molecular imaging of thrombosis, such as in myocardial infarction, as well as monitoring success or failure of pharmacological thrombolysis. cancer: cancer cells also express a specific set of receptors, mainly receptors that encourage angiogenesis, or the growth of new blood vessels. if microbubbles are targeted with ligands that bind receptors like vegf or activated glycoprotein iib/iiia, they can non-invasively and specifically identify areas of cancers. gene delivery: vector dna can be conjugated to the microbubbles. microbubbles can be targeted with ligands that bind to receptors expressed by the cell type of interest. when the targeted microbubble accumulates at the cell surface with its dna payload, ultrasound can be used to burst the microbubble. the force associated with the bursting may temporarily permeablize surrounding tissues and allow the dna to more easily enter the cells. targeted theranostic microbubbles (directed at vcam-1) have been employed to deliver mir126 in a preclinical setting to stop the development of aaa in vivo. drug delivery: drugs can be incorporated into the microbubble's lipid shell. the microbubble's large size relative to other drug delivery vehicles like liposomes may allow a greater amount of drug to be delivered per vehicle. by targeted the drug-loaded microbubble with ligands that bind to a specific cell type, microbubble will not only deliver the drug specifically, but can also provide verification that the drug is delivered if the area is imaged using ultrasound. ultrasound-guided drug delivery has been successfully applied in the treatment of pancreatic cancer. advantages on top of the strengths mentioned in the medical sonography entry, contrast-enhanced ultrasound adds these additional advantages: the body is 73% water, and therefore, acoustically homogeneous. blood and surrounding tissues have similar echogenicities, so it is also difficult to clearly discern the degree of blood flow, perfusion, or the interface between the tissue and blood using traditional ultrasound. ultrasound imaging allows real-time evaluation of blood flow. destruction of microbubbles by ultrasound in the image plane allows absolute quantification of tissue perfusion. ultrasonic molecular imaging is safer than molecular imaging modalities such as radionuclide imaging because it does not involve radiation. alternative molecular imaging modalities, such as mri, pet, and spect are very costly. ultrasound, on the other hand, is very cost-efficient and widely available. since microbubbles can generate such strong signals, a lower intravenous dosage is needed, micrograms of microbubbles are needed compared to milligrams for other molecular imaging modalities such as mri contrast agents. targeting strategies for microbubbles are versatile and modular. targeting a new area only entails conjugating a new ligand. active targeting can be increased (enhanced microbubbles adhesion) by acoustic radiation force using a clinical ultrasound imaging system in 2d-mode and 3d-mode. disadvantages in addition to the weaknesses mentioned in the medical sonography entry, contrast-enhanced ultrasound has the following disadvantages: microbubbles don't last very long in circulation. they have low circulation residence times because they either get taken up by immune system cells or get taken up by the liver or spleen even when they are coated with peg. ultrasound produces more heat as the frequency increases, so the ultrasonic frequency must be carefully monitored. microbubbles burst at low ultrasound frequencies and at high mechanical indices (mi), which is the measure of the negative acoustic pressure of the ultrasound imaging system. increasing mi increases image quality, but there are tradeoffs with microbubble destruction. microbubble destruction could cause local microvasculature ruptures and hemolysis. targeting ligands can be immunogenic, since current targeting ligands used in preclinical experiments are derived from animal culture. low targeted microbubble adhesion efficiency, which means a small fraction of injected microbubbles bind to the area of interest. this is one of the main reasons that targeted contrast-enhanced ultrasound remains in the preclinical development stages.