  Mechanisms for non-invasive target drug delivery using microbubbles and
ultrasound have attracted growing interest. Microbubbles can be loaded with a
therapeutic payload and tracked via ultrasound imaging to selectively release
their payload at ultrasound-targeted locations. In this study, an ultrasonic
trapping method is proposed for simultaneously imaging and controlling the
location of microbubbles in flow by using acoustic radiation force. Targeted
drug delivery methods are expected to benefit from the use of the ultrasonic
trap, since trapping will increase the MB concentration at a desired location
in human body.
  The ultrasonic trap was generated by using an ultrasound research system UARP
II and a linear array transducer. The trap was designed asymmetrically to
produces a weaker radiation force at the inlet of the trap to further
facilitate microbubble entrance. A pulse sequence was generated that can switch
between a long duration trapping waveform and short duration imaging waveform.
High frame rate plane wave imaging was chosen for monitoring trapped
microbubbles at 1 kHz. The working principle of the ultrasonic trap was
explained and demonstrated in an ultrasound phantom by injecting SonoVue
microbubbles flowing at 80 mL/min flow rate in a 3.5 mm diameter vessel.
