AJR 2004; 183:1653-1657
© American Roentgen Ray Society
CT Features of the Accessory Spleen
Koenraad J. Mortelé1,
Bart Mortelé2 and
Stuart G. Silverman1
1 Division of Abdominal Imaging and Intervention, Department of Radiology,
Brigham and Women's Hospital, Harvard Medical School, 75 Francis St., Boston,
MA 02115.
2 Department of Radiology, Ghent University Hospital, De Pintelaan 185, Ghent
B-9000, Belgium.
Received January 16, 2004;
accepted after revision March 8, 2004.
Address correspondence to K. J. Mortelé
(kmortele{at}partners.org).
Abstract
OBJECTIVE. The purpose of this study was to describe the prevalence
and CT features of the accessory spleen.
CONCLUSION. Accessory spleens are present in 16% of patients
undergoing contrast-enhanced abdominal CT. Typically, accessory spleens appear
on CT scans as well-marginated, round masses that are smaller than 2 cm and
enhance homogeneously on contrast-enhanced images. When accessory spleens are
smaller than 1 cm, their attenuation may be lower than that of the spleen
because of partial volume effects. Their most frequent location (22%) is
posteromedial to the spleen; anterolateral to the upper pole of the left
kidney; and lateral, posterior, and superior to the tail of the pancreas.
Familiarity with these characteristic features may differentiate them from
other pathologic findings in the upper abdomen.
Introduction
Accessory spleens, also known as supernumerary spleens, splenunculi, or
splenules, are congenital foci of healthy splenic tissue that are separate
from the main body of the spleen
[1]. They arise from the
failure of fusion of the splenic anlage, located in the dorsal mesogastrium,
during the fifth week of fetal life
[2]. Accessory spleens are
relatively common and are seen in 10–30% of patients at autopsy
[1–3].
Although usually asymptomatic and incidentally discovered, they are clinically
important in some patients
[4–14].
To our knowledge, no prior study has described the CT appearance of
accessory spleens in a large cohort of patients. CT is the imaging technique
most commonly used to evaluate the abdomen and therefore, familiarity with the
CT features of accessory spleens is helpful in differentiating them from other
splenic abnormalities and pathologic findings in the abdomen. We conducted a
retrospective study of the frequency and CT appearance of the accessory
spleen.
Materials and Methods
Subjects and CT Technique
Abdominal CT scans of 1,000 consecutive patients were performed on a
Somatom Plus 4 scanner (Siemens Medical Solutions). Unenhanced and
contrast-enhanced images were obtained using 8-mm collimation with a table
speed of 8 mm/sec (pitch = 1) during a single 24-sec breath-hold, extending
from the diaphragm to the level of the iliac crest. Dilute Gastrografin
(meglumine diatrizoate, Bristol-Myers Squibb) was administered orally to
opacify the gastrointestinal tract. Contrast-enhanced images were obtained
50–70 sec after IV administration of 120 mL of ionic iodinated contrast
material (meglumine ioxitalamate, 300 mg I/mL [Telebrix 30], Guerbet) using a
power injector at a rate of 3 mL/sec.
Data Analysis
CT scans were reviewed retrospectively in consensus by two radiologists.
The following characteristics of accessory spleens were recorded on the axial
CT images: presence or absence, total number per patient, size (in three
dimensions: anteroposterior, transverse, craniocaudal), shape (round, ovoid,
triangular, other), location (in relation to the spleen), enhancement pattern
(relative to the spleen), and vascular supply (whether splenic artery branch
was visible). Axial anteroposterior and transverse diameters were measured
using calipers on hard-copy images. Craniocaudal diameters were estimated by
counting the number of images on which the accessory spleen was visible. We
differentiated the locations of the accessory spleens by dividing the left
upper quadrant into discrete anatomic compartments (Fig.
1A,
1B,
1C).
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Fig. 1A. —Anatomic drawings of spleen. Locations of axial
(A), coronal (B), and sagittal (C) accessory
spleens are derived by dividing left upper quadrant into discrete anatomic
compartments. This was done by drawing line perpendicular to longest axis of
main spleen at level of spenic hilum, resulting in anteromedial (AM),
anterolateral (AL), posteromedial (PM), and posterolateral (PL) compartments.
Craniocaudally, main spleen was divided into superior third, middle third, and
inferior third. When accessory spleen was located anterior (A) to or posterior
(P) to main spleen, area was again divided into superior third, middle third,
and inferior third. Accessory spleens outside this region were considered
either superior (S) to or inferior (I) to main spleen.
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Fig. 1B. —Anatomic drawings of spleen. Locations of axial
(A), coronal (B), and sagittal (C) accessory
spleens are derived by dividing left upper quadrant into discrete anatomic
compartments. This was done by drawing line perpendicular to longest axis of
main spleen at level of spenic hilum, resulting in anteromedial (AM),
anterolateral (AL), posteromedial (PM), and posterolateral (PL) compartments.
Craniocaudally, main spleen was divided into superior third, middle third, and
inferior third. When accessory spleen was located anterior (A) to or posterior
(P) to main spleen, area was again divided into superior third, middle third,
and inferior third. Accessory spleens outside this region were considered
either superior (S) to or inferior (I) to main spleen.
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Fig. 1C. —Anatomic drawings of spleen. Locations of axial
(A), coronal (B), and sagittal (C) accessory
spleens are derived by dividing left upper quadrant into discrete anatomic
compartments. This was done by drawing line perpendicular to longest axis of
main spleen at level of spenic hilum, resulting in anteromedial (AM),
anterolateral (AL), posteromedial (PM), and posterolateral (PL) compartments.
Craniocaudally, main spleen was divided into superior third, middle third, and
inferior third. When accessory spleen was located anterior (A) to or posterior
(P) to main spleen, area was again divided into superior third, middle third,
and inferior third. Accessory spleens outside this region were considered
either superior (S) to or inferior (I) to main spleen.
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The accuracy of identifying accessory spleens was determined by reviewing
the patients' medical records and follow-up imaging. For example, in patients
with known or suspected malignancy, the stability of the size and other CT
features of the accessory spleen were validated by comparing them with prior
CT examinations and follow-up imaging tests, when available.
Results
Incidence
CT scans of 156 patients (15.6%) revealed the presence of at least one
accessory spleen. In 21 of those patients (13%), more than one accessory
spleen was detected, with a maximum of three per patient, resulting in a total
of 180 accessory spleens.
Size
The anteroposterior diameter varied between 4 and 29 mm, with a mean of
11.9 mm. The transverse diameter ranged from 4 to 25 mm, with a mean of 11.6
mm (Figs. 2 and
3). Craniocaudal diameters
varied between 8 and 32 mm, with a mean of 16.8 mm.
Shape and Location
All accessory spleens were well marginated and were round in 141 patients
(78.3%), ovoid in 27 (15%), and triangular in 12 (6.7%) (Figs.
4,
5,
6). The location of the
accessory spleens was variable (Table
1). However, the most common location was the inferior third of
the posteromedial compartment (21.8 %), with the accessory spleen located
anterolateral to the upper pole of the left kidney, and adjacent (lateral,
posterior, and superior) to the tail of the pancreas (Figs.
7 and
8). Intrapancreatic accessory
spleens were seen in two patients (Fig.
9).
Pattern of Enhancement and Blood Supply
All accessory spleens enhanced homogeneously. However, 57 of them (31.7%)
enhanced less than the spleen. All these hypodense accessory spleens were
smaller than 10 mm in the anteroposterior or transverse plane. Supplying
vascular branches arising from the splenic artery were detected in 67 patients
(43.3%).
Discussion
The detection and characterization of accessory spleens are important in
three clinical scenarios. First, an accessory speen may mimic lymphadenopathy
and tumors in other abdominal organs, such as the pancreas, the adrenal gland,
and the kidney
[4–7].
Second, accessory spleens occasionally may become symptomatic because of
torsion, spontaneous rupture, hemorrhage, and cyst formation
[8–11].
Third, a surgeon's awareness of their presence may be important when the
intention is to remove all functional splenic tissue (e.g., hematologic
disorders)
[12–14].
Other congenital and acquired splenic anomalies, including splenic clefts,
lobulations, polysplenia, and splenosis, should be differentiated from
accessory spleens [15].
Splenic clefts are a result of incomplete fusion of the splenic anlage and
usually are visible under the diaphragmatic surface
[1–3].
Splenic lobulations persist after fetal life
[1–3].
They usually are seen along the medial part of the spleen and sometimes are
supplied by an early branch of the splenic artery
[15]. Polysplenia is a
congenital syndrome seen in patients with bilateral left-sidedness, in which
two to 16 splenic nodules of equal size can be found in the right or left
upper quadrant (depending on the associated situs)
[15]. Cardiovascular and
pulmonary abnormalities are associated with polysplenia in most patients
[15]. Similar to accessory
spleens, splenosis is a condition in which isolated foci of heterotopic
splenic tissue are present
[1–3].
However, unlike accessory spleens, splenosis is an acquired condition and
originates from seeding or implantation of splenic cells after splenectomy or
splenic rupture (autotransplantation). Splenosis nodules usually are small as
a result of limited blood supply. They show a sessile growth pattern and are
found typically adjacent to small-bowel serosa, the greater omentum, the
parietal peritoneum, and the diaphragm. Their blood supply derives from
neovascularization and is not of embryologic origin, as is the case with
accessory spleens
[1–3].
To our knowledge, no CT criteria currently exist for diagnosing accessory
spleens, although they are relatively common. As a result, differentiation of
this benign splenic anomaly from pathologic disorders can be difficult. Our
data suggest that most accessory spleens have a characteristic appearance on
CT, appearing as well-marginated, round masses that are smaller than 2 cm.
Homogeneous enhancement on contrast-enhanced images is another important
feature. In our study, 21.8% of the accessory spleens were located near or in
the pancreatic tail. Differentiation from a hypervascular pancreatic neoplasm
(e.g., islet tumor) is, therefore, sometimes challenging. As described by
Hayward et al. [8], an
intrapancreatic accessory spleen can be suggested when an intrapancreatic mass
enhances in a manner identical to that of the spleen. Similarly, accessory
spleens may be differentiated from metastatic lesions or lymphadenopathy in
the splenic hilum when they enhance to the same degree as the spleen. However,
in our study, 32% of accessory spleens were hypodense compared with the main
spleen. Because all were smaller than 1 cm, their attenuation may have been
caused by partial volume effects. It is likely that, when using thinner
collimation (e.g., 
5 mm), accessory spleens will appear similar to the
spleen.
In 1,000 patients, no accessory spleens were found superior to the main
spleen. Also, they were rarely found lateral (either anterior or posterior) to
the spleen. Therefore, masses in these locations should be considered
suspicious for a pathologic process.
Our study had some limitations. First, the pelvis was not imaged in our
study population and, therefore, it is possible that pelvic accessory spleens
were missed. However, pelvic accessory spleens are considered rare
[3]. Second, it is possible
that if thinner collimation with MDCT had been used, more accessory spleens
may have been detected.
In conclusion, accessory spleens are common and their CT features
characteristic. Typically, they are well-marginated, homogeneously enhancing,
round masses that are smaller than 2 cm. Their most frequent location is
posteromedial to the spleen. When smaller than 1 cm, they may appear hypodense
relative to the spleen.
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Abstract
Introduction
