Using medical implants to wirelessly report physiological data is a technique that is rapidly growing. Ultrasound is
well-suited for implants -- it requires little power and this form of radiated energy has no ill effects on the body. We
report here on techniques we have developed in our experience gained in implanting over a dozen Doppler ultrasound flow-measuring implants in dogs. The goal of our implantable device is to measure flow in an arterial graft. To accomplish this, we place a Doppler transducer in the wall of a graft and an implant unit under the skin that energizes the 20 MHz Doppler transducer system, either when started by external command or by internal timetable. The implant records the digitized Doppler real and imaginary channels and transmits the data to a nearby portable computer for storage and evaluation. After outlining the overall operation of the system, we will concentrate on three areas of implant design where special techniques are required: ensuring safety, including biocompatibility to prevent the body from reacting to its invasion; powering the device, including minimizing energy used so that a small battery can provide long-life; and transmitting the data obtained.
Stroke arrives without warning 80% of the time, leading to death and disability in a large number of the 700,000 it strikes in the USA each year. After discussing the special characteristics of screening instruments and the particular challenges of imaging the carotid artery, we report on our progress in developing an instrument to screen for the carotid stenoses that cause the majority of these strokes.
This paper describes a new kind of clinical instrument designed to allow non-specialists to quantitatively measure blood velocity. The instrument's design utilizes vector continuous-wave (CW) Doppler. Vector CW Doppler insonates a volume with simultaneous multiple-angle beams that define a measurement region; within that region, the velocity vector of the blood can be measured independently of the probe orientation. By eliminating the need for simultaneous imaging and the specially trained technician required for the complicated instrument needed for such imaging, easy and inexpensive blood velocity measurements becomes possible. A prototype for a CW vector Doppler instrument has been used to measure blood velocity in several clinically important arteries: the radial and ulnar in the arm, the femoral in the leg, and the carotid in the neck. We report here on its first clinical use -- monitoring the flow in dialysis access grafts to prevent graft thrombosis. These early clinical results show accuracy and rapid learning of proper instrument use. The design approach presented shows much promise in creating instruments that will provide simple and low-cost-of-use procedures for measurement of blood velocity.
Diffracting grating transducers are interdigitated grating- like structures of thin elements spaced at a few wavelengths apart. By changing the phase or the frequency of the driving signal, two beams whose angle differs by a known quantity are produced. Doppler measurements in the blood stream at the two beam angles enable the estimation of angle- independent blood velocity. The performance of diffracting grating transducer in term of efficiency and bandwidth has been optimized by finite element analysis. Two different arrays operating at 5 MHz and 10 MHz were fabricated and modeled. The simulated and measured insertion losses for each transducer were compared. To broaden the bandwidth, matching layers were included in the design. The insertion losses for different driving modes were also computed. The results may shed light as to how the performance of the diffracting grating transducer may be improved to make better angle independent Doppler velocity measurements.
A new method of blood-flow measurement requires construction of transducers that act like diffraction gratings, producing beams at variable angles as a function of the signals driving them. A new transducer structure that eases fabrication and operation of such transducers is presented and its operation experimentally verified. This transducer is called a higher-order diffracting-grating transducer because, like a higher-order optical diffraction grating, the periodicity of its elements is based on a multiple number of wavelengths. The theory and experimental results for this new transducer structure are shown to agree.
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.