General contraindications to MR imaging also apply to 3D CE MR portography, which has several other limitations. First, there is a risk of allergic reactions to contrast media, although the incidence is low. Second, this technique is unable to demonstrate the flow direction of the portal venous system, unlike phase-contrast or time-of-flight MR angiography [27, 28]. Third, important portosys-temic collateral vessels may be overlooked when they are too anterior or posterior to the imaging slab or when the slab is positionedinappropriately. Fourth, if the interval between injection ofGd-based contrast agent and the start of imaging is too prolonged, the arteries and portal vein may not be differentiated. Fifth, artifacts from respiratory motion and peristaltic bowel movement degrade image quality, especially in debilitated patients who are unable to hold their breath for 12-24 seconds. Sixth, when subtraction techniques are used, respiratory misregistration also degrades image quality.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.