High resolution mapping of nonlinear MHz ultrasonic fields using a scanned scatterer

We show that a 2 MHz HIFU field can be measured in water and with high spatial resolution by placing a small scatterer in the field and sensing the scattered wave. We use a bistatic configuration placing a hydrophone at a safe distance. Glass and steel tips are fabricated using simple techniques and evaluated. Tapered tip geometry proves to be mechanically robust and not prone to internal resonance or scattering from other sites than the tip. Glass proves to be superior to polished carbide steel in its ability to delay the initiation of cavitation, and relatively easy to manufacture using common pipette drawing technology. In our tests, a highly nonlinear 2 MHz HIFU field is mapped in degassed water with a 0.1 mm glass tip. The fundamental, 2^nd, 3^rd, and 4^th harmonics are mapped with at least 25 dB dynamic range and are comparable with numerical results. Numerical simulations for nonlinear fields in water are obtained using a well established KZK equation based calculation.