Experimental characterization of fundamental and second harmonic beams for a high-frequency ultrasound transducer.

In the diagnostic frequency range, nonlinear imaging has been shown to improve image contrast and decrease artefacts. The extension of these techniques to high-frequency imaging (>15 MHz) was investigated. The second harmonic beam at 40 MHz of a high-frequency focused transducer (aperture 6 mm, focal distance 10 mm, f-number 1.67) was measured experimentally in water, in transmission and pulse-echo, and compared with the fundamental beams at 20 MHz and 40 MHz. Measurements were performed at peak negative pressures of 0.8 to 4.7 MPa. Transmission measurements were performed with a custom hydrophone with a 25microm spot size to limit beam averaging. Over the range of peak negative pressures, the transmitted harmonic (40 MHz) beam had an average lateral beam width (-3 dB) of 77 microm and an average depth-of-field of 0.93 mm, whereas the fundamental beam had a corresponding beam width of 137 microm and a depth-of-field of 1.59 mm. The harmonic beam showed a 3-dB decrease in side lobe levels. Preliminary second harmonic images of mouse tissue in vitro are presented and compared to fundamental imaging at 20 and 40 MHz.