The basics ofultrasomc transducer array design in the frequency range useful for medical imaging ( 1-10 MHz) are discussed. Performance parameters of importance in transducer design are considered, including sensitivity, coupling constant, band width, frequency downshift, pulse duration, beam focusing properties, and electrical matching. Two and three dimensional effects must also be taken into account. The advantages of computer modeling in three dimensions with finite element analysis code are highlighted. The principles of multi-element array transducers useful for two dimensional real-time scanning are reviewed. Two dimensional arrays provide the opportunity offocusing the elevation beam dimension in the short axis, or make possible full three dimensional scanning volumes.

Changes in ac hopping conductivity with electron irradiation have been measured in n-type silicon, p-type silicon, and high purity silicon. • Irradiations were carried out at both 4.8 and 1.6 degrees kelvin, with measurements made at reference temperatures of 4.2 and 1.3 degrees kelvin, respectively. In the p-type crystals, the changes in ac hopping conductivity depended strongly on the concentration of chemical acceptors, indicating a / /. concentration dependence on impurities in the defect production rate. The production rates at 1.6'K were generally very similar to those for a 4.8'K ' irradiation. No significant thermal annealing stages below room temperature were observed in any of the crystals. A small amount of "reverse annealing, " i.e. an increase in ac hopping conductivity was'observed below 30'K. No radiation annealing effects due to a beam of either .500 Mev or .350 Mev electrons were observed, either at 4.5'K or at 1.45°K.

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Various means of correcting gamma ray images for motion blurring have been investigated. An analogue circuit with unique features was developed which follows the movement of the centroid of an object distribution and stabilises the image display against any such movement.Motion correction with the centroid circuit will be of help in liver, spleen and kidney imaging.