Generation of regional strain inside object using acoustic radiation forces

Recently, there have been reports on assessing the mechanical properties of tissue by measuring the minute displacement induced by an acoustic radiation force generated by two ultrasounds at slightly different frequencies. However, when an object exists in soft tissue, there may only be a change in the object´s position, and its strain is hardly generated. Therefore, in such cases, the mechanical properties of the object cannot be measured. In such cases, we aim to generate the regional strain inside the object by using two phase-controlled acoustic radiation forces. We generate this strain 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 kept below the safety guideline of 1 W/cm². 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, the 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, a 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.