The radiated fields of focussing air-coupled ultrasonic phased arrays.

This paper presents an investigation into the fields radiated into air by ultrasonic phased arrays under transient excitation. In particular, it includes a theoretical prediction of spatial variations in amplitude throughout the both the near-field and far-field of such arrays. The approach has been used to predict the result of phasing to produce a focus in air, which can be seen to be particularly effective in the near-field of the array. Interesting features are observed, which are then described in terms of the performance of both individual elements and the resulting array. It is shown how some elements of design can be used to improve performance in focussing. The predictions are compared to the results of experiments in air using electrostatic arrays, where good focussing could be achieved provided the appropriate design principles were followed. The approach has been developed specifically for use in air, but the results would also hold for modelling in certain medical arrays where a focussing requirement might be needed close to the array itself.

A conical air-coupled capacitance transducer for surface imaging.

This paper describes the construction and operation of an air-coupled capacitance transducer with a conical backplate. This was designed to produce a focal region over an extended distance along the transducer axis. Measurements were performed to examine both the frequency response of the transducer in pulse-echo mode, and the lateral resolution for imaging purposes. The radiated field was measured and compared to theory, and the extent of the focal region determined. Images of surface topography are presented, to illustrate the range of application of the transducer.

The characterization of capacitive micromachined ultrasonic transducers in air.

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.

Novel, wide bandwidth, micromachined ultrasonic transducers.

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. This process allows inherent control of parameters such as membrane size and thickness, cavity size and the intrinsic stress in the membrane to be achieved. Devices fabricated using this process exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz when driven from a standard ultrasonic voltage source. Experiments have been performed with devices containing silicon nitride membranes of variable thicknesses over a 2 microm thick air cavity and with device dimensions of up to 5 mm square. This is much larger than has been reported for a device with a single membrane. Calibration measurements using 1/8 inch microphones in air, and miniature PVDF hyrdophones in water, have been performed. The dependence on d.c. bias voltage is examined, involving static membrane deflection measurements and received peak voltages. Pulse-echo and pitch-catch mode operation have been achieved. Interferometric measurements of membrane displacement have been performed in air to illustrate the membrane deflection characteristics. Operation in liquids is also discussed.

The role of breakdown of the blood-retinal barrier in cell-injection models of proliferative vitreoretinopathy.

Rabbits were given an intravitreous injection of 5.0 x 10(5) rabbit retinal pigment epithelial (RPE) cells, human RPE cells, or human dermal fibroblasts in one eye and an injection of vehicle alone in the other eye. Some rabbits were treated with retinal cryopexy or intravenous sodium iodate on the day before injection. Vitreous fluorophotometry (VFP) and fundus examinations were performed before and at various times after cell injections. Retinal detachments were graded by premortem ophthalmoscopic examinations and postmortem gross pathologic examinations. Eyes injected with cells had higher VFP readings than eyes injected with vehicle at all time points. Eyes injected with fibroblasts or rabbit RPE had significantly higher mean VFP values before the onset of retinal detachment than those injected with human RPE cells. Within each group, high levels of fluorescein leakage in the first week correlated well with severity of subsequent traction retinal detachment and the fibroblast and rabbit RPE groups had more severe detachments than the human RPE group. Treatment with cryopexy or sodium iodate resulted in higher VFP readings, a higher frequency of retinal detachments, and detachments that occurred earlier and that were more severe. These data demonstrate that intravitreous cells cause blood-retinal barrier breakdown in rabbits and that the amount and duration of this breakdown are important variables in retinal detachment formation.