Gas-coupled acoustic microscopy in the pulse-echo mode

We demonstrate the technical feasibility of a new scanning acoustic microscope (SAM) system. Unlike the traditional SAMs, which use liquids as couplants, the new system uses gases as couplants. To demonstrate that the new system can operate in the pulse-echo mode, we generated "dark-field" C-scans of the interior of solid objects at MHz frequencies. Previously, gas-coupled ultrasonic imaging was thought to be feasible only in the transmission and surface-reflection modes. In our experiments, coins encapsulated in polymethyl methacrylate (PMMA) were used as subjects. At present, 0.25 mm sub-surface lateral resolutions are attainable at 3 MHz in PMMA and even better performance should be possible at higher frequencies. To facilitate operation in the pulse-echo mode, we had to pressurize the coupling gases, typically nitrogen or argon. At present, pressures in excess of 30 atmospheres are required to reduce the absorption of ultrasound in the coupling gas, substantially increase the conversion efficiency and sensitivity of the transducers, and abate the dynamic-range and recovery-time requirements of the front-end receiver electronics. The image-formation mechanisms of gas-coupled acoustic imaging systems are impacted by the fact that sound propagates more slowly in gases than in liquids (330 m/s in air at standard pressure and temperature vs. 1480 m/s in water) and finite-amplitude saturation effects are observed at much lower sound-pressure levels in gases than in liquids