Magnetic resonance imaging (MRI) is establishing a role as a primary diagnostic technique with evidence showing MR to have advantages over computer tomography (CT) as regards diagnostic sensitivity and specificity for many pathologies of solid organs, bile and pancreatic ducts, bowel, peritoneum, and retroperitoneum. MRI is particularly well-suited to the evaluation of liver pathology due to an ability to generate contrast by a variety of mechanisms. This allows specific evaluation of important diffuse processes such as abnormal fat, as may be seen in non-alcoholic steatohepatitis, or iron accumulation as seen in hemochromatosis.
The use of intravenous gadolinium-based contrast agents allows evaluation of the vascular supply to benign and malignant tumors, yielding important diagnostic information. Similarly, perfusion and interstitial distribution of gadolinium contrast agent in the liver parenchyma may be valuable for sensitive evaluation of the acute and chronic changes of hepatitis, including cirrhosis.
Dynamic perfusion analysis is obtained by the acquisition of a series of scans at multiple times. No other imaging technique can provide the comprehensive evaluation of liver disease possible on MRI.
Use of contrast-enhanced CT for multiphase examinations is associated with an ionizing radiation burden that is proportional to the number of scans obtained during the study. There are increasing concerns regarding the risks of radiation and the iodinated contrast agents associated with CT imaging of the abdomen. For example, the National Academy of Science has released BEIR VII, the seventh in a series of consensus reports on radiation risks, which includes a section on radiation from diagnostic CT. In brief, assuming the demographic distribution of the U.S. population, a single dose of 100 mSv is associated with an estimated lifetime attributable risk (LAR) for developing a solid cancer or leukemia of 1 in 100 while a single dose of 10 mSv is associated with a LAR of 1 in 1000 for developing a cancer.
Given that it is estimated that 60 million CT examinations are performed per year and that utilization is increasing, the potential tumor burden to the population should be an important consideration in determining practice patterns for liver evaluation. The incidence of contrast-induced nephropathy associated with iodinated contrast
agents used for CT scanning is difficult to ascertain.
Although the risk of renal insufficiency for the general population is estimated at below 2%, risk factors including pre-existing impaired renal function, diabetes mellitus, and high contrast agent volume may significantly elevate the likelihood. Patients with diabetes and mild to moderate renal insufficiency have been estimated to have a 9%-40% risk, and this estimated risk has been reported to increase to 50%-90% in various
studies.Conversely, the injectable gadolin ium contrast agents used for MRI have an excellent safety history, with no significant nephrotoxic effects.