ABSTRACT
In vivo, the patient is breathing, the circulation and all physiologic processes are going on, and the patient may experience pain, dyspnea, and fear, all potential sources of motion artifacts. Also, side effects of the examination, whether due to radiation exposure or to contrast agents, must be considered and may cause major limitations. In vivo imaging, however, also has signicant advantages because it can show image contrast based on functional information in addition to the pure morphology. Most importantly, moving blood is detected by ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) for angiography, ow quantication, and direction analysis, for perfusion or diffusion measurements, and to differentiate tissues by their specic contrast enhancement. Contrast enhancement means the degree of signal increase or decrease as a function of time after injection of a contrast agent. With most contrast agents, enhancement reects perfusion, neovascularity, and interstitial diffusion, whether these are iodine-containing benzol derivates in x-ray imaging or gadolinium-containing chelates in MRI. They all have an extracellular distribution volume; from the intravascular space they are distributed into the interstitial space, reaching an equilibrium concentration between the two anatomic compartments and not entering into the cells. Since they are eliminated by glonumerular ltration in the kidneys (and to a minor degree through the biliary system), after a few minutes intravascular concentration starts decreasing, and according to the new concentration gradient, contrast agents diffuse back to the intravascular space.
