Role of magnetic resonance in bowel pathologies: Beyond inflammatory bowel disease

Abstract

Magnetic resonance imaging (MRI) has an established role for identifying and monitoring inflammatory bowel disease. This review summarizes key MRI findings and select technique considerations for bowel pathologies beyond inflammatory bowel disorders, emphasizing how imaging features within this modality can support diagnosis and assist with management. We highlight MRI features of inflammatory and infectious conditions, mechanical obstruction, neoplasms, and mesenteric ischemia. We also discuss how particular imaging techniques may improve diagnostic confidence, including diffusion-weighted imaging when contrast cannot be administered and rapid motion-robust sequences to reduce motion artifacts from peristalsis and breathing.

Key Words: Magnetic resonance imaging; Mesenteric ischemia; Bowel obstruction; Small bowel; Inflammatory bowel disease; Enteritis

Core Tip: Magnetic resonance imaging (MRI) is widely recognized for evaluating inflammatory bowel disease, but its utility extends well beyond this indication. This minireview highlights key MRI features of additional bowel pathologies, including infectious and inflammatory conditions, radiation-induced injury, fistulas, mechanical obstruction, intussusception, neoplasms, and mesenteric ischemia. We emphasize practical technique considerations, such as diffusion-weighted imaging when intravenous contrast is contraindicated and rapid motion-robust sequences like half-Fourier acquisition single-shot turbo spin-echo to mitigate peristaltic and respiratory artifacts. Recognizing these findings can enhance diagnostic confidence and guide management, particularly in pediatric, pregnant, and radiation-sensitive patients.

INTRODUCTION

Cross-sectional imaging is frequently used to evaluate patients with gastrointestinal symptoms and diagnose both small and large bowel pathologies. Initial imaging with ultrasound (US) and computed tomography (CT) can be helpful in the acute setting. However, magnetic resonance imaging (MRI) can serve as a valuable tool when further soft-tissue characterization is necessary. Furthermore, MRI can be particularly useful in patients for whom ionizing radiation or contrast administration are not recommended.

While the role of MRI has been well-characterized for identifying and monitoring inflammatory bowel disease (IBD), its role in evaluating other small and large bowel pathologies is less frequently emphasized in routine practice[1]. MRI offers increased soft tissue contrast, allowing for better characterization of lesions such as diverticula or tumors, thus improving treatment planning and patient follow-up evaluation[2]. Additionally, magnetic resonance enterography (MRE) allows for a more detailed evaluation of the bowel lumen, wall, and extra-intestinal complications, and its diagnostic role has expanded beyond IBD[3].

MRI exams consist of a variety of sequences, each providing valuable diagnostic information. Diffusion-weighted imaging (DWI) can be used in cases where contrast is not recommended, including known allergic reactions, pregnancy, or renal impairment[4]. Furthermore, cine motility and magnetization transfer MRI can detect adhesions, fibrosis, and strictures[4]. The use of MRI has increased due to its noninvasive nature, lack of radiation, and ability to visualize subtle areas of inflammation[5]. This narrative review discusses common bowel pathologies and presents key MRI findings and technique considerations for conditions beyond IBD. These include other inflammatory/infectious entities, postoperative/treatment-related conditions, obstruction, neoplasia and other related masses, and mesenteric ischemia.

The pathologies selected for this review were chosen because they represent a spectrum of non-IBD bowel conditions encountered in abdominal radiology practice for which MRI can provide clinically meaningful diagnostic or management-related information. Together, they illustrate how MRI extends beyond IBD as a problem-solving, characterization, and radiation-sparing tool across inflammatory, treatment-related, post-operative, obstructive, neoplastic, and vascular bowel disorders. By reviewing MRI’s role in these pathologies, we aim to provide a practical companion to the existing IBD-focused MRI literature. Table 1 summarizes the principal MRI sequences used in the evaluation of non-IBD bowel pathology, their key findings, contrast requirements, and main clinical indications.

Table 1 Magnetic resonance imaging sequences, contrast requirements, key imaging findings, and main clinical indications for non-inflammatory bowel disease bowel pathologies.

Condition	MRI sequences						IV contrast	Key findings	Main clinical indication
	T2/HASTE	T1	DWI	Post-Gd T1	MRE/enteric	MRA/flow
Celiac disease	√							Bowel dilatation; mural thickening; submucosal edema; jejunoileal fold pattern reversal; jejunization of ileum; lymphadenopathy	Not routine for diagnosis; problem-solving in nonspecific symptoms
Appendicitis	√		√					Dilated, fluid-filled appendix; wall thickening/T2 hyperintensity due to edema; periappendiceal inflammation; restricted diffusion	Equivocal US, particularly children or pregnancy
Diverticulitis	√	√	√					Diverticula; wall thickening; pericolonic fat stranding; phlegmon/abscess; perforation or fistula	Alternative in selected patients with suspected diverticulitis
Radiation enteritis	√			√	√		√	Wall thickening; mural enhancement; luminal narrowing; strictures; chronic obstruction	Post-radiotherapy abdominal symptoms or suspected chronic radiation enteropathy
Radiation colitis/proctitis	√			√			√	Wall thickening/T2 hyperintensity due to edema; mural enhancement; luminal narrowing; fibrosis/strictures; presacral or mesorectal fascial change	Suspected large-bowel radiation injury after pelvic irradiation
Fistulas	√	√		√			√	Fistulous tract; focal wall thickening; debris/air; fat stranding; abscess or fluid collections; pelvic-organ involvement	Define tract anatomy and associated complications
Bowel neoplasms	√	√		√	√		√	Annular/constricting lesion; polypoid intraluminal mass; exophytic mass; segmental thickening; obstruction, nodes, hemorrhage, necrosis	Detect, characterize, and stage bowel lesions and complications
Pouchitis	√	√		√			√	Pouch wall thickening > 2 mm; abnormal mural enhancement; peripouch fat infiltration; fluid collection or abscess; sinus tract or fistula	Evaluate mural inflammation and extraluminal complications after ileal pouch-anal anastomosis
Bowel obstruction	√	√			√			Dilated proximal loops; transition point; adhesions; collapsed distal bowel; visible obstructing cause when present	Radiation-sparing alternative in children, pregnancy, or high cumulative exposure risk
Intussusception	√				√			Bowel-within-bowel configuration; hyperintense intraluminal fluid; low/intermediate wall signal; perienteric edema; lead point	Problem-solving or incidental detection; assess obstruction or a lead point
Mesenteric ischemia	√			√		√	√	Mural thickening/edema; reduced or absent enhancement; mesenteric edema/ascites; vascular filling defect; pneumatosis or portomesenteric gas in severe cases	Best established for chronic mesenteric ischemia; MRA and flow MRI assess stenosis and postprandial flow

√: May be used for evaluation; Blank: Not routinely used or required for that indication; DWI: Diffusion-weighted imaging; Gd: Gadolinium; HASTE: Half-Fourier acquisition single-shot turbo spin-echo; IV: Intravenous; MRA: Magnetic resonance angiography; MRE: Magnetic resonance enterography; MRI: Magnetic resonance imaging; US: Ultrasound.

INFLAMMATORY AND INFECTIOUS CONDITIONS

Celiac disease

Celiac disease is an autoimmune enteropathy triggered by dietary gluten in people who are genetically predisposed, most often those carrying the human leukocyte antigen (HLA)-DQ2 or HLA-DQ8 haplotypes[6]. The prevalence of celiac disease in the United States has been reported to be approximately 1%[7]. Common symptoms include diarrhea, steatorrhea, malabsorption, weight loss, and bloating[6]. Diagnosis is typically established with serological testing, including anti-tissue transglutaminase and anti-endomysial antibodies. However, the gold standard for diagnosis continues to be small-bowel mucosal biopsy[6].

Because the diagnosis of celiac disease is established by serology and confirmed by small-bowel mucosal biopsy, small-bowel imaging is generally considered unnecessary in uncomplicated cases[8]. However, because celiac disease is not often considered in adults and cross-sectional abdominal imaging is frequently obtained in patients with nonspecific abdominal symptoms, recognizing imaging features is crucial[8,9]. Small-bowel findings on MRI associated with celiac disease include bowel dilatation, mural thickening, submucosal edema, jejunoileal fold pattern reversal, and jejunization of the ileum, but these are often nonspecific[9]. MRI can also reveal extraintestinal abnormalities such as lymphadenopathy, which may affect patient management[9]. Although MRI is not the primary diagnostic tool for celiac disease, Tomei et al[9] reported that abdominal MRI was becoming the first-choice imaging method, and often the only imaging modality performed, in patients with suspected celiac disease at their institution[9].

Appendicitis

Acute appendicitis is one of the most common causes of right lower quadrant abdominal pain in adult and pediatric patients that may require surgical intervention[10,11]. It is more frequently encountered in adolescents and adults with an estimated 87% of cases occurring in individuals over 15 years old[12]. Appendicitis frequently results from luminal obstruction of the appendiceal orifice, often related to an appendicolith or lymphoid hyperplasia, with neoplasm being less commonly observed[13]. When acute appendicitis is suspected, ultrasonography should be the first line imaging modality in many patients, particularly children, pregnant patients, and non-obese young adults[14]. When US findings are equivocal or there is clinic-radiological dissociation, imaging via CT or MRI is recommended[14]. The American College of Radiology Appropriateness Criteria notes that abdominopelvic CT with intravenous (IV) contrast is usually appropriate for initial evaluation of right lower quadrant pain in many adult patients[10]. However, for pediatric and pregnant patients with suspected appendicitis, MRI is often used after an inconclusive US due to the absence of radiation exposure and high diagnostic performance[10,14-16]. Furthermore, pregnant patients require timely and accurate diagnosis, as delays in management can increase the risk of fetal loss[11].

On MRI, a normal appendix can be seen as a blind-ended tubular structure arising from the cecum, usually with a diameter of less than 7 mm, a thin wall (less than 2 mm), and no periappendiceal inflammatory changes[11]. Findings of appendicitis on MRI include a dilated, fluid-filled appendix, wall thickening (often T2 hyperintense secondary to edema), and periappendiceal edema and inflammatory changes (Figures 1 and 2)[11]. Restricted diffusion of the appendiceal wall or lumen may also be observed on DWI[11]. Complications such as periappendiceal fluid collection/abscess or perforation can occur and should be assessed when present to guide patient management[11].

Figure 1 Acute appendicitis on magnetic resonance imaging. A: Coronal view of adult patient demonstrating a blind ending pouch with peripheral free high signal fluid, consistent with acute appendicitis (orange arrow); B and C: Coronal (B) and axial (C) T2 weighted magnetic resonance imaging views of pediatric patient with a thickened blind ending pouch in the right lower quadrant with surrounding enhancement and fluid, consistent with appendicitis (orange arrow).

Figure 2 Acute appendicitis in a pregnant patient. A and B: Coronal (A) and axial (B) T1 weighted images demonstrating a 30-week gestation patient with a thickened appendix and surrounding inflammation, consistent with acute appendicitis; C: Axial T1 fat suppressed weighted image of the same patient highlighting the enhancing fluid in the right lower quadrant surrounding the appendix.

Diverticulitis

Diverticulitis, or inflammation of diverticula, most commonly occurs within the sigmoid colon[17]. The demographic most affected by diverticulitis are older adults with low fiber consumption[15,17]. CT is typically the first-line imaging modality in the setting of acute abdominal pain[18]. However, MRI can be an effective alternative for some patients with suspected diverticulitis given its reported sensitivity and specificity of up to 94% and 92%, respectively[18].

A study by Buckley et al[17] provides a classification system that stages diverticulitis as mild, moderate, or severe[17]. MRI features of mild diverticulitis include evidence of bowel diverticula, bowel wall thickening, and peri-colonic fat stranding (Figure 3)[17]. Moderate diverticulitis findings include bowel wall thickening > 3 mm with phlegmon or abscess formation[17]. Severe diverticulitis findings include bowel wall thickening of > 5 mm, abscess formation > 5 cm, or perforation[17]. MRI evaluation should also include assessment of complications such as abscess, fistula, or free perforation, with DWI assisting in characterizing associated collections[18-20].

Figure 3 Sigmoid diverticulitis on magnetic resonance imaging. A: T1-fat suppressed axial magnetic resonance imaging (MRI); B: T2 MRI. Both demonstrate a thickened sigmoid colon with diverticula and surrounding edema (orange arrows). No evidence of rim enhancing fluid collection.

RADIATION-INDUCED BOWEL INJURY

Radiation-induced bowel injury refers to radiation-related inflammatory and fibrotic changes which affect the small or large intestine[21]. This type of injury most commonly occurs after abdominal or pelvic radiotherapy[21]. Different sections of bowel demonstrate variable radiosensitivity[22]. The small-bowel mucosa is particularly radiosensitive due to rapid cellular turnover, while the rectum is relatively radioresistant[22]. However, the rectum is frequently exposed during pelvic irradiation due to its fixed position and proximity to the pelvic organs[22]. Acute radiation-induced bowel injury typically develops within 3 months, while chronic injury can develop between 18 months and 6 years later[21,23].

Radiation enteritis

As cancer diagnoses continue to rise, increasing numbers of patients undergo radiotherapy and subsequently develop treatment-related complications such as radiation enteritis (acute or chronic)[22,24]. Radiation enteritis involves injury to the small intestine after pelvic or abdominal radiation therapy[23]. Clinical presentation includes abdominal pain, diarrhea, nausea, vomiting, or obstruction[23,24]. While CT is typically the initial imaging modality in emergent settings, particularly in patients presenting with obstructive symptoms, MRI offers a radiation-free alternative with superior soft tissue contrast, making it particularly valuable in patients with chronic radiation enteritis[22,24,25]. On MRI, findings may include bowel wall thickening, mural enhancement, and luminal narrowing or strictures (Figure 4)[22,24]. In the small bowel, wall thickening and mucosal hyperenhancement tend to predominate early, whereas luminal narrowing and fixed strictures are more prominent in chronic enteropathy contributing to obstruction[22,23].

Figure 4 Radiation enteritis. T1-weighted post-contrast fat-saturated magnetic resonance image highlighting a short segment of the ileum with luminal narrowing (orange arrow), submucosal enhancement and bowel wall thickening following pelvic radiation.

Radiation colitis or proctitis

Radiation colitis/proctitis refers to radiation injury of the large intestine, most commonly affecting the sigmoid colon and rectum following pelvic irradiation[23]. Evaluation is generally guided by presenting symptoms, with colonoscopy typically being performed for overt large-bowel complaints, and cross-sectional imaging with CT or MRI being preferred in patients with non-specific symptoms like abdominal pain or weight loss[21,23]. MRI findings similarly include bowel wall thickening, edema, mural enhancement, and luminal narrowing, with fibrosis and strictures representing chronic changes[24]. The earliest rectal MRI change may be increased T2 signal within the submucosa with relatively low signal in the outer wall and accompanied by strong mural enhancement[22]. With progressive injury, the rectal wall can thicken and the outer muscular layer may demonstrate increased T2 signal intensity[22]. Benign post-radiation processes such as proctitis and fibrosis may also demonstrate diffusion restriction and should be correlated with mural morphology and enhancement patterns to avoid overcalling a recurrent tumor[22]. Radiation-related changes may also extend beyond the bowel wall, including thickening of the mesorectal fascia and widening of the presacral space[22]. In the acute phase, this presacral space may appear T2 hyperintense secondary to edema, whereas in chronic phases it may demonstrate predominantly low T2 signal consistent with fibrosis[22]. Ultimately, the severity of imaging findings may correlate with clinical severity, particularly when strictures or complications such as ulceration with hemorrhage or fistula formation are present[21,24,26].

FISTULAS

Fistulas are pathologic tracts that connect adjacent epithelial surfaces and typically arise secondary to inflammatory processes involving the small or large bowel[27]. Other reported etiologies include diverticular disease, radiotherapy, rectosigmoid adenocarcinoma, or idiopathic causes[28]. Colovesical fistulas are the most common, but coloenteric and colouterine fistulas may also occur[17]. Although CT can identify colovesical fistulas, MRI is particularly useful for delineating the fistulous tract, defining its anatomic extent, and determining the underlying etiology because of its superior soft-tissue contrast and multiplanar capability[28]. On MRI, evaluation should focus on delineating the fistulous tract and associated inflammatory changes, and identifying associated fluid collections/abscess or pelvic organ involvement when present[27]. Imaging findings of a colovesical fistula include focal wall thickening, a demonstrable tract involving the bladder wall, and intraluminal debris and/or air[27]. Additional signs that support the diagnosis of a colovesical fistula include fat stranding, free fluid, and reactive lymphadenopathy[27].

MASSES

Small bowel malignancies are uncommon and account for approximately 1%-6% of all gastrointestinal tract malignancies[29]. When a small bowel neoplasm is suspected, both CT and MRI can be used for cross-sectional evaluation. However, when compared to CT, MRI offers better soft-tissue characterization, avoids ionizing radiation, and may provide more information about the nature of mesenteric small bowel tumors[29]. MRI, most commonly performed as MRE, can help detect and characterize bowel lesions, define the length of stenosis or obstruction, and evaluate extramural spread and complications[30]. Magnetic resonance enteroclysis can also be used in select cases to achieve more consistent small bowel distention and improve the visualization of subtle intraluminal lesions[29,31].

Bowel neoplasms on MRI are often described by their main growth pattern. This can include an annular constricting segment of wall thickening that narrows the lumen, a polypoid intraluminal mass, an exophytic mass arising from the bowel wall, or segmental thickening with occasional aneurysmal dilatation[29]. The differential diagnosis can then be narrowed down using the lesion’s T1/T2 signal, enhancement pattern, and additional supportive findings such as obstruction, mesenteric reaction, lymphadenopathy, and hemorrhage or necrosis[29]. The MRI features of common benign and malignant small bowel neoplasms are summarized in Table 2.

Table 2 Comparative magnetic resonance imaging features of common benign and primary malignant small bowel neoplasms.

Category	Lesion	MRI features
Benign	Adenoma	Small, well-defined, intraluminal soft-tissue lesion, usually < 2 cm, with moderate enhancement
	Lipoma	Signal intensity identical to fat, hyperintense on T1 and T2, with signal loss on fat-suppressed sequences
	Hemangioma	T2 hyperintense and enhancing; cavernous subtypes may appear polypoid or submucosal
	Leiomyoma	Well-circumscribed, submucosal smooth-muscle tumor; homogeneous focal mass with uniform enhancement greater than adjacent bowel
Malignant	Adenocarcinoma	Annular or constricting lesion narrowing the lumen, often with irregular wall thickening and obstruction
	Neuroendocrine tumor	Hyperenhancing primary lesion; often isointense to adjacent musculature on T1 and isointense to mildly hyperintense on T2; may be associated with a mesenteric mass or desmoplastic reaction
	GIST	Exophytic, heterogeneously enhancing mass that may demonstrate necrosis or hemorrhage
	Lymphoma	Variable appearance, including long-segment involvement and aneurysmal dilatation; splenomegaly and lymphadenopathy may support the diagnosis

MRI: Magnetic resonance imaging; GIST: Gastrointestinal stromal tumor.

Benign small bowel neoplasms

Common benign tumors in the small bowel include adenomas, lipomas, hemangiomas, and leiomyomas. Adenomas are typically small (< 2 cm), well-defined, intraluminal, soft tissue lesions that moderately enhance after IV contrast administration[29]. Lipomas demonstrate signal intensity identical to fat (hyperintense on T1 and T2) with signal loss on fat-suppressed sequences[29,32]. Hemangiomas appear hyperintense on T2 imaging and enhance, with cavernous subtypes sometimes presenting similarly to polypoid or submucosal lesions[33]. Lastly, small bowel leiomyomas are well-circumscribed, submucosal smooth-muscle tumors that can appear as a homogeneous focal mass with uniform enhancement greater than that of adjacent bowel[29].

Primary malignant small bowel neoplasms

Common primary malignant tumors found within the small bowel include adenocarcinoma, neuroendocrine (carcinoid) tumor, gastrointestinal stromal tumor (GIST), and lymphoma (Figure 5). Small bowel adenocarcinomas often appear as an annular or constricting lesion that narrows the small bowel lumen and may be associated with irregular wall thickening and obstruction[29]. Neuroendocrine tumors may be subtle at the primary site but can often be associated with a hyperenhancing primary lesion and a mesenteric mass or desmoplastic reaction[29]. On MRI, these tumors are often isointense to adjacent musculature on T1-weighted imaging and may be isointense to mildly hyperintense on T2-weighted imaging[29]. GISTs commonly present as an exophytic, heterogeneously enhancing mass that may demonstrate necrosis or hemorrhage[29]. Lymphomas can have variable appearances on MR imaging, including long-segment involvement and aneurysmal dilatation, with accompanying findings such as splenomegaly and lymphadenopathy further supporting the diagnosis when morphology is nonspecific[29].

Figure 5 Small bowel lymphoma on magnetic resonance imaging. A: Coronal T1 image; B: Axial T1 image; C and D: Axial T1-fat suppressed magnetic resonance imaging. All images of the abdomen of a 10-year-old pediatric patient show a homogeneously thickened small bowel loop measuring approximately 5 cm in thickness. This aneurysmal dilation of small bowel within the muscularis propria is consistent with small bowel lymphoma.

Technical considerations for MRI detection

Ultimately, small bowel masses can have variable enhancement on post-contrast MRI and be difficult to detect if their signal characteristics approximate those of adjacent bowel. Biphasic oral enteric contrast agents used for MR enterography/enteroclysis can increase luminal distention and improve bowel wall and lesion visibility by creating T1 contrast against enhancing mucosa and T2 contrast between intraluminal fluid and bowel lesions[5,31]. Peristalsis can degrade image quality and produce motion artifacts or transient pseudolesions, but using antiperistaltic agents such as glucagon can reduce motion-related artifacts and improve assessment[29,30].

POUCHITIS

Pouchitis is the most common complication of ileal pouch-anal anastomosis (IPAA)[34]. It occurs in nearly half of patients undergoing IPAA, with a higher prevalence seen in ulcerative colitis patients (Figure 6)[34]. Although the diagnosis of pouchitis is typically clinical and endoscopic, CT and fluoroscopic studies have traditionally been used to assess pouch integrity and related complications[35]. MRI serves as a useful radiation-free alternative, particularly in suspected complicated pouchitis, because it can evaluate mural inflammation and extraluminal disease[35]. Features found on MRI include pouch mural thickening greater than 2 mm, abnormal (increased) mural enhancement, peri-pouch fatty infiltration, associated fluid collections, sinus tracts, and fistulas (Figure 7)[35].

Figure 6 Mild pouchitis on magnetic resonance imaging. Axial T2 half-Fourier acquisition single-shot turbo spin-echo image of the pelvis in a female patient with a history of total proctocolectomy with ileal anal anastomosis showing a thickened ileal wall with surrounding free fluid, suggestive of mild pouchitis.

Figure 7 Active pouchitis on magnetic resonance imaging. Coronal T1 volumetric interpolated breath-hold examination fat-saturated contrast enhanced magnetic resonance imaging showing thickened and edematous submucosa of the J-pouch in a patient with acute flare of known Crohn’s disease, consistent with active pouchitis.

OBSTRUCTION

Small and large bowel obstruction account for approximately 15% of hospital admissions for acute abdominal pain in the United States and are an important cause of morbidity and mortality[36,37]. The etiology is based on an intrinsic mechanical obstruction of the lumen or extrinsic compression[37]. Common causes of small bowel obstruction include adhesions, hernias, neoplasms, and Crohn’s disease, while large bowel obstruction is typically caused by cancer, volvulus, and diverticular strictures (Figures 8 and 9)[37]. Conventional radiography and CT are usually the imaging modalities of choice when suspecting a bowel obstruction due to their speed and ability to visualize the obstruction's location, degree, and causes[36,37]. However, MRI may be a useful alternative particularly in children, pregnant patients, and patients who are at high risk for cumulative radiation exposure such as those with Crohn’s disease[36,37]. The diagnosis can be made when a transition point, or point of obstruction, distal to the dilated bowel loops is identified[36,38]. Adhesions in postoperative patients may also occasionally be visualized on T2-weighted imaging as soft-tissue bands traversing the mesenteric fat[15,36]. Oral contrast can further improve transition point visualization in suspected small bowel obstruction by distending proximal loops[36]. In addition, half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequence MRI allows for rapid scanning, reduces motion artifacts from peristalsis and respiration, and can detect bowel obstruction with high diagnostic performance[36,38]. However, large bowel obstruction can be harder to evaluate on HASTE due to the colonic lumen’s relatively low signal when compared to the typically bright small bowel lumen[36]. Other limitations of MRI include cost, availability, and the use of HASTE-only protocols being limited for evaluating the mesentery and complications such as bowel ischemia, pneumatosis, and pneumoperitoneum[36].

Figure 8 Gallstone ileus on magnetic resonance imaging. A: Coronal T1 fat suppressed magnetic resonance imaging of the abdomen demonstrating a common bile duct stone (orange arrow) and diffusely distended small bowel loops; B: Axial T2 image showing an additional stone in the distal ileal loop causing proximal obstruction (orange arrow). These findings are in keeping with gallstone ileus.

Figure 9 Small bowel obstruction secondary to adhesions. A and B: Coronal (A) and axial (B) T1-weighted magnetic resonance imaging of the abdomen demonstrating radial orientation of distended small bowel loops with mural enhancement (orange arrows) in an obstruction secondary to adhesions.

INTUSSUSCEPTION

Intussusception is the telescoping of one segment of bowel into an adjacent segment, which may result in bowel obstruction and potentially ischemia if the compression of blood vessels is prolonged or severe[39,40]. In children, intussusception is most often idiopathic with a peak incidence in early childhood[39]. In adults, intussusception is rare and is more commonly associated with a pathologic lead point such as a benign tumor or malignancy[39]. Although US is the modality of choice, particularly in children, the widespread use of CT and MRI has increased the detection of intussusception in adults and incidental transient cases[39,40]. When obtained, MRI can depict intussusception on fluid-sensitive sequences as a concentric “bowel-within-bowel” configuration with hyperintense intraluminal fluid and low-to-intermediate signal intensity of the bowel wall[39,40]. MRI may also demonstrate surrounding perienteric edema or an associated lead point, such as duplication cysts, solid masses, or polyps[39,40].

ISCHEMIA

Mesenteric ischemia occurs when there is inadequate perfusion of the small or large bowel and may be acute or chronic[41]. Major etiologies for acute mesenteric ischemia include arterial embolism, arterial thrombosis, mesenteric venous thrombosis, or non-occlusive mesenteric ischemia (Figure 10)[41,42]. If not quickly treated, acute mesenteric ischemia can progress to a transmural infarction of the bowel wall with necrosis and perforation[42]. In contrast, chronic mesenteric ischemia is mostly related to atherosclerotic mesenteric arterial stenosis and classically presents as postprandial abdominal pain and weight loss[42].

Figure 10  Mesenteric ischemia secondary to superior mesenteric vein thrombosis. A: T2-weighted fat-saturated axial magnetic resonance imaging of the abdomen demonstrating diffuse ascending and transverse colon wall thickening, submucosal edema and pericolonic free fluid at the level of the hepatic flexure; B: T2-weighted post-contrast fat-saturated axial image demonstrating occlusive thrombus in the superior mesenteric vein (orange arrow). Findings are concordant with wet ischemia secondary to superior mesenteric vein thrombosis.

While CT angiography remains the first-line test in acute mesenteric ischemia, MRI has a more established role in chronic mesenteric ischemia[42]. MR angiography can be used to assess mesenteric arterial stenosis and occlusions[42,43]. Additionally, time-resolved two dimensional phase-contrast MRI (2D PC-MRI) can quantify mesenteric flow at both baseline and after a meal challenge to evaluate impaired postprandial hyperemia[42,43]. Previously, 2D PC-MRI was not well adopted due to long scan times, operator variability, and implementation challenges, but the emerging four-dimensional flow MRI can perform scans in under 10 minutes without requiring IV contrast[42].

MRI findings of bowel ischemia can include mural thickening with submucosal edema, abnormal (reduced or absent) mural enhancement, associated mesenteric edema or ascites, and mesenteric vascular filling defects[44]. However, these findings are nonspecific and should be interpreted alongside mesenteric vascular findings and clinical presentation[44,45]. In cases with severe arterial occlusion, the bowel may appear thin with decreased or absent enhancement, reflecting infarction[41,42]. Severe ischemia may also show pneumatosis, portomesenteric venous gas, or intraperitoneal free air, although these gas-related findings can be less conspicuous on MRI than CT[42,44,45]. DWI may also demonstrate restricted diffusion in ischemic bowel when included in the protocol[46].

MRI LIMITATIONS

Despite its advantages in soft-tissue characterization and lack of ionizing radiation, MRI has important limitations that should be recognized. Compared with CT, MRI generally requires longer examination times, greater patient cooperation, and is more susceptible to motion-related degradation from bowel peristalsis and respiration[5]. Furthermore, residual intraluminal gas may also reduce image quality and routine use of MRI may be limited by cost, availability, and lower practicality in urgent or unstable clinical settings[5].

CONCLUSION

MRI can be used to evaluate a wide range of small and large bowel disorders. The lack of radiation and high soft tissue contrast make MRI a useful option in children, pregnant patients, and those requiring repeated imaging over time. Beyond IBD, MRI can aid in characterizing infectious conditions, mechanical obstruction, neoplasms, and mesenteric ischemia. Moreover, MRI can assist in identifying complications such as abscesses and detailing fistulous tracts. As scanner availability increases and imaging techniques continue to improve, the role of MRI in the evaluation of complex bowel pathology will grow.