Introduction:
MR venography or magnetic resonance venography (MRV) is used in the structural evaluation of the vascular system. Veins are the blood vessels that carry deoxygenated blood from the tissues back to the heart. Exceptions are the pulmonary and umbilical veins, as both carry oxygenated blood from the tissues to the heart. MRV is a highly accurate imaging technique that uses a magnetic field and radio waves to detect the venous system.
What Are the Uses of MR Venography (MRV)?
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Evaluation of thrombosis (occurs when blood clots block arteries or veins).
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A tumor (cancer) of the cerebral venous sinus.
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Drowsiness associated with headache.
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How Do You Prepare for an MR Venography Scan?
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You may be asked to remove any metal jewelry or accessories, as magnets are used in this procedure. If you have any of the metal inside your body, such as pacemakers or cochlear implants, you cannot undergo this procedure.
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Inform your doctor about your medical conditions and your regular medicines.
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Tell your doctor if you had allergic reactions to contrast material from your previous scan.
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Tell your doctor if you are afraid of closed spaces (claustrophobic). He may give you medicine that makes you feel comfortable and reduces your anxiety.
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Inform your doctor if you are pregnant. Injection of the contrast material during the scan is not recommended in pregnant women in their first trimester. It is considered safe during the second and third trimesters at 3.0 tesla (T) or less.
How Is MR Venography (MRV) Performed?
Magnetic resonance venography is done using the same equipment as magnetic resonance imaging (MRI). During the procedure, you may be asked to lie on the table that will be moved into the scanner. Sometimes, they may keep a pillow or blankets to make you comfortable. You may be provided with earphones to block the loud noise heard from the scanner during the imaging process. Your technician will communicate with you from a separate room through an intercom. Once you are ready, the MRI machine produces a strong magnetic field around you, and radio waves are directed to the target area to produce the images of the required area. You may be asked not to move during the procedure as it may affect the image quality. Sometimes, you may be asked to hold your breath for a few seconds to get better images. The final images will appear on the computer screen. The contrast material commonly used in this procedure is gadolinium. It rarely causes allergic reactions. After the test, you can continue your regular activities. You may be asked to drink plenty of water to flush out the contrast material from the body used in this procedure. Nausea (urge to vomit), vomiting, headache, and skin rashes may occur as a side effect of the procedure due to the contrast material used.
What Are the Contraindications of MR Venography?
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Magnetic surgical clips or staplers.
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Intracranial aneurysm clips (surgical treatment for brain aneurysm [enlargement of an artery caused by weakness in the arterial wall] by placing the metal clip).
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Any electrically or magnetically activated implant such as a cardiac pacemaker, insulin pump bio stimulator, neurostimulator, hearing aids, or cochlear implant.
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Metallic foreign bodies such as bullets in the body.
What Are the MR Venography Techniques?
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Two-dimensional (2D) time-of-flight (TOF) MR venography.
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Phase-contrast (PC) MR venography.
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3D contrast-enhanced MR venography.
Time-of-Flight (TOF) MR Venography:
Two-dimensional time-of-flight imaging is a specialized technique that shows the contrast between stationary tissues and flowing blood by controlling the magnitude of magnetization. Stationary (not moving) tissues such as vessel walls and connective tissue become easily saturated (the state of being full) by the application of repeated radiofrequency pulses (RF). The flowing blood remains unsaturated as it has not been exposed to prior radiofrequency pulses. Thus, the flowing blood appears bright against a dark background. In time-of-flight venography of the thoracic veins, an additional pulse is applied over the heart to hold back the signal from inflowing arterial blood and simplify the evaluation of the veins. The time-of-flight MR venography does not rely on contrast material, and it can produce maximum intensity projection (MIP) images which is helpful for the radiologist to interpret the results easily. Time-of-flight is affected by the movement of the patients, the artifacts from adjacent air or metal, or the magnetic field irregularities. Time-of-flight MRV is very sensitive in the detection of venous occlusion but lacks accuracy in the detection of nonocclusive (incomplete blockage) thrombosis or stenosis (narrowing of the vessel).
Phase-contrast MR Venography:
In phase contrast MRV, stationary background tissues will appear as a gray color. Blood flowing towards the feet (caudally) appears white, and blood flowing towards the head (cranially) appears as black color. The intensity of black or white represents the magnitude of velocity in the respective directions. When the radiofrequency pulse is applied to the artery, the signal is saturated before it reaches the image slice. Venous blood will generate a high-intensity signal, and the stationary tissue and arterial blood are suppressed and appear dark. Gadolinium is used as a contrast material in MR venography. Gadolinium is more accurate in the diagnosis of thrombosis and stenosis. Phase-contrast venography is sensitive to a range of velocities. Different velocities are used for different imaging vessels.
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High-velocity encoding for articles as the arterial flow is fast (40-70 cm/sec).
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Low-velocity encoding for veins as the venous flow is slow (10-20 cm/sec).
3D Contrast-enhanced MR Venography:
Indirect Approach:
In an indirect approach, contrast material is injected through an antecubital vein. A large dose of contrast material is required during this procedure, as there will be a dilution of material before it reaches the venous system. Images should be taken in the early equilibrium phase, which prevents the redistribution of contrast material into extra fluid compartments.
In order to improve the quality of three-dimensional images, image subtraction needs to be performed. Image subtraction is the process in which the digital numerical value of one pixel or the whole image is subtracted from another image. Image subtraction is less effective in the chest and abdomen due to artifacts caused by respiratory movements. The major advantage of this approach is that there is no need for direct cannulation (placing a thin tube into the vein).
Direct Approach:
For the direct approach, a diluted paramagnetic contrast agent is continuously injected upstream into the affected part. It shows a clear vision of the deep and superficial venous system. The CNR (contrast-to-noise ratio) values are superior, although the contrast material is less. The dilution factor should be in the ratio of 1: 10: 20. Imaging should start following the injection of the first 50 to 60 ml diluted contrast material, and simultaneously two 60 ml syringes are attached. This approach requires more time to fill venous collaterals in the presence of a venous blockage.
What Are the Limitations of MR Venography?
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In case of blockage in the venous system, the time of contrast injection might not be enough for complete collateral (secondary) veins filling. Therefore, images should be taken immediately after the injection of the contrast material. If the image quality is still not adequate, you should increase the volume of the contrast material to be injected along with a longer period of injection.
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Venous anatomy may be unclear as to the arterial anatomy, especially if collateral veins need to be viewed in the presence of post-thrombotic changes. In order to overcome this, all the veins are displayed in 3D volume.
Conclusion:
Though MR venography has limited effects on the visualization of the arterial system, they are a valuable tool in the assessment of portal and systemic venous systems. The contrast-enhanced 3D MR venography is used in case of unusually slow flow or tortuous venous anatomy. The contrast agent used in MRV does not affect the function of the kidney. The major advantage of MRV is that it does not use ionizing radiation for scanning, which is a drawback of most imaging techniques.
