Magnetic resonance imaging (MRI) and diseases of the liver and biliary tract. Part 1. Basic principles, MRI in the assessment of diffuse and focal hepatic disease

Magnetic resonance imaging (MRI) relies on the physical properties of unpai red protons in tissues to generate images. Unpaired protons behave like tin y bar magnets and will align themselves in a magnetic field. Radiofrequency pulses will excite these aligned protons to higher energy states. As they return to their original state, they will release this energy as radio wave s. The frequency of the radio waves depends on the local magnetic field and by varying this over a subject, it is possible to build the images we are familiar with. In general, MRI has not been sufficiently sensitive or speci fic in the assessment of diffuse liver disease for clinical use. However, b ecause of the specific characteristics of fat and iron, it may be useful in the assessment of hepatic steatosis and iron overload. Magnetic resonance imaging is useful in the assessment of focal liver disease, particularly in conjunction with contrast agents. Haemangiomas have a characteristic brigh t appearance on T-2 weighted images because of the slow flowing blood in di lated sinusoids. Focal nodular hyperplasia (FNH) has a homogenous appearanc e, and enhances early in the arterial phase after gadolinium injection, whi le the central scar typically enhances late. Hepatic adenomas have a more h eterogenous appearance and also enhance in the arterial phase, but less bri skly than FNH. Hepatocellular carcinoma is similar to an adenoma, but typic ally occurs in a cirrhotic liver and has earlier washout of contrast. The a ppearance of metastases depends on the underlying primary malignancy. Overa ll, MRI appears more sensitive and specific than computed tomography with c ontrast for the detection and evaluation of malignant lesions. (C) 2000 Bla ckwell Science Asia Pty Ltd.