Vasodilation—the widening of blood vessels—increases blood flow in a region. In tissue that is ischemic, vasodilation can be induced to enhance the effects of radiotherapy by increasing the delivery of oxygen and blood to the target.
Vasodilation can also aid drug treatments by increasing the amount of the drug delivered to a target.
Focused ultrasound can create a pressure change at a precise location, triggering the endothelium of targeted blood vessels to release nitric oxide, the chemical signal that causes smooth muscle relaxation and the dilation of blood vessels1. This is a reversible process, and blood vessels revert to their original size shortly after the end of the focused ultrasound treatment with no permanent damage to targeted tissue1,2. Thermal effects are minimal when pulsed focused ultrasound is used, however, local hyperthermia will also cause localized vasodilation.
Certain drugs have been shown to more easily diffuse across dilated vessels, increasing the bioavailability of these drugs in surrounding tissues. Furthermore, dilated blood vessels carry a larger volume of blood, potentially providing a further increase in the amount of drugs absorbed by tissue3. By using focused ultrasound to noninvasively induce local vasodilation, the delivery of drugs to targeted tissues could be enhanced without causing permanent damage to blood vessels.
Due to the enhanced efficacy of radiotherapy after vasodilation, this treatment is perfectly suited to be used as a neo-adjuvant treatment prior to radiation therapy for many different cancers. Furthermore, there are numerous clinical indications that could be more effectively treated with the enhanced drug delivery effects of vasodilation.
 A. Maruo, C. E. Hamner, A. J. Rodrigues, T. Higami, J. F. Greenleaf, and H. V. Schaff, “Nitric oxide and prostacyclin in ultrasonic vasodilatation of the canine internal mammary artery.,” Ann. Thorac. Surg., vol. 77, no. 1, pp. 126–132, Jan. 2004.
 F.-Y. Yang, W.-H. Chiu, S.-H. Liu, G.-L. Lin, and F.-M. Ho, “Functional changes in arteries induced by pulsed high-intensity focused ultrasound.,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control., vol. 56, no. 12, pp. 2643–2649, Dec. 2009.
 Wang S, Zderic V, Frenkel V. Extracorporeal, low-energy focused ultrasound for noninvasive and nondestructive targeted hyperthermia. Future Oncol. Lond. Engl. 2010;6:1497–511.
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