An additional risk of the administration of gadolinium is nephrogenic systemic fibrosis, a severe cutaneous sclerosing disease. Most instances occur in patients with preexisting renal failure, for which reason it has become common to obtain BUN and creatinine measurements before administering gadolinium. The problem had not been appreciated initially in part because of its rarity (the frequency has not been well established) and because of a delay in the appearance of sclerosis in the kidney and skin, of several days to months.
Many types of MRI image artifacts are known, most having to do with technical aspects of the electronic characteristics of the magnetic field or of the mechanics involved in the imaging procedure (for details, see Morelli et al). Among the most common and problematic are CSF flow artifacts in the thoracic spinal cord, giving the impression of an intradural mass; distortions of the appearance of structures at the base of the brain from ferromagnetic dental appliances; and lines across the entire image induced by vascular pulsations and patient movement.
The increasing use of MRI and the sensitivity of current machines have had the unintended effect of revealing a large number of unimportant findings that create undue worry and often trigger neurologic consultation. Moreover, many lesions are not referable to the clinical problem at hand. A surprising number of incidental brain lesions are exposed by indiscriminate use of imaging. For example, a large survey of asymptomatic adults who were being followed in the “Rotterdam Study” is in accord with several prior studies in which cerebral aneurysms were found in approximately 2 percent, meningiomas in 1 percent, and a smaller but not insignificant number of vestibular schwannomas and pituitary tumors; the meningiomas, but not the aneurysms, increased in frequency with age. One percent had the Chiari type I malformation, and a similar number had arachnoid cysts. In addition, 7 percent of adults older than age 45 years had occult strokes, mostly lacunar. Because this survey was performed without gadolinium infusion, it might be expected that even more small asymptomatic lesions could be revealed (Vernooij et al).
These are noninvasive techniques for visualizing the intracranial and cervical arteries. They can reliably detect intracranial vascular lesions and extracranial arterial stenosis and are supplanting conventional angiography. They approach the radiographic resolution of catheter-based angiography, but do not engender the risk of selective arterial catheterization (Fig. 2-4). Visualization of the cerebral veins is also possible by CT (Fig. 2-4D) and MRI.
CT angiography requires contrast administration. In comparison, MR angiography can be performed without contrast, using the “time-of-flight” technique. This data can be reconstructed into an image that reflects flow-related enhancement. The signal obtained from time-of-flight MRA represents flow through the lumen of a vessel, rather than the configuration as obtained by contrast opacification. The use of these and other methods for the investigation of carotid artery disease is discussed further below and in Chap. 35, on cerebral vascular disease.
This technique is a valuable method for the diagnosis and treatment of aneurysms, vascular malformations, narrowed or occluded arteries and veins, arterial dissections, and angiitis. To a large extent, CT and MRI angiographic techniques have supplanted the diagnostic role of catheter angiography, but the latter remains necessary for a variety of conditions, particularly small vascular malformations. It is also possible to introduce thrombolytic substances and mechanical devices through catheters for the treatment of cerebrovascular disease.