  Ultrasonic cavitation is being used in medical applications as a way to
influence matter, such as tissue or drug vehicles, on a micro-scale.
Oscillating or collapsing cavitation bubbles provide transient mechanical force
fields, which can, e.g., fractionate soft tissue or even disintegrate solid
objects such as calculi. Our recent study demonstrates that an ultrasonically
actuated medical needle can create cavitation phenomena inside water. However,
the presence and behavior of cavitation and related bioeffects in diagnostic
and therapeutic applications with ultrasonically actuated needles are not
known. Using simulations, we demonstrate numerically and experimentally the
cavitation phenomena near ultrasonically actuated needles. We define the
cavitation onset within a liver tissue model with different total acoustic
power levels. We directly visualize and quantitatively characterize cavitation
events generated by the ultrasonic needle in thin fresh bovine liver sections
enabled by high speed imaging. On a qualitative basis, the numerical and
experimental results show a close resemblance in threshold and spatial
distribution of cavitation. These findings are crucial for developing new
methods and technologies employing ultrasonically actuated fine-needles such as
ultrasound-enhanced fine-needle biopsy, drug delivery and histotripsy.
