Diagnostic Ultrasound - Abdomen and Pelvis

Approach to Sonography of Abdominal Wall/Peritoneal Cavity
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Diagnoses: Abdominal Wall/Peritoneal Cavity
Anatomy-Based Imaging Issues
Abdominal Wall
The anterolateral muscles of the abdominal wall are the
external oblique, internal oblique and the transversus
abdominis muscles. The aponeuroses of the internal oblique
and transversus abdominis join medially to form the rectus
sheath which contains the paired rectus abdominis muscles.
These muscle groups protect the abdominal organs and assist
in trunk flexion, twisting, walking and sitting as well as
increasing intraabdominal pressure.
The linea alba is a midline raphe between the two rectus
muscles. The linea semilunaris/spigelian fascia is a vertical
fibrous band at the lateral edge of rectus sheath. These are
sites of weakness and locations of abdominal wall hernias.
Midline hernias through the linea alba include epigastric,
umbilical and hypogastric hernias. Lateral hernias through the
spigelian fascia are spigelian hernias. Deep to muscle layer is
the transversalis fascia, followed by the extraperitoneal fascia
and fat and the parietal peritoneum.
The inguinal ligament is the inferior edge of the external
oblique aponeurosis, running from the anterior superior iliac
spine to the pubic tubercle. The inguinal canal runs between
the external oblique and the transversalis fascia, above the
inguinal ligament. The inguinal canal begins at the deep
inguinal ring and opens medially and inferiorly at the
superficial inguinal ring. The deep inguinal ring is a defect in
the transversalis fascia half way between the anterior superior
iliac spine and the pubic tubercle. It is lateral to the inferior
epigastric artery which is a key landmark for hernia evaluation.
The Hesselbach triangle lies medial to the inferior epigastric
artery, lateral to the rectus sheath and above the inguinal
ligament. Indirect inguinal hernias pass through the deep
inguinal ring to emerge at the superficial inguinal ring with a
neck lateral to the inferior epigastric artery. Direct inguinal
hernias emerge through Hesselbach triangle, medial to the
inferior epigastric artery. Femoral hernias pass through the
femoral canal, inferior to the inguinal ligament and medial to
the femoral vein.
The posterior abdominal wall muscles are the psoas major and
minor, iliacus and quadratus lumborum. They assist in hip
flexion, maintenance of posture and lateral trunk flexion.
Paraspinal muscles are the three columns of the erector
spinae: Iliocostalis, longissimus and spinalis which extend the
vertebral column. The latissimus dorsi arises from the
thoracolumbar fascia lateral to the erector spinae. Lumbar
hernias arise in two lumbar triangles: Superior (Grynfeltt)
below the 12th rib, lateral to erector spinae and medial to the
internal oblique muscle; inferior (Petit) above iliac crest
between latissimus dorsi medially and external oblique
laterally.
Peritoneal Cavity
The peritoneal cavity is a potential space between abdominal
visceral and parietal peritoneum. It is divided into greater and
lesser sacs which communicate at the foramen of Winslow.
Compartments include right and left supramesocolic and
inframesocolic spaces, paracolic gutters and pelvic cavity. The
peritoneal cavity normally contains a small amount of
lubricating fluid. Fluid collects in dependent pouches of the
peritoneal cavity such as the pouch of Douglas (rectouterine
pouch) and Morison pouch (hepatorenal pouch). This is where
fluid should be sought initially. With increasing volumes of
intraperitoneal fluid, fluid will spill over into all recesses and
spaces.
The greater omentum and lesser omentum are folds of
peritoneum that drape from the stomach. The greater
omentum extends to cover the transverse colon and the small
bowel and serves to contain inflammation or tumor. The
lesser omentum extends from the lesser curve to the liver and
proximal duodenum. The mesentery is a layer of peritoneum
that encloses mobile bowel, connecting it to the posterior
abdominal wall. The ascending and descending colon are not
mobile and are retroperitoneal. Ligaments connect viscera to
each other or to the abdominal wall, folds are peritoneal
reflections. In the absence of ascites, sonographic evaluation
of these normal structures is limited.
Pathologic Issues
Ultrasound has many advantages for imaging of the anterior
abdominal wall and peritoneal cavity. For the abdominal wall,
real-time, high-resolution ultrasound and the dynamic ability
to provoke hernias makes it an ideal imaging test. For fluid
detection, ultrasound is highly sensitive within limitations of
body habitus.
One of the most common indications for imaging the
abdominal wall and groin is for the evaluation of hernias. The
most common hernias are acquired inguinal and incisional
hernias. Risk factors include abdominal wall laxity and
increased intraabdominal pressure, prior surgery and trauma.
Femoral and umbilical hernias are not uncommon. Congenital
hernias are typically umbilical and indirect inguinal hernias.
Other than confirming the clinically suspected diagnosis,
ultrasound can determine the type of hernia and contents as
well as complications such as strangulation.
Ultrasound is often used for the evaluation of lumps and
swelling. Masses include lipomas and other soft tissue tumors,
fat necrosis, epidermal inclusion cysts, endometriosis,
desmoid tumors, and metastases. Careful search for tumor
blood flow and comparison to the other asymptomatic side
are recommended. Some lesions have characteristic findings,
others require further imaging or biopsy.
Other indications for ultrasound of the abdominal wall include
suspected fluid collections or abscesses. Ultrasound can be an
ideal modality for localized fluid collections, able to detect
internal contents and vascularity. Hematomas and abscesses
are more complex than seromas.
Ultrasound of the peritoneal cavity usually consists of a search
for ascites or acute hemorrhage (as in the FAST scan after
abdominal trauma). Complexity of ascites, peritoneal
nodularity or discrete soft tissue masses may suggest causes
for the ascites, such as carcinomatosis or bacterial peritonitis.
Imaging Protocols
Prior to imaging, a good clinical history is obtained. Specific
details regarding location and onset of symptoms and
exacerbating factors are very important. The study can then
be tailored to the patient.
Abdominal Wall
A high-frequency (8-15 MHz) linear transducer is required. The
trapezoid format and extended field of view/panoramic
format are very useful techniques for imaging larger
structures. Curvilinear lower frequency transducers may also
be required for larger lesions. More penetration and deeper
field of view are needed for deeper lesions. For complex and