Magnetic Resonance Imaging in Gastroenterology: Time to Say Good-bye to All That Endoscopy?

• Introduction
• Pancreaticobiliary Diseases
• Bowel Diseases
• Outlook
• References

Magnetic resonance imaging (MRI) has been described as the most important development in medical diagnosis since the discovery of the roentgen ray more than 100 years ago. The effectiveness of MRI has been extended to make it applicable in a wide variety of gastrointestinal disorders. The attention of gastroenterologists is currently focusing on pancreaticobiliary and bowel diseases. Magnetic resonance cholangiopancreatography (MRCP) has become a competitive alternative to diagnostic endoscopic retrograde cholangiopancreatography in a variety of hepatobiliary and pancreatic diseases. Magnetic resonance enteroscopy has the potential to become the preferable method for evaluating the entire small bowel; virtual colonoscopy, on the other hand, is far from the stage at which it could be promoted as a tool for general screening purposes in suspected colonic diseases. Its drawbacks include problems with standardization, implementation of the techniques in generalized settings, and patient acceptance.

Introduction

Magnetic resonance imaging (MRI) has been described as the most important development in medical diagnosis since the discovery of the roentgen ray more than 100 years ago. Even in its early stages, it became one of the major diagnostic tools in neuroradiology, and it is now being applied to virtually every part of the body. With the development of fast imaging sequences and specialized surface coils, and with improvements in the quality of abdominal images, MRI has come a long way in a short time. The effectiveness of MRI has been extended to make it applicable to a wide variety of gastrointestinal disorders. The attention of gastroenterologists is currently focusing on pancreaticobiliary and bowel diseases.

Pancreaticobiliary Diseases

The ability of magnetic resonance imaging to depict a dilated biliary tract was first demonstrated in 1986. Five years later, magnetic resonance cholangiopancreatography (MRCP) was described for the first time, representing a completely noninvasive application allowing visualization of the biliary and pancreatic ducts - similar to the images produced by endoscopic retrograde cholangiopancreatography (ERCP), but without the need for contrast media [1]. Recent technical advances have led to notable improvements in this field, with good tolerance by patients and a surprising degree of clinical acceptance by gastroenterologists.

MRCP was found to be highly sensitive (90 - 100 %) for visualization of the normal common bile duct (Figure [1]). In addition, dilation of the bile duct is always visible during MRCP [2]. In the diagnosis of choledocholithiasis, MRCP reaches a sensitivity of between 80 % and 100 % when modern techniques with strong gradients are used (Figure [2]). The sensitivity of MRCP for the detection of choledocholithiasis is superior to that of percutaneous ultrasonography and computed tomography. There are limitations to the routine use of magnetic resonance cholangiography with standard equipment for diagnosing common bile duct stones, particularly for stones smaller than 6 mm [3]. Another potential error is misdiagnosis of a stone as another type of intraluminal filling defect, such as an intraductal tumor, blood clot, or gas bubble.

Congenital biliary abnormalities are well depicted by MRCP. Extrahepatic and intrahepatic dilation is adequately visualized in patients with choledochal cysts [4]. MRCP has also proved to be useful in hepatolithiasis and for diagnosing primary sclerosing cholangitis [5].

There is currently no consensus regarding the precise role of MRCP in the clinical assessment of patients with suspected bile duct obstruction. Although the presence and site of biliary strictures can be identified by percutaneous ultrasonography, evaluating the cause of such strictures may be more difficult, and requires direct cholangiopancreatography. ERCP often only demonstrates the ducts below the site of obstruction (double duct sign); visualization of an obstructed part of the biliary tree is often not possible. In addition, opacification of undrained bile ducts places the patient at risk for cholangitis. MRCP can detect the presence of biliary obstruction and the level of the obstruction with a sensitivity of about 90 % and a specificity reaching almost 100 %. In addition, it routinely identifies the dilated biliary tree upstream of an obstruction, allowing synchronous strictures to be identified [6]. Nevertheless, the cause of such strictures may be more difficult to determine on the basis of MRCP alone. The differential diagnosis can be improved with the use of conventional cross-sectional magnetic resonance images. The role of MRCP in patients with an obstruction at the level of the ampulla has not been adequately studied. ERCP has obviously several advantages over MRCP in this group of patients, since it allows direct visualization of the ampulla, biopsy of lesions, and intraductal sonography.

Due to the small caliber of the normal pancreatic duct, evaluation of the duct has long been a major technical challenge for MRCP. Visualization of the pancreatic duct can be improved if imaging is performed using advanced techniques and after the administration of secretin. Administration of secretin is now recommended when the pancreatic duct is not evident on MRCP [7]. Secretin-enhanced MRCP can also allow visualization of possible communications between the pancreatic duct and pseudocysts in patients with advanced chronic pancreatitis. Some authors have even predicted that dynamic secretin-enhanced MRCP, with the acquisition of multiple images at short intervals after stimulation, will be able to enhance our understanding of the pathophysiology of pancreatitis. If dynamic evaluation of the bile ducts after choleretic stimulation also becomes a reality, performing an ERCP to measure sphincter pressures manometrically is likely to become obsolete.

As yet, MRCP has seldom been used in the setting of acute pancreatitis. Recent reports [8], however, have described similar results with contrast-enhanced computed tomography (CT) and nonenhanced MRCP in the diagnosis of pancreatic necrosis and acute fluid collections (Figure [3]). As techniques continue to improve, MRCP may become the principal imaging modality for patients with acute pancreatitis. MRCP has now been shown to be of clear value in the diagnosis of chronic pancreatitis [9]. In the diagnosis of pancreas divisum and cystic mucin-producing tumors of the pancreas, MRCP has been reported to be as good as, or even superior to, ERCP [10] [11] .

Undoubtedly, MRCP is the method of choice in all patients in whom ERCP examinations fail for technical reasons [12].

Bowel Diseases

Imaging of the entire small bowel is usually carried out using radiographic barium techniques, since enteroscopy is only able to examine a small section. Magnetic resonance imaging has many characteristics that would make it an ideal imaging technique for the small bowel. Unfortunately, its use is still limited due to technical and practical drawbacks. A range of both positive and negative contrast media have been investigated, but until recently, a contrast medium suitable for delineating the bowel has not been available. The lumen has to contain fluid with a long T₂ value in order to delineate the bowel when using heavily T₂-weighted techniques that are rapid enough to freeze bowel peristalsis. The value of water, with its long T₂ value, has recently been reported [13]. However, since 1 - 2 l of cold tap water need to be ingested within a very short time (about 5 min), vomiting and related aspiration remain a potential risk, particularly in patients with a small-bowel stricture. The problem of early water reabsorption might be addressed by adding nonabsorbable (methyl cellulose) material to the oral water load, in a similar fashion to small-bowel studies in conventional radiography. Alternatively, direct small-bowel intubation could be used, in an analogous fashion to conventional small-bowel enteroclysis. Once these technical problems have been solved, magnetic resonance enteroscopy is likely to become the preferred method for evaluating the entire small bowel.

Colonoscopy is the method of choice for the diagnosis of colon diseases. Continuing advances in the capabilities of the hardware used in MRI have made high-resolution data volumes available. With the aid of sophisticated algorithms and high-resolution two-dimensional imaging, three-dimensional images of the colon simulating conventional colonoscopy images can be reconstructed off-line [14]. This rapidly growing field of “virtual colonoscopy” has attracted multidisciplinary attention as a potential noninvasive test for colorectal polyps and cancer [15] [16] [17] , and it is hoped that it may overcome the many disadvantages associated with conventional endoscopy, including the method's invasiveness, patient discomfort, and the potential for iatrogenic perforation. Although substantial technological advances have been made, the challenges that lie ahead include standardization, implementation of the techniques in generalized settings, and patient acceptance [18]. As with other methods of examining the colon, virtual colonography requires standard oral colonoscopy preparation, followed by uncomfortable gas insufflation of the colon. The cleansing preparation would probably have to be repeated if conventional endoscopy was subsequently required. In addition, three-dimensional datasets are generally acquired with the patient in both the prone and supine positions, involving considerable radiation exposure in high-resolution CT colonography [19]. It is therefore not surprising that patients graded conventional colonoscopy as being better than virtual colonoscopy with regard to pain during the procedure, overall discomfort, and embarrassment [20]. Together with Debatin et al., we believe that if virtual colonoscopy has a future, it will lie with magnetic resonance (MR) rather than CT colonography [21]. Currently, MR colonography is available at only a handful of centers throughout the world. The current costs consist of the time required for a radiologist to perform the procedure and the cost of the contrast agent [22]. The need for subsequent colonoscopy for further evaluation of the many false-positive results (low specificity) and to resect polyps also has to be taken into account. Until these problems have been adequately addressed, virtual colonoscopy will remain a long way from becoming a tool for general screening purposes in suspected colon diseases. It might at best be recommended for preoperative assessment of the tumor stage of rectal cancer [23] (Figure [4]), and for evaluating the entire colon before surgery in patients with distal occlusive cancers (Figure [5]) that cannot be traversed endoscopically [24].

Outlook

Whether or not it affects the survival of endoscopy, the development of magnetic resonance imaging may well mark a historic turning-point in the field of gastroenterology. Radiologists have left the door open, inviting gastroenterologists to contribute their endoscopic expertise for the further development of virtual imaging [21]. MRI will open up tremendous diagnostic opportunities, and gastroenterologists should have a stake in it.

References

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H. E. Adamek, M.D.

Med. Klinik C Klinikum Ludwigshafen

Bremserstrasse 79 67063 Ludwigshafen Germany

Fax: Fax:+ 49-621-5034112

Email: E-mail:medclu@t-online.de

References

• 1 Wallner B K, Schumacher K A, Weidenmaier W, et al. Dilated biliary tract: evaluation with MR cholangiography with a T2-weighted contrast enhanced fast sequence.  Radiology. 1991;  181 805-808
  CrossrefPubMedSearch in Google Scholar
• 2 Soto J A, Barish M A, Yucel E K, et al. Magnetic resonance cholangiography: comparison with endoscopic retrograde cholangiopancreatography.  Gastroenterology. 1996;  110 589-597
  PubMedSearch in Google Scholar
• 3 Zidi S H, Prat F, Le Guen O, et al. Use of magnetic resonance cholangiography in the diagnosis of choledocholithiasis: prospective comparison with a reference imaging method.  Gut. 1999;  44 118-122
  PubMedSearch in Google Scholar
• 4 Adamek H E, Albert J, Weitz M, et al. A prospective evaluation of magnetic resonance cholangiopancreatography in patients with suspected bile duct obstruction.  Gut. 1988;  43 680-683
  PubMedSearch in Google Scholar
• 5 Kubo S, Hamba H, Hirohashi K, et al. Magnetic resonance cholangiography in hepatolithiasis.  Am J Gastroenterol. 1997;  92 629-631
  PubMedSearch in Google Scholar
• 6 Hintze R E, Adler A, Veltzke W, et al. Clinical significance of magnetic resonance cholangiopancreatography (MRCP) compared with ERCP.  Endoscopy. 1997;  29 182-187
  Thieme ConnectPubMedSearch in Google Scholar
• 7 Matos C, Metens T, Devière J, et al. Pancreatic duct: morphologic and functional evaluation with dynamic MR pancreatography after secretin stimulation.  Radiology. 1997;  203 435-441
  CrossrefPubMedSearch in Google Scholar
• 8 Lecesne R, Taourel P, Bret P M, et al. Acute pancreatitis: interobserver agreement and correlation of CT and MR cholangiopancreatography with outcome.  Radiology. 1999;  211 727-735
  PubMedSearch in Google Scholar
• 9 Sica G T, Braver J, Cooney M J, et al. Comparison of endoscopic retrograde cholangiopancreatography with MR cholangiopancreatography in patients with pancreatitis.  Radiology. 1999;  210 605-610
  PubMedSearch in Google Scholar
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  CrossrefPubMedSearch in Google Scholar
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  CrossrefPubMedSearch in Google Scholar
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  Thieme ConnectPubMedSearch in Google Scholar
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  PubMedSearch in Google Scholar
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  CrossrefPubMedSearch in Google Scholar
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  PubMedSearch in Google Scholar
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  CrossrefPubMedSearch in Google Scholar
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  CrossrefPubMedSearch in Google Scholar
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  PubMedSearch in Google Scholar
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  PubMedSearch in Google Scholar
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  PubMedSearch in Google Scholar
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  Thieme ConnectPubMedSearch in Google Scholar
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  PubMedSearch in Google Scholar
• 23 Brown G, Richards C J, Newcombe R G, et al. Rectal carcinoma: thin-section MR imaging for staging in 28 patients.  Radiology. 1999;  211 215-222
  PubMedSearch in Google Scholar
• 24 Fenlon H M, McAneny D B, Nunes D P, et al. Occlusive colon carcinoma: virtual colonoscopy in the preoperative evaluation of the proximal colon.  Radiology. 1999;  210 423-428
  CrossrefPubMedSearch in Google Scholar

H. E. Adamek, M.D.

Med. Klinik C Klinikum Ludwigshafen

Bremserstrasse 79 67063 Ludwigshafen Germany

Fax: Fax:+ 49-621-5034112

Email: E-mail:medclu@t-online.de