Cell membranes are semi-permeable. Just like your private dinner party, only certain molecules can cross the cell membrane. This allows the cell some control over what enters and leaves. Water, small molecules, and molecules without a charge can pass freely.
Magnetic Resonance Angiography (MRA) and Magnetic Resonance Venography (MRV) Aetna considers magnetic resonance angiography (MRA) medically necessary according to the selection criteria outlined below. MRA is considered appropriate when it can replace a more invasive test (e.g., contrast angiography) and reduce risk for members. While MRA is a rapidly evolving technology, its clinical safety and effectiveness for all anatomical regions have not been established by the peer- reviewed medical literature.
Magnetic resonance venography (MRV) is an accurate, noninvasive technique used to detect deep vein thrombosis. This application of MRI provides images of blood flow in diseased and normal veins. In patients who are allergic to iodinated contrast medium, MRV is used in place of venography or CT venography. MRV is particularly useful for visualizing vascular abnormalities, thrombosis, and other pathology. MRV can be accomplished with a contrast-enhanced (CE) or nonâcontrast-enhanced method. Special imaging sequences allow the visualization of moving blood within the venous system.