ANATOMY

Splenic Size and Shape

The typical spleen measures less than 13 cm in length, up to 7 to 8 cm in width, and 4 cm in thickness. Normal splenic volume is usually less than 300 ml. The spleen generally has a smooth convex posterior, superior, and lateral surface. Splenic clefts are relatively common and may be confused with lacerations in the setting of trauma (Fig. 15-1). Unlike most lacerations, congenital clefts are sharply and smoothly marginated and not associated with perisplenic blood. Splenic bulges are common medially and may cause mass effect on the stomach or be mistaken for a splenic or renal mass.

Figure 15-1 Enhanced axial computed tomographic scan of a patient with a splenic cleft (A) and a patient with a splenic laceration (B).

Pitfall: Splenic clefts are relatively common and may be confused with lacerations in the setting of trauma. Unlike most lacerations, congenital clefts are sharply and smoothly marginated.

Anatomic Relations and Peritoneal Reflections

The spleen usually resides within the left upper quadrant of the abdomen and is in close proximity to the diaphragm, left abdominal wall, greater curvature of the stomach, left kidney, tail of the pancreas, splenic flexure of the colon, and occasionally the lateral segment of the left hepatic lobe. The spleen forms within the dorsal mesogastrium and lies between the pancreas and stomach. It divides the dorsal mesogastrium into the splenorenal and gastrosplenic ligaments.

The splenorenal ligament connects the anterior left kidney to the splenic hilum and contains the splenic vessels and the tail of the pancreas. Pancreatic neoplasms and inflammation can spread from the pancreas directly to the spleen via this ligament. Between the spleen and greater curvature of the stomach, these same peritoneal folds form the gastrosplenic ligament, which contains the short gastric and left gastroepiploic vessels. This ligament facilitates spread of infection and tumor between the stomach and spleen. The spleen is also supported, in part, by the phrenicocolic ligament.

The spleen and its corresponding ligaments form the left lateral extent of the lesser sac. The spleen itself is invested in a fibroelastic capsule. This capsule is continuous with numerous trabeculae that extend into and compartmentalize the splenic parenchyma, and convey the trabecular vessels. As with the liver, a variably sized bare area is present that lacks a visceral peritoneal covering. The spleen may demonstrate considerable mobility on its mesentery. When the splenorenal ligament fails to fuse with the retroperitoneum, the spleen may be exceptionally mobile. This condition predisposes individuals to ectopic splenic location (i.e., “wandering” spleen) and torsion.

Vascular Anatomy of the Spleen

The blood supply to the spleen is via the splenic artery, which travels in the splenorenal ligament with the splenic vein. The splenic artery creates many smaller arteries in addition to the terminal splenic branches, including several pancreatic branches, short gastric arteries (which travel in the gastrosplenic ligament), the left gastroepiploic artery, and occasionally a posterior gastric artery.

The splenic parenchyma consists of white pulp and red pulp. The white splenic pulp consists of lymphatic tissue surrounding the arterioles that exit the trabeculae. White pulp causes most lymphatic neoplasms of the spleen. The red pulp consists of splenic sinuses and the splenic chords, which contain blood cells supported by a reticular network. Nonhematolymphoid tumors tend to arise from cells of the red pulp. The interface between red and white pulp is referred to as the marginal zone. Blood flowing through the spleen may bypass the splenic cords and flow directly to the venous sinuses (closed circulation) or enter the splenic cords (open circulation). As part of the reticuloendothelial system, the spleen contains abundant numbers of phagocytic cells.

The splenic vein runs posterior to the body and tail of the pancreas, and represents a useful sonographic landmark for the position of the pancreas. In addition to draining the spleen, the splenic vein receives venous blood from short gastric veins, the left gastroepiploic vein, pancreatic veins, and usually the inferior mesenteric vein before joining the superior mesenteric vein at the portal confluence. The lymphatic drainage of the spleen is to the hilar and, ultimately, celiac lymph nodes.

Accessory Splenic Tissue

During development, several splenic islands coalesce within the dorsal mesogastrium. When some of these islands fail to join the bulk of the coalescing splenic tissue, accessory spleens or splenules result. Splenules are typically round, smooth, smaller than 2 cm, and functional. Despite being completely separate from the spleen, splenules share similar imaging characteristics with the normal spleen (Fig. 15-2). Occasionally, the branch of the splenic artery that supplies the splenule can be seen on imaging studies (Fig. 15-3). More than 95% of splenules are found in the splenorenal ligament (near or in the pancreatic tail), gastrosplenic ligament, splenic hilum, gastrocolic ligament, or greater omentum. However, accessory spleens have been localized as far away as the pelvis, and they are occasionally mistaken for adrenal, pancreatic, and peritoneal tumors (Fig. 15-4). Accessory spleens rarely reside lateral to the main spleen.

Figure 15-2 Unenhanced axial computed tomographic (A), T2-weighted magnetic resonance (MR) (B), and gadolinium-enhanced, fat-suppressed, T1-weighted MR image (C) of splenule.

Figure 15-3 Enhanced axial computed tomographic scan through the spleen shows arterial supply to a splenule. This vessel could be traced back to the splenic hilum on the complete data set.

Figure 15-4 Enhanced axial computed tomographic scan through the upper abdomen shows a splenule in the region of the pancreatic tail. A splenule such as this can be mistaken for a pancreatic mass.

Pitfall: Splenules can mimic adrenal, pancreatic, and peritoneal tumors.

Pathologic processes that affect the spleen may also involve splenules (Fig. 15-5). Accessory spleens are important to identify before surgery in patients undergoing splenectomy for such disorders as idiopathic thrombocytopenic purpura. Failure to prospectively identify and remove accessory spleens in such a setting may result in return of hypersplenism. After splenectomy, residual accessory spleens often enlarge (Fig. 15-6). Accessory spleens may occasionally undergo torsion, infarction, or spontaneous rupture.

Figure 15-5 Unenhanced axial computed tomographic scans through the spleen of a patient who was diagnosed with lymphoma between the time of the first scan (A) and the second scan (B). Note increase in size of both the spleen and the splenule.

Figure 15-6 Axial computed tomographic scans through the upper abdomen of a patient before (A) and after (B) splenectomy. Note interval enlargement of splenule after splenectomy (B).

Splenosis is a condition that can result when splenic tissue gains entrance to the peritoneal cavity, usually caused by trauma, splenic rupture, or splenectomy (Fig. 15-7). Once within the peritoneal cavity, splenosis can migrate to any peritoneal recess similar to the peritoneal spread of tumor or infection. Splenosis does not receive blood supply from the splenic artery, resulting in different enhancement characteristics and poor function compared with the normal spleen and splenules. Splenosis may be confused with pathologic processes such as peritoneal metastatic disease or lymphadenopathy. Polysplenia, a congenital condition associated with a variety of other congenital abnormalities, can be distinguished from multiple accessory spleens by the absence of a normal dominant spleen in the former condition (Fig. 15-8).

Figure 15-7 Sagittal reformation from unenhanced computed tomographic scan (A) and lateral single-photon emission computed tomographic (SPECT) image from technetium-99m–labeled, heat-damaged, red blood cell study (B) in a patient with splenosis in the pelvis.

Figure 15-8 Axial, gadolinium-enhanced, T1-weighted, magnetic resonance imaging of a patient with polysplenia, abnormal situs, and azygous continuation of the inferior vena cava.

Single-photon emission computed tomographic scanning after administration of technetium 99m (Tc-99m) sulfur colloid may demonstrate uptake of the radiopharmaceutical in an accessory spleen or splenosis. Similarly, uptake of superparamagnetic iron oxide by accessory spleens or splenosis may be demonstrated with MRI. Therefore, these tests may be beneficial when attempting to distinguish between ectopic splenic tissue and other causes of soft-tissue mass.

NORMAL IMAGING APPEARANCE OF THE SPLEEN

With sonography, the spleen is usually of homogeneous echogenicity. It is more echogenic than normal renal cortex and liver parenchyma. On unenhanced CT images, the spleen is normally slightly less dense (approximately 5-10 HU less) than the normal liver, ranging from 40 to 60 HU in attenuation. On unenhanced MRI, the spleen is lower in signal intensity than the normal liver on T1-weighted images and normally brighter than the liver than on T2-weighted images. The unenhanced T1 and T2 relaxation times of the spleen on MRI closely resemble those of many types of liver metastases.

During the arterial phase of dynamic multiphase, enhanced imaging with CT or MRI, the normal spleen enhances with a characteristic heterogeneous pattern that sometimes resembles the stripes of a zebra or tiger (Fig. 15-9). This heterogeneous enhancement pattern presumably results from differences in the rate of contrast (blood) flow through the different splenic compartments, with a portion of the blood percolating slowly through the splenic cords (open circulation of the spleen), and a percentage of splenic blood flow passing more rapidly through the splenic sinuses and into the venous sinuses that join to form the trabecular veins (closed circulation). Normal splenic enhancement usually becomes homogeneous after the first minute. The spleen avidly accumulates technetium sulfur colloid and heat-damaged red blood cells. Splenic uptake of superparamagnetic iron oxide particles results in significant signal loss on T2- and T2*-weighted MRI sequences.

Figure 15-9 Axial arterial phase-enhanced computed tomographic scan through the upper abdomen of a patient with normal spleen shows normal heterogeneous early enhancement pattern.

Accessory spleens have the same imaging characteristics on ultrasound (US), CT, and MRI as the normal spleen (see Fig. 15-2). Enhancement of accessory spleens after intravenous contrast administration on CT and MR images may be similar in character and intensity to that of the normal spleen, a feature that helps distinguish accessory spleens from pancreatic, adrenal, or peritoneal neoplasms. However, when accessory spleens enhance to a lesser degree than the spleen, they may be more difficult to distinguish from other types of masses. They can be correctly identified with the same nuclear medicine techniques used to image normal spleen or with ferumoxides-enhanced MRI.