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 25-mum 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 mum and an average depth-of-field of 0.93 mm, whereas the fundamental beam had a corresponding beam width of 137 mum 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. (C) 2002 World Federation for Ultrasound in Medicine Biology.