Role of Abdominal Imaging in the Diagnosis of IBD Strictures, Fistulas, and Postoperative Complications

Computed Tomography Enterography

CTE technique is tailored to maximize small bowel wall assessment. Adequate technique requires that after a 4-hour fast, the patient ingests a large volume of neutral enteric contrast material that contains sugar alcohols or osmotic laxatives, agents that fill the bowel lumen and prevent absorption of water and ultimately provide optimal bowel distension. Nearly 1300–1800 cc of oral contrast material is ingested over 30–60 min. These neutral enteric contrast agents have the attenuation of water and thus increase the conspicuity of the enhancement of actively inflamed bowel wall following the administration of intravenous (IV) contrast material while positive oral contrast material such as is often used for routine CT abdomen and pelvis can obscure pathologic mural enhancement. Additional differences from a routine CT abdomen and pelvis include images obtained at 45 s after infusion of contrast material begins (enteric phase), use of thinner axial images, sagittal and coronal reformations, maximum intensity projection images, and extension to the perineum (alerts to perianal disease). CTE has a high sensitivity (>90%) and specificity (∼90%) for the detection of small bowel CD, and it has been validated against clinical, histologic, and endoscopic assessments. The diagnostic accuracy in patients with suspected or established CD is comparable to that of MRE, with CTE reported to have higher and consistent image quality. In addition to accurate disease detection in patients with suspected CD, CTE has been shown to play a vital role in the management of patients with known CD as it can detect clinically occult inflammation and can detect and help map out complications such as fistulas, abscesses, and strictures. Ultimately the findings at CTE frequently alter treatment plans and impact corticosteroid usage.

Magnetic Resonance Enterography

Standard CD MRE technique is less standardized across the country than CTE, but in general the most useful sequences include unenhanced T2-weighted images and gadolinium-enhanced T1-weighted sequences in multiple planes of acquisition. Similar to CTE, patients are asked to ingest a large-volume oral agent, and IV contrast is utilized. Post-gadolinium images are obtained 45 s after injection followed by dynamic images. Spasmolytics are useful for reducing bowel peristalsis and motion artifacts. The most frequently used oral contrast agents for MRE are considered “biphasic”, that is, they are low signal intensity on T1-weighted images (thus increasing conspicuity of inflammation following administration of contrast material) and they are hyperintense on T2-weighted images, allowing for better assessment of the bowel wall thickness. The T2-weighted images are often obtained both with and without fat saturation; those with fat saturation allow for better detection of high T2 signal mural and perienteric inflammation/edema, and those without fat saturation help with accurate bowel wall assessment and allow better visualization of the vasa recta. Images are initially obtained during the enteric phase at 45 s after the initiation of the contrast material infusion. Because of the lack of ionizing radiation, many sequential acquisitions can be obtained before and after injection followed by dynamic images. Delayed imaging at 7 min may allow differentiation of inflammatory and fibrotic components of strictures. Gadolinium-enhanced T1-weighted images allow interrogation of bowel wall enhancement characteristics, transmural ulcers, fistulas, sinus tracts, comb sign (dilated vasa recta), and perienteric abnormalities, whereas bowel wall thickening and mural edema are better evaluated in the T2-weighted images. MRE, similar to CTE, has high sensitivity (>90%) and specificity (>90%) for the diagnosis of active small bowel inflammation.

MRE protocols may now also include diffusion-weighted images (DWIs). This technique provides quantifiable information that may complement T1/T2-weighted images and can be acquired without IV contrast material. High signal intensity on DWIs and corresponding decrease in the apparent diffusion coefficient (ADC) suggest restricted diffusion of water molecules due to active inflammation. A prospective, noninferiority study in 44 patients with known or suspected CD compared precontrast sequences alone with DWIs to conventional MRE with gadolinium contrast. In this study, precontrast sequences with DWIs identified active inflammation in the terminal ileum with a sensitivity and specificity of 93% and 67%, respectively. DWIs can be considered in CD patients who require serial magnetic resonance (MR) assessments, are pregnant, or have impaired renal function and therefore a desire to limit gadolinium exposure.

MRE scoring systems have been developed to allow quantitative assessments of lesions for target-directed medical therapy in clinical practice and trials. These have been developed in comparison to an external reference of either ileocolonoscopy (Magnetic Resonance Index of Activity [MaRIA] score and Nancy score) or histopathology of a resected specimen (Crohn's disease MRI Index [CDMI] score). Of these, the MaRIA and CDMI have been validated, with only the MaRIA score shown to be responsive and reliable in assessing the response to therapy in patients with CD. An MRE-based scoring system utilizing DWI has also been developed, the Clermont score, which can be obtained without bowel preparation or colonic enema. Finally, recent efforts have also centered on the development of a scoring system that accounts for clinical, endoscopic, and radiological information to assess the burden of disease, the Lémann index. This index accounts for previous operations, strictures, and penetrating lesions across the entire gastrointestinal tract. This system will likely need to be simplified before widespread use in clinical practice if feasible and practical.

Ultrasonography

Ultrasonography (US) in IBD has been extensively studied in Europe and selective North American IBD academic centers. The unenhanced gray-scale US protocol is often performed as a two-step process with 3–8 mHz probes followed by higher frequency linear probes (7–9 mHz). The lower frequency probes provide a better overview of the abdomen and can sometimes more completely image larger lesions in the intestine. The curved or linear higher frequency transducers offer a more detailed transabdominal evaluation of the small intestine, allowing visualization of the bowel wall layers. A common imaging approach is to examine the entire colon in a retrograde fashion from the rectum to the cecum, followed by evaluation of the distal ileum, and subsequent systematic surveillance of the remainder of the small bowel in all four quadrants of the abdomen. If pathology is detected, bowel wall thickness should be documented, as well as presence or absence of mural stratification, luminal narrowing/stenosis, and/or bowel dilatation. In addition, the motility pattern and the presence or absence of fibrofatty proliferation, and perienteric lymph nodes should be noted. The pooled per-patient sensitivity and specificity for the diagnosis of CD are 85% (95% confidence interval [CI], 83%–87%) and 98% (95% CI, 95%–99%), respectively, at IBD centers with experience with bowel US, with highest accuracy at the terminal ileum and left hemicolon. Sonographic technology has since evolved with the expanded use of Doppler ultrasound. This tool quantitates the amount of blood flow in the wall of the bowel and surrounding mesentery by the absolute velocity of flow and density of mural blood vessels. US measurements have been used to assess response to medical treatment.

Contrast enhanced ultrasonography (CEUS) protocols contain the intravenous administration of contrast containing microbubbles filled with sulfur hexafluoride (SonoVue and Lumason, SV, Bracco, Italy), octafluoropropane (Definity and Luminity, Lantheus Medical Imaging, USA), perflutren (Optison, GE Healthcare, USA), or perfluorobutane (Sonazoid, GE Healthcare, USA). The contrast agents demonstrate tissue perfusion with time blood-pool imaging. These agents have not yet been approved by the Food and Drug Administration in the United States for use in IBD patients. CEUS has been shown to perform better than gray-scale US or color Doppler at detecting active CD, with 93.5% sensitivity, 93.7% specificity, and 93.6% overall accuracy. A recent meta-analysis of eight studies utilizing CEUS for the detection of active CD demonstrated a pooled sensitivity of 0.94 (95% CI 0.87–0.97) and specificity of 0.79 (95% CI 0.67–0.88).

Small intestine contrast ultrasonography (SICUS) is another modification of unenhanced gray-scale US involving the ingestion of oral contrast material (usually 250–800 mL of polyethylene glycol), after an overnight fast. This has been shown to improve the detection of proximal small bowel inflammatory lesions and strictures compared to conventional US. A scoring system, the quantitative sonographic lesion index for CD has been developed, that has been shown to be responsive to medical therapy with anti-TNF-α agents.