Imaging description

Susceptibility artifacts adjacent to vessels can result in pseudostenoses on MR angiography. Susceptibility artifacts occur when material within the body, usually metal, distorts the local magnetic field in or near the vessel of interest, resulting in a region of low signal intensity on the subsequent MR images. On maximum intensity projection (MIP) subtraction images, this low signal intensity creates the false appearance of a vascular stenosis or occlusion. These artifacts can vary in size depending on the amount of material and the pulse sequence used. For instance, very large artifacts are seen with knee and hip replacements, whereas smaller areas of signal loss are seen around surgical clips. Gradient echo sequences, which are used for gadolinium-enhanced MR angiography, will have more severe artifacts than spin echo-based sequences. Pseudostenoses on MRA due to surgical clips can be seen anywhere in the body, not uncommonly in the superficial femoral artery of the medial thigh due to prior saphenous vein harvesting or in the common femoral artery related to prior groin catheterization. Surgical clips adjacent to lower extremity bypass grafts can also mimic stenosis. Ductus clips can result in large artifacts that obscure the aortic arch (Figure 51.1). Knee pros-theses will result in an appearance simulating popliteal artery occlusion, whereas hip prostheses can simulate femoral artery occlusions (Figure 51.2). Susceptibility artifact from metal in vascular stents can obscure the lumen of the stented vessel, giving the false appearance of occlusion or stenosis. One particularly difficult artifact is a pseudostenosis of the subclavian artery due to susceptibility effects of highly concentrated gadolinium in the adjacent subclavian vein (Figure 51.3). These artifacts will be seen only on the side of injection and may resolve on subsequent venous phase images. Occasionally repeat imaging with injection on the contralateral side may be required to exclude the possibility of a true stenosis.