic B-mode image, digital techniques for dynamic focusing and steering have been receiving a great deal of attention for a long time. In this paper, we present the theory and preliminary experimental results for a new architecture of a fully digital B-mode imaging system with this desired feature. The system employs the Pipelined-Sampled-Delay-Focusing (PSDF) scheme to eliminate the bulky memory addressing and interpolation circuits which are needed in conventional digital imaging systems. To reduce hardware requirements and the bandwidth required for digital processings, we describe an efficient method to generate the sampling clocks for dynamic focusing and steering and some modified bandwidth sampling techniques to reduce the sampling rate for signal digitization. Using these methods, it is possible to achieve dynamic focusing and steering in real-time sector imaging using a linear phased array.

The echo signal obtained from a homogenous and isotropically scattering medium can be described as a Poisson time series which is convolved with the transmission pulse of the transducer. The probability density function (pdf) of this signal approximates to a Gaussian pdf for a narrowband pulse waveform. Methods to derive the phase-derivative (PD) signal from the complex envelope and the preenvelope of the echo signal are described. The first order pdf of the PD asymptotically becomes a Gaussian pdf by smoothing. Since the rectified PD is employed to obtain 2-dimensional grey scale images, the first order pdf as well as the signal-to-noise ratio (SNR) of this signal are also derived. The rectified PD is further smoothed by a cosine time window prior to the imaging. The SNR and the autocorrelation function (in the axial direction) of this latter signal can be derived under the assumption of a Gaussian spectrum of the transmission pulse. These first and second order characteristics of the PD images are calculated for the conditions employed in simulations and experiments reported previously and are quantitatively compared to the values obtained from these.

We describe a new method for the estimation of the speed of sound in soft tissues in the pulse-echo mode. A transducer imparts an accurate transaxial compression to the tissue, and the corresponding change in the arrival time of an echo feature is measured. The ratio between the compressed depth and the difference in arrival time is taken as the estimate of the speed of sound. A second, noncompressing transducer is used to correct for distal tissue movement. We show theoretically and experimentally that accurate speed of sound estimations can be made in overlying and underlying tissue mimicking layers.

This paper deals with the estimation of the slope of attenuation in human breast tissue. The measurement is done in the reflection mode with a short time Fourier analysis. All important factors such as diffraction effect, tissue depth and specular reflectors are taken into account. A population of 49 normal women shows large inter-individual variations of the attenuation coefficient. A multiple linear regression allows correlation of this variation with the duration of the woman's genital life and pregnancies. A preliminary study is done on 10 benign diffuse breast diseases and shows a weak correlation with the type of the breast: normal or pathological. The utility of the quantification is discussed for one case of large fibrocystic disease.

In previous work, we demonstrated that accurate backscatter coefficient measurements are obtained with a data reduction method that explicitly accounts for experimental factors involved in recording echo data. An alternative, relative processing method for determining the backscatter coefficient and the attenuation coefficient is presented here. This method involves comparison of echo data from a sample with data recorded from a reference phantom whose backscatter and attenuation coefficients are known. A time domain processing technique is used to extract depth and frequency dependent signal ratios for the sample and the reference phantom. The attenuation coefficient and backscatter coefficient of the sample are found from these ratios. The method is tested using tissue-mimicking phantoms with known scattering and attenuation properties.