Magnetic Resonance Venogram
Magnetic Resonance Venogram
Magnetic Resonance Venogram
A magnetic resonance venogram uses the equipment and technology of magnetic resonance imaging (MRI) to assess the body’s venous system. Unlike other radiologic techniques (x-rays, CT scans), magnetic resonance imaging does not involve radiation. Instead, MRI employs a combination of magnetic fields and radio waves to generate images. The magnets cause hydrogen atoms in the subject’s body to line up in a particular way; the radio waves then bounce off of these aligned hydrogen atoms. This signal is captured and recorded by a computer, which uses the information to create a two- or three-dimensional image of the tissue being studied. In order to be able to adequately image the veins during a magnetic resonance venogram, radioactive contrast is injected in the patient. This circulates throughout the venous system, and “lights up” the venous system. In this way, the outline of the veins can be visualized, and any blockages, narrowing (stenosis), leaks, or other abnormalities can be evaluated.
A magnetic resonance venogram can be performed to assess a variety of conditions involving the venous system throughout the body. MRV is particularly useful for the diagnosis of thrombosis (obstruction by blood clots) in the inferior vena cava (one of the very large major veins into which many of the veins in the body drain), renal (kidney) vein, and portal vein (a major vein in the liver). A thromboembolism (a blood clot that has traveled through the venous system to a point distant from its origination) in the pulmonary system can also be visualized. MRV can be used to demonstrate deep venous thrombosis anywhere in the body, such as the major leg veins or veins deep in the pelvis. Pelvic vein varicosities (enlarged, twisted, tortuous varicose veins) can also be assessed with MRV. MRV is an important method used to evaluate cerebral sinus venous thrombosis, a serious condition in which a clot blocks the drainage of blood from the brain. Lesions that occur in multiple sclerosis may also be visualized with MRV.
Prior to beginning the MRV scan, radioactive contrast is injected through an IV. The classic MRI unit consists of an examination table on which the patient lies, and a doughnut-shaped scanner into which the table slides. During the course of the MRV, which may take between thirty minutes and two hours, the patient must lie very still, and may at times be asked to hold his or her breath. Some people are bothered by the sounds that the MRI scanner makes, which include a variety of tapping, bumping, and fan sounds. Although no one is in the room with the patient, the patient can usually communicate with the MRI technician through a two-way sound system installed within the MRI unit.
Multiple sclerosis— A chronic degenerative neurological disease in which demyelination of the nerves causes progressive weakness and loss of motor function.
Thromboembolism— A blood clot that originates in one area of the body, but travels through the venous system to another area, where it obstructs blood flow. This is particularly problematic when the thromboembolus lodges in the lung.
Thrombus— A blood clot that is blocking a blood vessel.
Varicose vein— A vein that is abnormally enlarged, swollen, and/or dilated, and may be twisted or tortuous.
Because the strong magnetic field employed in MRV can interact with anything else that contains metal, it is crucial that the patient remove any jewelry, including from any piercings, prior to undergoing MRV. Other personal objects that should be removed include hearing aids, dentures, eyeglasses, hairpins. Pockets should be emptied of any metal-containing items, including coins, credit cards, Patients should inform the radiologist about any potentially metal-containing objects or medical devices that they have, such as tattoed eyeliner, a pacemaker, implanted defibrillator, aneurysm clips, cochlear implant, artificial limb, bone pin, medication patch, artificial heart valve, stent, infusion pump, or intrauterine device. People who have occupations in which they work frequently with metal should also inform the radiologist of this fact. In some cases, the MRV cannot be performed due to the presence of metal that cannot be removed and would be unsafe to expose to the magnetic fields of an MRI scanner. Sometimes, an x-ray will be ordered prior to an MRV in order to verify that there is not other metal in the body that would preude performing the test.
Some patients with a strong history of anxiety or claustrophobia find it difficult to be enclosed in the doughnut-shaped MRI machine. There are some open machines available that may cause less anxiety. Sometimes, a sedative can be used to help the patient relax during the MRV.
Women who are pregnant or who think they may be pregnant are advised against undergoing MRV. Women who are breastfeeding and who require MRV should feed their baby with formula for two days following the procedure, and should pump and discard their breast milk, since it will be contaminated with the radioactive dye.
Most MRI units have an upper limit of weight that they can hold. Patients over 300 pounds may not be able to undergo MRI, or may need to seek an open MRI unit for their study.
There is no aftercare necessary following an MRV. The patient can return to a normal diet and normal activities.
An MRV poses very little risk to the patient. Rarely, a patient may have an allergy to the radioactive contrast utilized.
Normal results of an MRV would reveal normal venous architecture, with fully patent arteries throughout the venous tree. No narrowing, blockages, reduced blood flow, or outpouchings of the vein walls are visualized in a normal MRV.
An MRV is abnormal if the veins appear to be dilated, enlarged, tortuous, or if there is reduced blood flow through any part of the venous tree. If a thrombus is completely obstructing a vein, the vein may not be visualized at all, since the obstruction will prevent any blood flow (and therefore any radioactive contract) from reaching that part of the venous system. When a thrombus is blocking a vein, the MRV may show the vein to be abnormally dilated, with a rim of increased radioactive intensity around the actual thrombus.
Grainger, R. G., et al. Grainger & Allison’s Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. Philadelphia: Saunders, 2001.
Mettler, F. A. Essentials of Radiology, 2nd ed. Philadelphia: Saunders, 2005.
Rosalyn Carson-DeWitt, MD
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