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Biomedical subjects

K K Shung

Publications and source records attributed to K K Shung.

At least 37 records · Page 2Linked to original sources
PubMed1994

A computer model for simulating ultrasonic scattering in biological tissues with high scatterer concentration.

Scattering of ultrasonic waves by biological tissues at different scatterer concentrations is investigated using one- and two-dimensional computer simulation models. The backscattered power as a function of scatterer concentrations is calculated using two types of incident waves, a Gaussian shaped pulse and a continuous wave (CW). The simulation results are in good agreement with the Percus-Yevick packing theory within the scatterer concentrations, from 0% to 100% in one-dimensional (1D) space, and 0% to 46% in two-dimensional (2D) space. In all cases, the simulation results from a pulsed incident wave show a much smaller standard deviation (SD) than those from an incident CW. The simulation can serve as a useful tool to verify scattering theories, simulate different experimental conditions, and to investigate the interaction between the scatterer properties and the scattering of ultrasonic waves. More importantly, the 2D simulation procedure serves as an initial step toward the final realization of a true three-dimensional (3D) simulation of ultrasonic scattering in biological tissues.

Blood↗
PubMed1994

Analysis of ultrasonic scattering in blood via a continuum approach.

Since the pioneering work by Reid et al. on measuring ultrasonic scattering in blood, this phenomenon has been extensively studied both theoretically and experimentally. The knowledge on ultrasonic scattering properties of blood is needed for the design of ultrasonic methods for measuring blood flow, and a better interpretation of ultrasonic images. The development of high frequency intravascular or intracardiac imaging devices raises the possibility of measuring blood properties, e.g., erythrocyte aggregation and fibrinogen concentration, in situ. A number of theoretical approaches have been developed to analyze this phenomenon where in general ultrasound wavelength is much greater than the erythrocytes. These results show that the backscattering coefficient of blood, defined as power backscattered by a unit volume of blood per steradian per unit incident intensity, is proportional to variance of the erythrocyte number fluctuation and backscattering cross-section of a single erythrocyte. In this paper, we will show that similar results can also be obtained by taking a continuum approach.

Blood↗
PubMed1994

High frequency ultrasonic backscatter from erythrocyte suspension.

Previous studies have shown that ultrasonic backscattering from red blood cells suspended in saline is proportional to the fourth power of frequency for frequencies below 15 MHz, as predicted by Rayleigh scattering theory. Recently, we have extended the measurements up to 30 MHz, because scattering of ultrasound by red blood cells may no longer be negligible at these frequencies and can affect, to a great degree, the operation of intravascular imaging devices. The experimental results show that the fourth power dependence on frequency of the backscattering coefficient for porcine erythrocytes suspended in saline solution appears to be valid up to 30 MHz. To confirm this, backscattering cross-section of porcine red cells was computed as a function of frequency using the T-matrix method. Since at higher frequencies the shape of the scatterers may also play a significant role, its effect was investigated by treating the red cell as a sphere, a disc, and a biconcave disc of the same volume. Good agreement was obtained between the experimental and theoretical results.

Animals↗
PubMed1994

Ultrasound scattering from blood with hematocrits up to 100%.

The backscattering coefficient of saline suspensions of porcine red blood cells was measured for hematocrits up to about 90%. It was found that the coefficient peaks at approximately 15%, but then, contrary to what a simple "gap theory" might suggest, it decays smoothly to zero, without showing another peak at high hematocrits. A one-dimensional (1-D) slab scattering model, in which the number of slabs per unit length represents the hematocrit and whose thickness and acoustical properties are similar to red cells/plasma, was also used to investigate the relation between the backscattered power and hematocrit. Monte-Carlo simulations performed for randomized boundary conditions show a similar relation to that of the 3-D system. The experimental data is compared to the Percus-Yevick theory for the packing of hard spheres, and the simulated data is compared to the Percus-Yevick theory for infinite slabs.

Animals↗
PubMed1993

A feasibility study on quantitating myocardial perfusion with Albunex, an ultrasonic contrast agent.

Quantitating regional myocardial perfusion has been the much sought-after but still elusive goal of many intensive investigations over the years. Videodensitometry of the variation of myocardial echogenicity in two-dimensional (2-D) echocardiograms as a function of time in conjunction with the injection of a bolus of an ultrasound contrast agent has been used clinically as a tool for a direct assessment of regional myocardial perfusion, despite that the precise relationship between tissue echogenicity observed on an image and the echoes detected by the ultrasonic probe is unknown. A study was undertaken to determine whether ultrasonic backscatter calculated from unprocessed radio frequency (RF) echoes returned from myocardium could be used to quantitate regional myocardium perfusion. A real-time ultrasonic scanner has been modified and interfaced to a microcomputer to acquire RF data at a rate up to 10 frames per second. Preliminary experimental data were obtained from four open-chest dogs following intracoronary injection of a bolus of Albunex and two dogs following intravenous injection with this modified scanner. On one hand, these results indicate that the integrated backscatter measured from the region of myocardium perfused by the coronary artery where Albunex is injected and selected for monitoring initially increases, reaches a peak, and then decreases as the contrast agent is washed out and that the magnitude of the peak is approximately linearly proportional to the volume concentration of Albunex microspheres injected, clearly demonstrating the feasibility of this approach for quantitating region myocardial perfusion. On the other hand, intravenous injections did not result in any appreciable change in myocardial backscatter in the left ventricle although a response could be observed in the left ventricular blood pool.

Albumins↗
PubMed1993

Cyclic variation of the power of ultrasonic Doppler signals backscattered by polystyrene microspheres and porcine erythrocyte suspensions.

Factors affecting the power of the ultrasonic Doppler signal within the flow cycle have been evaluated experimentally using a pulsatile flow loop model. Polystyrene microspheres and porcine red cells suspended in saline solution for hematocrits between 2 and 40% were used as scattering fluid in the flow model. Experiments were performed at mean flow velocities of 11, 64, and 76 cm/s. In laminar flow experiments performed at a mean velocity of 11 cm/s, no variation of the Doppler power was found for both polystyrene microspheres and red cell suspensions (40% hematocrit). When turbulence was induced in the flow model, the power increased during systole, a maximum was observed early after peak systole, and a decrease was obtained in diastole during deceleration of flow. At higher mean flow velocities (64 and 76 cm/s), a significant cyclic variation of the Doppler power was also measured for all values of hematocrits (between 2 and 40%). The power of the signal scattered by microspheres and red cell suspensions at 4% hematocrit dropped in systole, reached a minimum at peak systole, and then increased during early diastole. For red cells suspended in saline at 40% hematocrit, a slightly different pattern of variation was obtained. The cyclic variations observed at high flow velocities and in the presence of turbulence are believed to be associated with cyclic changes in the correlation among particles. In the present study, the effect of red cell aggregation on the cyclic variation has not been addressed.

Animals↗
PubMed1992

The effects of hematocrit, shear rate, and turbulence on ultrasonic Doppler spectrum from blood.

Previous studies of ultrasonic scattering properties of blood using a pulse-echo experimental arrangement show that ultrasonic backscatter from blood is influenced by a number of factors including hematocrit, shear rate, and the nature of flow (J. Acoust. Soc. Amer., vol. 75, p. 1265, 1984 and J. Acoust. Soc. Amer., vol. 84, p. 1, 1988). Since the Doppler frequency spectrum from a Doppler flowmeter is derived from echoes backscattered by red blood cells in the flowing blood, it is also undoubtedly a function of these parameters. The effects of these parameters on Doppler spectrum from blood have been investigated using a pulsed Doppler flowmeter. The results agree well with those obtained in previous studies. One important conclusion of this study is that the assumption that the Doppler spectral power density at a frequency in Doppler spectrum is linearly proportional to the number of red cells flowing at that velocity used in many theoretical models developed to explain the Doppler phenomenon may be erroneous. An alternative is proposed. It is shown that conclusions derived from these theoretical models would remain valid by making this assumption.

Animals↗
PubMed1990

Nonlinear energy exchange among harmonic modes and its applications to nonlinear imaging.

Nonlinear energy exchange and redistribution among the harmonic modes have been investigated. A problem with the pre-bias boundary condition, which means that in addition to the fundamental harmonic, nonzero higher harmonics with adjustable amplitudes and phases are also applied to the input simultaneously, has been considered. The results show that the pre-bias technique that is proposed in this paper may offer a better means for detecting the nonlinear properties of the medium. The feasibility of the approach was validated by computer simulation and experimental results.

Acoustics↗
PubMed1990

A novel method for the measurement of acoustic speed.

Traditional methods for measuring acoustic speed require knowledge of either the specimen thickness or the distances between the transducers and the specimen. In general, the accuracy in measuring these quantities determines the accuracy of the experimental technique for measuring speed. This problem is particularly acute in measuring sound speed in biological specimens. A new method for measuring acoustic speed of materials, which eliminates the need for determining these quantities, has been developed. The technique, which necessitates the use of only one transducer, requires measurement of four times of flight of a sound pulse and the knowledge of the speed of sound in a reference fluid medium in which the specimen is placed. Ultrasonic speed in stainless steel and Plexiglas was measured using this method to verify its validity. Results on measurements on porcine liver, myocardium, and soft fat are also reported.

Acoustics↗
PubMed1989

Echoicity of whole blood.

A clear relationship was found to exist when the echoicity of whole blood under different flow conditions was determined and related to its corresponding ultrasonic backscattering properties. The results indicate that the echoicity of whole blood is shear rate and species dependent. Echoicity of porcine whole blood was found to decrease as the shear rate was increased, whereas the echoicity of bovine whole blood was found to be shear rate independent. At the same shear rate and hematocrit, the echoicity of human and porcine whole blood was found to be higher than that of bovine whole blood. These observations can be readily understood when red cell aggregation is considered. In imaging porcine whole blood under steady laminar flow, under certain conditions a hypoechoic region was observed to appear near the center of the flow conduit. The behavior of this hypoechoic region and the mechanisms responsible for its appearance are not entirely clear at present.

Animals↗
PubMed1988

Ultrasonic backscatter from flowing whole blood. II: Dependence on frequency and fibrinogen concentration.

Earlier studies showed that ultrasonic backscatter from erythrocytes suspended in saline is a function of hematocrit and frequency and that it can be affected by flow disturbance. The experimental data agree well with the theories. Recently, studies have been extended to flowing whole blood. The results indicated that ultrasonic backscatter from flowing whole blood differs from that from saline suspensions of erythrocytes in that it is shear-rate dependent and species dependent. In the present article, data on the dependence of ultrasonic backscatter from flowing whole blood on frequency and on fibrinogen concentration are reported. It was found that ultrasonic backscatter from flowing whole blood also depends on fibrinogen concentration when red blood cell (RBC) aggregation exists. Moreover, when the blood is under conditions that favor RBC aggregation such as low shear rates, high fibrinogen concentration, or high hematocrits, Rayleigh scattering apparently is no longer sufficient to describe its scattering behavior.

Animals↗
PubMed1988

Ultrasonic backscatter from flowing whole blood. I: Dependence on shear rate and hematocrit.

Previous results show that ultrasonic backscatter from red blood cells (RBCs) suspended in saline is a function of hematocrit and frequency and that it can be affected by flow disturbance. The experimental data agree well with the theories. In the present article, results on ultrasonic backscatter from flowing whole blood are reported. Studies have been conducted on porcine, bovine, and human blood. Ultrasonic backscatter of flowing whole blood differs from that of RBC suspensions in that it is shear-rate dependent, which means that it is a function of spatial position of the blood in the flow conduit. Moreover, the results indicate that it is also species dependent. This behavior can be readily understood when red cell aggregation is considered.

Animals↗
PubMed1987

Ultrasonic backscatter from bovine tissues: variation with pathology.

This is the second in a series of two papers reporting results collected on ultrasonic propagation properties, namely, velocity, attenuation, and backscatter of various mammalian tissues. In the first paper [J. Acoust. Soc. Am 78, 871 (1985)], the experimental results of five different types of normal bovine tissues were given. In this paper, results obtained for the most common bovine disease in the liver, liver abscess, and in the heart, lymphosarcoma, will be presented. These data show that the ultrasonic properties of tissues with these two types of pathologies are significantly deviated from those of normal tissues, thus providing further evidence that ultrasonic properties may be used as quantitative indicators of a certain disease for diagnostic purposes.

Animals↗
PubMed1987

Recent developments in diagnostic ultrasound.

Ultrasound is being used increasingly in clinical diagnosis throughout the world in many medical specialties. Its major advantages over other imaging modalities are that it is minimally invasive, less expansive, and portable. Its resolution rivals that of X-ray computerized tomography and magnetic resonant imaging. Its limitations are that certain organs, such as lung and bone, are inaccessible by ultrasound, and that certain organs such as heart can only be accessed through limited acoustic windows. In recent years, great advances have been achieved in ultrasound imaging technology that make it even more versatile. Color Doppler flow imaging, parallel processing, Duplex scanning, and transesophageal imaging are just a few examples. In this paper, those developments, among others, are reviewed and their implications in clinical practice are discussed.

Animals↗
PubMed1986

Further studies on ultrasonic properties of blood clots.

Two-dimensional echocardiography has been found to be an effective clinical tool in diagnosing intracardiac thrombi. Misdiagnosis may, however, still frequently occur because of the difficulty in differentiating the thrombi from other intracavitary masses based only on the echographic appearance of these structures. Ultrasonic tissue characterization techniques have been used in attempts to minimize this diagnostic uncertainty. Previously, we have shown that all ultrasonic parameters of blood, including ultrasonic backscatter, a quantitative measure of echogenicity, at 7.5 MHz increase rapidly following clotting. In this article, we report recent results on the measurements of attenuation and backscatter of thrombi as a function of time following clotting over the frequency range of 3 MHz to 8 MHz. These results indicate that ultrasonic backscatter from thrombi 12 h old is at least 18 dB higher than that of unclotted blood over the frequency range of 3 MHz to 8 MHz, and the slope of the attenuation coefficient is increased to 0.43 dB/cm-MHz. Comparison with the backscatter of bovine myocardium shows that the myocardium is more echogenic than fresh thrombi and is less echogenic than thrombi 12 to 24 h old. Similar results were also obtained for integrated backscatter measurements over the same frequency range.

Animals↗
PubMed1986

Pulsed ultrasonic Doppler velocity measurements inside a left ventricular assist device.

In this study we have employed a single channel, pulsed ultrasonic Doppler velocimeter to measure instantaneous velocity distributions within the pumping chamber of a ventricular assist device. Instantaneous velocities have been decomposed into periodic mean and turbulent fluctuating components from which estimates of Reynolds stresses within the chamber and mean shear stresses along the wall of the chamber have been obtained. A review of the complete data set indicates a maximum value of the mean wall shear stress of 25 dynes/cm2 and a maximum Reynolds stress of 212 dynes/cm2. These values are lower than those measured distal to aortic valve prostheses in vitro and are well below levels known to damage blood components. Core flow patterns, wall washing patterns and flow stagnation points are also revealed.

Assisted Circulation↗