Measurement of spatial cross sections of ultrasound pressure fields by optical scanning means.

The measurement of spatial cross sections of ultrasound pressure fields is an essential element of exposimetry of ultrasonic medical equipment. An optical technique is presented that allows the two-dimensional (2-D) determination of ultrasound pressure using an optical multilayer hydrophone in which a laser beam with suitable wavelength is partially reflected from a dielectric optical multilayer system. By detecting the change in reflectivity of the multilayer coating induced by the incident ultrasound, the pressure time waveform can be determined. A 2-D data acquisition covering an area of at least 15 mm x 5 mm was realized by two complementary approaches. A serial detection scheme was set up by scanning the sensing point across the area of interest by a micromechanically engineered scanning mirror and acquiring pressure time waveforms sequentially and pointwise. This allows the measurement of repeating ultrasonic waveforms with a spatial resolution of better than 70 microm and a minimal detectable pressure of 50 kPa (bandwidth: 50 MHz) in a few seconds. In an alternative approach exploiting the parallel processing capabilities of a charge-coupled devices (CCD) camera chip, the probe was strobe-illuminated by a large-diameter collimated beam of a pulsed laser diode. The 2-D pressure distribution at a particular moment was derived from captured reflectivity distributions with a spatial resolution of 75 microm. By successive delaying of the laser pulse with respect to the ultrasound pulse, the complete 2-D pulse waveform was acquired with high spatial resolution. Measurement results on ultrasound fields from plane and focusing transducers are presented and compared to simulation results. Individual advantages and drawbacks of both approaches are discussed. A combined setup merging both detection schemes into a single device was developed and represents a milestone on the way toward constructing an optical ultrasound measuring camera.