Vascular pathology of the spleen, part I

Abstract

Quite often, the spleen is regarded as the ‘‘silent and forgotten’’ organ of the abdomen. Although primary splenic diseases are rare, the spleen is a frequent site of secondary manifestations in a wide range of hematologic, immunologic, oncologic, infectious, vascular, and systemic disorders. Despite this broad spectrum of splenic diseases, the radiologic literature on splenic pathology is rather sparse. The introduction of cross-sectional imaging modalities opened up new diagnostic horizons. Ultrasound (US) rapidly became a suitable method for screening abdominal pathology. Despite a low specificity, US provides a high sensitivity for the identification of focal splenic lesions. During the 1980s, due to its superior contrast resolution, computed tomography (CT) became the gold standard for splenic imaging. Currently, helical, volumetric CT scanning with bolus injection of contrast material is considered the state-of-the-art imaging modality for evaluation of the spleen. Magnetic resonance imaging (MRI) has shown great promise due to its superior tissue-characterization ability in selected cases. The spleen is surrounded by a capsule that sends out trabeculae that divide the parenchyma or splenic pulp into incomplete compartments. The white pulp contains arterioles surrounded by a sheath of densely packed, small lymphocytes that are subdivided into central, intermediate, and peripheral marginal zone that form a lattice for the red pulp, which consists of large, thinwalled sinusoids lined by fenestrated endothelium that are filled with blood and separated by thin plates of cords of lymphoid tissue, the cords of Billroth. The structure of the spleen is built around its blood supply. There are two models of circulation within the spleen, open and closed. In the closed circulation model, central arteries supply the white pulp, blood runs through a system of marginal zone sinuses that drain directly into the venous sinuses. In the open circulation model, central arteries supply the red pulp and form sheathed capillaries that drain into the splenic parenchyma, which consists of a stellate network of the splenic cords. Venous sinuses flow together to become trabecular veins that unite at the hilus to form the splenic vein. The closed circulatory system consists of a small (10%) fast flow compartment, Whereas the open system consists of a large (90%) slow flow compartment. The mixture of the two circulation models is most likely the reason for the nonhomogeneous, flamelike enhancement of the spleen on the early arterial phase of CT or MRI (Fig. 1), whereas the spleen will enhance homogeneously on the late arterial and venous phases. This pictorial essay refers to lesions involving the splenic vessels and to the consequent splenic parenchymal pathology of such lesions. The spleen also plays a major role in portal hypertension, and splenic vessels are involved in the development of collateral pathways.