MAGNETIC-RESONANCE-IMAGING IN THE EVALUATION OF THE PORTAL VENOUS SYSTEM

Magnetic resonance (MR) imaging is used in assessing the portal venous system through qualitative and quantitative methods. Magnetic resonance angiography can be performed using time-of-flight or phase-contrast techniques. Time-of-flight techniques (which use gradient echo images to display bright blood or spin echo images to display black blood) are relatively standardized and commercially available. These techniques are used to display liver morphology, portal vein patency, portal venous collaterals, and surgically created portosystemic shunts. Magnetic resonance is equivalent to angiography in the detection of varices, according to a preliminary study. Time-of-flight flow imaging using gradient echo techniques (in which a thrombus appears as absence of bright signal in the portal vein) and spin-echo techniques (where thrombus appears as a bright signal) can become combined to increase specificity for diagnosis of portal vein thrombosis. Phase-contrast techniques provide flow information based on phase shifts induced by flow through magnetic gradients. Phase-contrast angiography is less widely available than time-of-flight angiography. However, phase-contrast methods allow imaging of very slow flow that is not possible using time-of-flight methods. Quantitation of flow is possible, both with time-of-flight techniques using bolus tracking and with phase-contrast techniques using quantitative measurement of phase shifts. Calculations of flow velocity correlate well with Doppler ultrasound estimations, MR flow quantitation does not, at present, rival ultrasound in terms of cost or availability. However, MR is not limited by obesity or overlying bowel gas which can prevent adequate ultrasound evaluation. With further technical advances, clinical applications of MR imaging and MR angiography in hepatology will increase. Prospective, controlled studies will be necessary to provide data on efficacy and cost effectiveness.