Techniques necessary for measurement of the second harmonic of the insonifying frequency backscattered from ultrasonic contrast agents are described, and used to determine this characteristic for the agents Albunex and FSO69. The results confirm theoretical predictions that scattered second harmonic pressure is proportional to the square of the incident pressure. Because contrast agents of the type investigated improve discrimination of blood echoes against tissue echoes by means of the second harmonic of the insonifying frequency, these results allow a comparison of the relative merits of Albunex and FSO69 for harmonic imaging.
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