19 Contrast-Enhanced Ultrasound (CEUS) in Splenic Diseases

The spleen is comprised of two functionally and morphologically distinct compartments, the red pulp and the white pulp, separated by the marginal zone. While the red pulp functions as a blood filter for removing foreign material and destroying aged or defective erythrocytes, the white pulp consists of lymphatic cells representing the largest secondary lymphoid organ. The white pulp is composed of three subcompartments: the periarteriolar lymphoid sheath (PALS), the follicles, and the marginal zone. The spleen has—unlike the liver—a single vascular supply. The splenic artery divides into segmental and trabecular arteries. From here, small arterioles enter the red pulp where they become central arterioles, which are surrounded by lymphoid tissue.¹ On contrast computed tomography (CT), contrast magnetic resonance imaging (MRI), and contrast-enhanced ultrasound (CEUS), an open circulation can be differentiated from a closed one. Blood is pushed into the red pulp and the erythrocytes have to squeeze through fenestrated sinusoid epithelium. The red pulp harbors an entirely open circulatory system making the spleen the only human organ where blood passes through spaces not lined by endothelia or other barrier-forming cells.² On CEUS, zebra-like spleen in the early arterial phase of enhancement, representing the open circulation, may be seen during the first minute after contrast injection after which the spleen is homogenously enhanced (Fig. 19.1a). B-mode ultrasound (US) is the method of choice in evaluation of its size and anatomic variations. A homogeneous gray scale pattern is in most cases a reliable sign for ruling out focal lesions.

19.2 Examination Technique, Normal Findings, and Indications for CEUS

A contrast bolus of 1 mL Lumason will in most cases be sufficient to enhance the spleen for at least 10 minutes. A 1 to 6 MHz transducer is the preferred probe. The first 20 s after the bolus injection, with the arrival of the contrast agent the following 20 s should be digitally stored as a sweep. Additionally, short clips should be stored after 1, 2 and 3 minutes. If the local finding is still unclear store a short clip (sweep) even later. In case B-mode does not show any focal lesions, one should start with a long axis view with the feeding vessels entering the spleen. In case of a focal lesion, the scan plane should cover the lesion and its surrounding splenic tissue. Due to its anatomical and functional characteristics, the contrast agent first enhances the splenic artery and the intrasplenic arteries. The total uptake in the spleen is constant over at least 5 minutes, while there is a reduction of microbubble concentration within the liver and both kidneys during this time.³ The enhancement may last for 15 to 20 minutes probably due to phagocytosis by macrophages (Kupffer cells) or trapped bubbles in the splenic tissue (sinusoids) or both. It is assumed that the long enhancement time is a specific feature of splenic tissue. This is also true for accessory spleen or splenic tissue that may be found anywhere in the abdominal cavity after a splenic trauma or surgery. Therefore, this feature alone allows differentiation between accessory spleen and perisplenal lesions^{4 ,​ 5} like splenosis or diseased lymph nodes, neoplasms especially of the pancreatic tail, and the adrenal glands (Fig. 19.2).

Fig. 19.3. shows an accessory spleen after splenectomy, with a prolonged enhancement over 7min. Splenosis represents ectopic splenic peritoneal implants line after trauma or surgery with the same long lasting homogeneous contrast behavior as an accessory spleen.

Ectopic splenic tissue such as accessory spleen (found in 10% of individuals) or splenomas can be characterized as splenic tissue by their long enhancement time and thus be differentiated from malignant lesions (Fig. 19.3).

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