V.L. Newhouse

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.

Ultrasound Doppler Probing of Flows Transverse with Respect to Beam Axis

It is an accepted fact that transverse Doppler effects of the first order in v/c are nonexistent for all physical wave phenomena, including acoustics, i.e., the Doppler effect is zero for radiation normal to the direction of motion. However, this statement assumes that the incident field is a plane wave, which is not true in general for finite aperture sources. Consequently, the probing of flows transverse to the axis of finite diameter beams, particularly focused beams, is feasible. This geometry will be advantageous in many applications where the classical orientation of the sound beam, oblique to the flow, is not possible. With this motivation in mind, the theory and experimental feasibility of measuring Doppler spectra in transverse geometries is presently investigated.

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.

Theory of ultrasound Doppler-spectra velocimetry for arbitrary beam and flow configurations

In conventional ultrasound Doppler systems, the velocity component along the beam axis is derived from the observed frequency shift. Recently, it was verified that by using a pulsed-Doppler system with the beam transversely oriented with respect to the flow, the velocity component transverse to the beam can be derived from the edges of the spectrum. These results are generalized to take into account arbitrary angles of incidence, effects of velocity gradients, arbitrary apertures, and arbitrary source pulses. For uniform apertures and transverse flow, it has been previously shown that the Doppler output spectrum is symmetrical about zero frequency, with its width depending on the Doppler effect due to the transverse velocity and the geometry of the problem. For a beam direction oblique to the velocity, it is shown that the spectrum is now shifted, and is centered about the classical Doppler frequency. For velocity gradients and transverse flows the spectrum remains symmetrical, with the edges corresponding to the maximal velocity; however, the profile becomes peaked at the center. For oblique incidence, an asymmetrical spectrum is obtained and its edges are related to the maximal and minimal velocities within the sampling volume.<<ETX>>

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.

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.

Three-dimensional vector flow estimation using two transducers and spectral width

Current ultrasonic blood flow measurement systems estimate only that component of flow which is parallel to the incident ultrasound beam. This is done by relating the mean backscattered frequency shift to the axial velocity component through the classical Doppler equation. A number of ultrasonic techniques for estimating the two-dimensional (2D) blood velocity vector have been published, both Doppler and non-Doppler. Several three-dimensional (3D) blood velocity vector techniques have also been proposed, all of which require a multiplicity of transducers or lines of sight. Here a technique is described for estimating the total velocity vector, using only two transducers. This is achieved by measuring not only the frequency shifts but also the bandwidths of the backscattered spectra, making use of the fact that the bandwidth of a Doppler spectrum has been shown to be proportional to the velocity component normal to the sound beam. Partial experimental verification of the proposed vector flow estimation scheme is demonstrated by using a constant velocity thread phantom.<<ETX>>

Split-spectrum processing: analysis of polarity threshold algorithm for improvement of signal-to-noise ratio and detectability in ultrasonic signals

A polarity thresholding algorithm that has recently been developed for split-spectrum processing for ultrasonic coherent noise reduction is theoretically analyzed to evaluate its performance. The probability density function (PDF) of the output of the algorithm is derived and used to calculate the theoretical signal-to-noise ratio (SNR) enhancement and the receiver operating characteristics. The performance limits of the algorithm are also established. Some experimental results of SNR enhancement obtained with the polarity thresholding algorithm are presented.<<ETX>>

Split spectrum processing: optimizing the processing parameters using minimization☆

Abstract A major limitation to ultrasonic NDE is the noise produced by interfering scatterers such as grain boundaries, etc. effectively masking the flaw. The effect is more severe in some types of material, thereby making it necessary to use some forms of signal processing techniques to improve the signal-to-noise ratio. Split spectrum processing is one such technique introduced about half a decade ago. However, even though the technique has proved useful in signal-to-noise ratio enhancement, its application appears to have been limited perhaps because of the prevailing ambiguity in optimizing the processing parameters. This Paper provides experimental confirmation of a recently postulated procedure for optimizing the choice of processing parameters in split spectrum processing and demonstrates its application to signal-to-noise ratio enhancement in welds, centrifugally cast stainless steel and carbon-epoxy composites.

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.