Functional studies.

For the first time, functional studies with ultrasound have been made possible by the introduction of microbubble contrast agents. That they are confined to the circulation and that the small volume boluses that are effective are particular advantages when compared to iodinated and gadolinium-based agents. Classical indicator dilution calculations can be applied to the transit curves that can be measured as the contrast washes into and out of a region of interest. In addition, it is sometimes possible to measure the transit of the bolus through the supply artery and the draining vein, e.g. of a kidney and thus calculate the true transit time. Similar timing measurements have proved particularly useful for measuring the portosystemic shunts that are a feature of liver metastases and cirrhosis. A unique feature of microbubble contrast agents is their fragility when insonated at higher (but still permissible) powers. This can be used to create a negative bolus and the reperfusion of a slice of tissue can then be followed. From the exponential refill curve, features that relate to true tissue perfusion can be extracted.