The introduction of microbubble contrast agents has enabled functional studies that go beyond what was possible with Doppler. By giving an agent such as SonoVue as a bolus and tracking its passage through a region of interest, for example an organ or a lesion or through important blood vessels (such as the hepatic artery and vein), useful measurements of haemodynamic alterations can be made. The wash-in and wash -out curves generated can be analysed using the classical indicator-dilution approaches developed for iodinated CT agents. Where both the artery and vein can be accessed, curve-fitting may be applied that allows the transit time to be measured. The delay between injection or arrival in the hepatic artery and the first appearance of contrast in a hepatic vein can be measured; these hepatic vein transit times have proved useful in assessing porto-systemic shunting to provide non-invasive markers of liver metastases and of cirrhosis. Microbubbles have particular advantages when used for functional studies. The injected volumes are small, 0.6, 1.2 or 2.4 mL usually being sufficient for SonoVue (the smaller doses work well for large vessel studies, and larger doses for organ or lesion studies), and they can be administered in one or two seconds. The injection is followed by a saline flush, which results in a tight bolus compared to those produced by iodinated agents where 100 mL or more of contrast is required. The fact that they are non-toxic is an obvious advantage, especially in patients with renal failure, and this allows studies to be repeated as required for monitoring. The fact that ultrasound contrast-specific modes operate in real time means that the entire transit is available for study. A unique feature of microbubbles that can be exploited for functional studies is their fragility. By deliberately destroying the bubbles in a slice and then observing the refill, unique haemodynamic information can be extracted from the refill time-intensity curve. On the other hand, functional studies with microbubbles suffer from a number of disadvantages. Fundamental to most functional studies, particularly those that measure the amount rather than the timing of the enhancement, is that there is a predictable (ideally linear) relationship between the microbubble concentration and the signal strength they return. At least a relative correspondence has been demonstrated when using spectral Doppler with Levovist but non-linear modes with perfluoro microbubbles such as SonoVue have not been systematically investigated as yet. An additional problem is the microbubble's decay, both spontaneous (as the gas diffuses out and dissolves in blood) and resulting from ultrasound-induced destruction. The insonating power is not uniformly distributed through the volume under study, making for inhomogeneities in the received signal strength, and tissue movement may cause errors as the region being studied changes, for example, with breathing, though tracking algorithms can adjust for this, at least within the scan plane. Use of the raw RF data rather than the log compressed video data gives more useful results and the ability to store raw data is available on newer systems. Clinical functional studies include transit timing of the flow across the kidney and the liver: the latter was originally implemented using spectral Doppler to measure the time taken from injection until appearance in the hepatic veins and was shown to be capable of distinguishing between grades of diffuse liver disease, especially cirrhosis. It also promises to be useful in staging liver metastases, which alter the liver's blood, supply in a similar way to cirrhosis. Subsequently, a 2D version was developed that made use of the non-linear modes that were under development. For this much simpler method, a hepatic vein is targeted and kept in the field of view before the injection is given. The arrival in the hepatic artery is readily observed anywhere in the liver, and the delay from this moment until the appearance of contrast in the hepatic vein is noted. Though much simpler to perform, the discrimination between the different degrees of chronic hepatitis and cirrhosis is less complete. Functional studies of tumours treated with antiangiogenesis agents show great promise in demonstrating early response and predicting when treatment should be changed. The quantification of the amount of enhancement seems to be the most sensitive feature and is much more sensitive if RF data is used.
Any of the components of time intensity curves can be turned into functional images by allocating a colour to the feature and presenting it as an overlay on the B-mode image. They have the potential to depict spatial variations of the original numbers