Diagnostic Ultrasound - Abdomen and Pelvis

Bowel
– Forms an incomplete ring in females
○ External anal sphincter
– Thick ring of skeletal muscle around internal anal
sphincter
– Under voluntary control
– 3 parts from superior to inferior: Deep, superficial, and
subcutaneous
○ Longitudinal muscle
– Thin muscle between internal and external anal
sphincters
– Conjoined muscle from muscularis propria of rectum
and levator ani
Histology
• Bowel wall throughout GI tract has uniform general
histology, comprised of 4 layers
○ Mucosa
– Functions for absorption and secretion
– Composed of epithelium and loose connective tissue
– Lamina propria
– Muscularis mucosa (deep layer of mucosa)
○ Submucosa
– Consists of fibrous connective tissue
– Contains Meissner plexus
○ Muscularis externa
– Muscular layer responsible for peristalsis or propulsion
of food through gut
– Contains Auerbach plexus
○ Serosa
– Epithelial lining continuous with peritoneum
IMAGING ANATOMY
Overview
• GI tract extends from mouth to anus
• Esophagus, which is intrathoracic, is difficult to visualize
with external ultrasound due to rib cage and air-containing
lungs
○ Endoluminal ultrasound performed to assess mural
pathology
• Stomach to rectum lie within abdomen and pelvis
• Stomach, 1st part of duodenum, jejunum, ileum, transverse
colon, and sigmoid colon suspended within peritoneal
cavity by peritoneal folds and are mobile
• 2nd to 4th part of duodenum, ascending colon, descending
colon, and rectum are typically
extraperitoneal/retroperitoneal
○ Retroperitoneal structures have more fixed position and
are easy to locate
• Stomach located in left upper quadrant
○ Identified by presence of rugae/mural folds
○ Prominent muscular layer facilitates identification of
pylorus
• Small bowel loops are located centrally within abdomen
○ Abundant valvulae conniventes helps identify jejunal
loops
○ Jejunalization of ileum seen in celiac disease to
compensate for atrophy of folds in proximal small bowel
○ Contents of jejunal loops are usually liquid and appear
hypoechoic/anechoic
• Cecum and colon identified by haustral pattern
○ Located peripherally in abdomen
○ Contain feces and gas
○ Haustra seen as prominent curvilinear echogenic arcs
with posterior reverberation
○ Cecum identified by curvilinear arc of hyperechogenicity
(representing feces and gas) in right lower quadrant
blind-ending caudally
○ Not uncommonly, cecum high-lying and may be
horizontally placed
○ Sigmoid colon variable length and mobile
○ Junction of left colon with sigmoid colon identified in
left iliac fossa by tracing descending colon
○ Rectosigmoid junction has fixed position and is identified
with full bladder, which acts as acoustic window
• Appendicular base normally located in right lower quadrant
○ Length and direction of tip vary
○ Retrocecal appendix and pelvic appendix can be difficult
to locate transabdominally
– Transvaginal ultrasound examination useful to identify
pelvic appendix
• Normal measurements of bowel caliber
○ Small bowel < 3 cm
○ Large bowel
– Cecum < 9 cm
– Transverse colon < 6 cm
• Stratified appearance of bowel wall on histology is depicted
by 5 distinct layers on ultrasound as alternating
echogenic/sonolucent (hypoechoic) appearance (gut
signature)
○ Interface of lumen and mucosa: Echogenic
○ Muscularis mucosa: Hypoechoic
○ Submucosa: Echogenic
○ Muscularis propria/externa: Hypoechoic
○ Serosa: Echogenic
• Normal bowel wall thickness < 3 mm
Bowel Motility
• Bowel is hollow viscus and is constantly mobile due to
peristalsis
○ Assessing direction of flow of contents often challenging
○ When visibility permits, direction of flow can be
determined by following long segments of bowel in
continuous fashion
• Fixed points of bowel are easy to assess with
transabdominal ultrasound
○ Pylorus, "C loop" of duodenum, and ileocecal junction
useful landmarks to assess direction of content flow
• Different bowel pathologies have potential to alter normal
gut motility
• Real-time dynamic ultrasound provides useful information
regarding bowel mobility, which can aid in diagnosis of
underlying condition
○ Cine function useful to store dynamic images for review
• Abnormal bowel identified as thickened or dilated
segments
○ Thickened bowel demonstratesreduced peristalsis
– Stands out among normally peristalsing loops of
normal bowel
Anatomy: Abdomen
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