Upcoming non‐invasive alternative methods are mainly based on cross sectional imaging, such as CT and MRI.^4,5 By means of post‐processing software, acquired CT or MRI data can be reconstructed into a virtual endoscopic view or fly through. Because of the higher clinical availability of scanners and lower costs, most approaches to date have focused on CT colonoscopy (CTC). Despite promising diagnostic results, the long term impact of CTC as a screening method remains uncertain, with the associated ionising radiation burden raising the possibility of a public health concern.^6,7 A compelling rationalisation for pursuing magnetic resonance colonography (MRC) is safety and avoidance of risks associated with exposure to ionising radiation of an otherwise mostly healthy screening population. While the diagnostic effectiveness of CTC has already been assessed in a screening cohort,⁵ MRC approaches have been evaluated only in high risk populations with a limited number of subjects.^8,9,10 In addition, most MRC protocols in the past required bowel purgation similar to the preparation for colonoscopy. However, this part of the procedure is considered very unpleasant by the vast majority of patients^11,12 and negatively impacts on acceptance levels. The requirement for bowel cleansing can be reduced by adding contrast agents to regular meals prior to the magnetic resonance examination, thereby modulating the signal characteristics of faecal material (faecal tagging).

The aim of the present study was to assess the diagnostic accuracy of a faecal tagging based MRC protocol in a screening population compared with colonoscopy serving as the gold standard.

Colonoscopy has been considered the gold standard for colorectal cancer screening. Apart from a high diagnostic accuracy, it provides the possibility of simultaneously performing polypectomy or histological sampling. However, there are some drawbacks inherent in this technique. Depending on the experience of the endoscopist, the procedure may be incomplete in up to 19% of cases, not allowing for a full assessment of the large bowel.^13,14 Furthermore, there is a small, but existing, risk of bowel perforation, which was seen in one of our patients. Hence it is controversial whether colonoscopy really represents an ideal gold standard. The underlying rationale is related to the fact that a considerable number of polyps, mostly close to haustral folds, may be missed even in back to back colonoscopies.^5,15 Thus there is a general interest in finding alternatives to colonoscopy.

Virtual colonography has been used increasingly over the past decade. There are different indications for this modality. Above all, its implementation in patients with incomplete colonoscopy is widely accepted.^16,17 Even in the presence of high grade stenoses or elongated bowel loops, which might not be passed by an endoscope, the pre‐stenotic bowel segments can be assessed by virtual colonography. However, there are only few data on the impact of virtual colonography as a device for colorectal cancer screening. The largest number of screening patients to date has been evaluated in a trial using CT technology.⁵ Pickhardt et al examined more than 1200 healthy subjects using CT and compared their results with colonoscopy. The sensitivity rates determined for the detection of adenomatous polyps >5 mm and >10 mm were 88.7% and 93.8%, respectively. These findings are almost in agreement with the results of our trial. However, multicentre trials report much lower accuracy of CT based virtual endoscopy, with a sensitivity ranging from 55% to 59% for the detection of colorectal lesions >1 cm.^18,19 However, consistency of expertise and technical uniformity may be questioned in a multicentre study.

Also, there still remains the burden of radiation exposure despite the use of low dose protocols for CT colonography.²⁰ This is even more relevant as data acquisition for CT colonography is performed both in supine and prone positions. Recently, lifetime attributable risk estimates for developing a radiation induced malignancy in a regularly screened population using CT have been as high as 1 in 50 patients.²¹ Furthermore, most current CT protocols obviate the administration of intravenous contrast because of the potential risk of anaphylactic reactions or impairment of renal function.^22,23 However, the use of intravenous contrast compounds has been shown to increase specificity rates as colorectal lesions can be more accurately distinguished from residual faecal material.^24,25 This is one of the main advantages of MRI, because intravenously applied contrast agents have a far better safety profile and can even be used in patients with renal failure.^26,27

MRC was first described by Luboldt et al in 1997.²⁸ Initial approaches were based on so‐called “bright lumen” techniques, which applied gadolinium containing enemas. Ever since, the modality has been modified and improved. One important innovation was the introduction of “dark lumen” MRC.²⁹ An enema consisting of tap water was used and additional intravenous contrast was given. It could be shown that this dark lumen technique was more accurate than the initial bright lumen approaches because a more reliable differentiation between residual faecal material and polyps was possible.^29,30 Furthermore, examination times were reduced as data acquisition only needed to be performed either in the prone or supine position. The diagnostic potential of dark lumen MRC has been assessed in two larger trials involving patients with increased risks of colorectal cancer and/or gastrointestinal symptoms.^8,9 Ajaj et al examined 122 patients using MRC and compared their findings with subsequent conventional colonoscopy.⁸ While colorectal lesions <5 mm could not be depicted in this trial, sensitivity and specificity for visualisation of lesions >5 mm were 93% and 100%, respectively. Similar findings were reported by Hartmann et al.⁹ In their study, 100 patients were included and conventional colonoscopy was again used as the reference. Adenomatous polyps were detected with a sensitivity of 84% for lesions between 5 and 10 mm (100% for lesions >10 mm). Despite the lower prevalence of polyps in our screening population, the results of the present study confirm the findings of Hartmann et al.

In addition, most hyperplastic lesions were not detected by MRC, even in retrospective re‐analysis, which is concordant with previous studies.^9,31 However, we have to consider adenomatous lesions as the main screening target as hyperplastic lesions show no or only little potential for malignant transformation.^32,33 Similarly, the relatively poor overall sensitivity of MRC in our trial needs to be discussed in this context; an overall sensitivity of less than 50% for a screening method is not acceptable. However, we found that over two‐thirds of the polyps detected by the reference standard were smaller than 5 mm (independent of their histology). Most authors claim little importance of these small lesions because of their reduced risk of malignant transformation.^21,34 In other clinical trials the benchmark for relevant lesions was even considered to be 10 mm.^5,35 Taking these guidelines into account, the sensitivity for the detection of adenomatous polyp >5 mm was more than 80% in our study. Thus the outlined MRC concept may be regarded as a suitable screening tool. However, one limitation will be flat adenomas, which are likely to remain elusive.

An important prerequisite for screening modalities is high patient acceptance. Although patient preferences were not particularly evaluated in this trial, virtual colonography may be considered an alternative to colonoscopy. Apparently, patients are less likely to undergo a screening test when negative expectations, including pain and discomfort, are involved.³⁶ Only moderate participation was determined in some screening programmes for colonoscopy in the past.^37,38 This can be indirectly confirmed by our data as almost 25% of the initial 414 patients eventually refused to undergo colonoscopy. There have been several studies evaluating patient preferences between conventional colonoscopy and virtual colonography. The outcome has been very inconsistent, with some trials claiming virtual colonography to be more accepted and other studies preferring colonoscopy.^39,40 However, there seems to be a definite potential for virtual non‐invasive modalities as there will always be a certain proportion of patients who will refuse to be screened by conventional colonoscopy. This plays an even greater role as bowel cleansing has been determined to be the most unpleasant part of colonic examinations.⁴¹ Thus the faecal tagging protocol used for MRC in the present study, which obviates bowel purgation, may be one key criterion for subjects to accept virtual MRC as a screening modality. The present technique is not without its limitations. Whenever a colorectal polyp is found by faecal tagging based MRC, patients have to undergo bowel cleansing for subsequent colonoscopy and polypectomy. However, we found that only approximately 10% of subjects showed an adenomatous polyp >5 mm who then would have had to undergo additional colonoscopy outside the clinical trial.

Clearly, there are certain limitations regarding the outlined MRC concept with faecal tagging. First and foremost, we have to be aware that MRC was not diagnostic in approximately 5% of all colonic segments because of failure of faecal tagging. It is still speculative as to whether these patients were non‐compliant with ingestion of the tagging agent or if other factors, such as bowel transit times, may be the reason for this limitation. Nevertheless, there is still a need for improving the faecal tagging outcome. Similarly, some colonic segments could not be interpreted because of motion artefacts. This was assumedly due to the length of the acquisition times and the need to perform data collection under breath‐hold conditions. With the implementation of new sequence types and image reconstruction algorithms, including parallel imaging, these artefacts will probably impede image interpretation less often in the future. Finally, a relatively large number of patients refused to undergo colonoscopy after MRC. Again, one can wonder if there is a certain bias for this patient group and whether results would have been different if these subjects had been included. However, this fact underlines the fear that some patients have of undergoing an invasive screening test.