MRI of the small-bowel: how to differentiate primary neoplasms and mimickers

Abstract

MRI of the gastrointestinal tract is gaining clinical acceptance and is increasingly used to evaluate patients with suspected small-bowel diseases. MRI may be performed with enterography or enteroclysis, both of which combine the advantages of cross-sectional imaging with those of conventional enteroclysis. In this paper, MRI features of primary small-bowel neoplasms, the most important signs for differential diagnosis and the diseases that can be considered as mimickers of small-bowel neoplasms, are discussed.

Tumours of the small-bowel are rare, accounting for approximately 3–6% of all gastrointestinal (GI) neoplasms [1]. The diagnosis is complicated by non-specific clinical presentation, and a low index of clinical suspicion makes the detection of these tumours a challenge for both the physician and the radiologist. Major symptoms include abdominal pain, anaemia or obscure GI bleeding, leading clinicians to investigate many different conditions with a delay of several months from first clinical manifestation to diagnosis [1]. Conventional enteroclysis and capsule endoscopy are the most common procedures to visualise mucosal abnormalities, but are limited in the evaluation of mural and extramural extent of small-bowel tumours [2-4]. Therefore, radiologists assume a major role in the detection of small-bowel tumours. However, inadequate radiological studies may cause incorrect interpretation of radiological findings and important delay in diagnosing primary malignancies of the small intestine. MRI examination is rapidly increasing the clinical acceptance for evaluating the small-bowel and can be the initial imaging method to investigate small-bowel diseases [5-9]. Therefore, MR examination may provide the initial opportunity to detect and characterise tumours of the small-bowel. The excellent soft-tissue contrast of MRI and its ability to combine the morphological information of cross-sectional imaging with functional information can help in the diagnosis and evaluation of small-intestinal neoplasms [8,9]. In this article we describe the MR findings of the primary small-bowel neoplasms, discuss the MR signs useful in evaluating the origin of the neoplasms (mucosal vs extramucosal and extraparietal) and discuss the MR findings for the differential diagnosis.

Technical consideration and methodology

MRI can be performed using two different approaches: MR enteroclysis or MR enterography [2-9].

In enterography, bowel distension is achieved by oral administration of large volumes of enteric contrast agent, while in enteroclysis, enteric contrast agent is administered through a nasoenteric tube, as described in previous reports [8].

An enteric contrast agent is used in both techniques to distend the lumen, aiding conspicuity of the small-bowel walls; in fact, because bowel loops tend to collapse when unfilled, normal post-gadolinium enhancing walls could be misinterpreted as pathologically thickened walls [6-10].

For suspected of small-bowel neoplasms it is recommended to use biphasic agents, such as water, Volumen, sorbitol and polyethylene glycol (PEG), which produce a low signal intensity on T₁ weighted images and a high signal intensity on T₂ weighted images. These contrast agents provide conspicuous distinction between the bowel wall and the lumen, both on T₂ weighted images (e.g. by enabling the detection of mural ulcers, which could otherwise be missed, being darkened if a negative—hypointense—contrast agent is used) and on T₁ weighted post-gadolinium images (e.g. by increasing the conspicuity of hyperenhancing masses) [8].

For MR enteroclysis protocol, MR fluoroscopy is performed continuously during the infusion of the enteric contrast agent in order to allow functional evaluation of the small-bowel mobility, which is helpful to determine the grade of the stenosis. The MR enterography pulse sequence protocol is the same as for MR enteroclysis, except for the MR fluoroscopy sequence, which is repeated only once to assess small-bowel distension. Table 1 shows the small-bowel imaging protocol. The preference for performing an enterography over enteroclysis examination is controversial [6-10]. MR enteroclysis combines the advantages of cross-sectional MR with those of conventional enteroclysis and enables optimal distension of the entire small-bowel, which results in accurate visualisation even of a lesion causing low-grade obstruction [8]. Moreover, MR enteroclysis delineates superficial changes better than MR enterography [7-12]. MR enteroclysis allowed detection of small-bowel neoplasms with an accuracy of 96.6% and can be an effective diagnostic technique in patients with a suspected small-bowel neoplasm [9].

Table 1. MRI parameters.

Parameter	TrueFISP		T2 HASTEa		T1 3D sequence (VIBE)b
	Axial	Coronal	Axial	Coronal	Coronal	Axial
Repetition time/echo time (ms)	3.31/1.36	3.51/1.41	800/85	800/87	4.27/1.56	4.35/1.61
Flip angle (degrees)	60	60	120	120	10	10
Field of view (mm)	320–400	320–400	320–400	430	320–400	320–400
Matrix	256×192	320×270	320×216	384×256	256×256	256×192
Parallel imaging factor	2	No/2	2	2	3	2
Section thickness (mm)	4–5	4–5	4–5	4–5	3	3
Intersection gap (mm)	0.9–1.0	0	0.9–1.0	0	0	0
Number of sections per stack	35	20–24	35	20–24	40	72
Breath-hold time per stack (s)	20–15	18–10	20–15	16	21–18	17
Number of stacks	2–3	1	2–3	1	1	2

3D, three-dimensional; HASTE, half-Fourier acquired single-shot turbo spin-echo; TrueFISP, true fast imaging with steady state precession; VIBE, volumetric interpolated breath-hold examination.

^aAxial sequence was also acquired with fat saturation achieved with the chemical shift-selective fat-suppression technique.

^bDynamic T₁ weighted imaging was performed with coronal images obtained before contrast agent administration, coronal images obtained in the arterial phases, coronal and axial images obtained in the venous phases and coronal images obtained in the delayed phase—0.1 mmol kg^–1 gadopentetate dimeglumine (Dotarem; Guerbet, Paris, France) as a bolus, at a rate of 2 ml s^–1, followed by bolus injection of 20 ml isotonic saline. Fat saturation was achieved with the chemical shift-selective fat suppression technique or frequency-selective adiabatic inversion pulse (SPIRE).

Image interpretation

The transit of the PEG solution through the small-bowel is considered normal when unimpeded flow of intraluminal solution from the duodenojejunal junction to the ascending colon is observed, with no evidence of transit delay or stenosis during MR fluoroscopy.

Bowel wall thickness >3 mm must be considered abnormal [2-12]. MR is able to assess the morphology of the lesion and the endoluminal, mural and extramural abnormalities (Figure 1); these findings are relevant for the differential diagnosis of small-bowel neoplasms.

Figure 1.

The different origins of intestinal neoplasms.

Small-bowel tumours are commonly mildly hypointense or isointense to the intestinal wall on pre-contrast T₁ weighted sequences and show a variable grade of enhancement on post-gadolinium images, and are better depicted with the use of fat suppression techniques, which increase the conspicuity between the enhancing mass and the surrounding mesenteric fat. The high signal intensity of the intraluminal fluid and mesenteric fat on true fast imaging with steady state precession (TrueFISP) and half-Fourier acquired single-shot turbo spin-echo (HASTE) images allows the depiction of tumours exhibiting intermediate signal intensity [12]. High contrast between the tumours and surrounding bright fat enables MRI to accurately demonstrate the local extension of the lesions [8].

MR fluoroscopic sequences allow the evaluation of the grade of the stenosis and the visualisation of findings similar to those obtained with barium studies, such as loop distortion, which are useful in the differentiation between mucosal, submucosal and extraparietal neoplastic origin [12]. The location and origin of a lesion, mucosal or extramucosal, can be evaluated on MR fluoroscopy images (Figure 2).

Figure 2.

MR fluoroscopy findings of small-bowel neoplasms. (a) Annular constriction with an irregular lumen (arrow) with a destroyed normal mucosa pattern (small arrows), indicative of malignant mucosal lesion (adenocarcinoma; same case as Figure 7). (b) Rounded intraluminal defect (arrow), smoothly marginated, indicative of submucosal growth (leiomyoma; same case as Figure 4). The smooth surface is a result of stretching of the overlying normal mucosa. (c) Smoothly circumscribed intraluminal defect with significant mass effect (arrow) on the small-bowel loop, which is compressed and dislocated, indicative of submucosal extramural growth (gastrointestinal stromal tumour; same case as Figure 18).

MR appearance of small-bowel tumours

Small-bowel neoplasms are histologically classified according to the World Health Organization International Classification of Tumours, which is divided into benign and malignant tumours for each kind of histological pattern [1] (Table 2).

Table 2. Classification of primary tumours of the small-bowel.

Origin tissue	Benign	Malignant
Epithelial tumour	Adenoma	Adenocarcinoma
		Mucinous adenocarcinoma
		Signet-ring cell carcinoma
		Small cell carcinoma
		Squamous cell carcinoma
		Adenosquamous carcinoma
		Medullary carcinoma
		Undifferentiated carcinoma
Endocrine tumour		Carcinoid
Mesenchymal tumour	GIST	GIST
	Lipoma	Liposarcoma
	Vascular: haemangioma, lymphangioma	Angiosarcoma
		Kaposi's sarcoma
	Leiomyoma	Leiomyosarcoma
Lymphoid		Lymphoma
Neurogenic	Schwannoma, neurofibroma ganglioneuroma

GIST, gastrointestinal stromal tumour.

Benign tumours

Adenoma

Adenomas are the most common benign tumours of the small-bowel. Grossly, adenomas present with three possible patterns: a polypoid pedunculated mass on a stalk; a sessile mass (broad-based and without a stalk); or a mural nodule within the mucosa. Adenomatous small-bowel polyps appear as intraluminal homogeneous enhancing masses on gadolinium-enhanced fat-suppressed images, confined within the boundaries of intestinal lumen. On HASTE and TrueFISP sequences, polyps appear as rounded low-signal-intensity intraluminal masses (Figure 3). MR fluoroscopy sequences show an intraluminal filling defect with mild narrowing of the lumen, which is not usually associated with proximal dilatation [8,12]. Despite the benignancy, adenomas are considered pre-cancerous lesions. Although it may not be possible to detect a focus of carcinoma within the polyp, extraserosal extension of a polyp is suggestive of malignant degeneration.

Figure 3.

MR enterography images of a 46-year-old male with ileal adenoma. (a) Transverse true fast imaging with steady state precession sequence shows a soft-tissue intraluminal mass (arrow), which appears moderately low in signal intensity, with no sign of bowel wall infiltration; note that the lesion is well circumscribed with lobulate borders and protrudes within the lumen of terminal ileum distended by polyethylene glycol solution. (b) Axial contrast T₁ weighted fat-suppressed image shows intense enhancement of the lesion (arrow) with a slender stalk (short arrow).

Leiomyoma

True leiomyomas of the small-bowel are rare mesenchymal neoplasms, and they can manifest with haemorrhage, anaemia and pain [1]. At MRI leiomyomas may have variable appearances, such as polypoid intraluminal, submucosal or extraluminal masses. However, the lesion shows benign features, with its typically homogeneous and sharp margins, and uniform post-gadolinium enhancement (Figure 4), and it is not associated with mesenteric involvement or metastases. MR fluoroscopy shows a filling defect, with round or crescent morphology (Figure 2b).

Figure 4.

MR enteroclysis images of a 48-year-old male with ileal leiomyoma. (a) Axial true fast imaging with steady state precession sequence shows a well-defined soft-tissue lesion, smoothly outlined (arrow), extending outside the intestinal wall of an ileal loop. (b) The lesion shows marked uniform enhancement after gadolinium axial contrast fat saturation T₁ weighted image.

Lipoma

Lipomas are mature adipose tissue proliferations that arise in the submucosa of the bowel wall. They usually grow intraluminally but can occasionally extend outward in the mesenteric surface, and the ileum is the more frequent location [1]. At MR they appear as sharply demarcated, sessile, small intraluminal lesions showing high signal intensity on T₁ weighted images and have signal intensity comparable to intra-abdominal fat on T₂ weighted images, with a typical loss of signal intensity on T₁ and T₂ weighted fat-suppressed images and without enhancement on post-gadolinium images [4] (Figure 5).

Figure 5.

MR enterography images of a 45-year-old female with lipoma of the ileo-coecal valve. (a) Axial T₂ half-Fourier acquired single-shot turbo spin-echo (HASTE) weighted image shows a high-signal-intensity mass (arrow) at the level of the ileo-coecal valve. (b) On an axial HASTE fat-suppressed image the mass (arrow) shows characteristic loss of signal intensity.

Haemangioma

Intestinal haemangiomas are congenital submucosal tumours, mostly located in the jejunum. They can be sessile or pedunculated [1]. Intestinal haemangiomas show low signal intensity on T₁ weighted images and marked hyperintensity on T₂ weighted images; marked nodular enhancement is seen within the tumour in the arterial phase and homogeneous enhancement in the delayed phase (Figure 6).

Figure 6.

MR enterography images of a 25-year-old female with jejunal haemangioma. (a) Axial half-Fourier acquired single-shot turbo spin-echo (HASTE) and (b) axial HASTE fat saturation images show a marked hyperintense mass (arrow) located in the jejunum, which compresses the small-bowel lumen [small arrow in (a)] with no loss of signal at fat saturation sequence. (c) Coronal post-gadolinium T₁ weighted fat-suppressed image shows marked homogeneous enhancement.

Malignant tumours

Adenocarcinoma

Adenocarcinomas are malignant tumours of the glandular epithelium. Despite their low incidence (<1% of all primary GI malignancies), they are the most common primary malignancy of the small-bowel and occur in the majority of cases in the proximal intestine, duodenum and jejunum [1]. They often metastasise to regional lymph nodes, liver or peritoneum. At MRI, adenocarcinomas may appear as infiltrative lesions, causing luminal stenosis and obstruction with pre-stenotic dilatation (Figure 7), while polypoid intraluminal masses are less common (Figure 8); ulceration is a common feature. Post-gadolinium MRI typically demonstrates heterogeneous and moderate enhancement. MR fluoroscopy sequences show sharply demarcated, circumferential narrowing of the lumen, with shouldering of the margins and mucosal destruction (Figure 2a) [12]. Adenocarcinomas tend to infiltrate the entire bowel wall and extend into the surrounding mesenteric fat tissue, inciting a local desmoplastic reaction, which is easily assessed at MRI. MRI is also able to stage liver metastases, lymph node involvement and peritoneal carcinomatosis. The comparative characteristics favouring the diagnosis of primary adenocarcinoma include that it is a solitary lesion, it is most frequently located proximally, it involves short segments of bowel and it infiltrates perivisceral fat.

Figure 7.

MR enterography images of a 63-year-old male with infiltrative adenocarcinoma of the proximal ileum. (a) Coronal half-Fourier acquired single-shot turbo spin-echo sequence shows irregular wall thickening involving a short segment of ileal small-bowel loop with intraluminal soft-tissue mass (arrow). (b) Coronal T₁ gradient echo sequence shows heterogeneous enhancement of the mass with the presence of liver metastasis (short arrow).

Figure 8.

MR enterography images of a 48-year-old male with polypoid adenocarcinoma of the jejunum. Axial true fast imaging with steady state precession sequence shows a polypoid mass of a jejunal loop (arrowhead) causing a short stenosis with ulcerations (arrow).

Carcinoid

Carcinoids are well-differentiated endocrine tumours that arise from the enterochromografin cells at the base of the crypts of Lieberkuhn, accounting for nearly 25% of primary malignant small-bowel neoplasms [5]. At MRI, carcinoid tumours appear as nodular mural thickening (Figure 9), usually associated with linear soft-tissue strands radiating towards the surrounding mesentery in a stellate appearance. On unenhanced sequences, these lesions are isointense to muscle on T₁ weighted images and isointense or mildly hyperintense to muscle on T₂ weighted images. The primary lesion shows hypervascular contrast enhancement.

Figure 9.

MR enteroclysis images of a 50-year-old male with jejunal carcinoid neoplasm. (a) Coronal true fast imaging with steady state precession shows irregular thickening of the wall with an extraluminal soft-tissue mass (arrow). Note the linear soft-tissue strands radiating towards the surrounding mesentery, indicative of desmoplastic reaction and presence of a mesenteric lymph node (short arrow). (b) Contrast coronal T₁ weighted fat saturation sequence during the arterial phase shows marked enhancement of the jejunal mass (arrow) and of the lymph node (short arrow). The primary lesion shows hypervascular contrast enhancement.

MR fluoroscopy shows solitary or multiple round, intramural or intraluminal filling defects encroaching on the intestinal lumen; if a secondary mesenteric mass is present, it can manifest with stretching, rigidity and fixation of ileal loops. Carcinoid neoplasm causes kinking of the bowel wall, with secondary narrowing of the lumen, rather than annular stenosis [3-5]. Tumours secreting serotonin usually induce a typical sclerosis and retraction of the adjacent mesenteric stroma, thus producing a sharp bend in the lumen.

Secondary manifestations include liver metastases, typically hypervascularised in the arterial phase, enlarged lymph nodes and ascites due to peritoneal seeding.

Calcifications are frequently present both in the primary mass and in the lymph node; they are easily detected at CT, but can also be observed at MRI as voids of signal.

Carcinoid neoplasm can also present as a mesenteric mass that is typically isointense to muscle on both T₁ and T₂ weighted images (Figure 10) [1,3-5].

Figure 10.

MR enteroclysis images of a 69-year-old female with ileal carcinoid neoplasm. (a) Coronal half-Fourier acquired single-shot turbo spin-echo (HASTE) sequence shows a hypointense mesenteric mass (arrow) with multiple spiculated bands (short arrows), indicative of desmoplastic reaction, and mesenteric nodes. Note that the mesenteric tissue is well demonstrated on the HASTE image because of the high contrast between the hypointense fibrous tissue and the surrounding high signal intensity of intraperitoneal fat. (b) Coronal T₁ weighted fat-suppressed image shows the heterogeneous enhancement of the mass (arrow) with prominent peripheral stranding creating a stellate pattern and infiltration of multiple small-bowel loops. (c) MR fluoroscopic sequence shows that the carcinoid tumour causes multiple irregular intramural filling defects (arrows) with stretching, rigidity and fixation of ileal loops due to desmoplastic reaction. (d) The surgical finding shows the spiculated mass involving the serosal layer causing kinking of ileal loops (arrow).

Sclerosing mesenteritis can mimic the mesenteric involvement from carcinoid tumours. Sclerosing mesenteritis commonly appears as a soft-tissue mass in the small-bowel mesentery, which may envelop the mesenteric vessels, and collateral vessels may develop over time. There may be preservation of fat around the mesenteric vessels, a phenomenon that is called the “fat ring sign” (Figure 11). This finding may help distinguish sclerosing mesenteritis from neoplastic mesenteric processes.

Figure 11.

MR enterography of a 47-year-old female with sclerosing mesenteritis. (a) Coronal true fast imaging with steady state precession sequence shows the presence of a mesenteric mass (arrow) involving small-bowel loops. (b) Coronal and axial half-Fourier acquired single-shot turbo spin-echo sequences show the preservation of fat around the mesenteric vessels—the so-called “fat ring sign”(arrow), characteristic of sclerosing mesenteritis.

Lymphoma

Lymphomas originate from lymphoid intestinal tissue (mucosa-associated lymphoid tissue) or may involve the bowel from widespread systemic disease. They represent about 20% of primary malignancies of the small intestine [6,11]. Risk factors for small-bowel lymphoma include coeliac disease, previous extraintestinal lymphoma, chronic lymphocytic leukemia and immunoproliferative small intestinal disease [6]. The terminal ileum is the most frequent site, owing to the increased amount of lymphoid tissue normally present relative to the duodenum and jejunum.

GI lymphomas manifest in three principal patterns: polypoid; infiltrating, which causes wall thickening, destroying parietal folds; or exophytic, which tends to ulcerate and fistulise (Figures 12 and 13) [11]. One characteristic feature of lymphoma is the aneurysmal dilation of the lumen, due to the loss of tone of intestinal musculature invaded and destroyed by pathological tissue [6,11]. At MR fluoroscopic sequences, an infiltrative lesion with patency of bowel lumen or a non-stenotic bowel mass is suggestive of lymphoma. Another helpful feature suggesting lymphoma is the frequent presence of mesenteric involvement with enlarged lymph nodes, which may become confluent and cause progressive narrowing of the lumen of the affected loop. Moreover, the association of splenomegaly and mesenteric and retroperitoneal lymphadenopathy support the diagnosis [6].

Figure 12.

MR enterography images of a 16-year-old male with follicular B-cell non-Hodgkin's ileal lymphoma. (a) Coronal contrast and (b) axial fat saturation T₁ gradient echo images show minimal enhancement of the mass (arrows) infiltrating the distal ileal small-bowel, which appears dilatated (short arrows). (c) Surgical specimen confirms this “aneurysmal dilatation” of the small-bowel (arrow), characteristic of lymphoma, due to loss of the muscle tone of the intestinal wall caused by lymphomatous invasion and destruction of the muscle layers and neural plexuses.

Figure 13.

MR enterography of mucosa-associated lymphoid tissue lymphoma of a 27-year-old male. (a) Axial half-Fourier acquired single-shot turbo spin-echo image shows a bulky polypoid mass (arrow) arising from the wall of distal ileum loop with ulcerations (small arrow). (b) Coronal contrast T₁ gradient echo image shows the homogeneous enhancement of the mass (arrow). (c) Photograph of the resected specimen shows a large polypoid mass with ulceration.

Smooth mural contour, diffuse or segmental bowel loop aneurysmal dilatation and absence of a distinct mesenteric or antimesenteric distribution are highly suggestive of the presence of lymphoma in coeliac disease patients (Figure 14) [6].

Figure 14.

MR enterography of jejunal T-cell non-Hodgkin's lymphoma in a 48-year-old male with a background of coeliac disease. Axial true fast imaging with steady state precession sequence shows a long segment of jejunum (arrow), with abnormal thickening, smooth margins and luminal narrowing with loss of normal mucosal folds.

A large, aggressive, ulcerated adenocarcinoma can easily be mistaken for lymphoma; however, infiltration of mesenteric fat is more likely to be seen in adenocarcinoma. Moreover, lymph node metastases in lymphoma are usually more bulky than in adenocarcinoma.

Small-bowel wall thickness and luminal stenosis are found in both neoplastic and inflammatory diseases.

Specific MRI features of inflammatory small-bowel diseases are the presence of bowel wall oedema, ulcerations, increased mesenteric vascularisation (comb sign), enhancing mesenteric lymph nodes and increased mesenteric fat. In acute inflammation, the bowel wall can have a layered pattern due to submucosal oedema, which is not seen in neoplastic diseases (Figure 15). Functional information can be used to assess the grade of bowel wall stenosis and the distensibility of the stenosis that can occur in inflammatory conditions. An increased prevalence of small-bowel carcinoma has been reported in patients with long-standing Crohn's disease involving the small-bowel mostly in the terminal ileum. A pre-operative radiological diagnosis of Crohn's cancers is almost always impossible because of the absence of characteristic features.

Figure 15.

MR enterography of a 38-year-old female with Crohn's disease. (a) Coronal T₁ gradient echo image obtained with fat suppression and intravenous contrast material shows stratified bowel wall enhancement (arrow), a finding characteristic of active inflammation with a benign cause. Moreover, engorgement of the vasa recta is present (small arrow) with increased flow to the actively inflamed bowel segment, a finding known as the comb sign. This finding may be helpful for diagnosing active inflammation. (b) MR fluoroscopy image shows long, segmental, tubular stricture of terminal ileum (arrow) with small ulcers (short arrows); this finding, known as the string sign, is indicative of stenosis due to inflammation.

Infectious diseases often cause a diffuse involvement of the jejunum and ileum, characterised by thickening of all jejunal wall layers with wall oedema (Figure 16); the findings resolve completely after therapy.

Figure 16.

MR enterography of a 46-year-old female with enteritis. (a) Coronal true fast imaging with steady-state precession and (b) axial half-Fourier acquired single-shot turbo spin-echo image show diffuse thickened wall of multiple jejunal and ileal small-bowel loops with thick valvulae conniventes (arrows) and hypersignal wall due to oedema. Fluid collection is also present. These findings are suggestive of infectious enteritis.

In intestinal tuberculosis, MRI may show circumferential wall thickening of the coecum and terminal ileum, associated with adjacent mesenteric lymphadenopathy. Wall ulcerations and peritoneal involvement can be seen; moreover, the lymph nodes demonstrate central areas of colliquative necrosis (Figure 17).

Figure 17.

MR enterography of a 51-year-old male with terminal ileum tubercolosis. (a) Axial true fast imaging with steady state precession sequence shows thickening of the terminal ileum and of the medial wall of the coecum (arrows). (b) In the axial contrast T₁ gradient echo image, note the presence of enhancement of the coecum (arrow), peritoneal micronodularity (short arrow) and adenopathy with central necrosis (arrowhead).

Gastrointestinal stromal tumours

GI stromal tumours (GISTs) can occur anywhere along the GI tract (most commonly in the stomach, followed by small intestine) and constitute the major subset of GI mesenchymal neoplasms, encompassing most tumours previously classified as smooth muscle tumours [13-18]. GISTs share some microscopic features with other mesenchymal tumours of the small-bowel (e.g. smooth muscle and neurogenic tumours) [18] but differ from the others for the typical immunohistological expression of the KIT (CD117) protein, a tyrosine kinase growth factor receptor, which is the only specific for GIST. In fact, other mesenchymal tumours do not express the KIT antigen [13-16]. Accordingly, true benign leiomyomas and also leiomyosarcomas are not derived from GIST precursor cells, and are different from GISTs in biological behaviour and immunology [13-18].

GISTs show a slow growth and can assume a broad spectrum of clinical behaviour, from benign, through incidentally detected, to frankly malignant [13].

At MRI, different growth patterns are observed: GISTs may appear as a mass protruding in an intraluminal, extraluminal or bidirectional fashion relative to the intestinal wall and lumen [15]. These tumours may exhibit significant mass effect, displacing the adjacent bowel loops [3-5], and are well shown at MR fluoroscopy (Figures 2c and 18).

Figure 18.

MR enteroclysis images of a 52-year-old female with jejunal gatrointestinal stromal tumour (GIST). (a) Axial true fast imaging with steady state precession image shows a large lobulated mass (arrow) arising from a jejunal loop (small arrow) with exophytic growth. Areas of focal high signal intensity indicate a haemorrhage present within the mass (arrowhead). (b) Coronal contrast fat saturation T₁ gradient echo image shows heterogeneous intense enhancement of the tumour; areas of low signal intensity are regions of tumour necrosis (arrows). The eccentric location is characteristic of GIST. (c) Photograph of the resected specimen shows areas of necrosis and of haemorrhage within the tumour.

Size is an important factor, as lesions >10 cm are generally malignant [13]. Malignant GISTs show slow growth, predominantly extraluminally and eccentrically, and frequently develop necrosis, haemorrhage, calcifications, fistula or infection. In contrast to other primary small-bowel tumours, if a mesenteric mass is present it usually shows a sharp outline without straining or retraction of the mesentery. The presence of necrosis and haemorrhage greatly affects the signal-intensity pattern. The solid portions of tumour are typically of low signal intensity on T₁ weighted images, high signal intensity on T₂ weighted images and enhance after administration of gadolinium. Areas of haemorrhage within the tumour will vary from high to low signal intensity on both T₁ and T₂ weighted images, depending on the age of the haemorrhage. A homogeneous pattern of signal intensity is less common.

Unlike adenocarcinoma and lymphoma, lymphatic spread does not usually occur in patients with GIST.

Leiomyosarcoma

Leiomyosarcomas arise from the smooth muscle of the wall or from small blood vessels. The appearance can be similar to that of leiomyomas except for the size which usually exceed 5 cm. Leiomyosarcomas grow slowly, predominantly extraluminally and eccentrically, and on MRI appear as large heterogeneous masses with central necrosis and haemorrhage. MR fluoroscopy shows a large extrinsic mass, displacing or distorting adjacent loops. In comparison with their benign counterparts leiomyomas, MR criteria favouring malignancy include a large size (>6 cm) and areas of necrosis or heterogeneous tissue intensity (Figure 19) [3-5].

Figure 19.

MR enterography images of a 78-year-old female with leiomyosarcoma. Contrast axial T₁ weighted image shows a large heterogeneous mass in the abdomen displacing adjacent loops of intestine (small arrows), with areas of ulceration and cavitation (arrow) within the tumour.

Kaposi's sarcoma

Kaposi's sarcoma is a systemic, multifocal neoplasm characterised by pigmented cutaneous lesions and visceral manifestations, which affects the GI tract in immunocompromised patients, occurring in up to 40% of individuals affected with human immunodeficiency virus [1,3-5]. Lesions are often multicentric and are typically seen as elevated submucosal lesions in the jejunum and ileum. Diffuse thickening of small-bowel folds can also be seen (Figure 20). Differentiation from lymphoma can be difficult; multiple polypoid nodules with retroperitoneal and mesenteric lymphadenopathy are frequently seen in patients with Kaposi's sarcoma. MR features of small-bowel tumours illustrated in this paper are summarised in Table 3.

Figure 20.

MR enterography images of a 32-year-old male with Kaposi's sarcoma. (a) Coronal half-Fourier acquired single-shot turbo spin-echo and (b) axial true fast imaging with steady state precession show diffuse nodular thickening [arrows in (a)] of jejunal loop. Note multiple submucosal lesions and broad mural ulcerations [arrows in (b)]. Distinguishing between lymphoma in human immunodeficiency virus patients and Kaposi's sarcoma at imaging may be very difficult, although the growth pattern of Kaposi's sarcoma is more focal and nodular.

Table 3. Histology, location and MRI appearance of small-bowel neoplasms.

Histology	Common site	T1 signal	T2 signal	Contrast enhancement	MR fluoroscopy
Adenoma	Jejunum	Low	High	Marked homogeneous	Intraluminal submucosal
Leiomyoma	Proximal ileum	Low	Low	Marked homogeneous	Intraluminal submucosal or extraluminal
Lipoma	Ileum	High	High; low at FS	Absence	Intraluminal
Haemangioma	Jejunum	Low	High	Marked heterogeneous	Normal
Adenocarcinoma	Jejunum	Low	Low	Mild heterogeneous	Annular narrowing
Carcinoid	Distal ileum	Low	Low	Marked homogeneous	Intraluminal submucosal narrowing–kinking
GIST	Jejunum	Low	High heterogeneous	Marked heterogeneous	Extramural
Lymphoma	Ileum	Low	Low	Mild heterogeneous	Stenosis or dilatation
Kaposi's sarcoma	Jejunum	Low	Low	Mild heterogeneous	Submucosal nodules

FS, fat saturation; GIST, gatrointestinal stromal tumour.

Conclusion

MRI can provide exquisite anatomical, functional and real-time information without the need for ionising radiation in the evaluation of small-bowel tumours. MR enteroclysis and enterography are accurate non-invasive modalities in assessing the intraluminal, parietal and extraluminal neoplastic manifestations. MR signal appearances of the lesions, combined with the contrast enhancement behaviour and the characteristic of the stenosis, can help in differentiating from other non-neoplastic diseases of the small-bowel. Radiologists should therefore be familiar with the MR appearance of various small-bowel neoplasms and their mimickers.