Microscopic visualization of high-frequency ultrasound fields using a new method of Schlieren photography

We have proposed a new technique for visualization of an ultrasound field that enables acquisition of Schlieren images by subtraction of two shadowgraphs taken in the presence and absence of ultrasound exposure. This technique can be easily applied to other optical methods because there is no need to use a conventional Schlieren optics. This paper shows the possibility of applying the technique to microscopic observation of high-frequency ultrasound fields. A light source of short pulsed light of 0.11 ns and a CCD camera with a 15 bit dynamic range were installed in an inverted-type microscope. High-frequency ultrasound fields of a miniature 20-MHz transducer for intravascular ultrasound imaging and a 50-MHz wideband transducer were visualized using x4 and x10 objective lenses. Observation results showed that the high-frequency fields were successfully visualized at the maximum spatial resolution of 0.93 ! m. Transmission and reflection of pulsed ultrasound at a water Plexiglas interface were also observed, and the acquired movie image of ultrasound propagation demonstrated the importance of this technique for direct understanding of complex acoustic fields.