Multiparametric Ultrasound in Budd-Chiari Syndrome: From Venous Outflow Obstruction to Microcirculatory Dysfunction

Budd-Chiari syndrome (BCS) is a rare and potentially severe disorder of hepatic venous outflow with a heterogeneous anatomical and etiological spectrum. Obstruction may range from focal or segmental involvement of a single hepatic vein to multifocal hepatic venous disease with inferior vena cava (IVC) involvement. Although BCS is traditionally diagnosed by demonstrating macroscopic venous obstruction, its clinical severity also depends on the resulting changes in sinusoidal pressure, hepatic perfusion, collateral circulation, and the intrahepatic microcirculation. Paroxysmal nocturnal hemoglobinuria (PNH), although less common than myeloproliferative neoplasms as an underlying cause of BCS, represents a particularly severe prothrombotic model in which macrovascular thrombosis may coexist with systemic vascular and microcirculatory abnormalities. The purpose of the study is to provide a pathophysiology-oriented review of BCS, with emphasis on the relationship between hepatic venous outflow obstruction, altered hepatic hemodynamics, tissue perfusion, microcirculatory dysfunction, and the complementary role of conventional and advanced imaging techniques, using PNH as a model of severe thrombotic BCS. A narrative literature review was performed using PubMed/MEDLINE and related biomedical databases for publications from January 2000 to March 2026, supplemented by relevant earlier landmark studies. Literature addressing the etiology and pathophysiology of BCS, PNH-associated thrombosis, hepatic hemodynamics, conventional and advanced ultrasound techniques, computed tomography (CT), and magnetic resonance imaging (MRI) was reviewed. Particular attention was given to Doppler ultrasound, contrast-enhanced ultrasound (CEUS), parametric perfusion imaging, microvascular flow imaging, and elastography. BCS results from a broad spectrum of primary and secondary causes, including myeloproliferative neoplasms, PNH, inherited and acquired thrombophilia, malignancy, trauma, and iatrogenic conditions. The anatomical extent and duration of venous obstruction determine the degree of sinusoidal hypertension, hepatic congestion, redistribution of portal and arterial flow, and development of intrahepatic collateral pathways. Conventional Doppler ultrasound remains the foundation of diagnosis and provides assessment of venous patency, the extent of obstruction, waveform abnormalities, portal and arterial hemodynamics, and collateral circulation. CEUS and parametric perfusion imaging provide complementary information regarding regional and temporal hepatic perfusion, while microvascular flow imaging may demonstrate slow residual flow and small collateral pathways not readily visualized with conventional Doppler. Elastography provides additional information regarding congestion-related tissue stiffness and chronic remodeling. CT and MRI remain essential for comprehensive anatomical characterization, identification of secondary causes, assessment of parenchymal complications, and treatment planning. BCS should be regarded as a dynamic disorder of hepatic circulation rather than solely as an anatomical abnormality of the hepatic veins or IVC. A multiparametric imaging approach integrating morphology, macrovascular hemodynamics, tissue perfusion, microvascular flow, and tissue mechanics may provide a more comprehensive assessment of disease phenotype and treatment response than assessment of venous patency alone. PNH provides a particularly informative model for understanding the interaction between macrovascular thrombosis and microcirculatory dysfunction. However, advanced ultrasound techniques remain complementary and require further prospective validation, standardization, and correlation with hemodynamic measurements, hepatic functional reserve, and clinical outcomes.