|ECR 2018 / C-2081|
|A Neglected Organ: Spleen|
Findings and procedure details
We classified lesions affecting the spleen in six major categories: (a) Normal variants, (b) congenital diseases, (c) trauma, (d) infectious/ inflammatory diseases, (e) vascular and hematologic disorders, (f) benign and malignant tumors, and (g) other diseases. All cases confirmed by surgical resection, or radiological follow-up study. This article presents a description and representative radiological images for many of these disorders.
Splenic clefts, notches and lobulations
The fetal spleen is lobulated and they normally disappear before birth. Lobulation of the spleen may persist after fetal life and results as a variation in normal shape. They are typically seen along the medial part of the spleen and are sometimes supplied by a branch of the splenic artery.1 They should not be misinterpreted as a mass originating from the kidney when they have close relation to the kidney.
The notches and clefts are usually located on the diaphragmatic surface of the spleen and represent remnants of the grooves that seperated the fetal lobules. These clefts are occasionally as deep as 2-3 cm and may cause misinterpretations as splenic laseration. Absence of free abdominal fluid and persistence of this appearance on delayed images are useful differentiating clefts from laceration.2
Accessory spleen is the most common congenital anomaly of the spleen.1 It arises from nonfusion of the splenic anlage and can be solitary or multifocal. It is usually located near the splenic hilum (75%) and the tail of the pancreas (25%).2 However it can occur anywhere in the peritoneal cavity. Diagnosing an accessory spleen in the tail of pancreas may be a challenge, because some pancreatic tumors can share similar imaging characteristics with intrapancreatic accessory spleen.3 Accessory spleen is appear as a round or ovoid mass with well-defined borders measures from a few milimeters to centimeters. On all imaging modalities, it shows similar imaging characteristics as a normal spleen.
Reporting it is important in a patient with a hematologic or autoimmune disorder, as they may be responsible for disease recurrence after splenectomy.4
Splenopancreatic fusion which is a form of splenopancreatic field anomalies may result from disturbances in the embryogenesis, as the two organs arise from dorsal mesogastrium near each other.2 Splenopancreatic fusion is a rare anomaly that mainly associated with trisomy 13. Splenopancreatic fusion is asymptomatic, but it is important to diagnose in patients scheduled to undergo distal pancreatectomy or splenectomy, to avoid possible intraoperative or postoperative complications such as bleeding or pancreatic ductal leaks.5
Asplenia and Polysplenia Syndrome
Asplenia is characterized by complete absence of the spleen and ambiguus location of the abdominal organs. There is a wide range of anomalies including congenital heart diseases, eparterial bronchi, trilobed lungs, bilateral right atria, midline liver and intestinal malrotation. It has a very high mortality rate of up to 95% of patients in the first year of life because of related congenital heart disease.1
Polysplenia is associated with multiple highly variable cardiovascular and visceral anomalies. Multiple discret spleens are seen either on the left or right side of the upper quadrent and always on the same side as the stomach. Other anomalies are right-sided stomach, midline or left-sided liver, intestinal malrotation, short pancreas, absence of gallbladder, inferior vena cava anomalies, hyparterial bronchi and bilobed lungs. Most patients die by the age of 5 years, yet 5 to 10% of the patients reach adulthood due to a normal heart or minor cardiac defects.1
Hematoma and Laceration
Spleen is the most commonly injured solid organ after blunt trauma. Trauma may cause subcapsular hematoma, intraparenchymal hematoma and parenchymal laceration. On CT, hematomas appear as poorly defined areas of decreased attenuation and do not enhance. A contrast blush which is secondary to the extravasation of contrast media and persistence on delayed images is diagnosed as active hemorrhage. Lacerations usually appear linear or branching hypodensity usually with subcapsular hematomas.
Splenosis in an acquired anomaly secondary to seeding of splenic cells after splenic trauma or splenectomy. It is usually in the peritoneal cavity, often in the splenic fossa and left upper quadrant but occasionally it occurs extraperitoneal.6 Although both accessory spleen and splenosis are ectopic splenic tissues, their microscopic architecture, the blood supply and origins are differant.2
Splenic implants are usually multiple and in different sizes. Splenosis is usually asymptomatic and detected incidentally. Splenosis has similar imaging characteristics as a normal spleen. If normal spleen is not seen and history of splenic trauma or splenectomy, splenosis gains weight as the most probable diagnosis.
Pyogenic abscesses are localized collection of pus with high mortality rates. Escherichia coli and Salmonella are the most frequently encountered pathogens. They are most commonly caused by hematogenous spread of infection. Endocarditis is the most commonly associated primary site.7 Also 15% of cases occur after trauma and 10% are develop after preexisting splenic infarction.8
They are usually unilocular but they can be multifocal. On US, it appears ill-defined hypoechoic masses. Gas is seen as high echogenicity associated with distal dirty shadowing. CT is the most reliable tecnique. A solitary abscess is a well-defined and low attenuated lesion with density values ranging from 20-40 HU. The presence of gas is diagnostic for pyogenic abscesses but most of the cases do not contain air. Abscesses are hypointense on T1-weighted images (WI) and a moderate to high signal intensity on T2-WI with smooth or irregular margins. Peripheral contrast enhancement may be seen both on CT and MRI when the capsule has developed. There can be septations of various thickness from 1 to 10 mm.
Hydatid cysts are usually caused by Echinococcus granulosus and commonly seen in endemic areas. Splenic involvement is rare, occuring less than in 2%. Concurrent liver cysts are frequently seen in most cases of the splenic hydatid diseases. Systemic dissemination and intraperitoneal spread from a ruptured liver cyst are two main reasons of splenic involvement.
Hydatid cysts are usually sharply demarcated, round or oval lesions. They can be unilocular or have a multilocular appearance secondary to small peripheral daughter cysts. They have attenuation equal to that of water but higher attenuation may be seen due to intracystic debris, protein or inflammation. They are hypointense on T1-WI and hyperintense on T2-WI. Typically there is no rim enhancement.7 Ringlike calcifications may be seen in the periphery mostly in the dead cysts.7
Fungal abscesses usually occur in immunocompromised patients. Candida albicans is the most common infecting organism. Microabscesses are typically multiple and smaller than 2 cm in size. On US, ‘wheel within a wheel’ pattern is seen in early disease due to perypheral hypoechoic zone, inner echogenic zone and central hypoechoic nidus. ‘Target’ or ‘bull’s eye’ appearance due to central hyperechoic inflammatory core surrounded by hypoechoic fibrotic tissue is seen when the patients neutrophil count returns to normal. On CT, the most common finding is multiple rounded hypodense lesions or calcifications. MRI is superior to CT for the detection and characterization of fungal microabscesses. They appear as hypointense on T1-WI and hyperintense on T2-WI with perypheral ring enhancement on gadolinium-enhanced images.
There are two main types of splenic tuberculosis: micronodular (miliary) and macronodular. Micronodular form is the most frequent that usually manifest as hyperechoic splenomegaly on US. Miliary nodules may appear as hypodense on CT. Calcifications may occur in late stage disease. The macronodular is extremely rare and may be either solitary or multiple. CT findings are nonspecific and variable. CT demonstrates noncalcified low-density lesion or calcified high-density lesion with or without rim enhancement. Both micronodular and macronodular tuberculous lesion are hypointense on T1-WI and hyperintense on T2-WI, yet these MR findings are usually nonspecific.
Sarcoidosis is a multisystem granulomatous disease that is characterized by non-caseating granuloma on histology. The lung, mediastinal and hilar lymph nodes are primarily affected. Splenic involvement is up to 59% of cases, although isolated splenic disease is quite rare.5,9
Splenomegaly with associated lymphadenopathy and hepatomegaly are the most common appearance both on US and CT.10 Less commonly, there is a nodular form. Nodules are multiple, diffusely scattered, 1 mm to 3 cm lesions. They are hypoechoic on US and hypodense on CT. On MRI, nodules are hypointense on all MRI sequences, and this is most conspicuous on fat-suppressed T2-WI. Nodules are hypovascular with only minimal or delayed enhancement.
Histoplasmosis is caused by Histoplasma capsulatum and usually seen in endemic areas. It is most commonly seen in immunocompromised patients. MRI demonstrates scattered hypointense lesions on both T1- and T2-WI. Multiple punctate calcifications may be seen in old granulomas.11
VASCULAR AND HEAMATOLOGIC DISORDERS
Splenic infarcts are mostly caused by tromboembolic process, occuring in any type of hemolytic anemia, endocartitis, atrial fibrillation, portal hypertension, or vascular collagen diseases.11 The typical but infrequent image of the infarct is peripheral, wedge-shaped hypoattenuating lesion that becomes more visible after intravenous contrast administration. In less frequent cases, infarct areas may appear as heterogeneous, poorly marginated and massive hypoattenuating lesions.
In the hyperacute phase, a hemorrhagic infarct may cause a mottled pattern that appears hyperintense on T1-WI. In the acute and subacute phases, infarcts become more focal and better demarcated. In the chronic phase, they have low signal intensity on T1-WI and high signal intensity on T2-WI with/without fibrous contraction.
Sclerosing Angiomatoid Nodular Transformation
Sclerosing angiomatoid nodular transformation (SANT) is a rare benign vascular splenic lesion consisting of multiple angiomatoid nodules surrounded by dense fibrous stroma that form a central scar. Most of the cases are asymptomatic and detected incidentally.
SANT presents as a solitary, lobulated, well-circumscribed nodule that is distinct from the surrounding splenic parenchyma. On US, it has an isoechoic appearance with hypoechoic rim and increased vascularity on color Doppler. On noncontrast CT, it is slightly hypodense relative to normal spleen. The mass is T2 hyperintense at the periphery and T2 hypointense radiating bands corresponding with areas of fibrosis. On postcontrast images, on both CT and MRI the mass shows peripheral enhancement during arterial and venous phase and delayed enhancement in a radiating pattern at the central area due to fibrous tissue. This appearance has been described as spoke-wheel pattern.
Myelofibrosis is a myeloproliferative disorder that may be primary or secondary and usually affects adults. On imaging, hepatosplenomegaly, osteosclerosis and lymphadenopathy may be seen. Splenomegaly is the most common finding, and it is usually massive.12 Associated splenic infarction and extramedullary hematopoesis may occur.
BENIGN AND MALIGNANT TUMORS
Cysts are the most frequent focal splenic lesions. They can be classified as primary (true cyst) or secondary (false cyst). True cysts have an epithelial lining and are either parasitic or nonparasitic (epidermoid). False cysts are lack an epithelial lining, and may result from trauma, infarction or infection. False cysts are much more common. They have mural calcifications in the wall in 50% of cases and they are usually located close to the splenic capsule.4 On imaging, cysts are typically appear as spherical, well-defined nonenhancing lesions with a thin wall. On MRI, they are hypointense on T1-WI and strongly hyperintense on T2-WI. Increased density on CT or signal intensity on T1-WI may correspond to hemorrhagic or proteinaceous content. Differentiating a benign cystic lesion from a malignant lesion is important. Absence of wall-thickening or adjacent solid contrast-enhancing component is in favour of benign lesions.
Hemangiomas are the most common benign primary neoplasm of the spleen. They form by proliferation of vascular channels filled with slow-flowing blood. According to the size of these channels, they are characterized as capillary and cavernous types. Cavernous hemangiomas are more common. Solitary hemangiomas are usually seen, but occasionally they can be multiple. Multiple splenic hemangiomas can be associated with angiomatosis syndromes such as Kasabach-Merritt syndrome or Klippel-Trenaunay-Weber syndrome.
Most hemangiomas are less than 2 cm in diameter with well-defined borders and homogeneous appearance. On US, capillary hemangiomas are typically hyperechoic, however cavernous hemangiomas can be hypoechoic.4 Most hemangiomas are hypodense on unenhanced CT. After contrast administration, cavernous hemangiomas show early peripheral enhancement extending toward the centre on portal-venous and delayed phases. Although smaller hemangiomas may show homogeneous enhancement. Capillary hemangiomas enhance homogeneously. On MRI, hemangiomas are hypo- or isointense on T1-WI and hyperintense on T2-WI. The signal intensity can vary due to presence of hemorrhage and proteinaceous content that appear high signal intensity on T1-WI. The enhancement pattern on MRI is similar as on CT.
Splenic hamartomas are rare benign tumors that consist of disorganized stroma and vascular channels. Hamartomas are usually well-defined, rounded, solid lesions but may contain cystic or necrotic components. They are mostly seen solitary, although occasionally can be multiple.
On US, hamartomas are typically homogeneous hyperechoic lesions. Color Doppler US demonstrates increased blood flow. They usually appear iso- or hypodence on noncontrast CT and occasionally may contain calcification or fatty components. On MRI, they are isointense to splenic parenchyma on T1-WI and heterogeneously hyperintense on T2-WI. After contrast administration, there is usually diffuse heterogeneous early enhancement on both CT and MRI, which is useful to differentiate hamartomas from hemangiomas. On delayed images, prolonged hyperenhancement may be seen and help distinguish hamartoma from lymphoma.7
Lymphangiomas are rare benign tumors usually seen in children. They are composed of malformations of the lymphatic system. They are usually located close to the splenic capsule and may be isolated or may be part of a systemic lymphangiomatosis syndrome.8
Cystic lymphangiomas which are the most common type, typically appear multilocular, thin-walled, well-circumscribed cysts containing lymph-like clear water.11 CTs demonstrate nonenhancing low attenuated lesion. Enhancement of the septa and tiny curvilinear peripheral calcifications may be detected. On MRI, multilocular lesions are homogeneously hyperintense on T2-WI and hypointense on T1-WI with hypointense fibrous septations. There may be high signal intensity on T1-WI due to proteinaceous or hemorrhagic content.
The most common malignant tumor of the spleen is lymphoma that can be either primary or secondary. Primary involvement is quite rare, most of which are Non-Hodgkin lymphomas.4 Secondary involvement of the spleen occurs as a part of systemic disease.
Splenic lymphoma can present as splenomegaly without a discrete mass (80%), a solitary mass, multiple focal nodular lesions or miliary lesions. Necrosis may cause a cystic appearance. Calcifications are rare, probably they occur secondary to treatment. Nodular lesions or focal masses usually appear hypoechoic on US and hypodense on CT. Lymphomas are isointense to splenic parenchyma on T1-WI and T2-WI, although they may be hypointense on T2-WI. Lesions are best detected after contrast administration as hypoenhancing lesion than normal splenic parenchyma.
Imaging findings of leukemia include hepatomegaly, lymphadenopathy, splenomegaly and focal nodular lesions in the liver and spleen.12 Splenic involvement is more common with chronic forms of the disease. Massive splenomegaly may cause spontaneous splenic rupture in 25% of patients.12
The spleen is a rare site for metastatic disease with only 7% occurence.11 Metastatic lesions are usually secondary to hematogeneous spread in malignant melanoma, gynecologic malignancies, breast, lung and stomach cancers.
Solitary or multiple lesions with variable sizes usually demonstrate hypoechoic appearance on US. Cystic splenic metastasis are most commonly seen in melanoma. On CT, splenic metastasis are hypodense with homogeneous or heterogeneous enhancement. Calcifications may be noted if the primary tumor is mucinous adenocarcinoma. MRI demonstrates hypo- to isointensity on T1-WI and mild hyperintensity on T2-WI. Melanoma metastasis can be hyperintense on T1-WI due to hemorrhage or the intrinsic paramagnetic proporties of melanin.
Splenomegaly is a common manifestation in many diseases. This may result most frequently result from portal hypertension, often associated with liver cirrhosis.4 Other causes of splenomegaly may include hematological disorders, neoplastic diseases and inflamatory diseases. Although the size of spleen varies, the upper limit defined as 15 cm length, 10 cm width and 6 cm depth.