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++++++++++++++++++ + +++ + RADIOLOGY U +++ ++++++++++++++++++ Magnetic Resonance Elastography A Novel Technique for Evaluation of Liver Disease By Benjamin Tubb, M.D., Ph.D. M AGNETIC RESONANCE (MR) elastography is an emerging imaging technology that allows sensitive, quantitative assessment of tissue stiffness. The most promising clinical application for MR elastography is the assessment of liver stiffness as a surrogate marker of liver fibrosis, the primary pathologic process in cirrhosis. Incidence of liver disease and cirrhosis is increasing, especially cirrhosis related to chronic viral hepatitis and nonalcoholic fatty liver disease (NAFLD, also known as nonalcoholic steatohepatitis, or NASH). When incorporated into a comprehensive liver MRI exam, elastography provides a sensitive test for early diffuse liver disease, potentially identifying liver fibrosis long before the morphologic changes of more advanced cirrhosis. Management of patients with liver disease involves careful monitoring for the development and progression of liver inflammation, fibrosis and cirrhosis. This is usually achieved through a combination of physical exam, laboratory testing and imaging, including abdominal ultrasound and CT or MRI exams. These traditional imaging exams 16 | San Antonio MD NEWS I MDNEWS.COM may appear normal during early stages of liver disease, versus more advanced stages of cirrhosis when morphological abnormalities become evident, such as nodular liver contour, segmental atrophy and secondary complications including portal venous hypertension with varices, splenomegaly and ascites. In patients with potential early liver disease, percutaneous biopsy is often used to assess liver tissue for inflammation and fibrosis. Liver biopsy is an invasive procedure with associated risks and inherently entails sampling error; given that only one site is biopsied, results sometimes underestimate or overestimate a patient's stage of generalized liver disease. MR elastography was developed in part by Richard Ehman, M.D., and colleagues at the Mayo Clinic as a noninvasive mechanism for evaluating tissue stiffness. Dr. Ehman's group and others have demonstrated that patients with different stages of diffuse liver disease show corresponding progressively increased stiffness of liver tissue, as detected and quantified by MR elastography, with excellent correlation between elastography measurements and degree of fibrosis identified in liver biopsies from the same patients. Importantly, researchers have shown MR elastography can sensitively detect early liver inflammation and fibrosis (stage 0–1 fibrosis) years before patients develop more severe fibrosis and gross anatomic abnormalities detectable by other imaging modalities. MR elastography requires some specialized equipment and software. The necessary equipment is relatively simple: a small disc-shaped transducer placed over the patient's upper abdomen at the start of the exam, connected through a flexible plastic tube to a subwoofer-like device, which creates low frequency sound waves. The sound waves cause small repetitive vibrations within the patient's abdomen without discomfort, creating pressure waves within liver tissue detectable through sensitive phase contrast MRI sequences. In patients with progressive liver disease, as their liver tissue becomes stiffer, the wavelength of the induced pressure waves becomes longer. The pressure waves can be visualized in color-coded images and their wavelengths can be measured, providing quantitative estimates of liver tissue stiffness, reported in units of elasticity called kilopascals (kPa).