Biomedical subjects

K K Shung

Publications and source records attributed to K K Shung.

At least 19 recordsLinked to original sources

PubMed2006

High frequency broadband PZT thick film ultrasonic transducers for medical imaging applications.

A modified sol-gel method is used to prepare PZT thick film on Pt-coated silicon substrate. A new method of vacuum filling sol-gel precursor solution is introduced to improve film quality. The effects of the filling on PZT thick film structure and ferroelectric properties are discussed. The fabrication of a high frequency transducer with the PZT film as the actuating layer is described. The performance of the transducer is measured and results show that the transducer backed by E-Solder without a matching layer has a center frequency of 103 MHz and a bandwidth of 70%. Beam profile measurements show that the transducer has an axial resolution of 9.2 microm and a lateral resolution of 33 microm.

Electromagnetic Phenomena↗

PubMed2002

The principle of multidimensional arrays.

Echocardiography is one of the most important diagnostic tools in cardiology today. One-dimensional phased arrays have been used extensively because they have a small footprint and allow beam steering. Their major limitation lies in that these devices can only be used to acquire images of two-dimensional slices in real-time and that the slice thickness cannot be controlled. To allow real-time three-dimensional imaging of the heart and focusing of the ultrasonic beam in two-dimensional, two-dimensional arrays, the design and fabrication of which are enormous engineering challenges, are required. Before reaching this ultimate goal, limited focusing in the elevational plane can be achieved with 1.5-dimensional arrays. Focusing in the elevational plane allows a reduction in slice thickness and thus an improvement in the image quality over a larger depth of view.

Echocardiography↗

PubMed2001

Doppler power variation from porcine blood under steady and pulsatile flow.

Although a number of recent studies have demonstrated that the echogenicity of blood varies as a function of time under pulsatile flow, the fundamental mechanisms responsible for it are still uncertain. To better understand this phenomenon, the Doppler power from porcine blood and polystyrene microsphere suspensions was measured at the center of the tube as functions of two crucial parameters, flow velocity and stroke rate (for pulsatile flow), under steady and pulsatile flow in a mock flow loop. In the present study, the experimental results were obtained with a 10-MHz pulsed Doppler system with a frequency response estimated more accurately by electronic injection, and validated by comparing to the radiofrequency (RF) signal acquired from the same Doppler instrument. The results show that the Doppler power from microspheres and porcine red blood cell (RBC) suspensions did not vary appreciably (< 2 dB), with either the speed or stroke rate (for pulsatile flow only) under steady and pulsatile flow. It was found that the Doppler power from porcine whole blood under steady flow decreased with the speed by approximately 13 dB from 3 to 33 cm/s and was only 3 dB higher than that from RBC suspension at 33 cm/s, suggesting minimal RBC aggregation in whole blood at this speed. The apparent cyclic variation from whole blood was observed at 20 and 40 beats/min (BPM). The cyclic variation became more obvious as the speed and stroke rate decreased. The mean Doppler power over a cycle increased as the peak speed decreased. The Doppler power reached a maximum near peak systole and a minimum at late diastole at the center of the tube. This pattern cannot be explained by RBC aggregation due to the shear rate alone, and may be attributed to acceleration and deceleration along with aggregation. The cyclic variation was not observed at 60 BPM, probably because of a lack of time for aggregation to occur.

Activity Cycles↗

PubMed2001

High frequency properties of passive materials for ultrasonic transducers.

The acoustic properties of passive materials for ultrasonic transducers have been measured at room temperature in the frequency range from 25 to 65 MHz using ultrasonic spectroscopy. These materials include alumina/EPO-TEK 301 composites and tungsten/EPO-TEK 301 composites. Experimental results showed that the acoustic impedance of the composites monotonically increased with the volume fraction of the particle filler, which is in agreement with the Denavey model. The attenuation, however, peaked between 7 and 9% volume fraction of particle filler. For comparison, several other passive materials were also fabricated and measured. The results suggest that materials that possess a higher attenuation also appear to have a larger velocity dispersion.

Aluminum Oxide↗

PubMed2001

In vivo measurements of ultrasonic backscattering in blood.

Ultrasonic backscattering in blood including its dependence on the hematocrit, plasma proteins, shear rate, and flow disturbance, has been studied extensively theoretically and experimentally in vitro. However, much of the result has never been validated in vivo. To do so, backscattering measurements were made on pigs using a 10-MHz non-focused intravascular transducer in direct contact with blood. The probe was placed in either the abdominal aorta or the inferior vena cava. The backscattering coefficient (BSC) of blood flowing in these vessels as well as downstream from a stenosis was measured using an approach that was originally developed for measurements with focused transducers. With this approach, 6% porcine red cell saline suspensions prepared immediately after each in vivo measurement were used as the reference medium. Result from seven pigs at hematocrits ranging from 29 to 36% (31.9 +/- 2.5%) demonstrated that BSC of blood in the vena cava, (4.62 +/- 2.06) x 10(-5) cm-sr-1, is consistently higher than that in the aorta, (2.65 +/- 1.22) x 10(-5) cm-sr-1. The difference has been attributed to the lower shear rate and the formation of red cell aggregation in venous blood. These in vivo results are in agreement with those obtained in vitro. In response to stenoses created by ligating the aorta, backscattering of the blood measured downstream from the stenosis showed that the closer the site of measurement relative to the stenosis, the higher the backscatter, presumably resulting from the higher degree of flow disturbance. In vitro backscattering results on porcine whole blood were also acquired at 20 MHz with a Diasonics intravascular scanner.

Animals↗

PubMed2001

The "black hole" phenomenon in ultrasonic backscattering measurement under pulsatile flow with porcine whole blood in a rigid tube.

The "black hole" phenomenon was further investigated with porcine whole blood under pulsatile flow conditions in a straight rigid tube 120 cm long and of 0.95 cm diameter. A modified Aloka 280 commercial scanner with a 7.5 MHz linear array was used to collect the radio frequency (RF) signal of backscattering echoes from the blood inside the tube. The transducer was located downstream from the entrance and parallel to the longitudinal direction of the tube. The experimental results showed that higher hematocrits enhanced the black hole phenomenon, leading to a more apparent and larger diameter black hole. The black hole was not apparent at hematocrits below 23%. The highest hematocrit used in the experiment was 60%. Beat rates of 20, 40 and 60 beats per minute (bpm) were used, and the black hole became weaker in amplitude and smaller in diameter when the peak flow velocity was increased at each beat rate. These results are consistent with the suggestion in previous work that the black hole arises from insufficient aggregation of red blood cells (RBCs) at the center of the tube because of the low shear rate. At 20 and 40 bpm, the peak flow velocity ranges were 10 approximately 25 cm/s and 18 approximately 27 cm/s, respectively. The black hole was very clear at the minimal peak flow velocity but almost disappeared at the maximal velocities for each beat rate. At 60 bpm, experiments were only performed at one peak flow velocity of 31 cm/s and the black hole was clear. The results showed that the black hole was more pronounced at higher beat rates when the peak velocity was the same. This phenomenon cannot be explained by previous hypotheses. Acceleration seems to be the only flow parameter that varies at different beat rates when peak velocities are the same. Therefore, the influence of acceleration on the structural organization and orientation of RBC rouleaux might be another factor involved in the formation of the black hole in addition to the shear rate. As the entrance length was changed from 110 to 15 diameters (D) in seven steps at the hematocrit of 60%, it was found that a position farther downstream yielded a black hole with a greater contrast relative to the surrounding region, while the backscattering power at the central hypoechoic zone did not increase with increasing entrance length.

Animals↗

PubMed2000

High-frequency backscatter and attenuation measurements of selected bovine tissues between 10 and 30 MHz.

There are now diagnostic ultrasonic imaging devices that operate at very high frequencies (VHF) of 20 MHz and beyond for clinical applications in ophthalmology, dermatology, vascular surgery, endoluminal imaging and small animal imaging. To be able to better interpret these images and to further the development of these devices, knowledge of ultrasonic attenuation and scattering of biological tissues in this frequency range is crucial. Attenuation and backscatter coefficients (BSCs) of bovine tissues in the frequency range of 10 to 30 MHz were measured, respectively, using a standard substitution method for attenuation measurements and a modified narrow-band substitution method for scattering measurements. A modified substitution method for scattering measurements has to be used at high frequencies because unfocused transducers due to their decreased sensitivity cannot be used in the simple substitution method. In the modified method, the flat reflector is substituted by a particulate reference medium whose BSC is well-known and documented; in this case, a red cell suspension. In this paper, experimental results on BSC and attenuation coefficient measured between 10 and 30 MHz are reported. The frequency dependence of backscatter of the selected bovine tissues ranges from 2.4 to 3.5, whereas attenuation is observed to be still approximately linearly proportional to frequency. The BSC measured with the modified method is in good agreement with those obtained with the standard method between 10 and 20 MHz.

Animals↗

PubMed1999

The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: an in vitro study.

Previous in vitro studies in rigid tubes under pulsatile flow conditions have reported a lack of a cyclic variation in blood echogenicity that contradicts in vivo results. To investigate whether or not these variations can be attributed to the compliance of the vessel wall, a series of in vitro experiments with compliant tubes, under pulsatile flow conditions, was performed. Two important factors that may affect the Doppler power were investigated: 1. the dependence on hematocrit and 2. the effect of the vessel wall elasticity. In the present study, it is shown that, at the low beat rates, the peak of the mean Doppler power within the flow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shifted from the early to the late systole. Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant vessels. At a higher pulsation rate of 37 beats/min, a different variation is observed. A drop in the power occurs near peak systole in compliant tube experiments and is more pronounced as the vessel becomes more constricted. The observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest that proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of the vessel wall, propagation velocity or, possibly, the phase angle of input impedance.

Animals↗

PubMed1999

Ultrasonic backscattering from porcine whole blood of varying hematocrit and shear rate under pulsatile flow.

It was shown previously that ultrasonic scattering from whole blood varies during a flow cycle under pulsatile flow both in vitro and in vivo. It has been postulated that this cyclic variation may be associated with the dynamics of red cell aggregation because the shearing force acting on the red cell aggregates across the lumen is a function of time during a flow cycle. In all studies, the local shear rate variation as a function of time is unknown. The effect of shear rate on the red cell aggregation and, thus, on ultrasonic scattering from blood can only be merely speculated. One solution to this problem is to estimate the shear rate in a flow conduit by finite element analysis (FEA). An FEA computational fluid dynamics (CFD) tool was used to calculate local shear rate in a series of experiments in which ultrasonic backscattering from porcine whole blood under pulsatile flow was measured as a function of hematocrit and shear rate intravascularly with a 10-MHz catheter-mounted transducer in a mock flow loop. The results show that, at 20 beats per min (BPM), the magnitudes of the cyclic variation for hematocrits at 30, 40, and 50% were approximately 4 dB. However, at 60 BPM, the magnitude of cyclic variation was found to be minimal. The results also confirm previous findings that the amplitude and the timing of the peak of ultrasonic backscattering from porcine whole blood under pulsatile flow during a flow cycle are dependent upon the shear rate and hematocrit in a complicated way.

Animals↗

PubMed1999

Interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements.

In a study involving 10 different sites, independent results of measurements of ultrasonic properties on equivalent tissue-mimicking samples are reported and compared. The properties measured were propagation speed, attenuation coefficients, and backscatter coefficients. Reasonably good agreement exists for attenuation coefficients, but less satisfactory results were found for propagation speeds. As anticipated, agreement was not impressive in the case of backscatter coefficients. Results for four sites agreed rather well in both absolute values and frequency dependence, and results from other sites were lower by as much as an order of magnitude. The study is valuable for laboratories doing quantitative studies.

1-Propanol↗

PubMed1998

In situ measurements of Doppler power vs. flow turbulence intensity in red cell suspensions.

Whereas previous studies have shown that ultrasonic backscatter and Doppler power from blood are affected by flow turbulence, turbulence level has only been inferred from the flow Reynolds number and not directly measured. In this study, both ultrasonic Doppler power and flow turbulence intensity were measured in situ to quantify the relationship between Doppler power and flow turbulence. Three grid meshes of different geometries were used in a steady-flow mock loop to generate controlled levels of flow turbulence in porcine red blood cell saline suspensions. Doppler power was measured by a 10-MHz PW Doppler flowmeter, and the turbulence intensity by using constant-temperature hot film anemometry. We showed that Doppler power is affected by turbulence and hematocrit in a complex way. At a fixed hematocrit, Doppler power increases nonlinearly with turbulence intensity and, at fixed turbulence intensity, Doppler power peaks at an optimal hematocrit level that increases with turbulence level. The shape factor, introduced by Lucas and Twersky (1987) to take into account effects of shape and orientation of the scatterers in a dense distribution of small and tenuous scatterers, was estimated by fitting the experimental data to the theoretical model. The results indicate that shape factor decreases with increasing turbulence intensity.

Animals↗

PubMed1997

An approach for measuring ultrasonic backscattering from biological tissues with focused transducers.

When the standard substitution method is used with a focused transducer to measure the backscattering coefficient from biological tissues including blood, it yields erroneous results. Extending the backscattering measurements to frequencies beyond 15 MHz necessitates the use of focused transducers because of the worsened signal-to-noise ratio--caused by the increased attenuation and the smaller transducer aperture size--in order to make the measurements close to the transducer. An approach which allows the use of focused transducers in backscattering measurements has been developed. It has been used to measure the backscattering coefficient of red cell suspensions of hematocrit ranging from a few percent to 30% in the frequency range from 5 MHz to 30 MHz. The results at hematocrits below 20% agree well with those obtained with the standard substitution method, although they differ as the hematocrit is increased beyond 20%. The experimental results also show that the fourth-power dependence of backscatter on frequency is in general approximately valid for suspended erythrocytes of hematocrit between 6% and 30%.

Animals↗

PubMed1996

Some considerations on the measurements of mean frequency shift and integrated backscatter following administration of Albunex.

Ultrasonic contrast agents have been of heightened interest in recent years. More success has been achieved by agents consisting of micro bubbles, since only a few of these agents are capable of producing very strong ultrasonic backscattered signals for the enhancement of certain tissue structures. Recent investigations also demonstrate that an analysis of the radio frequency (RF) backscattered echoes by the contrast agents may lead to quantitative means for assessing tissue perfusion. In these studies, a parameter, mean frequency shift (MFS) of the RF signal, along with integrated backscatter (IB) has received the most attention. In an effort to better understand the physical mechanisms responsible for the observed mean frequency shift, we have performed experiments on 10 dogs following injections of Albunex (Molecular Biosystems, Inc.) into the left atrium, coronary artery and abdominal aorta, respectively, for investigations in the heart and kidney. The integrated backscatter and mean frequency (MF) of a region of interest (ROI) were calculated from the RF signal acquired with a modified real-time ultrasonic scanner. The results show consistently that the RF signals acquired from all regions of interest are greatly affected by the presence of the contrast agent in the path between the transducer and the ROI, which can cause either an upward or a downward shift of the MF. This could not be observed by video densitometry or a measurement of the IB alone. The MFS is the result of the resonant behavior of the micro bubbles, which is related to the frequency, ambient pressure, and physical properties of the bubbles including size distribution, surface tension and concentration. On the other hand, when there is no contrast agent present in the path, a downward frequency shift is seen.

Albumins↗

PubMed1996

Cyclic variation of Doppler power from whole blood under pulsatile flow.

The echogenicity and Doppler power from whole blood under pulsatile flow have been found to vary during the flow cycle in previous studies both in vitro and in vivo. The present study was undertaken to better understand this phenomenon. Doppler power from whole blood under pulsatile flow was measured with a pulsed Doppler flowmeter as a function of the flow cycle, radial position and compliance of the vessel in a mock flow loop. It was found that the cyclic variation is more pronounced if the stroke rate is less than 56 beats/min and that the peak of the Doppler power from whole blood flowing near the center stream coincided with the peak of the flow velocity. However, it began to lead the velocity peak as the measurement site was moved away from the center stream. The lead increased as the radial distance was increased. The results also show that the compliance of the vessel can affect, to a certain extent, the magnitude of the cyclic variation. Results from intravascular Doppler measurements rule out the possibility that the cyclic variation is primarily due to the variation in attenuation caused by vessel wall during a flow cycle.

Animals↗

PubMed1996

Quantitative measurements of second harmonic Doppler using ultrasound contrast agents.

Quantitative measurements of second harmonic and first harmonic Doppler were carried out using two ultrasound contrast agents, Albunex and FS069. The RMS amplitudes of the Doppler shift spectra were measured as a function of the concentration of the agents, frequency and transmitted acoustic pressure. The results showed that, for a given lot of contrast agent investigated, FS069 was able to produce higher levels of first and, especially, second harmonic signals compared to Albunex. Under the same experimental conditions, the RMS Doppler amplitude (RDA) of FS069 was 3.8 +/- 0.8 dB higher than Albunex at first harmonic and 12.6 +/- 0.8 dB higher at second harmonic. The ratio of the second harmonic to first harmonic RDA, which we called R2/R1, decreases at a rate of 7 dB/MHz for both agents with increasing frequency. The difference in the value of R2/R1 between FS069 and Albunex at any frequency was approximately 4.5 dB. R2/R1 was found to increase linearly as a function of the transmitted acoustic pressure for both agents. Simulations using the Rayleigh-Plesset equation show a decrease of R2/R1 at a rate of 5 dB/MHz. Comparison of experimental results with theory indicates that the shell elasticity parameter may be an increasing function of the mean diameter of the bubbles.

Albumins↗

PubMed1996

Hydrogels with enhanced mass transfer for transdermal drug delivery.

The sonophoretic transport rates of monomeric insulin and vasopressin across human skin in vitro in the presence of a 20 kHz ultrasound field are shown to differ substantially depending on whether molecules enter the skin from a saline solution or from a viscous ultrasonic coupling medium (specifically, a methyl cellulose hydrogel or viscous sol). Theoretically, the reduction in sonophoretic transport caused by the hydrogels can be explained by boundary layers that form within the hydrogel owing to the relatively rapid rate of molecular transport across the (ultrasonically) permeated stratum corneum as well as poor diffusive mass transfer between the skin and gel. The results of in vitro experiments performed with an ac current accompanying the ultrasound show that the mass-transfer barrier posed by the hydrogel can be eliminated for both vasopressin and insulin by suppressing the diffusive boundary layers, indicating that relatively high rates of sonophoretic molecular transport across human skin are achievable when hydrogels are used as the ultrasound coupling medium as long as method is used to induce molecular mixing within the gel.

Biological Transport↗

PubMed1995

Time-domain ultrasonic contrast blood flowmetry.

Time-domain ultrasonic blood flow estimation methods have recently received considerable attention because of their advantages over conventional Doppler methods. Among them are that they may yield better spatial resolution and that methods based on frame-to-frame speckle tracking do not require a knowledge of the angle between directions of blood flow and the ultrasound beam. These methods, however, suffer from an intrinsic problem of poor signal-to-noise ratio, since the echoes scattered back by blood are much weaker than those of the surrounding structures. In addition, the maximal velocity that can be estimated by frame-to-frame tracking via conventional ultrasonic scanners is limited by the frame rate of the scanner. In this article, we will present experimental results to show that these problems may be alleviated by using a high frame rate scanner in conjunction with the injection of an ultrasonic contrast agent. In this study the contrast agent used was Albunex.

Albumins↗

PubMed1995

A study of the relationship between mechanical and ultrasonic properties of dystrophic and normal skeletal muscle.

A study has been made of the application of radio frequency (RF) ultrasound to the detection of muscular dystrophy by monitoring passively stretched skeletal muscle. The tests included detection of integrated backscatter changes in response to both static loading, in which muscle samples were stretched and allowed to relax, and stress relaxation. In both static and step strain loading conditions, the dystrophic muscle was found to exhibit little change in backscatter power while normal muscle responded to loading with significant changes in integrated backscatter. The backscatter response is compared with mechanical properties of the tissue (time constants and stress-strain constants). Both mechanical and ultrasonic time constants of relaxation are not significantly different between normal and dystrophic tissue, but stress-strain constants do differ. The difference in response of dystrophic and normal tissue appears to be due to a repression of motion of the constituent anatomy of dystrophic muscle which is responsible for the change of echogenicity with passive stretch.

Animals↗