Ultrasonic strategies to monitor drug delivery

Medical ultrasonography enables non-invasive, real-time and non-ionizing monitoring of a wide range of modifications in tissue during therapy. Many strategies are available, each sensitive to different aspects of disease evolution. B-mode imaging and the different modes of Doppler are clinically used to assess morphology and blood flow, respectively. Stabilized microbubble contrast agents with strong, nonlinear acoustic response are used to follow microvascular function in the clinic. Techniques for targeting contrast and nanoscale agents are under development to assess molecular expression and to enable agent accumulation within the interstitial space of tumors. The complex interaction between drug-laden agents and ultrasound is used by novel methods associating therapy and imaging. This review describes clinically-established therapy monitoring and developing techniques including dynamic contrast enhanced-ultrasound (DCE-US), elastography and photoacoustic imaging. Particular attention is given to contrast enhanced techniques to monitor cancer therapy. The variety of ultrasound-based strategies available makes ultrasonography a particularly flexible tool with which to examine and influence therapeutic response.