The signal backscattered by blood cells crossing a sample volume produces a Doppler power spectrum determined by the scatterers¿ velocity distribution. Because of intrinsic spectral broadening, the peak Doppler frequency observed does not correspond to the peak velocity in the flow. Several methods have been proposed for estimating the maximum velocity component--an important clinical parameter--but these methods are approximate, based on heuristic thresholds that can be inaccurate and strongly affected by noise. Reported here is a method of modeling the Doppler power spectrum of a flow, and from that model, determining what Doppler frequency on the descending slope of the power spectrum corresponds to the peak velocity in the insonated flow. It is shown that, for a fully insonated flow with a parabolic velocity distribution, the peak velocity corresponds to the Doppler frequency at the half-power point on that slope. The method is demonstrated to be robust with regard to the effects of noise and valid for a wide range of acquisition parameters. Experimental maximum velocity measurements on steady flows with rates between 100 and 300 mL/min (peak velocity range 6.6 cm/s to 19.9 cm/s) show a mean bias error that is smaller than 1%.

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Catheters and other interventional medical devices are presently guided by X-ray imaging, despite the advantages of ultrasound imaging over X-ray imaging in cost, safety, and availability. X-ray imaging is used because ultrasound reflects specularly from catheters and similar devices; their visibility is highly angle-dependent. With an omni-directional receiver mounted on a device, the receiver's location in the ultrasound image can be deduced from knowing which acoustic ray struck the receiver and the time from transmission of the imaging pulse to its reception by the receiver. This information is independent of specular reflection. The location of the device can then be indicated in the ultrasound image by an arrow pointing to the sensor, making possible ultrasound guidance of these devices. This paper describes the technical and practical considerations in the design and construction of the device-mounted receiver and associated electronics, and describes some clinical uses.

Ultrasound blood peak velocity estimates are routinely used for diagnostics, such as the grading of a stenosis. The peak velocity is typically assessed from the Doppler spectrum by locating the highest frequency detectable from noise. The selected frequency is then converted to velocity by the Doppler equation. This procedure contains several potential sources of error: the frequency selection is noise dependent and sensitive to the spectral broadening, which, in turn, is affected by the Doppler angle uncertainty. The result is, often, an inaccurate estimate. In this work we propose a new method that removes the aforementioned errors. The frequency is selected by exploiting a mathematical model of the Doppler spectrum that has recently been introduced. When a very large sample volume is used, which includes all the vessel section, the model is capable of predicting the exact threshold to be used without the need of broadening compensation. The angle ambiguity is solved by applying the threshold to the Doppler spectra measured from two different directions, according to the vector Doppler technique. The proposed approach has here been validated through Field II simulations, phantom experiments, and tests on volunteers by using defocused waves to insonify a large region from a linear array probe. A mean error lower than 1% and a mean coefficient of variability lower than 5% were measured in a variety of experimental conditions.

We have developed a blood flow measurement system using Doppler ultrasound flow sensors fabricated of thin and flexible piezoelectric-polymer films. These flow sensors can be wrapped around a blood vessel and accurately measure flow. The innovation that makes this flow sensor possible is the diffraction-grating transducer. A conventional transducer produces a sound beam perpendicular to its face; therefore, when placed on the wall of a blood vessel, the Doppler shift in the backscattered ultrasound from blood theoretically would be 0. The diffraction-grating transducer produces a beam at a known angle to its face; therefore, backscattered ultrasound from the vessel will contain a Doppler signal. Flow sensors were fabricated by spin coating a poly(vinylidene fluoride–trifluoroethylene) copolymer film onto a flexible substrate with patterned gold electrodes. Custom-designed battery-operated continuous wave Doppler electronics along with a laptop computer completed the system. A prototype flow sensor was evaluated experimentally by measuring blood flow in a flow phantom and the infrarenal aorta of an adult New Zealand White rabbit. The flow phantom experiment demonstrated that the error in average velocity and volume blood flow was less than 6% for 30 measurements taken over a 2.5-hour period. The peak blood velocity through the rabbit infrarenal aorta measured by the flow sensor was 118 cm/s, within 1.7% of the measurement obtained using a duplex ultrasound system. The flow sensor and electronics operated continuously during the course of the 5-hour experiment after the incision on the animal was closed.

In quantitative measurements of the attenuation of ultrasound in 18 samples of variously normal, benign, and malignant breast tissue, we found significantly different ranges of attenuation. As observed in previous in vivo studies, with the exception of medullary carcinoma, malignant tissue produces the most attentuation of ultrasound in the frequency range of 1.5 to 3.0 MHz we employed. The apparatus, measurement method, results, and the possible causes of the differences in attenuation are presented.

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