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 transducer's response must be almost omnidirectional, so that it detects the imaging system's 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.