The involvement of the liver by certain disease processes is non-uniform and may be related to hepatic anatomy and the dynamics of portal blood flow. These differences may result from inhomogeneous delivery of hepatic toxins or trophic substances, tumor secretion products or tumor metastases to different portions of the liver. In addition to effects on the natural history of hepatic disease, these features may affect the performance (planning/methods) and interpretation of diagnostic studies, such as hepatic venous sampling for measurement of tumor secretion products. This study will combine non-invasive quantitative in vivo measurements of blood flow with in vitro modeling of the vascular system. The purpose of the study is to define the normal hemodynamics of hepatic blood flow, particularly portal flow, using Magnetic Resonance Angiography (MRA) and to draw inferences regarding the role of hemodynamics in the natural history of hepatic disease.