Last advances in ultrasound molecular imaging

Molecular imaging with ultrasound contrast agents relies on the detection of targeted microbubbles or acoustically active nanoparticles. Microbubbles employed for these purposes are retained in the targeted diseased tissue, where they produce an acoustic signal as a response to the incident ultrasound field. Targeting is generally accomplished through one of the two following strategies: attachment of disease-specific ligands to the microbubble shell or chemical modification of the shell itself, in order to facilitate its adherence to cells encountered in disease areas. As microbubbles cannot leave the intravascular space, the targeted disease process must be characterized by molecular changes in the vascular compartment to be imaged. Examples of such disease processes are inflammation, angiogenesis and thrombus formation. Recently, the introduction of smaller nanoparticle-based contrast agents has opened new perspectives for targeted imaging of diseases expressing their molecular markers beyond the vessel endothelial layer, including in particular various type of cancer cells. Techniques for microbubble targeting are reviewed in this chapter, together with the currently available results of their employment in molecular imaging. Most promising directions for next future developments deriving from the use of nanoparticle agents are also illustrated.