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MRCP: examining the obstructed bile duct
  1. Royal Liverpool University Hospital,
  2. Prescot Street, Liverpool L7 8XP, UK

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Clinicians have been taken aback by the rapid developments in magnetic resonance cholangiopancreatography (MRCP), including elegant computer reconstructions of the biliary and pancreatic ducts from images acquired by magnetic resonance scanning. Pioneer endoscopists of the late 1970s and 1980s struggled to produce diagnostic retrograde cholangiograms, at some risk to the patient, and now in the 1990s images of almost comparable quality can be produced without an endoscope, without contrast and even without radiation. The technical aspects are so complex that the non-radiologist is unlikely to understand the variations such as T2 spin weighting, half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences, and rapid acquisition with relaxation enhancement (RARE). For those wishing to try, they are clearly covered in a recent leading article in this journal.1 Yet the technique is so beguiling and the images so similar to those obtained with contrast injection that clinicians will have to be wary of serious errors of misinterpretation, particularly when viewing an image in just one plane or with one technique. For instance, there are artefacts produced by flow and by adjacent structures such as vessels that can mimic luminal filling defects, and some beautifully illustrated examples have been published recently.2 Also, it should be emphasised that many clinicians will be disappointed when they demand such images from their radiology department. Technology is moving so fast that only the most recent and versatile machines are likely to have the facilities available for high quality MRCP.

Already there are reports of its use in every conceivable condition, from carcinoids3 to cholangiomas.4 There are also many direct comparisons between MRCP and endoscopic retrograde cholangiopancreatography (ERCP) where the newer technique produces sensitivities and specificities above 70%5 or even 90%,6 ,7 but the evidence of how and when MRCP should be used is emerging more slowly. The study by Adamek et al in this issue (see page 680) goes some way to defining this in the common clinical scenario of suspected bile duct obstruction. Clearly, one aim should be to avoid unnecessary ERCPs and in this context their results and those of others7 show a high negative predictive value—a normal MRC makes bile duct pathology unlikely. Unfortunately, in Adamek et al’s series there were only three patients in 60 who turned out to have ductal calculi, and we know that in clinical practice these are the most problematic ones because of the difficulty in imaging the duct by ultrasonography8 and the observation that the biliary system may well not be dilated—in distinction to malignant obstruction. Every endoscopist will tell you that stones may emerge after a sphincterotomy undertaken because of a high index of clinical suspicion in the face of a normal, high quality retrograde cholangiogram.

MRCP may be non-invasive and completely safe, but it is an expensive use of a valuable resource. Before we reach for the “MR request form” we should make sure we are already using existing evidence to exclude biliary disease by a combination of careful ultrasonography and biochemistry.9 We should not be undertaking ERCP now in patients with non-specific abdominal pain, normal liver function tests and normal ultrasonography, and so we should not be substituting MRCP either. At the other end of the spectrum, there is no point in doing MRCP in patients with unequivocal biochemical and ultrasonographic evidence of extrahepatic cholestasis—for example, the typical elderly patient with jaundice and dilated intrahepatic bile ducts. Such patients are going to benefit from the therapeutic potential of the endoscopic approach and no benefit is likely to accrue from costly duplication. Most surgeons like endoscopic stenting and biliary drainage as a preliminary to resection of pancreatic and bile duct tumours and unless this changes, ERCP will remain the method of choice for preoperative cholangiography. The quality of magnetic resonance is likely to continue to improve, and it may have a future role as a single, “one-stop” assessment of operability in such patients because of the potential to show blood vessels elegantly, as well as other structures.10 An area where MRCP has already shown itself to be invaluable in clinical decision making is the patient where endoscopic access to the bile duct is impossible, particularly because of previous surgery.11

The report from Adamek et al, together with at least 15 other publications in the first quarter of this year, helps to define the current position of MRCP. The algorithm for investigating suspected bile duct pathology, even for those clinicians fortunate to have the full range of high quality imaging and ERCP available, still starts with biochemical liver function tests and ultrasonography. Where both are normal, further biliary investigations are rarely justified. If there is clear evidence of extrahepatic obstruction and likely therapeutic potential, ERCP is the next investigation of choice, but for those in the equivocal group or where ERCP has failed, MRCP may provide a very useful alternative.


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