David Vilkomerson

Quasi-omnidirectional transducers for ultrasonic electronic-beacon guidance of invasive devices

A rapidly expanding area of medical treatment is using small invasive devices, e.g., balloon angioplasty catheters, to eliminate the need for conventional open surgery. The usual x-ray guidance requires patient and physician irradiation and the injection of contrast media, both undesirable. Ultrasound guidance, which would eliminate these hazards, has not been used because of the difficulty in determining with certainty the exact location of a particular point on the invasive device. By placing a transducer at such a point to act as a beacon, exact positioning by ultrasound imaging has been achieved. The required transducers response must be almost omnidirectional, so that it detects the imaging systems beam independently of angle; the size of the transducer must be small, so that the device can penetrate into the body easily; finally, the cost of the transducer must be low, so that it may be thrown away after one use. We show how the transducer is designed to achieve the required angular response and size, and outline how the required transducers can be fabricated at low cost.

A New Wire Phantom for Accurate Measurement of Acoustical Resolution

This article describes a phantom which is easier to use and gives more accurate results than a phantom made from several pairs of solid wires. This phantom employs two concepts: spatially modulated wires and diverging wires. Spatially modulated wires, such as helically twisted wires, reflect the sound into a broad range of angles. This has two advantages. One, it is easier to align the transducer with the reflected beam. Two, the diffuse reflection is similar to the scattering from tissue, and the system gain need not be turned abnormally low. Diverging wires improve the accuracy and convenience of the measurement. Rather than many pairs of parallel wires, a single pair of wires which diverge from an apex is used. By scanning across this pair until the wires are just resolved, the resolution can be easily calculated.