Stricturing Crohn’s disease of the small bowel: Magnetic resonance enterography activity index

 

ACTIVITY INDEX ENTEROGRAPHY

In Crohn’s disease it is difficult to establish objective indexes for assessing inflammatory activity. Thus, it is common practice to combine clinical, laboratory, endoscopic and imaging techniques to this effect. Nevertheless, it is universally agreed that a standardized method would be desirable, allowing the evaluation of disease activity in a noninvasive and well tolerated manner, within the setting of routine clinical practice.

At our center, the imaging technique of choice for studying the small bowel is magnetic resonance enterography (MRE), and we have developed our own index for assessing the degree of activity.

 

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Figure 1. Table for calculating the disease activity index used at our center.
 

This index combines intestinal (wall thickness, enhancement after intravenous contrast injection, degree of stricture, motility, mucosal alterations, wall edema) and extraintestinal alterations (adenopathies, fistulas, inflammatory masses – abscesses). The possible scores range from 0 to 12, with the aim of ensuring that they coincide with the scores of the endoscopic index which we use (the SES-CD), while likewise ranges from 0 to 12. Disease inactivity is accepted for scores of ≤2, while mild activity corresponds to a score of 3-6, and moderate to severe activity is considered for scores of ≥7. This allows rapid interpretation of the clinical conditions by all members of the departments involved in patient management. The index is based upon the following factors:

Wall thickness

Under physiological conditions, the wall thickness of the small bowel is about 2 mm, with values of under 3 mm being considered normal. Thickness is easily measured with the tools available at any workstation. In most publications analyzing the role of imaging methods in the detection of activity, wall thickness is the most useful isolated parameter.

 
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Figure 2. Measurement of intestinal wall thickness with the workstation tools (T2-weighted axial view).

 

Enhancement with gadolinium contrast

Relative enhancement of the intestinal wall in principle is regarded as an indicator of inflammatory activity. However, it has recently been shown that such enhancement is often related to microcirculatory changes associated with chronification of the lesions.

Quantitative analysis of enhancement may vary according to the magnetic field intensity, non-homogeneity, local artifacts, etc. The usual practice is to obtain several measurements in the region of interest (ROI), as well as external to the patient, in order to correct for background signal or noise. The following is a frequently used formula:

Relative enhancement = (WSI postgadolinium – WSI pregadolinium / WSI pregadolinium) x 100 x (SDB pregadolinium / SDB postgadolinium), where WSI = wall signal intensity and SDB = standard deviation of the background signal or noise.

 
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Figure 3. Definition of the regions of interest for calculating relative enhancement. T1-EG coronal sequence with fat suppression, postgadolinium.
 

In many workstations relative enhancement can be calculated automatically. It is also possible to plot color coded maps of the enhancement of the abdominal structures and enhancement curves with gadolinium contrast. In studies of this kind enhancement appears to be greater in the early phases when the disease is active.

 
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Figure 4. Automatic quantification of relative enhancement: a) postgadolinium series; b) color map of the degree of enhancement and definition of regions of interest; c) enhancement curves corresponding to the bowel segment with active disease (red) and normal intestine (blue).

 

Percentage stricture

The inflammatory process causes thickening of the wall and therefore a decrease in intestinal lumen. Percentage stenosis or stricture is easily measured with the tools available at any workstation.

 
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Figure 5. Measurement of percentage stricture with the workstation tools (T2-weighted axial view).
 

Strictures of >60% are considered significant, particularly in the presence of prestenotic dilatation. A common problem when evaluating a given patient is to determine whether the stricture is of an inflammatory or fibrotic nature. Although the contrast uptake pattern can help, it is very common for both types of condition (inflammatory and fibrotic) to coexist. Thus, identifying important strictures with disease activity may be misleading and the global score may be overestimated as a result.

Motility

MRI also allows us to obtain functional information by means of MRI fluoroscopy sequences which can be visualized as Cine-MRI. This methodology involves assessment similar to that found in the classical bowel transit technique, and coherent gradient echo sequences can be used (B-FFE, FISP, FIESTA, depending on the system) and visualized in cine format.

In general, the affected segments are characterized by increased rigidity, loss of distensibility and diminished peristalsis (Video 1-3). In some cases the functional changes are the most notorious findings for the early assessment of improvement in response to patient treatment.

 

Video 1. Normal intestinal motility in Cine-MRI.
 

 

Video 2. Reduced intestinal motility in Cine-MRI.
 

 

Video 3. Absent intestinal motility in Cine-MRI.

 

Edema

The presence of signal hyperintensity in the wall of the inflamed bowel loops in T2-weighted sequences constitutes a sign described from the first studies of Crohn’s disease using MRI.

No absolute pathological value or cutoff point has been established; as a result, signal intensity is usually evaluated in reference to that of other structures such as normal bowel loops, the psoas muscle or cerebrospinal fluid. In our case we use the psoas muscle as a reference.

 
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Figure 6. Thickening of the ileal wall (arrows) with increased intensity due to edematization (T2-weighted sequence).

 

Mucosal alterations

Mucosal anomalies are the most characteristic findings of the disease, though in principle the resolution capacity of magnetic resonance enterography (MRE) is unable to detect aphthous ulcerations. The detectable findings range from the follicular hyperplasia to ulcers (Figure 7), pseudopolyps, or generation of the cobblestone pattern (Figure 8).

 
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Figure 7. Small ulcerations of the ileal mucosa in a bowel loop presenting wall thickening
(T2-weighted axial sequence).

 

 
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Figure 8. Ulceronodular pattern (arrows) corresponding to an ileal segment in T2-weighted sequencing.

 

Adenopathies

Although subject to controversy, the analysis of adenopathies (size and enhancement after gadolinium contrast injection) may indicate inflammatory activity and be of use in distinguishing between inflammatory and stricturing presentations.

 
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Figure 9. Large adenopathies in the ileocecal region, showing enhancement with gadolinium contrast (enterographic coronal sequence).

 

Fistulas and sinus tracts

The presence of deep sinus tracts or fistulas is indicative of penetrating disease. These lesions are seen in about 15% of all patients and are correlated to the disease activity indexes.

 
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Figure 10. T2-weighted coronal sequence with fat suppression showing an enterovesical fistula (arrows).

 

Inflammatory masses – abscesses

These findings constitute clear signs of disease activity, though they are present in only 15-20% of all patients.
Inflammatory masses or phlegmons are poorly defined edematous mesenteric lesions (Figure 11), without data indicative of abscessification. Abscesses usually appear between bowel loops, though in other cases they can extend towards other spaces such as the psoas muscle (Figure 12), etc.

 
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Figure 11. Appearance of an inflammatory mass or mesenteric phlegmon (enterographic coronal sequence).
 

 
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Figure 12. Abscess of the psoas muscle (asterisk), produced by an ileal fistular tract (arrows), in T2-weighted axial sequencing with fat suppression.

 
 

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