Small Bowel Imaging from Stepchild of Roentgenology to MR Enterography: Part I: Guidance in Performing and Observing Normal and Abnormal Imaging Findings

Abstract

MRE has become a standard imaging test for evaluating patients with small bowel pathology, but the indications, interpretation of imaging findings, methodology, and appropriate use must be standardized and widely known. Several signs of small bowel damage in inflammatory and non-inflammatory small bowel pathology include strictures, abscess, inflammatory activity, sinus tract, wall edema, fistula, mucosal lesions, and mesentery fat hypertrophy, all of which are widely and accurately explained by MRE. MRE is a non-invasive modality that accurately assesses the intra-luminal, parietal, and extra-luminal small bowel. The differential MRE appearance allows us to distinguish between different small bowel pathologies, such as neoplastic and non-neoplastic small bowel diseases. The purpose of this paper is to present the MRE technique, as well as the interpretation of imaging findings, through the approach of a rigorous stepwise methodology.

Keywords: Crohn’s disease; MR enterography; bowel wall enhancement patterns; small bowel.

Figure 1

Surgical images (A,B,D) show that compared to the ileum, the jejunum has less fatty mesentery (asterisk in A,C) and longer and straighter vasa recta (green arrowheads in B,D).

Figure 2

Normal MRE features of small bowel–coronal gradient echo sequences (FIESTA) (A): Compared to the ileum, the jejunum has less fatty mesentery (A) and more valvulae conniventes (B,C); yellow area, mesentery of the jejunum; green area, mesentery of the ilium. Red line, root of the mesentery.

Figure 3

Normal MRE features of small bowel–coronal gradient echo sequences (FIESTA) (A) Compared to the ileum, the jejunum has less fatty mesentery and prominent valvulae conniventes. In (A), an imaginary curved line with a left convexity divides the abdomen in the region hosting the jejunum from that hosting the ileus. Coronal (B) and axial (D) FIESTA sequences (B): jejunum prominent valvulae conniventes (yellow arrowhead). Coronal (C) and axial (E,F) FIESTA sequences: The wall is normally <3 mm thick.

Figure 4

Axial (A) FIESTA sequence: Wall thickening of 9mm (green arrowhead). Coronal T2-weighted (B,C) wall thickness (green arrowhead in B,C). Normal wall thickness <3 mm (yellow arrowhead in A–C).

Figure 5

Coronal (A) FIESTA sequences do not show fibrofatty proliferation of normal mesenteric fat with kissing bowel loops. Coronal T2-weighted images (B) show fibrofatty proliferation (creeping fat), i.e., hypertrophy of the mesenteric fat, which separates bowel loops as a sign of mesenteric inflammation in Crohn’s disease. Affected bowel loops are separated by focal/regionally increased fat (fibrofatty proliferation or creeping fat): Opposed green and yellow arrowheads in (B).

Figure 6

Normal small bowel parameters on MR enterography: The number of folds per 2.5 cm varied from 4.6 in the jejunum to 1.5 in the terminal ileum [15]. Folds: red spheres. Normal small bowel diameter: yellow line [15].

Figure 7

This scheme was borrowed from the enhancement patterns observable in CT enterography after the administration of an intravenous contrast medium: (1) target sign: indicates inflammation or ischemia of the bowel where the inner and outer high-enhancement layers correspond to the hyperemic mucosa and serosa, respectively, while the poor enhancement central layer represents the edematous or fat submucosa. (2) Homogeneous hyperenhancement or white pattern: global wall intense enhancement equal to or greater than that of venous vessels. (3) poor enhancement of the bowel wall is considered when the bowel wall is enhanced to be similar to that of the muscle. In 1A: the axial T2-weighted MRE image shows small-bowel wall thickening, mural edema (hyperintense mural signal intensity), and luminal narrowing (active Crohn’s disease phase). 1B,1D: the axial T2-weighted MRE image (1B) and fat-suppressed T2-weighted image (1D) show small bowel poor wall thickening and submucosa fat (hypointense mural signal intensity in fat-suppressed T2-weighted image) in the chronic Crohn’s disease phase. (1C) Axial contrast-enhanced fat-suppressed T1-weighted show stratified mural hyperenhancement. (2): coronal contrast-enhanced fat-suppressed T1-weighted MRE images (2E,F) show global small-bowel mural hyperenhancement. (3): poor enhancement. 3G: Crohn’s disease with active inflammation; axial contrast-enhanced fat-suppressed T1-weighted images show the resolution of the mural edema and hyperenhancement.

Figure 8

Non-functionally significant stenosis: there is an upstream bowel dilatation of less than 3 cm. The coronal FIESTA MRE image shows a perivalvular thick-walled ileal segment (approximately 6 cm long) with evident luminal narrowing and rigidity, not expandable with peristalsis but constantly fixed, albeit without upstream bowel dilation.

Figure 9

Condition of the “damned loop” in functional stenosis. We show the decomposition of the frames (1–8) of the Cine-MRE, which demonstrates how the peristaltic activity fails to overcome the stricture (yellow arrowheads in 1,2), despite the attempts of the upstream loop to promote intestinal transit by contracting energetically, quickly, and vigorously as a species of a damned loop (green arrowheads).

Figure 10

Very functionally significant stenosis: upstream bowel dilatation greater than 3 cm with constant and permanent ectasia of upstream bowel loops before high-grade bowel stenosis. Contrast-enhanced fat-suppressed T1-weighted image (A): The freehand red curve represents a relatively long, markedly stenotic segment of the ileum with marked enhancement, rigid and non-expandable. Permanent ectasia of upstream bowel loops before high-grade bowel stenosis in coronal (B,C) and sagittal (D) MRE in the same patient.