Lymph nodes within the retroperitoneum are named and grouped according to their relation to the inferior vena cava and the abdominal aorta: paracaval, precaval, retrocaval, aortocaval, preaortic, and para-aortic. The lymph nodes of the abdominal viscera ultimately drain into the retroperitoneum and into the cisterna chyli. The lymph nodes associated with the abdominal viscera are situated along the distribution of the celiac, superior mesenteric, and inferior mesenteric arteries. Lymph nodes within the abdomen can be broadly divided into those related to the abdominal viscera and retroperitoneal nodes ( Figures 61-1 to 61-3 ). The thoracic duct is the final lymphatic pathway for the entire pelvis, retroperitoneum, and peritoneal cavity. The lymphatics follow the course of the parietal and visceral branches of the abdominal aorta and ultimately drain into the venous stream via the thoracic duct. The visceral lymphatics initially drain into nodes located close to the viscera, then into intermediate nodes situated along the peritoneal ligaments and mesenteries, and finally into the larger group of nodes situated along the major branches of the abdominal aorta. The parietal lymphatic system has a superficial component that drains the skin and subcutaneous tissue and a deep component draining the muscles and fasciae of the abdominal wall. The lymphatics of the abdomen have been divided into parietal and visceral systems composed of the lymphatic channels and nodes that drain the walls and contents of the abdominal cavity. There are approximately 230 nodes in the abdomen and pelvis and 400 to 500 lymph nodes in the entire adult body. It provides protection against organisms, particulate matter, and neoplastic cells by the processes of phagocytosis, cell-mediated and antibody-mediated immune complexes, production of B and T lymphocytes, and antibody-producing plasma cells. The lymphatic system is a complex network of lymph nodes connected by lymphatic capillaries and ducts that play an important role in the immune system. Additionally, various clinical manifestations of lymphadenopathy in the abdomen and pelvis will be individually discussed. This chapter will discuss the various imaging modalities and describe their role in imaging of lymph nodes of the abdomen and pelvis. Development of investigational lymphotropic nanoparticles for the diagnosis of lymph node metastases has added a new dimension to the scope of MRI of lymph nodes as well. Fluorodeoxyglucose (FDG)-labeled PET has further revolutionized oncologic imaging by recognizing benign and malignant nodes based on their FDG uptake. However, reliance on size criteria for characterization has limited the accuracy of these modalities in nodal staging of malignancies. The superior soft tissue resolution of CT and MRI has improved the detection and assessment of even small lymph nodes. However, with the advent of imaging techniques such as ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and PET/CT, the role of lymphangiography has now faded and has only a few valid indications. Lymphangiography was once considered the preferred imaging modality for evaluation of nodal disease. A wide range of imaging techniques is available for evaluation of the lymph nodes in the abdomen and pelvis. Therefore, accurate identification and characterization of abnormalities of the lymph nodes in the abdomen and pelvis are crucial to achieve optimal diagnosis and plan treatment strategies. This has special relevance in the abdomen because the lymph node system in this region is not readily accessible for clinical examination or tissue sampling. Imaging evaluation of lymph nodes forms an integral component of staging of various malignancies, including lymphomas, and is also helpful in the evaluation of certain infective and inflammatory processes within the abdomen.
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