Multiple-frequency ultrasonic imaging by transmitting pulsed waves of two frequencies

The aim of this study was realization of a broadband measurement system that is capable of effectively carrying out a frequency compound method. In the present method, the secondary wave components of difference and sum frequencies are generated along with the higher harmonic components through the nonlinear interaction of two-frequency ultrasound. A multiple-frequency beam is generated together with the initially radiated frequency components.

For the structure of a transducer capable of simultaneously radiating two sound waves with different frequencies, a coaxial arrangement of a circular-disc piezoelectric transducer and a ring piezoelectric transducer was designed. The radiating frequencies chosen were 2 and 8 MHz. In addition to the 4-MHz second harmonic sound of the 2-MHz primary sound, sounds of the 6-MHz difference frequency and the 10-MHz sum frequency can be generated.

By measuring the acoustic pressure distribution, the formation of a multiple-frequency beam was confirmed. The signal-to-noise ratio in an agar-gel phantom image was increased by 5-6 dB with application of the frequency compound method. The validity of the proposed method was demonstrated through the generation of a human finger image. Further, it was found that the influence of the Doppler effect was small enough that almost all the secondary waves were attributable to the nonlinear propagation of sounds.

A multiple-frequency sound beam was realized by radiating a two-frequency sound. The effectiveness of the presented method was demonstrated through actual imaging.