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P M Shankar

Publications and source records attributed to P M Shankar.

26 records · Page 2Linked to original sources
PubMed1998

Using phase information in ultrasonic backscatter for in vivo liver analysis.

In recent studies, it has been shown that information about scatterer spacing can be obtained from analyzing the phase of the ultrasound echo from various media. Such information proves to be useful when examining the ultrasonic backscatter from well-organized tissue, such as the liver. By quantifying the deviations in scatterer spacing and/or varying degrees of regularity, conclusions may be drawn about the underlying pathology of the tissue. This paper examines the physical basis of how the scatterer locations affect the phase of the data. Computer simulations were performed that mimic various scattering conditions and that display the effects of differing degrees of regularity, as well as increases in a diffuse random background scattering component. Results of studies on a phantom are also included to investigate and display the phase response under well-controlled scattering conditions. Finally, in vivo data taken from liver scans were analyzed. In this work, it was shown that the phase of the backscattered signal holds valuable information regarding the pathological state of liver tissue. It is suggested that this simple examination of the phase can be refined into a technique to be used as a method to consistently detect the onset of pathological change.

Algorithms↗
PubMed1998

Advantages of subharmonic over second harmonic backscatter for contrast-to-tissue echo enhancement.

It is shown experimentally that backscatter from two ultrasonic contrast agents suspended in water or saline contains subharmonics of the incident frequency that are stronger than those backscattered at the same incident pressure from chicken breast. It is also shown that the ratio of subharmonic backscattered from contrast to that backscattered from tissue, is stronger than the ratio of backscattered second harmonic. In consequence, blood that contains contrast should be more easily detectable with respect to tissue if the subharmonic, rather than the second harmonic, is used for imaging.

Albumins↗
PubMed1996

Studies on the use of non-Rayleigh statistics for ultrasonic tissue characterization.

The research groups at Drexel University and Thomas Jefferson University had proposed the use of non-Rayleigh statistics for tissue characterization. Previous work based on the hypothesis that the envelope of the backscattered echosignal from abnormal regions of the tissue is more likely to be K-distributed than Rayleigh distributed, used the parameter of the K-distribution, M, to distinguish between regions containing benign or malignant masses and normal ones. In this work the B-scan breast images of 19 patients were studied using this approach. Previous studies have also been extended to exploit the existence of non-uniform phase characteristics of the echosignal from scatterers with some regular spacings, such as those in a periodic or quasi-periodic alignment. Computer simulations were carried out to show that the phase statistics deviate significantly from uniform in the range of (0, 2 pi) if the imaging region contained a number of periodically aligned (regular lattice) scatterers along with a collection of randomly distributed scatterers resulting in a quasi-periodic arrangement. This methodology was then applied to B-scan images of the breasts to distinguish between benign and malignant masses. If benign lesions show some sort of quasi-periodic or regular structures in the tissue, they will present non-uniform phase characteristics while more randomly structured malignant masses will have uniform phase characteristics. It is seen that the K-distribution may be used to identify the abnormal regions in the breast images and information on the phase may be used to further separate the abnormal regions into benign and malignant ones.

Breast Neoplasms↗
PubMed1995

Characterization of ultrasonic B-scans using non-Rayleigh statistics.

The envelope of the ultrasonic echo backscattered from tissues is modeled using non-Rayleigh statistics, namely the K-distribution. We show that the K-distribution can be used to describe the envelope of the echo, and its parameters may be used to distinguish between different regions in ultrasonic B-scan images. The validity of the model is tested using phantoms. Preliminary results indicate that the parameters of the K-distribution may be used to separate targets, in which the number of scatterers or the scattering cross-sections differ from that of the background. The method of employing the parameters of the K-distribution appears to be better than the methods based solely on the signal-to-noise ratio.

Humans↗
PubMed1995

A model for ultrasonic scattering from tissues based on the K distribution.

A model for the backscattered ultrasound echo from tissues is presented. The model takes into account the fact that the range cell being insonified may contain only a few scatterers and the number may not be large enough to justify the use of a Gaussian model which results in Rayleight statistics for the echo. Furthermore, the model also considers the case where the echogenicity of the scatterers in the range cell may not be uniform, the lack of uniformity resulting from variations in scattering cross-sections produced by the chemical as well as biochemical changes brought on by the presence of disease, growth of benign or malignant tumours, etc. The model is developed from the fundamental principles of scattering using the results available in radar. This new model results in a two-parameter distribution, namely the K distribution for the echo, thereby making it possible to gain information on the number as well as scattering cross-sections of the scatterers in the range cell. The model is extended to include the effects due to the presence of scatterers having some regular or periodic orientation in the range cell, resulting in the so-called generalized K distribution which approximates to Rayleight, Rician, or Gaussian under various limiting cases. Results of computer simulations and experiments on tissue-mimicking phantoms are also provided, which strongly suggest that this new model offers potential for tissue characterization.

Humans↗
PubMed1994

Characterization of ultrasonic transducers using a fiberoptic sensor.

The PVDF (polyvinylidene fluoride) hydrophones, commonly used to measure the characteristics of ultrasonic transducers, suffer from a number of drawbacks. They disturb the field distribution to be measured and cause spatial averaging effects because of their finite aperture. In addition, they are very delicate and susceptible to damage. To overcome some of these problems, the authors previously proposed the use of an optical fiber-based probe to measure the ultrasonic fields. In this paper, this fiberoptic ultrasonic sensor is used to measure the characteristics of six transducers, focused as well as unfocused, covering a frequency range of 2.25 MHz to 20 MHz. Results obtained using the fiberoptic sensor are compared with those obtained using a calibrated PVDF needle hydrophone with an effective diameter of 0.5 mm. The temporal responses as well as the beam profiles of the transducers measured using the fiberoptic sensor show excellent agreement with the results obtained using the PVDF needle hydrophone.

Fiber Optic Technology↗
PubMed1991

Ultrasound speckle analysis based on the K distribution.

The departure of speckle magnitude from Rayleigh statistics was applied to examine insonated phantoms with both low and high concentrations of scatterers. A mathematical model, the K distribution of Jakeman, was used to characterize non-Rayleigh statistics. This model contains a parameter, alpha, which characterizes the clustering of the scattering sites in a medium. It is shown from phantom experiments that alpha is linearly proportional to the log-scaled scatterer concentration in a range from about 1 to 30 scatterers per sample volume.

Mathematics↗