PubMed Health⌕ Search

Biomedical subjects

K J Parker

Publications and source records attributed to K J Parker.

At least 19 recordsLinked to original sources
PubMed1992

Sonoelasticity of organs: shear waves ring a bell.

Sonoelasticity is the use of ultrasonography to visualize, in real time, the hardness of stiffness of tissues and organs by depicting the tissue's motion in response to an applied vibration source. The applied vibration source is usually of low amplitude and low frequency (less than 0.1 mm displacement and less than 2000 Hz). Under these conditions, the natural vibration response of tissues and whole organs is revealed as a standing wave pattern determined by the low-frequency elastic constants of the tissues and their boundary conditions, factors that are not related to the ultrasonic echogenicity. As a result, hard or dense isoechoic tumors that are undetectable by conventional ultrasonography often can be visualized in sonoelasticity imaging by virtue of their altered vibration response. In this report, we demonstrate the appearance of organs such as the breast, liver, and kidney during real-time, in vivo sonoelasticity imaging. The results show that the shape and location of vibration patterns are dependent on the tissues and vibration frequencies; thus, information about the basic elastic properties of tissues should be obtainable.

Breast↗
PubMed1991

Ultrasound properties of liver with and without particulate contrast agents.

Basic acoustic parameters are examined in rabbit liver with and without a solid contrast agent used for tumor detection. In normal liver, backscatter, attenuation, and sound speed are found to decrease with increasing water content. The addition of micron-sized particles made from iodipamide ethyl ester (IDE) can increase backscatter and attenuation depending on size and concentration. A discrepancy of the increased backscatter from theoretical predictions based on random scatterers is attributed to the particle's biodistribution in the liver.

Absorption↗
PubMed1991

Absorption of finite amplitude focused ultrasound.

Predictions of the absorption of focused finite amplitude waves based on weak shock theory have been tested experimentally. The characteristics of this absorption are qualitatively different from those associated with small signal losses. Under appropriate conditions, the absorption of finite amplitude ultrasound is determined largely by source amplitude, field geometry, and the nonlinear properties of the medium and is only weakly dependent upon the small signal absorption coefficient of the material. These effects are seen most dramatically in sharply focused sound fields. To emphasize nonlinear absorption in an experimental test of these predictions, measurements of heating were made in agar which has a very small linear absorption coefficient. Under appropriate conditions, nonlinear losses can make the effective absorption coefficient of this poorly absorbing material somewhat greater than the soft tissues of the body.

Acoustics↗
PubMed1991

New approaches to nonlinear diffractive field propagation.

In many domains of acoustic field propagation, such as medical ultrasound imaging, lithotripsy shock treatment, and underwater sonar, a realistic calculation of beam patterns requires treatment of the effects of diffraction from finite sources. Also, the mechanisms of loss and nonlinear effects within the medium are typically nonnegligible. The combination of diffraction, attenuation, and nonlinear effects has been treated by a number of formulations and numerical techniques. A novel model that incrementally propagates the field of baffled planar sources with substeps that account for the physics of diffraction, attenuation, and nonlinearity is presented. The model accounts for the effect of refraction and reflection (but not multiple reflections) in the case of propagation through multiple, parallel layers of fluid medium. An implementation of the model for axis symmetric sources has been developed. In one substep of the implementation, a new discrete Hankel transform is used with spatial transform techniques to propagate the field over a short distance with diffraction and attenuation. In the other substep, the temporal frequency domain solution to Burgers' equation is implemented to account for the nonlinear accretion and depletion of harmonics. This approach yields a computationally efficient procedure for calculating beam patterns from a baffled planar, axially symmetric source under conditions ranging from quasilinear through shock. The model is not restricted by the usual parabolic wave approximation and the field's directionality is explicitly accounted for at each point. Useage of a harmonic-limiting scheme allows the model to propagate some previously intractable high-intensity nonlinear fields. Results of the model are shown to be in excellent agreement with measurements performed on the nonlinear field of an unfocused 2.25-MHz piston source, even in the near field where the established parabolic wave approximation model fails. Next, the model is used to compare the water path and in situ fields of a medical ultrasound device. Finally, the model is used to calculate the spatial heating rate associated with a nonlinear field and to simulate the phenomenon of saturation-induced beam broadening.

Fourier Analysis↗
PubMed1991

New approaches to the linear propagation of acoustic fields.

New algorithms are described that provide insight into linear field propagation and offer significant reductions in computational complexity. The developments presented here include the usage of a recently developed discrete Hankel transform to implement two single step, planar propagation algorithms for baffled, radially symmetric, acoustic pressure or velocity fields; an update on the single step approaches that reduce computational complexity through geometrically determined spatial frequency limitations; and algorithms for extending to multistep propagation. Two equivalent means of introducing arbitrary medium attenuation into the above schemes are presented. Finally, a planar boundary crossing algorithm that accounts for refraction and reflection (but not multiple reflections) is added to one of the multistep propagating algorithms. The resulting algorithm is then used to examine the differences between the corresponding fields of a focused piston source operating in water and in a layered fat/liver (biomedical imaging) medium. The results yield computationally efficient algorithms that can be used for linear propagation of focused or unfocused beams in attenuating, multilayer media, and also provide the basis for a novel nonlinear propagation algorithm.

Acoustics↗
PubMed1991

Sonoelasticity imaging: results in in vitro tissue specimens.

The authors present a method for imaging tissue stiffness (sonoelasticity) that has been developed and tested in a laboratory setting by using in vitro canine and human prostate glands. A low-frequency acoustic source was used to induce vibration in tissue under examination, and a color Doppler ultrasound (US) instrument was modified to detect vibration amplitude. The resulting image is a color "map" of tissue vibration superimposed on conventional gray-scale US images. Stiffer tissues vibrated less in response to audible sound, regardless of echogenicity. Normal human and canine prostate glands demonstrated a uniform vibration pattern. Four of four human prostatic adenocarcinomas and two stiff inclusions injected into canine prostate glands demonstrated a lack of vibration in comparison with normal surrounding tissue. The authors conclude that while further study is necessary, sonoelasticity imaging may enhance the detection of neoplasms by enabling their identification solely on the basis of stiffness.

Adenocarcinoma↗
PubMed1991

Segmentation of speckle images based on level-crossing statistics.

When imaging is performed by using a coherent signal, the result is frequently a realization of the stochastic process known as speckle. The information sought from this process is often the mean value of its envelope or intensity at each point in the image plane. When only a single realization of the process is available, ergodicity is required within a sufficiently large region for accurate estimation of the mean. The identification of these regions is the segmentation problem that is addressed. The approach presented clips the speckle image at a constant threshold level and analyzes the resulting bilevel image based on the level-crossing statistics of the speckle process. An analysis of the level-crossing process leads to a decision rule for identifying or segmenting distinct regions of the image based on the sizes of the fades and the excursions in the clipped speckle. The measurement of these sizes is accomplished by using the morphological transformations of opening and closing. This new approach has been applied to computer-generated speckle images and may prove useful in laser, ultrasound, and radar imaging, in which speckle phenomena are manifest.

Image Processing, Computer-Assisted↗
PubMed1990

"Sonoelasticity" images derived from ultrasound signals in mechanically vibrated tissues.

A method has been developed for detecting and imaging the relative "stiffness," or elasticity of tissues. Externally applied vibration at low frequencies (10-1000 Hz) is used to induce oscillations within soft tissues, and the motion is detected by Doppler ultrasound. The results are displayed in a format resembling conventional Doppler color flow mapping, and are termed "sonoelasticity images." Preliminary experiments indicate that these novel images may be useful for detecting hard tumors in the prostate, liver, breast, and other organs.

Animals↗
PubMed1990

Tissue response to mechanical vibrations for "sonoelasticity imaging".

The goal of "sonoelasticity imaging" is to differentiate between normal soft tissues and hard lesions. This is done by measuring and then displaying the ultrasound Doppler spectrum of regions within tissues which are mechanically forced with low frequency (20-1000 Hz) vibrations. The resolution and sensitivity of the technique ultimately rest on the spatial resolution of ultrasound Doppler detection, the low frequency mechanical properties of tissues, and the vibration response of layered, inhomogeneous regions with hard tumor inclusions and complicated boundary conditions set by the presence of skin, bones and other regions. An initial investigation has measured some tissue stiffness parameters, and applied these in a NASTRAN finite element analysis to simulate a prostate tumor in the pelvic cavity. The measurements show a wide separation between the elastic modulus of tumors and soft tissues such as muscle and prostate. NASTRAN analyses show the ability to delineate regions of different elasticity based on the pattern of vibration amplitudes. The ability to change vibration frequency within the 100-300 Hz band seems particularly helpful in simulations and experiments which visualize small stiff inclusions in tissues. Preliminary results support the postulate that sonoelasticity imaging can provide useful information concerning tissue properties that are not otherwise obtainable.

Computer Simulation↗
PubMed1990

Lysis of cells in Elodea leaves by pulsed and continuous wave ultrasound.

Resonance lysis of the cells in the leaves of the aquatic plant Elodea originally reported by Miller have thresholds at intensities of a few W/cm2. With pulsed ultrasound, the resonance behavior vanishes and the thresholds are at much higher amplitudes. This is similar to the characteristics of the thresholds for killing of Drosophila larvae by pulsed ultrasound. Both organisms contain small gas bodies within the tissues which may serve as nuclei for a cavitation related phenomenon. The results suggest that the response of these bubbles to continuous wave fields and very short pulses is qualitatively different.

Cell Survival↗
PubMed1990

Ultrasound contrast for hepatic tumors using IDE particles.

Iodipimide ethyl ester (IDE) can be formulated as dense spherical particles with narrow diameter distribution. When IDE particles are injected intravenously, the Kupffer cells of the hepatic sinusoids accumulate particles within 10 to 20 minutes, after which the clearance and excretion of IDE takes place. During the uptake phase, the dense particles act as scattering sites, increasing the echogenicity of normal liver tissue. In comparison, tumors and other lesions remain at pre-injection echogenicity, as they lack Kupffer cells and therefore do not retain particles. This report provides initial studies of contrast enhancement in rabbit livers with implanted VX2 tumors, scanned in vivo and evaluated ex vivo using pulse-echo techniques. The distribution of particles within hepatic lobules may explain why the observed echogenicity is greater than that predicted by single-particle backscatter theory. Directions for future improvements are discussed.

Animals↗
PubMed1989

Contrast agents in diagnostic ultrasound.

We review the field of contrast agents in diagnostic ultrasound. The progress in the development of various classes of contrast agents such as free and encapsulated gas bubbles, colloidal suspensions, emulsions, and aqueous solutions is described. The mechanisms for production of backscatter contrast, as well as attenuation contrast and speed of sound contrast are explained. Finally, the potential advantages and disadvantages of various classes of contrast agents are compared.

Animals↗
PubMed1989

Liver glycogen and water storage: effect on ultrasound attenuation.

Glycogen has been shown in vitro to have a high specific absorption coefficient (ultrasound absorption in water per concentration) compared to other proteins. Depending on the amount of water which may accompany glycogen flux into and out of liver cells, the ultrasound attenuation coefficient of the liver may rise and fall with stored hepatic glycogen. This paper reports ex vivo studies on rats and in vivo studies on normal human volunteers before and after fasting. The results show a statistically significant difference in liver attenuation between well fed and fasted individuals. Generally, the attenuation difference is greater than 10%, and indicates that liver wet weight may not be strictly constant over glycogen storage cycles, as suggested in classic works. In contrast, no significant change in ultrasound backscatter is noted. The results point to the possible role of ultrasound attenuation measurements as a sensitive indicator of tissue physiology, and suggest that glycogen (feeding or fasting) must be controlled in tissue characterization experiments which compare liver attenuation coefficients of individuals and groups.

Animals↗
PubMed1989

Finite-amplitude effects on ultrasound beam patterns in attenuating media.

Some problems relevant to medical ultrasonics are addressed through experimental measurements of focused, pure-tone beam patterns under quasilinear conditions where significant nonlinearities are manifested. First, measurements in water provide a comparison of the beam patterns of the fundamental and nonlinearly generated harmonics against recent theoretical predictions of others. The radial beamwidths, presence and spacing of sidelobes, axial distances to peak pressures, focal shock parameter, time-domain waveform asymmetry, and post-focal falloff of the fundamental through fifth harmonics are discussed relative to various models under preshock conditions (sigma less than 1). Second, the focused sources are placed in a more attenuating fluid to mimic the behavior of these fields in tissue. The changes in beam characteristics are examined relative to measurements at the same intensities in water, and relative to theoretical predictions. The results suggest that, given a known linear (low-intensity) focused beam pattern in water, guidelines can be followed to predict the beam pattern of the fundamental and higher harmonics at higher intensities in water, and then in attenuating media such as tissue.

Ultrasonics↗
PubMed1988

In-vivo measurements of ultrasound attenuation in normal or diseased liver.

Ultrasonic attenuation coefficients of liver have been derived from echoes received by a modified commercial B-scan imaging instrument. Values have been measured from selected regions within liver scans of 59 individuals, of which 15 cases were presumed normal (based on medical histories), and the remainder were involved with diffuse liver disease such as alcoholic cirrhosis, chemotherapy toxicity, chronic hepatitis, and liver metastases. Medical histories on most individuals include the results of serum liver function enzymes, conventional B-scan examinations, and exposure to drugs and alcohol. The results of CT abdominal scans (N = 13) and/or liver biopsy (N = 12) were also available. The results show that normal attenuation values for human liver are 0.054 +/- 0.009 Np/cm-MHz (0.47 dB/cm-MHz) with a frequency dependence of fn, where n = 1.05 +/- 0.25, in agreement with in vitro studies of mammalian liver. In diffuse liver disease, no relationship was found between the attenuation coefficient and the results of CT or conventional ultrasonic examination. A trend towards higher attenuation with increased fibrosis and fat, as graded from liver biopsies, was noted, but the results were generally not statistically significant. However, a significant correlation was found between high values of attenuation and abnormal liver function tests. High attenuation is also found with ingestion of alcohol, chemotherapeutic agents, and steroids, all of which may affect liver composition.

Adolescent↗
PubMed1988

Particulate suspensions as ultrasonic contrast agents for liver and spleen.

Ultrasonic backscatter and attenuation coefficients of a medium can be increased by the addition of solid, micron-size inhomogeneities. A potentially useful agent for ultrasonic contrast of liver images has been identified. Iodipamide ethyl ester (IDE) particles can be produced in the form of dense, relatively incompressible solids with high impedance mismatch to water. The chemical, biochemical, and pharmacologic properties of the small, uniform diameter IDE particles permit safe intravenous injection followed by rapid accumulation of reticuloendothelial (RE) cells of the liver and spleen, and later elimination from these organs. Since the particles are phagocytized by RE cells, present in normal liver but not in tumors and many lesions, the selective enhancement of ultrasonic backscatter should improve detectability of lesions that are hypoechoic or isoechoic compared with surrounding tissue. The mechanisms of particle-ultrasound interaction may be described by relative motion attenuation, and scattering from a cloud of dense, incompressible spheres for the case of IDE particles in agar. Thus, values of attenuation and backscatter can be controlled by choice of ultrasound frequency and particle concentration and size. When the particles are accumulated in rat and rabbit livers, additional mechanisms induce attenuation and backscatter in excess of that predicted by IDE in agar. This preliminary work demonstrates that solid, biocompatible particles may be useful as an ultrasonic contrast agent.

Animals↗