Abstract

Contrast-enhanced ultrasound (CEU) is the application of specific contrast agents during ultrasonography. Ultrasound contrast agents are gas-filled microbubbles that are administered intravenously to the systemic circulation. Microbubbles have a high degree of echogenicity which can produce a unique sonogram with high difference in echogenicity between the microbubbles and the tissues in surroundings. CEU is widely used in clinical practice to image perfusion  in the microvasculature in different parenchymatous organs. Targeting ligands that bind to specific receptors, characteristic of intravascular diseases, can be conjugated to microbubbles. Ligands enable the microbubble complex to bind with specific receptors and accumulate selectively in areas of interest. This form of molecular imaging, known as targeted contrast-enhanced ultrasound, will only generate a strong ultrasound signal if  targeted microbubbles bind specific receptors in the area of interest. Targeted contrast-enhanced ultrasound can potentially have applications in clinical practice imaging cells of intravascular inflammation and cancer cells. Microbubbles can be vectors for gene delivery and targeted transporters to deliver drugs to specific cells. Low  targeted microbubble adhesion efficiency is one of the main reasons that targeted contrast-enhanced ultrasound has remained in the preclinical development stage. The targeted contrast-enhanced ultrasound technique is currently under preclinical research and development but has high potential in molecular imaging.