Diagnostic Ultrasound - Abdomen and Pelvis

Spleen
Anatomy: Abdomen
GROSS ANATOMY
Overview
• Spleen is largest lymphatic organ
○ Normal size is variable; no universal consensus
– Generally, normal adult spleen considered 12 cm
length x 4 cm thickness x 7 cm width
– Length = longest diameter in longitudinal plane;
thickness = transverse measurement from hilum;
width = longest transverse diameter
– Splenic index (product of length, thickness, and
width): Normally 120-480 cm³
– Size correlates with height and can exceed these limits
in tall, healthy people
○ Functions
– Manufactures lymphocytes, filters blood (removes
damaged red blood cells and platelets)
– Acts as blood reservoir: Can expand or contract in
response to changes in blood volume
• Histology
○ Soft organ with fibroelastic capsule entirely surrounded
by peritoneum, except at splenic hilum
○ Trabeculae: Extensions of capsule into parenchyma; carry
arterial and venous branches
○ Pulp: Substance of spleen; white pulp = lymphoid
nodules; red pulp = sinusoidal spaces containing blood
○ Splenic cords (plates of cells) lie between sinusoids; red
pulp veins drain sinusoids
• Relations and vessels
○ Spleen contacts posterior surface of stomach and is
connected via gastrosplenic ligament (GSL)
– GSL is left anterior margin of lesser sac; carries short
gastric and left gastroepiploic arteries and venous
branches to spleen
○ Contacts pancreatic tail and surface of left kidney and is
connected by the splenorenal ligament (SRL)
– SRL is left posterior margin of lesser sac (omental
bursa); carries splenic arterial and venous branches to
spleen
○ Splenic vein runs in groove along dorsal surface of
pancreatic body and tail
– Receives inferior mesenteric vein (IMV)
– Combined splenic vein and IMV join superior
mesenteric vein to form portal vein
○ Splenic artery arises from celiac axis in 90%; 8% directly
from aorta; often very tortuous
IMAGING ANATOMY
Internal Contents
• Echo pattern
○ Homogenous, similar to liver
○ Echogenicity: Pancreas > spleen > liver > kidney
• Architecture
○ Radiating pattern of segmental arteries and veins
○ Splenic vein
– Normal diameter 5-10 mm; peak systolic velocity
(PSV): 9-18 cm/s
– Splenic vein at midline is useful landmark for locating
pancreas; pancreas lies anterior to splenic vein
– Diameter increases between 50-100% from quiet
respiration to deep inspiration; increase of < 20%
suggests portal hypertension
– Spectral Doppler waveform typically shows band-like
flow profile with minimal respiratory fluctuations
○ Splenic artery
– Low-resistance waveform; tortuosity of vessel results
in turbulence and spectral broadening
– Normal diameter: 4-8 mm; PSV: 25-45 cm/sec
ANATOMY IMAGING ISSUES
Imaging Recommendations
• Patient positioned supine or right decubitus position (left
side up) with left arm raised
• Place transducer parallel to ribs in 10th or 11th intercostal
space at left midaxillary line, searching for best window
○ Due to rib angle this results in oblique view, which by
convention is called longitudinal or transverse
(depending on transducer orientation)
○ Transverse US view of spleen does not correlate directly
to axial CT view
• End expiration may be helpful; lung base may obscure
spleen in full inspiration
• Spleen poorly accessed from posterior (obscured by left
lung base), anterior, or subcostal approach (obscured by
stomach and colon)
• Assess splenic vein at hilum and midline for patency and
flow direction
Key Concepts
• Spleen has highly variable size and shape
○ Easily indented and displaced by masses and even
loculated fluid collections
EMBRYOLOGY
Practical Implications
• Accessory spleen (splenunculus, splenule): Found in 10-
30% of population
○ Usually small, near splenic hilum
○ Can enlarge and simulate mass, especially after
splenectomy
○ Ectopic intrapancreatic splenule can mimic pancreatic tail
mass; should not be more than 3 cm from tail tip
• Wandering spleen: Spleen may be on long mesentery;
found in any abdominopelvic location; risk of torsion
• Asplenia and polysplenia (heterotaxy syndromes)
○ Situs ambiguus; rare congenital conditions of altered
left/right orientation of organs; associated with
cardiovascular anomalies, intestinal malrotation, etc.
• Splenosis: Peritoneal implantation of splenic tissue after
traumatic splenic injury, can mimic polysplenia
SELECTED REFERENCES
1. Benter T et al: Sonography of the spleen. J Ultrasound Med. 30(9):1281-93,
2011
2. Kim SH et al: Intrapancreatic accessory spleen: findings on MR Imaging, CT,
US and scintigraphy,pathologic analysis. Korean J Radiol. 9(2):162-74, 2008
3. Gillen MA : The spleen. In McGahan JP, Goldberg BB (eds) Diagnostic
Ultrasound, 2nd ed, New York, NY: Informa Healthcare:2008: 801-822
4. Applegate KE et al: Situs revisited: Imaging of the heterotaxy syndrome.
RadioGraphics 19:837-852, 1999
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