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Biomedical subjects

V L Newhouse

Publications and source records attributed to V L Newhouse.

At least 19 recordsLinked to original sources
PubMed2001

Velocity magnitude estimation with linear arrays using Doppler bandwidth.

The dependence of pulsed wave Doppler bandwidth on parameters typical of linear transducer arrays used in commercial Duplex and color flow mapping systems is investigated experimentally. For a single flow line it is observed that this bandwidth generally depends not only on the scatterer velocity and the beam-to-flow angle, but also on the flow line range and orientation. This is due to the fact that in Duplex and color flow systems the transducer is differently focused in the scan and elevation planes and its aperture and focal lengths are often made to vary, depending on the distance of the flow line from the transducer. It is however experimentally demonstrated that, at points where the ultrasound beamwidths in the scan and elevation planes are both comparable to the sample volume length, the Doppler bandwidth is independent of the beam-to-flow angle. It is also shown that this invariance can be extended to other ranges by appropriately modifying the array aperture. Finally, as an application of this independence, the flow-line velocity magnitude in these beam regions is estimated with better than 5% uncertainty through a simple bandwidth measurement.

Ultrasonics↗
PubMed1999

Subharmonic generation from ultrasonic contrast agents.

Ultrasonic contrast agents are used to enhance backscatter from blood and thus aid in delineating blood from surrounding tissue. However, behaviour of contrast agents in an acoustic field is nonlinear and leads to harmonic components in the backscattered signal. Various research groups have investigated second-harmonic emissions. In this work, the subharmonic emission from contrast agents is investigated with a view towards potential use in imaging. It is shown that the microbubbles with various surface properties, such as contrast agents, generate significant subharmonics under various insonating conditions. Theoretical results as well as experimental results using Optison indicate the generation of strong subharmonics with burst insonation at twice the resonant frequency of the microbubble. It is suggested that subharmonic imaging may provide a better modality than second-harmonic imaging to delineate blood from tissue and will be of significant importance for imaging deep vessels, such as in echocardiography and vascular diseases, due to the high signal-to-clutter ratio of the subharmonic imaging.

Albumins↗
PubMed1999

Subharmonic backscattering from ultrasound contrast agents.

The ultrasonic contrast of blood in tissue, which is needed for ultrasonic estimation of tissue perfusion, can be increased by injecting the blood with bubbles or hollow microspheres. It has been shown that an even greater improvement in contrast can be obtained by using the subharmonic generated by irradiated microspheres. By obtaining analytical solutions to the modified RPNNP equation for a coated microbubble, the relationship between the physical parameters of the encapsulated bubble and the threshold pressure is established. The observed increase in the resonance frequency of a coated microsphere is explained by introducing the concept of "acoustic radius" of the encapsulated bubble. It is predicted that subharmonic generation in contrast agents requires a threshold insonifying pressure, and should be a minimum when microspheres are insonated at twice their resonance frequency. Experiments confirm the existence of this optimum incident frequency and of a reasonably low threshold pressure for the generation of the subharmonic. The existence of the low threshold pressures for subharmonic generation in contrast agents may prove to be very valuable in ultrasonic contrast imaging.

Contrast Media↗
PubMed1998

Onset delay of acoustic second harmonic backscatter from bubbles or microspheres.

A well-known method of enhancing blood detectability in ultrasound imaging of tissues detects the second harmonic of the incident radiation, which is generated by ultrasound contrast agents in the form of bubbles or microspheres that may have been injected into the blood. We report here a delay in the onset of the backscattered second harmonic with respect to the backscattered first harmonic for these agents. This effect, which should limit the axial resolution attainable with harmonic imaging, is investigated by simulation as well as experiment, and its dependence on the incident ultrasonic amplitude and microsphere parameters is established.

Albumins↗
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↗
PubMed1997

Second harmonic characteristics of the ultrasound contrast agents albunex and FSO69.

Techniques necessary for measurement of the second harmonic of the insonifying frequency backscattered from ultrasonic contrast agents are described, and used to determine this characteristic for the agents Albunex and FSO69. The results confirm theoretical predictions that scattered second harmonic pressure is proportional to the square of the incident pressure. Because contrast agents of the type investigated improve discrimination of blood echoes against tissue echoes by means of the second harmonic of the insonifying frequency, these results allow a comparison of the relative merits of Albunex and FSO69 for harmonic imaging.

Albumins↗
PubMed1997

Estimation of axial blood velocity using the Doppler equation corrected for broadening.

To estimate axial velocity for vessels that are so narrow that their width is comparable to the minimum length of the range cell, it has been customary to substitute the maximum Doppler frequency fmax into the classical Doppler equation [eqn (1)]. It was shown here that this ignores transit time broadening, which can lead to significant errors at large beam-to-flow angles. We use a relation [eqn (2)], which does take this broadening into account, and give in vivo experimental proof of this allows accurate estimation of the axial velocity even when the range cell extends across the whole vessel lumen. It may be concluded, therefore, that, by using the above procedure for taking transit time spectral broadening into account, more accurate velocity estimates can be obtained for laminar flow than is possible with the current method using the classical Doppler equation uncorrected for broadening.

Algorithms↗
PubMed1995

Improved blood velocity estimation using the maximum Doppler frequency.

In vessels whose diameter is smaller than the length of the range cell or measurement volume, the maximum blood velocity is often calculated from the maximum frequency of the Doppler spectrum, using the classical Doppler equation. It is shown that the accuracy of this procedure is significantly improved at large beam-to-flow angles, if a correction for transit time broadening is made. This finding is based on the demonstration that the maximum frequency of the Doppler spectrum depends only on the maximum velocity passing through the measurement volume, but in a manner which is a function both of the Doppler shift frequency as well as the transit time broadening associated with the passage of scatterers through the beam width.

Algorithms↗
PubMed1995

Demonstration of three-dimensional vector flow estimation using bandwidth and two transducers on a flow phantom.

A method of performing three-dimensional (3-D) velocity vector estimation with two transducers is demonstrated on a flow phantom using the Doppler spectra's mean frequencies and bandwidths. The results are compared with 3-D vector estimates computed from Doppler mean frequencies obtained with a five-transducer system. It is shown that the two-transducer vector Doppler system which uses bandwidth can improve on the accuracy of a three-transducer vector Doppler system which relies only on Doppler mean frequencies.

Blood Flow Velocity↗
PubMed1994

On the spectral properties of Doppler thread phantoms.

It is shown that some of the threads used in Doppler phantoms have a repetitive structure which leads to peaks in the angular distribution of the backscattered power at beam axis-to-flow angles of theta = 90 degrees and approximately theta = 70 degrees. This nonuniform scattering does not significantly interfere with modelling the Doppler spectrum peak as a function of velocity and beam-to-thread angle, but makes it impossible to model the spectral width as a function of these parameters. A new plaited structure is described which has a periodicity too small to lead to subsidiary reflection peaks, and which has a more uniform backscattering profile than the other threads studied.

Blood Flow Velocity↗
PubMed1993

Second harmonic ultrasonic blood perfusion measurement.

In vitro and in vivo testing of a recently introduced method of evaluating blood perfusion is presented, where the Doppler shift of the second harmonic component of the backscattered echo is measured. Central to this measurement is the administration of a galactose-based contrast agent (Schering AG, Berlin, Germany, SHU-508 or derivative) which has been shown in vitro to exhibit extraordinary nonlinear backscattering properties. Two types of experiments are described: in vitro studies on excised sheep kidneys and in vivo studies on living rabbits. In the animal model, blood perfusion was manipulated by various mechanisms to obtain some indication of the quantitative ability of the measurement. Comparisons between measurements made at the fundamental component of the backscattered echo and at the second harmonic show that use of the second harmonic measurement results in a much improved ratio of blood echo intensity to tissue echo intensity (signal-to-clutter ratio), allowing detection of blood flowing in smaller vessels and opening up the potential for real-time determination of blood volume fluctuations in tissue.

Animals↗
PubMed1993

Comparison of conventional and transverse Doppler sonograms.

When measuring flow velocity using the conventional ultrasonic Doppler effect, beam axis-to-flow angles approaching 90 degrees are avoided as the Doppler spectrum frequency shift is known to go to zero at this angle. In this paper, the conventional Doppler technique is compared with the transverse Doppler method, in which the Doppler spectrum bandwidth is used to estimate flow, allowing flow to be probed at 90 degrees. The comparison is made using a moving thread flow phantom capable of executing various velocity profiles. This technique may allow the probing of vessels that are inaccessible to conventional oblique probing, thus complementing the conventional Doppler technique.

Blood Flow Velocity↗
PubMed1992

Doppler ultrasound technique for measuring capillary-speed flow velocities with strong stationary echoes.

Ultrasound Doppler is widely used for low measurement in both medicine and industry, having the advantages of being non-invasive and comparatively simple and therefore inexpensive. The technique has not however been used for capillary blood flow measurement, because of the relatively low velocities encountered and because of the presence of strong interfering signals from the encompassing tissue. An ultrasound Doppler system capable of measuring flow velocities of one millimetre per second in the presence of one thousand times stronger interfering signals is described, as well as test results using both thread and flow phantoms.

Blood Flow Velocity↗
PubMed1992

Simulated capillary blood flow measurement using a nonlinear ultrasonic contrast agent.

This paper proposes a system for measuring slow, small volume blood flow, such as that found in the capillary beds. The method relies on the injection of a strongly nonlinear echocardiographic contrast agent, whose echoes are then analyzed by a modified Doppler process. The contrast agent is necessary to increase the signal-to-clutter ratio from the small blood volume, and to distinguish the blood movement from other moving structures, such as vessel walls or surrounding tissue. The nonlinear properties of this saccharide-based contrast agent are described, as evidenced by experimental evaluation. The contrast agent and signal processing modality are then tested in a blood flow simulator, at velocities as low as 1.5 mm/s, with good results. In addition, an analysis for applying this technique in vivo is presented, accounting for the dynamic and acoustic parameters of the physiological environment. Finally, some specific applications for this method are discussed, including its limitations.

Acoustics↗
PubMed1991

Invariance of the Doppler bandwidth with flow displacement in the illuminating field.

It is known that if single frequency continuously transmitted ultrasound or electromagnetic energy is reflected from "straight line flow," defined here as one or more scatters moving with constant velocity along an infinite straight line, the Doppler effect will shift the echo spectrum center frequency from the transmitted value, and broaden its bandwidth. It is proved that if such straight line flow is shifted laterally or in range anywhere in the field, i.e., without change of orientation, its Doppler bandwidth remains unchanged. (The "Doppler bandwidth" is here defined as the frequency difference between the extrema of the echo power spectrum.) The theorem holds true even though the time domain echo changes dramatically with motion of the flow path, and is believed to be valid for electromagnetic as well as ultrasound waves. Its implications with respect to flow measurement, as well as preliminary experimental and computational confirmation, will be discussed.

Blood Flow Velocity↗