Ultrasound and computed tomography in the evaluation of mesenteric lesions: A pictorial review.

Ultrasound is the primary imaging modality in the evaluation of mesenteric lesions. It helps in characterising mesenteric lesions as solid or cystic. Ultrasound guided percutaneous biopsy offers real time assessment of the needle tract to avoid vascular injury while performing biopsy. 20 Evaluation of lesions is limited on USG because of operator-dependence, obscuration of the mass in the presence of a gaseous abdomen and obese patients, and inadequate visualisation of the extent of large mesenteric lesions. Contrast enhanced CT is the primary workhorse for evaluation of mesenteric lesions. It helps in determining the origin of the lesions, providing thoughtful differential diagnosis of masses, for selecting the site of biopsy, determining the extent of pathology and accessibility for resectability of lesion. CT, however, has several limitations including poor soft tissue contrast resolution, exposure to ionising radiation and contrast reactions. MRI can be used as the next step in characterising mesenteric lesions. It has higher soft tissue contrast resolution and helps in characterising fluid content of cysts as serous, mucinous, chylous or haemorrhagic and the soft tissue component of solid lesions. Fluoro-deoxyglucose (FDG) positron emission tomography (PET) CT is presently used to detect lymph nodes and distant metastases in mesenteric tumours and also for response assessment to chemotherapy.

Lymphangiomas are benign lymphatic proliferations that usually occur in childhood. They appear as thin-walled cystic lesions with multiple thin septa (honeycomb or cobweb pattern) on USG and as a unilocular or multilocular cyst with enhancement of the wall or septations on contrast enhanced CT. 10 They show variable attenuation depending on their contents (chylous, serous or haemorrhagic contents) and may insinuate between the mesenteric vessels ( Figure 7 ). Mesenteric lymphangioma. Transverse grey scale ultrasound images (a, b) of the abdomen in a 3-year-old male child reveal an ill-defined multiloculated irregular-shaped thin-walled cystic lesion in the mesentery showing posterior acoustic enhancement. The lesion is located anterior to bilateral kidneys, pancreas (white arrow) and aorta. Multiple thin septations are noticed within the lesion creating a ‘cobweb’ appearance. Echogenic contents are seen within few loculi resulting in differential echogenicity of loculi. No calcification is noticed within the lesion. Axial contrast enhanced CT images (c, d) in the same patient reveal a multilobulated thin-walled fluid density cystic lesion in the mesentery anterior to pancreas and aorta. The lesion is insinuating between leaves of mesentery and bowel loops. Mesenteric vessels can be seen traversing through the lesion. These appear as thin-walled anechoic unilocular cysts with posterior acoustic enhancement on USG and unilocular fluid attenuation lesions with no discernible wall on CT. 11 Benign cystic mesotheliomas are also known as peritoneal inclusion cysts and they usually occur in premenopausal women. They are often associated with trauma, surgery, infection and endometriosis. 2 They occur because of entrapment of fluid (secreted by the ovaries during ovulation) between peritoneal adhesions. 7 They are seen as a fluid attenuation multilocular cystic lesion, multiple unilocular thin-walled cysts or unilocular cystic mass on CT with enhancing but non-calcified septa, which often surround the ovaries. On USG, they appear as unilocular or multilocular cystic lesions, which may be anechoic or may display contents of variable echogenicity ( Figure 8 ). Benign cystic mesothelioma. Transverse grey scale ultrasound images (a, b) of the abdomen in a 38-year-old female reveal a well-defined cystic lesion around the right ovary with internal low-level echoes and a ground glass appearance suggestive of an endometrioma (white thick arrow in a). Another ill-defined thin-walled multiloculated cystic lesion (white thin arrow in b) is seen in midline and left side of lower abdomen and pelvis encasing the left ovary in its posterior aspect. Coronal CT image (c) of the abdomen reveals a well-defined hyperdense cyst of mean attenuation ~ 33 HU in the right adnexa (thick white arrow in c). Multilobulated fluid attenuation (~18 HU) cystic lesion with thin imperceptible walls is noticed in left lower abdomen and pelvis (thin white arrow). Sagittal reformatted CECT image (d) reveals a thin-walled multilobulated cystic lesion anterior and superior to uterus, encasing the left ovary posteriorly (arrowhead). Minimal fluid is observed within the endometrial cavity. No calcification or mural nodule is noted within the lesion. They usually arise from implantation of ovarian tissue during migration or metaplasia of mesothelial cells into ovarian tissue. They appear as thin-walled fluid attenuation unilocular cystic lesions in the mesentery ( Figure 9 ). Mesenteric cyst. Grey scale ultrasound image (a) of the abdomen in a 26-year-old female reveal a well-defined thin walled unilocular anechoic cystic lesion (asterisks) showing posterior acoustic enhancement in the mesentery. No evidence of internal septations, calcification, mural nodule or internal vascularity noted. Axial contrast enhanced CT image of the abdomen (b) shows a well-defined thin walled unilocular cystic lesion (asterisks) anterior to lower pole of left kidney. Coronal reformatted contrast enhanced CT image (c) shows a well-defined unilocular cystic lesion (asterisks) in the left lumbar and iliac region displacing the descending colon medially (white arrow). Photomicrograph of the histopathological specimen (d) reveals the cyst wall lined by flattened and mucin secreting tall columnar epithelium. There is no atypia, mitosis or necrosis.

Secondary involvement of the mesentery by tumours elsewhere is much more common than primary mesenteric tumours. These neoplasms spread to the mesentery by the following routes: Several abdominal malignancies including biliary, pancreatic, gastric and colon cancers may invade directly into the mesentery. 12 Direct spread into the mesentery occurs in small bowel carcinoid, which appears as an intensely enhancing soft tissue mass with linear fibrous bands radiating in the surrounding fat. Calcification occurs in 70% cases. 13 Associated thickening of the bowel wall and angulation of bowel loops might be noticed ( Figure 10 ). Carcinoid tumour of ileum with mesenteric metastases. Axial (a, b) and coronal (c) contrast enhanced CT images of the abdomen in a 56-year-old male reveal nodular enhancing wall thickening of the distal ileum (thick white arrow) causing obstruction and dilation of proximal small bowel loops. A homogenously enhancing soft tissue lesion (thin white arrow) is seen in the mesentery adjacent to the bowel thickening. Numerous fibrous strands are seen radiating from its periphery to the ileal loops with resultant kinking of ileal loops. An intensely enhancing nodular lesion is noted in the mesentery (arrowhead in b) likely a lymph node. Photomicrograph of the histopathological specimen from the mesenteric mass (d) reveals regular small polygonal cells in an insular pattern with round nuclei, salt and pepper chromatin and lacking atypia and mitosis. Lymphoma appears as a lobulated sonolucent mass surrounding vascular trunks with maintained perivascular echogenic fat giving the ‘sandwich appearance’. 14 Lymph nodes may be small and discrete in early disease and later coalesce to form a conglomerate soft tissue mass, which grows around and displaces surrounding structures. Involved lymph nodes have soft tissue attenuation (40 HU – 50 HU) and demonstrate homogeneous enhancement following contrast administration ( Figure 11 ). Besides lymphoma, many other tumours such as lung, breast, colon, ovarian cancers, chronic lymphoid leukaemia, and others, can involve the mesentery via the lymphatics. Non-Hodgkin’s lymphoma-follicular type. Transverse grey scale ultrasound image of the abdomen (a) in a 62-year-old female reveals a well-defined homogenously hypoechoic mass encasing the mesenteric vessels anterior to the common iliac arteries. A few foci of calcification are seen within the lesion (thick white arrow). Axial (b) and coronal (c) contrast enhanced CT images reveal a homogenously enhancing lymph nodal mass encasing the SMA (thin white arrow in c), superior mesenteric vein (thick white arrow in c) and their branches without causing luminal attenuation (sandwich sign). Another homogenously enhancing lymph nodal mass lesion is seen encasing the common iliac arteries (curved arrows in b). A few foci of calcification are seen within the lesion (asterisk in b). Gross ascites is also seen. Photomicrograph of the histopathological specimen (d) shows variable sized follicles replacing the entire lymph node. The follicles are comprised of follicular centre cells with an admixed large number of centroblasts (H&E, 200x). Melanoma, breast and lung carcinoma can involve the mesentery via the hematogenous route. 12 These lesions are usually seen along the antimesenteric border because of the abundant submucosal vascular plexus. Four patterns of peritoneal seeding are noted: 12 Infiltrative pattern leading to a misty mesentery. Nodular pattern/caking ( Figure 12 ). Retractile pattern seen as small bowel retraction, angulation and kinking. Stellate mesentery : The straightened mesenteric blood vessels are held rigid within a thickened sheet such as the mesentery, producing the characteristic stellate appearance on CT because of infiltration of tumour along the mesenteric vessels. Bilateral ovarian granulosa cell tumour with metastatic deposits. Axial contrast enhanced CT image (a) of the abdomen in a 45-year-old female reveals multiple heterogeneously enhancing predominantly solid nodular deposits in Morison’s pouch (thick white arrow) and along the lateral surface of the liver (arrowhead). Axial section in the pelvis (b) reveals heterogeneously enhancing, predominantly solid masses in both ovaries (asterisk) along with a nodular deposit in the mesentery (thin white arrow). Sagittal reformatted CT image on bone window (c) shows a fracture and collapse of the L2 vertebral body (short thick white arrow), likely a pathological fracture related to metastases. Photomicrograph of the histopathological specimen (d) of the mesenteric lesion [H & E stain] reveals scattered and clustered round plasmacytoid cells with central to eccentrically placed nuclei and a prominent microfollicular pattern. Some of the follicles show central pinkish material (Call–Exner bodies) and moderate focal pleomorphism.

Although histopathology is the gold standard for diagnosis of mesenteric lesions, imaging plays an important role in diagnosis of mesenteric neoplasms, detection of complications and in deciding appropriate treatment options. Ultrasound is inadequate for optimal visualisation of mesenteric neoplasms and in detecting their relation with other structures because of shadowing from bowel gas, calcification or inadequate penetration of sound beams in obese patients. CT is an excellent imaging modality in the characterisation of mesenteric lesions, detecting their extent and relations with surrounding structures, which is useful for surgical planning. However, CT too has several limitations and other imaging modalities such as MRI and FDG-PET can be helpful in further characterisation of mesenteric lesions.

Abdominal lymphadenopathy is the most common finding associated with tuberculosis. Involved lymph nodes appear enlarged, conglomerate and show ring enhancement (necrosis). Other findings observed in cases of abdominal tuberculosis include mesenteric nodularity, fat stranding, peritoneal thickening, localised collections, free fluid, clumping of bowel loops, bowel wall thickening and solid organ involvement in the form of hepatomegaly, splenomegaly, liver and splenic granulomas. 15 It is an infectious condition caused by Gram-positive bacterium, Tropheryma whipplei . On imaging, small bowel wall thickening associated with enlarged mesenteric lymph nodes demonstrating hypoattenuating centres because of fat deposition can be seen. Diagnosis is made by small intestinal mucosal biopsy. 16 Actinomyces israelii is an anaerobic Gram-positive bacterium and is a normal inhabitant of the oral cavity, gastrointestinal tract and female genital tract. It spreads to involve the mesentery when a mucosal breach is present such as inflammation, surgery, trauma or intrauterine contraceptive device (IUCD) use. 17 On imaging, an ill-defined heterogeneously enhancing soft tissue mass infiltrating the mesentery and adjacent organs can be seen. Septic embolism leading to hepatic, renal and splenic abscesses can occur. 2 Diagnosis is made based on aggressive imaging features and subtle clinical symptoms in predisposed individuals. Biopsy should be performed in indeterminate imaging findings, which reveal sulphur granules representing bacterial colonies. These usually occur because of intraperitoneal rupture of liver or splenic hydatid cysts ( Figure 13 ). Hydatid cysts can appear as either unilocular cysts (Type I cyst), multiloculated cystic lesions with daughter cysts at the periphery (type IIA), multilocular lesions with irregular daughter cysts occupying almost the entire volume of the maternal cyst (rosette appearance – type IIB) or high attenuation lesion containing occasional calcification and daughter cysts (type IIC). Type III cysts are calcified cysts while complicated hydatid cysts are included in the type IV category. 18 Hydatid cysts. Axial contrast enhanced CT images (a, b) of the abdomen in a 6-year-old child reveal multiple, well-defined peripherally enhancing unilocular cystic lesions in the mesentery and pelvis (white arrows) with no internal septations, mural nodule or calcification within the lesions. Coronal reformatted CT image (c) reveals well-defined unilocular cysts in the liver and spleen with no internal septations, calcifications or mural nodule.

Management of mesenteric lesions depends on symptoms and imaging findings. Mesenteric lesions detected incidentally on imaging need classification as benign or malignant. Definite diagnosis of a few pathologies such as mesenteric panniculitis, lymphoma, desmoid tumour in patients with FAP and benign cystic lesions can be made on imaging. However, lesions with indeterminate imaging findings need to be biopsied to rule out malignancy. Imaging helps in determining the site for percutaneous biopsy in such tumours. Lesions that are inaccessible to percutaneous biopsy or show indeterminate histopathology findings on percutaneous biopsy require surgical biopsy to confirm the diagnosis. 21 Once confirmed by biopsy, management of the mesenteric lesions depends on the histopathology. Lymphomas and desmoid tumours are managed conservatively with chemotherapy and hormonal therapy or imatinib, respectively. Well-circumscribed lesions located in the periphery of the mesentery can be resected completely, without sacrificing significant bowel length or major mesenteric vessels. Infiltrative mesenteric lesions, located in the root of mesentery are usually managed conservatively because of involvement of major vessels and the need for sacrificing a major portion of bowel, which may lead to small bowel syndrome. Well-defined malignant tumours and symptomatic benign tumours are treated surgically. The goal of surgery is R0 resection (grossly as well as microscopically negative tumour margins), while R1 resection followed by adjuvant chemoradiotherapy can also be performed. Well-defined asymptomatic benign lesions can be followed up and resection is performed when they become large or cause symptoms. Symptomatic infiltrative benign or malignant lesions can be managed by debulking surgery to reduce tumour load and prevent complications such as bowel obstruction or mesenteric ischemia.

Mesenteric changes in inflammatory bowel disease include mesenteric fibrofatty proliferation, mesenteric lymphadenopathy, fat stranding ( Figure 14 ), abscess and prominent vasa recta (Comb sign). Crohn’s disease. Axial (a, b) and coronal (c) contrast enhanced CT images of the abdomen in a 45-year-old female reveal mesenteric fibrofatty proliferation in the right iliac fossa (curved white arrow), mesenteric lymph nodes (thin white arrow), thickening of the caecal wall with submucosal fat proliferation ([fat target sign] – thick white arrow) and prominent and straightened vasa recta ([Comb sign] – arrowheads). Photomicrograph of the ileal biopsy specimen (d) shows transmural infiltration of the ileum by inflammatory cells with loss of the villous architecture. Findings in acute mesenteric ischaemia related to superior mesenteric artery (SMA) thrombosis include dilated bowel loops with paper-thin walls, pneumatosis intestinalis, air within mesenteric vessels, mesenteric fat stranding and free fluid on CECT 19 ( Figure 15 ). Superior mesenteric artery (SMA) thrombosis with acute mesenteric ischaemia. Axial contrast enhanced CT images (a–d) reveal thrombus in the superior mesenteric artery (thick white arrow in a) at the level of its origin from aorta. The large and small bowel loops appear dilated and show paper-thin walls. Few air foci are seen within wall bowel loops suggestive of pneumatosis intestinalis (thin white arrow in b). Air is also noticed along the mesenteric vessels (arrowheads in c and d). On USG, tangled serpentine vessels can be seen, which show pulsatile waveforms on spectral doppler while on CECT, tangled serpentine vessels can be seen with early opacification of venous channels in the arterial phase. Direct communication between branches of the SMA and superior mesenteric vein (SMV) can be visualised on volume rendering technique (VRT) images.

The mesentery is a broad, fan-shaped fold of peritoneum that suspends the loops of small intestine from the posterior abdominal wall. Secondary involvement of the mesentery from tumours elsewhere is much more common than primary mesenteric neoplasms such as desmoid tumour, inflammatory myofibroblastic tumour (IMFT), and others. Most patients with mesenteric lesions present with non-specific symptoms of abdominal pain, tenderness, palpable abdominal swelling, abdominal distension and weight loss. These symptoms are shared by pathologies of other abdominal organs and it is therefore very difficult to identify mesenteric lesions clinically. Some mesenteric diseases present with distinctive imaging findings while others have similar findings, thereby complicating their differential diagnosis. Understanding the characteristic radiological patterns on ultrasound (USG) and CT offers valuable insights for differential diagnoses of mesenteric lesions and their treatment.