Introduction
Morrison's pouch or hepatorenal pouch is the most dependent intraperitoneal space
in the supramesocolic compartment. Because of its dependent location and close relation
to various organs, numerous pathological entities can involve this space either directly
or indirectly by peritoneal spread. In this review, we would like to present a brief
radiological anatomy of Morrison's pouch and describe various pathological conditions
involving this peritoneal space.
Peritoneal Anatomy
Peritoneum is a complex serous membrane derived from the mesoderm. It forms a closed
sac in males, but in females it is open at lateral ostia of the fallopian tubes. One
of its major functions is to limit the spread of a disease, but it is also a potential
pathway for intraperitoneal and subperitoneal disease spread.[1]
Peritoneum has two layers, parietal and visceral, which are lined by the mesothelium.
Abdominal wall is lined by parietal peritoneum, whereas abdominal and pelvic organs
are covered by visceral peritoneum. The space between parietal and visceral peritoneum
is called peritoneal cavity, which is a single continuous fluid-containing space.
Peritoneum folds to form ligaments, mesentery, and omentum. Mesentery, which is a
double fold of peritoneum, is further divided into dorsal and ventral portions by
primitive gut tube. Dorsal mesentery extends from the foregut to hindgut, whereas
ventral mesentery exists only in the foregut. Development of the liver divides the
ventral mesentery into lesser omentum and falciform ligament. These peritoneal reflections
and mesentery divide the peritoneal cavity into several communicating spaces.
Transverse mesocolon divides the peritoneal cavity into two main compartments, supramesocolic
and inframesocolic. Supramesocolic compartment is subdivided into right and left by
falciform ligament. Right supramesocolic space is further divided into right subphrenic
space, right subhepatic space (hepatorenal pouch or Morrison's pouch), and lesser
sac. The focus of this article is to highlight the radiologic anatomy of right subhepatic
space and imaging appearances of its various pathologies.
Morrison's Pouch (Hepatorenal Pouch/Right Subhepatic Space)
Morrison's pouch lies between the upper pole of the right kidney and inferior surface
of the right lobe of the liver ([Fig. 1]). Superiorly, it is bounded by the inferior layer of coronary ligament, posteriorly
by the anterior surface of the upper pole of the right kidney, laterally by parietal
peritoneum lining the right lateral abdominal wall, inferomedially by hepatic flexure
of colon, transverse mesocolon and second part of duodenum. It is the most gravity-dependent
space in supramesocolic compartment.[2] Hepatorenal space is separated from anterior pararenal space by parietal peritoneum.
It communicates with the right paracolic space inferiorly and right subhepatic space
superiorly.
Fig. 1 Representative image of the peritoneal space. Right subhepatic space is the Morrison's
pouch. Dotted line represents the direction of peritoneal flow through which the intraperitoneal
spread of malignancy or infection can occur.
Imaging Modalities
Ultrasonography (USG) is a readily available, safe, and cost-effective imaging tool
for investigation of intraperitoneal pathologies. It is more sensitive than computed
tomography (CT) for the detection of intraperitoneal fluid and is widely used in trauma
settings for the same.[4] Right upper quadrant view or Morrison's pouch view is one of the four views used
for the detection of hemoperitoneum during eFAST.
Multidetector CT allows for a more comprehensive evaluation of intraperitoneal pathologies.
Good-quality sagittal and coronal reformats aid in delineating the full extent of
the disease.[2]
Magnetic resonance imaging (MRI) has superior contrast resolution and better characterizes
the contents of fluid collections. However, lower spatial resolution and motion artifacts
are setbacks.[2]
Positron emission tomography (PET) scan has a role in the detection of peritoneal
carcinomatosis. Fusion FDG PET/CT imaging has higher sensitivity in its detection
than PET or CT imaging alone.[5]
Pathologies of Morrison's pouch
Fluid Collections
Ascites
In healthy individuals, the peritoneal cavity contains ∼50 to 75 mL of clear fluid.[4] Abnormal collection of fluid between parietal and visceral peritoneum is called
ascites that may be caused by a variety of pathological entities. Ascitic fluid can
have low protein concentration (transudate) or high protein concentration (exudate).
Common causes of transudative ascites include cirrhosis, congestive cardiac failure,
hypoproteinemia, and renal failure. Several conditions can result in exudative intraperitoneal
collection such as peritonitis (bacterial or tubercular), malignant ascites, hepatic
vein outflow tract obstruction, pancreatitis ([Fig. 2]), chylous ascites, etc. Peritoneal, mesenteric thickening, and abnormal peritoneal
enhancement on contrast-enhanced CT/MRI favors exudative causes. Infectious causes
result in smooth peritoneal thickening and enhancement, whereas peritoneal and omental
nodularity favors malignant causes.[6] However, tuberculosis can also result in nodular lesions and omental thickening
mimicking malignant causes.[7] Unless loculated, ascitic fluid usually accumulates in Morrison's pouch ([Fig. 3]).
Fig. 2 Ascites. Contrast-enhanced axial (A) and coronal (B) CT images in a 40-year-old patient in a patient with acute pancreatitis showing
a large pseudocyst (central walled off cystic lesion) and ascitic fluid (arrow) accumulating
in Morrison's pouch (*).
Fig. 3 CT Anatomy of Morrison's pouch. Axial CECT image (A) shows intraperitoneal contrast outlining the peritoneum. Morrison's pouch (*) is
the space between the right kidney and the right liver lobe. Hepatic flexure of the
colon (arrowhead), transverse mesocolon (TM), and duodenum (line arrow) are seen anteromedially.
Posteriorly, the parietal peritoneum separates the anterior pararenal space (APS)
from Morrison's pouch (*). Coronal reformatted CT image (B) shows Morrison's pouch (*) between the inferior surface of the right lobe of the
liver and the upper pole of the right kidney. Laterally, limited by parietal peritoneum
(block arrow) lining the right lateral abdominal wall. Sagittal-reformatted CT image
(C) shows Morrison's pouch (*) between the inferior surface of the right lobe of the
liver and upper pole of the right kidney. Parietal peritoneum separates the anterior
pararenal space (APS) from Morrison's pouch. Anteroinferiorly, hepatic flexure of
the colon (arrowhead) is seen.
Hemoperitoneum
Hemoperitoneum can result from traumatic, nontraumatic, or iatrogenic causes. Common
causes include trauma, complications of surgery, coagulopathies, rupture of the ovarian
cyst or ectopic pregnancy, aneurysm rupture, or tumoral bleeding.
USG appearance is variable ranging from anechoic/hypoechoic fluid collections to echogenic
organized hemorrhage. CT shows high attenuation fluid (>30 HU) with variable appearance
depending upon the age, extent, and location of hemorrhage. Acute hemorrhage is hyperdense
with attenuation of 30 to 45 HU. Organized hematoma is even more hyperdense with attenuation
value of 45 to 70 HU ([Fig. 4]). However, these values may differ depending upon the hemodynamic status of patient
as hemoperitoneum in patients with low hematocrit level may have lesser attenuation
values even during acute stage.[8]
Fig. 4 Hematoma. Contrast-enhanced arterial phase axial (A) and coronal (B) computed tomography images in a 28-year-old male patient with history of trauma
showing a high-density hematoma in the perihepatic space (arrow) and in hepatorenal
pouch (*).
In the setting of liver trauma or rupture of liver tumor, blood flows caudally from
perihepatic space and gets accumulated in Morrison's pouch ([Fig. 5]).
Fig. 5 Hemoperitoneum. Contrast-enhanced coronal (A) and axial (B) CT scan in a 41-year-old patient with history of road traffic accident and positive
extended focused assessment with sonography in trauma (eFAST) showing grade 4 liver
injury (arrow) along with high-density fluid in Morrison's pouch (*) suggestive of
hemoperitoneum.
Adrenal Hemorrhage
A large adrenal hematoma can result in mass effect on surrounding organs and may bulge
into Morrison's pouch. Trauma and nontraumatic conditions such as hypovolemia, stress,
anticoagulants, bleeding diathesis, and adrenal tumors can result in adrenal hemorrhage.
Its appearance varies depending upon the age. USG during acute stage shows a predominantly
solid avascular echogenic mass in the suprarenal location. As it ages, central areas
of hematoma liquefy and demonstrate mixed echogenicity. On CT, acute or subacute hematoma
has higher attenuation. Attenuation decreases over time. The MR appearance of hematoma
([Fig. 6]) changes according to the stage of hemoglobin degradation.[9]
Fig. 6 Adrenal hematoma. Axial T1-Weighted (A) and T2W FS (B) image showing a well-defined heterogeneously hyperintense lesion (*) in the right
suprarenal location causing mass effect on the right lobe of the liver. Subtracted
post contrast sequence (C) shows no enhancing component in this lesion. Coronal T2-weighted fat-saturated image
(D) showing that the lesion is causing inferior displacement of the right kidney (white
arrow), effacement of hepatorenal space, and mass effect on the liver (black arrow).
Infectious Conditions
Peritoneal Tuberculosis
Peritoneum is commonly involved in tuberculosis. Mycobacterium tuberculosis may reach the peritoneum by hematogenous or lymphatic route, direct extension from
bowel, discharge of caseous material from the lymph nodes or peritoneal spill from
tubercular salpingitis. Three types have been described: wet type, fibrotic type,
and dry type.[10]
Imaging features vary according to the type of peritoneal involvement although considerable
overlap has been described in their imaging appearance.[10] Free or loculated ascites may be seen in the wet type of peritoneal tuberculosis.
Free ascites favors dependent peritoneal spaces, including Morrison's pouch, and it
may be accompanied by peritoneal thickening and enhancement ([Fig. 7]). Fibrous adhesions, mesenteric thickening, and caseous nodules may be seen in the
dry type. Fibrotic type shows omental and mesenteric thickening with cakelike masses.[10]
Fig. 7 Peritoneal involvement in a microbiologically proven case of disseminated tuberculosis.
(A). Axial CT sections show loculated ascites, smooth peritoneal thickening, and enhancement
(white arrow), omental thickening (black arrow) and clumped bowel loops (arrowhead).
Ascitic fluid can be seen extending into Morrison's pouch (*). (B) Axial section of the chest shows numerous centrilobular nodules in a tree and bud
configuration (*), further supporting the imaging diagnosis of tuberculosis in this
case. (C) Coronal reformatted image better demonstrates clumped small bowel loops (arrowhead)
along with peritoneal thickening and enhancement (arrow).
Imaging differential diagnosis includes peritoneal carcinomatosis and primary peritoneal
malignancies such as peritoneal mesothelioma. Irregular peritoneal thickening and
nodular implants suggest peritoneal carcinomatosis, while smooth peritoneal thickening
favors peritoneal tuberculosis. However, omental thickening and nodular peritoneal
thickening in tuberculosis commonly occur and render differentiation from malignant
causes difficult.[7]
Abscess
An abscess is a localized collection of pus caused by bacterial, fungal, or amoebic
infections. Spread of infection from the inframesocolic compartment through the right
paracolic space or from supramesocolic pathologies such as ruptured hepatic abscess,
subdiaphragmatic abscess, and retained gall stones can result in abscess formation
in Morrison's pouch ([Fig. 8]). Its radiological appearance is variable according to the stage and amount of liquefactive
necrosis. In early stages, it appears as a soft tissue mass with surrounding fat stranding.
Mature abscesses have a well-defined, irregular, and heterogeneously enhancing wall
with central nonenhancing necrotic debris.
Fig. 8 Ruptured hepatic abscess. (A) Sagittal sections of contrast-enhanced CT in a 52 year-old-male patient presenting
with high-grade fever and pain in the abdomen showing hepatic abscess in segments
VI and VII (*) with discontinuity in hepatic surface, subdiaphragmatic (black arrow),
hepatorenal pouch fluid (white arrow) and, ipsilateral pleural effusion (arrowhead).
(B) Axial sections of the same patient better demonstrate the hepatorenal pouch fluid
collection (white arrow).
Inflammatory Conditions
Acute Cholecystitis
Acute cholecystitis is the most common inflammatory condition of the gallbladder.
It can be seen in the setting of cholelithiasis, known as calculous cholecystitis
or in the absence of gall stone disease in critically ill patients, known as acalculous
cholecystitis. CT features of acute cholecystitis include gall bladder wall thickening,
mural hyperenhancement, pericholecystic fat stranding, and pericholecystic fluid.
This fluid may remain limited to pericholecystic location or may show extension posteriorly
into Morrison's pouch ([Fig. 9]).[11]
Fig. 9 Acute cholecystitis in a 75 year-old patient presenting with upper abdominal pain.
Axial contrast-enhanced CT images show a distended gall bladder, enhancing wall, layering
of the hyperdense sludge (arrow) and pericholecystic fluid extending into the hepatorenal
pouch (*).
Retained Gall Stones
Retained gall stones is a frequent complication of cholecystectomy. Perforation of
the gallbladder during the procedure results in gall stone spillage, incidence of
which has increased with the use of laparoscopic technique. Morrison's pouch is one
of the common sites. Patient may remain asymptomatic or may develop complications
such as abscess and fistula formation.[12] Pigmented gallstones are more likely to be infected and, therefore are likely to
result in abscess formation.[13] Abscesses associated with retained gall stones require surgical removal for definitive
cure.[12]
Endometriosis
Endometriosis is the presence of endometrial glands and stroma outside the uterus.
These ectopic glands are hormonally responsive and undergo repeated bleeding resulting
in inflammation and fibrosis. Pelvis is the most common site for these ectopic glands.
Extra-pelvic endometriosis is a relatively rare condition.[14] Morrison's pouch implants are commonly associated with diaphragmatic endometriosis.[15] Ultrasound and CT features are non-specific. MR imaging is the preferred modality
for the diagnosis of endometriosis. Endometriotic implants have a high signal intensity
on T1-weighted images and low signal intensity on T2-weighted images due to the presence
of methemoglobin.[14]
Space-Occupying Lesions
Local Extension of Malignant Tumors
Morrison's pouch can be involved by neoplastic pathologies due to the contiguous spread
from adjacent organs via serosal invasion or noncontiguous spread via intraperitoneal
seeding along the flow of ascitic fluid.
Hepatic malignancies involving the right lobe, renal malignancies, adrenal tumors,
and bowel malignancies from the colon/duodenum can spread to Morrison's pouch along
the visceral peritoneal surface ([Fig. 10]). Sometimes, abdomino-pelvic masses can be large enough to extend directly into
supramesocolic spaces ([Fig. 11]).
Fig. 10 Renal cell carcinoma. Axial (A) and coronal (B) sections of nephrographic phase in a 55-year-old patient with incidentally detected
right renal lesion on ultrasound showing a ball-type partially exophytic lesion (*)
in the inferior pole of the right kidney with extension into the perinephric space
(white arrow) and seen bulging into the hepatorenal pouch (black arrow).
Fig. 11 Infantile fibrosarcoma in a 9-month-old male child. Coronal (A and B) and axial (C) sections of contrast-enhanced CT shows an ill-defined heterogeneously enhancing
abdominopelvic soft tissue mass with large necrotic areas (*) extending into the hepatorenal
pouch (arrows in B and C).
Metastatic Deposits
Gastrointestinal and gynecological malignancies in their advanced stages reach the
peritoneal surface and are disseminated throughout the peritoneal cavity. Gravity,
gastrointestinal tract peristalsis, and diaphragm movement are responsible for the
spread via the peritoneal fluid.[16] Morrison's pouch being the most dependent intraperitoneal space in the supramesocolic
compartment is a common site of metastatic deposits ([Fig. 12]). CT/MR features include irregular peritoneal thickening and enhancement, enhancing
peritoneal nodules or masses. Ascites may or may not be present.
Fig. 12 Peritoneal carcinomatosis from high-grade serous adenocarcinoma of the ovary. Axial
sections of intravenous and oral contrast material-enhanced computed tomographic images
showing the right adnexal mass (arrowhead in A), soft-tissue deposits in hepatorenal pouch (* in B), and omental nodularity (white arrow in C) along with free fluid in pelvis (black arrows).
Metastatic Mature Teratomas
Metastatic mature teratomas are residual tumors that consist of mature teratomatous
tissue elements at the sites of metastasis of malignant non-seminomatous germ cell
tumors. They appear as partially cystic masses with enhancing septations. Fat and
calcifications are also seen, which are more common in females ([Fig. 13]).[17] Some of the proposed theories for this condition include “chemotherapeutic retro-conversion,”
where there is induction of somatic maturation in malignant cells by chemotherapy
and “selective destruction,” in which a chemotherapeutic agent destroys immature elements
and mature elements that are resistant to chemotherapy are left behind.[18] When there is proliferation of these mature tissue elements during or after chemotherapy,
in the setting of normal tumor markers, the condition is known as growing teratoma
syndrome. Imaging features include an increase in the size and cystic component of
metastatic mature teratoma.[17] In cases of ovarian germ cell tumors, the peritoneum has been described as the most
commonly involved site in growing teratoma syndrome.[19]
Fig. 13 Metastatic mature teratoma. Axial (A) and coronal (B) contrast-enhanced CT sections of a 16-year old girl previously treated for immature
teratoma showing a soft tissue deposit in Morrison's pouch (arrow) consisting of areas
of fat and calcifications. Tumor markers were normal in this patient.
Lymphatic Malformations
Abdominal lymphatic malformations are rare benign vascular malformations, most commonly
occurring in mesentery. Retroperitoneal lymphatic malformations are less common and
renal lymphatic malformations are even rarer. Sonographically, their appearance varies
from multilocular cystic lesions to almost solid-appearing lesions due to the presence
of numerous internal septations creating multiple echogenic interfaces. CT shows insinuating
mass with enhancing wall, internal septations and homogeneous low-attenuation fluid
content ([Fig. 14]). The MR appearance of internal contents resembles fluid.[20] However, the presence of proteinaceous contents or hemorrhage may result in a variable
appearance. Mesenteric lesions may extend into Morrison's pouch due to their insinuating
nature while retroperitoneal and perirenal lesions may bulge into hepatorenal space
from perirenal space.
Fig. 14 Lymphatic malformation. Contrast-enhanced axial (A) and coronal (B) computed tomographic images showing a large insinuating cystic lesion in the retroperitoneum
causing distortion of the left renal parenchyma (white arrows in A and B), crossing the midline to reach the contralateral perirenal space bulging into hepatorenal
pouch (*).
Conclusion
Morrison's pouch is an important intraperitoneal space that may be involved in a wide
variety of pathologies. Detailed knowledge of its anatomy, relationship with nearby
organs, and mechanisms of spread of distant pathology are important for comprehensive
diagnosis. Familiarity with the imaging spectrum of various conditions allows accurate
diagnosis and aids in adequate patient management.
