Recently, there are some reports on assessment of the mechanical properties of tissue by measuring its minute displacement induced by acoustic radiation force which is generated by two ultrasounds at slightly different frequencies. However, when an object exists in the soft tissue, these methods may generate only the change in position of the object, and the strain of the object is hardly generated. Therefore, in such cases, the mechanical properties of the object can not be measured. Even in such cases, we aim to generate the regional strain inside the object by using two phase-controlled acoustic radiation forces. We realize the generation of the strain inside the object by setting the phase difference between two cyclic acoustic radiation forces, which are radiated at two different depth points along the ultrasonic beam in the object, to be 180 degrees. The intensity of the applied continuous ultrasonic wave is suppressed to be lower than the safety guideline (1 W/cm2) recommended by the Japan Society of Ultrasonics in Medicine (JSUM). A minute strain with amplitude of less than 1 μm is cyclically generated by the radiation force with a low frequency Δf of several Hertz. For simultaneous measurement of the minute strain with ultrasound, cyclic radiation force is applied intermittently by maintaining its envelope waveform of the low-frequency component of Δf Hz. At the same time, an ultrasonic correlation-based method, namely, the ultrasonic phased tracking method, is employed to measure the minute strain. In basic experiments, the minute strain of several micrometers was generated in a gel phantom by applying radiation forces, and was successfully measured by the ultrasonic phased tracking method.
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