Ultrasound-stimulated vibro-acoustic imaging in vivo

The time dependent radiation force produced by modulating an ultrasonic beam can be used to excite mechanical waves in an insonified object. The amplitude and phase of the resulting acoustic emission can be measured with a sensitive hydrophone, and are dependent on the reflectivity and stiffness of the insonified region. To test the hypothesis that this method would provide images of biologic materials, we scanned a pig artery. A spherically focused, 3.0 MHz, 45 mm diameter transducer with a central disc and a surrounding annulus was raster scanned over the subject. The disc and annulus were separately driven by rf signals separated in frequency by approximately 35 kHz. Interference of the two beams at the focal point, produced cyclic displacement of excised and in vivo arteries at the difference frequency. Recording the amplitude and phase of the emitted sound resulted in images showing arteries with very little speckle. The method appears to be sensitive to sub-nanometers of mechanical displacement.