“…However, the high attenuation in air which increases with frequency is a great challenge (Magori, 1994;Yano, Tone & Fukumoto, 1987). It is reported that 1 MHz signal can theoretically measure distance up to 20 cm (Yano, Tone & Fukumoto, 1987), while 2 MHz signal can measure up to few centimeters (Noble et al, 1995). Recently, very high frequency ultrasonic transducers are reported for various high resolution biomedical applications.…”