Concern with health effects of extremely low frequency magnetic fields has been raised by epidemiologic studies of childhood cancer in relation to proximity to electric power distribution lines. This case-control study was designed to assess the relation between residential exposure to magnetic fields and the development of childhood cancer. Eligible cases consisted of all 356 residents of the five-county 1970 Denver, Colorado Standard Metropolitan Statistical Area aged 0-14 years who were diagnosed with any form of cancer between 1976 and 1983. Controls were selected by random digit dialing to approximate the case distribution by age, sex, and telephone exchange area. Exposure was characterized through in-home electric and magnetic field measurements under low and high power use conditions and wire configuration codes, a surrogate measure of long-term magnetic field levels. Measured magnetic fields under low power use conditions had a modest association with cancer incidence; a cutoff score of 2.0 milligauss resulted in an odds ratio of 1.4 (95% confidence interval (CI) = 0.6-2.9) for total cancers and somewhat larger odds ratios (ORs) for leukemias (OR = 1.9), lymphomas (OR = 2.2), and soft tissue sarcomas (OR = 3.3). Neither magnetic fields (OR = 1.0) nor electric fields (OR = 0.9) under high power use conditions were related to total cancers. Wire codes associated with higher magnetic fields were more common among case than control homes. The odds ratio to contrast very high and high to very low, low, and buried wire codes was 1.5 (95% CI = 1.0-2.3) for total cases, with consistency across cancer subgroups except for brain cancer (OR = 2.0) and lymphomas (OR = 0.8). Contrasts of very high to buried wire code homes produced larger, less precise odds ratios of 2.3 for total cases, 2.9 for leukemias, and 3.3 for lymphomas. Adjusted estimates for measured fields and wire codes did not differ from crude results, indicating an absence of confounding. Limitations to the study are nonresponse (especially for field measurements), differential mobility of cases and controls, and a presumably nondifferential exposure misclassification from the use of imperfect surrogates for long-term magnetic field exposure history. In spite of these concerns, the results encourage further examination of the carcinogenic potential from this form of nonionizing radiation.

Performance results are presented for an autonomous navigator using refraction measurements of starlight passing through the upper atmosphere. To obtain a realistic performance indication, a full-state simulation was developed comprising a truth model with an 8 x 8 gravity field, lunar and solar perturbations, atmospheric drag, real star distributions, and the effect of atmospheric density variations on the measurements. Some data on actual observations of refraction by the satellite HEAO-2 are presented to show the variability in density within -+ 20" latitude. The navigation portion of the simulation is capable of handling gravity fields up to degree 4 and order 4, and utilizes the extended Kalman filter to estimate satellite position and velocity.

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In 1970, we embarged on a project to use ultrasonic energy for the recanalization of blocked arteries. This study was prompted by a suggestion by a patient with severe coronary artery disease, who is a member of the Department of Electrical Engineering. At that time we were aware that efforts had been made to use ultrasonic energy for therapeutic purposes, and have since become awrare of others. For example, Dotter (1) and others had attempted recanalization of femoral arteries using simple mechanical methods of dilatation with catheters of increasing size; such procedures had achieved limited success in the alleviation of gangrene of the foot.Our initial efforts were directed toward designed and constructing an apparatus t o transmit ultrasonic energy t o the tip of a flexible line which could be passed through a catheter. We rejected the concept of placing the transducer within the body -i.e., at the tip of the linebecause of the difficulties of ensuring adequate insulation at potentially lethal voltages. Accordingly, our efforts were concentrated on the design of an external transduction system. Because there was a considerable reservoir of information on ultrasonic transducers, our main problem was therefore the development of an efficient transmission system with optimal impedance matching.* The requirements to be met were: selection of the best materials, minimizing heat production, and preventing mechanical failure of the line or its connections. High strength aircraft aluminum (7075-T6) was selected for the quarter-wave matching stubs and concentrating cone (Fig. 1, 1 and 2). The transducers, of lead-zirconium titanate , are located at a stress maximum between the quarter-wave matching stubs and are pre-stressed by an aluminum collar to -30 tons/sq. in. ( 4 x l o 9 dynes/sq. cm).Electrical contact between the silvered faces of the t,wo transducers is made by means of a brass disc (solid in Figure 1, l ) , and the two quarter-wave matching stubs are symmetrical about this. The concentrating cone (Fig. 1, 2) is catenoidal in design and one-half wavelength in length at 25 kHz. Thus each quarter-wave matching stub is about 2 ins. long and the cone 4 ins. Design equations for matching stubs and cone are well-known (2), and are of the following general form :where p = density of material used R = radius at any given point z = longitudinal displacement variable x = independent displacement variable E = Young's modulus Boundary conditions are zero stress at one termination of the quarter stubs and at both ends of the cone.The above equation governs dispersive propagation in the stubs and cone. For non-dispersive propagation in the transmission line (Fig. 1, 3), equation (1) yields X' L = p w h e r e A= wavelength and % = frequency.Cobalt nickel alloy (Elgiloy) was selected for the transmission line because of its excellent P VOLUME 2, NUMBER 3 217

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