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Value of MR colonography for assessment of inflammatory bowel disease? Believe what you see—see what you believe
  1. C Gasche1,
  2. K Turetschek2
  1. 1Department of Medicine, Medical University Vienna, Austria
  2. 2Department of Radiology, Medical University Vienna, Austria
  1. Correspondence to:
    Professor C Gasche
    Medical University Vienna, Neues AKH Klinik Innere Medizin 4, Vienna A-1090, Austria; christoph.gaschemeduniwien.ac.at

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Should magnetic resonance colonography be used to assess colonic inflammation in known inflammatory bowel disease or for assessment of inflammatory bowel disease?

libenter homines id quod volunt credunt.” (Men willingly believe what they wish). Gaius Julius Caesar, De Bello Gallico, Book 3.

Imaging methods in inflammatory bowel disease (IBD) are used to serve two purposes: firstly, to establish the diagnosis in suspected IBD, and secondly, to gain information for correct management in known cases of IBD. To date, colonoscopy with biopsy remains the method of first choice to diagnose IBD. Discrete morphological alterations such as erythema, oedema, and granularity of the mucosa, small erosions, or aphthous ulcers can be reliably depicted by videoendoscopy and subsequently confirmed by histopathology. In established IBD however, patients and physicians are reluctant to perform repeated colonoscopies because of the invasive nature of the test and the inability to assess extraluminal complications in Crohn’s disease, including enteric fistulae or abscesses. For this purpose, cross sectional imaging methods have gained increasing importance in the past years.1 The question arises as to whether advancement in technology (multislice computed tomography or magnetic resonance imaging (MRI) scans) may also enable assessment of the mucosal inflammation in IBD, thereby potentially replacing endoscopy and biopsy at some point in the future.

This question was approached by two independent groups from Germany in this issue of Gut.2,3 The group from Essen2 reported on a two phase investigation (see page 257).2 Firstly, they established the method of MR colonography (an MRI examination with large bowel enema for luminal distension) in healthy subjects and precisely scored several imaging parameters, such as bowel wall thickening, contrast media enhancement of the bowel wall, number of lymph nodes or haustral folds, as well as imaging artefacts. Next they tested its diagnostic accuracy in patients with known IBD. Sixty eight of 73 segments were identified as inflamed (by applying a specific MRI based score and comparing it with the results of histopathology as reference); there were no false positives, resulting in sensitivity and specificity values of 87% and 100%, respectively. The authors concluded that MR colonography is a promising alternative to endoscopy in monitoring IBD patients.

The second paper in this issue of Gut3 reached a somehow different conclusion. Applying an authentic prospective design, the group from Regensburg compared MRI colonography with conventional endoscopy using bowel wall thickening and contrast enhancement as MRI determinants of inflammation (see page 250). From 154 total bowel segments investigated, the authors achieved sensitivity and specificity values of 58.8% and 91.4% in ulcerative colitis, and 31.6% and 100% in Crohn’s disease, respectively. The authors concluded that MR colonography is not suitable to adequately assess colonic inflammation in patients with IBD, with the exception of severe Crohn’s disease.

It is likely that these contrary conclusions may confuse gastroenterologists as well as radiologists. Can we use MR colonography to adequately assess colonic inflammation in known IBD or not? Do radiologists believe what they see and or do they see what they believe? Can gastroenterologists still trust radiology reports?

Both studies were done in the same geographic region, in a comparable university hospital based setting, using a similar study design. Delving into the details of both studies, however, a series of small differences arises that may have contributed to the optimistic conclusion in Essen and at the same time to the pessimistic view in Regensburg.

The group in Essen2 took advantage of the latest technological MRI equipment. They used a stronger gradient system (compared with the Regensburg group) in conjunction with the latest developments in the software sector (3D-VIBE sequence). By testing the feasibility of their technique in healthy subjects, they established sensitive measures for correctly defining normal colonic segments. Both issues may have improved discriminating normal from diseased bowel.

However, the superior results from the Essen radiologists2 may have been caused by a certain amount of selection bias in their study. Firstly, the investigators limited their trial to highly active (applying both clinical and serological markers of active disease) individuals with known large bowel inflammation. Hence they studied only sick and symptomatic cases with a high probability of significant bowel changes. Secondly, they excluded any colonoscopic normal segments from their comparative analysis by not taking biopsies from normal appearing tissue (although this is recommended in their national guidelines).4,5 Thereby, their analyses were limited to endoscopically and histologically inflamed segments in a series of active colitis patients. Lastly, the report is unclear on the method of histological grading, on the rational and procedures for developing the radiological score of inflammation, and on the numerical results in the IBD population.

The histological features of Crohn’s disease differ from ulcerative colitis. Therefore, a single histological grading system as a gold standard of inflammation is questionable. The mismatch between endoscopic disease activity and histopathology has been known for several years. Specifically in Crohn’s disease, physicians are facing a multidimensional dilemma: clinical activity correlates poorly with endoscopic activity or histopathology. In fact, most endoscopic studies failed to demonstrate a relationship between disease activity, disease severity, endoscopic findings, or the degree of inflammation in biopsies (reviewed by Geboes and Dalle6). With regard to the novel MRI based score for quantification of bowel inflammation, two independent groups of IBD patients should have been studied: one for establishing and another for validating this score.

The Regensburg group3 avoided such selection bias by enrolling consecutive IBD patients, regardless of IBD subtype, Crohn’s disease location (including non-colonic Crohn’s disease), or disease activity. Thereby, they appropriately mimicked the true clinical situation and tested MR colonoscopy rigorously against conventional colonoscopy. However, they could have improved their sensitivity in two ways: firstly, by studying the normal cut off values in a healthy population, and secondly, by using quantitative measures for the contrast to noise ratio and contrast enhancement.

When critically interpreting the findings from Essen,2 MR colonography was able to reliably depict inflamed bowel segments in patients with known and systemically active colitis, a conclusion that is quite similar to that of the Regensburg group.3 In addition, MR colonography is certainly useful in identifying extraluminal disease complications, such as fistulae, enlarged mesenteric lymph nodes, or abscesses, with the advantage of a non-invasive and radiation free examination.

Will MR colonography replace colonoscopy in the future? We do not believe so. However, it is human nature that men willingly believe what they wish. So too does the group from Essen, by blaming colonoscopy and biopsy as sources of bowel perforation in IBD.2 The fact is that bowel perforation is a rare complication that occurs in approximately 0.045% of patients undergoing diagnostic colonoscopy7 and is not related to biopsy of inflamed and thickening bowel walls. On the other hand, the technique and spatial resolution of MR colonography will be further advanced in the future and thus successfully used in IBD patients. Instead of replacing colonoscopy in the future however, we believe that MR colonography will be complementary, similar to the situation with MR cholangiography and ERCP.

Should magnetic resonance colonography be used to assess colonic inflammation in known inflammatory bowel disease or for assessment of inflammatory bowel disease?

REFERENCES

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Footnotes

  • Conflict of interest: None declared.

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