The Role of MR Enterography in Assessing Crohn's Disease Activity and Treatment Response

Abstract

MR enterography (MRE) has become the primary imaging modality in the assessment of Crohn's disease (CD) in both children and adults at many institutions in the United States and worldwide, primarily due to its noninvasiveness, superior soft tissue contrast, and lack of ionizing radiation. MRE technique includes distention of the small bowel with oral contrast media with the acquisition of T2-weighted, balanced steady-state free precession, and multiphase T1-weighted fat suppressed gadolinium contrast-enhanced sequences. With the introduction of molecule-targeted biologic agents into the clinical setting for CD and their potential to reverse the inflammatory process, MRE is increasingly utilized to evaluate disease activity and response to therapy as an imaging complement to clinical indices or optical endoscopy. New and emerging MRE techniques, such as diffusion-weighted imaging (DWI), magnetization transfer, ultrasmall superparamagnetic iron oxide- (USPIO-) enhanced MRI, and PET-MR, offer the potential for an expanded role of MRI in detecting occult disease activity, evaluating early treatment response/resistance, and differentiating inflammatory from fibrotic strictures. Familiarity with MR enterography is essential for radiologists and gastroenterologists as the technique evolves and is further incorporated into the clinical management of CD.

Figure 1

MR enterography features of active Crohn's disease. (a) Coronal T2-weighted image demonstrates wall thickening (arrow), axial T1-weighted fat-suppressed postcontrast images obtained in enteric (b) and delayed (c) phases demonstrate early mucosal ((b), arrowhead) with progressive transmural ((c), arrow) enhancement; coronal balanced steady-state free precession image (d) demonstrates mesenteric hypervascularity (arrowhead).

Figure 2

Mucosal healing on serial MRE in a patient with Crohn's disease affecting the transverse colon. (a) Coronal T1-weighted postgadolinium with fat saturation and (b) axial T2-weighted images demonstrating bowel wall thickening, mural edema, mucosal hyperenhancement, and the comb sign (arrows) on initial imaging. Subsequent MRE performed after treatment ((c) and (d)) demonstrates normalization of imaging findings (arrowheads) correlating with mucosal healing at endoscopy.

Figure 3

Diffusion-weighted imaging of active Crohn's disease. (a) Axial single-shot T2-weighted image demonstrating wall thickening and edema in the terminal ileum; (b) DWI and (c) ADC images (b = 600) demonstrating hyperintense signal on DWI and low ADC (arrow) in the same region of the terminal ileum indicating restriction of diffusion. There is also restricted diffusion in the distal ileum and appendix (arrowheads) consistent with additional areas of inflammation.

Figure 4

USPIO-enhanced MR imaging of the bowel in a patient with Crohn's disease who received ferumoxytol infusion 24 hours earlier as iron replacement therapy. (a) Coronal T2-weighted and (b) coronal T1-weighted fat-saturated postgadolinium images demonstrating wall thickening and hyperenhancement of the terminal ileum (arrows). (c) Coronal T2^∗-weighted image depicts nanoparticle accumulation in the wall of the cecum and ascending colon (low signal indicated by the arrowheads) indicating inflammatory involvement not visible on conventional MR sequences. Images courtesy of Mukesh Harisinghani, MD.

Figure 5

Simultaneous ¹⁸F-FDG PET/MR in a patient with known Crohn's disease. (a) Axial T1-weighted fat-saturated postcontrast images with mucosal hyperenhancement of several loops of small bowel in the lower abdomen consistent with active inflammation. (b) Attenuation-corrected PET images demonstrate intense ¹⁸F-FDG uptake in these bowel loops. (c) Fusion overlay images demonstrate localization of ¹⁸F-FDG avidity to the enhancing small bowel loops. Images courtesy of Onofrio Catalano, MD.