Metastatic breast cancer lesion (a) disappears (right) due to improperly timed scan. In colon cancer patient, the consequences of delayed imaging are a fill-in from the periphery (b), leading to underestimation of lesion size. Images courtesy of Paul Silverman, MD, MD Anderson Cancer Center.

Demand for gastrointestinal imaging was long inhibited by the accessibility of much of the tissue to endoscopy. Recently, however, improvements in imaging technology and the drive to minimally invasive (and lower cost) procedures has encouraged recognition of gastrointestinal imaging as a specialty.1 Today, endoscopy, including such traditional studies as retrograde cholangiopancreatography, is being supplanted by radiologic studies if the purpose is purely diagnostic. Further changes (and uncertainty) can be expected as new technology comes online.

CT: An indispensAble tool

As in so many other areas of the body, CT is now the standard examination for most gastrointestinal conditions, a dominance brought about by greater scanner speed, the availability of software for three-dimensional reconstruction, and oral contrast agents. Helical CT "has changed radiologists' approach to liver imaging."2 Small-bowel obstruction and ischemia are routinely diagnosed by CT,3 which also is commonly used at some centers for gastric and mesenteric angiography.3,4 Enteroclysis with CT has become standard for imaging of the small bowel at many medical centers because the optimal distention of the intestinal loops improves the sensitivity and specificity, and CT allows close examination of the bowel wall and the neighboring anatomy.5

Multidetector-array scanners, with their greater speed and thinner collimation, have caused a further revolution in gastrointestinal imaging, permitting higher-resolution images, real-time three-dimensional reconstruction, and multiphasic studies while virtually eliminating motion artifacts.4 One clear result is superior assessment and staging of cancers.

At The University of TexasMD Anderson Cancer Center in Houston, 11 multidetector CT scanners perform 320 examinations per day. "Every patient either comes to us having had a CT scan or has one here," reports Paul M. Silverman, MD, professor of diagnostic radiology and director of academic development at UTAnderson. "The main considerations are the speed and comfort of the examination and the ability to accommodate the large number of patients we see and will continue to see as the Baby Boomers age. Imaging is critical for patients with cancer, because the clinicians use the images along with the histologic findings to select the treatment regimen. And that is the point of coming to a cancer center: to have your cancer staged accurately and get the best treatment for your particular situation."

Optimizing MDCT protocols is a focus of radiology research at UTAnderson. "Multidetector scanners permit us to detect lesions more effectively, and contrast medium lets you see the lesions even better. But this is assuming you use contrast appropriately. If you do notif the scan is not timed properly in relation to contrast deliveryyou can easily overlook lesions. It therefore is critical for radiologists to know the features and basic contrast dynamics of the target organ and where a particular type of cancer is most likely to metastasize. For example, if you are looking for a pancreatic tumor, your target is small, so you need thin slices. Ovarian cancer spreads to the omentum, so you need to look there very closely. In other words, you tailor your image acquisition and your reading closely to the specific situation. It's a sort of Willie Sutton rule: you spend your efforts where the moneyor, in this case, the canceris." The UT-Anderson team has published a series of articles on imaging of specific cancers,6-8 and Silverman is the author of two books on protocols for helical9 and multidetector-array CT.10

Ultrasonography: In flux

For gastrointestinal imaging, ultrasonography is largely an adjunct to endoscopy. The introduction of 6F to 10F catheter-mounted probes and linear-array echoendoscopes has "revolutionized the diagnosis and treatment of gastrointestinal diseases that affect the submucosal bowel wall and adjacent extramural structures."11 High-resolution three-dimensional imaging is now possible.12 Among the possibilities are measurement of esophageal varices,12 evaluation of submucosal lesions12 or chronic pancreatitis,13 and staging and resectability assessment of esophageal, rectal, and pancreatic cancers.13,14 However, there are questions about whether this invasive procedure is warranted when endoscopic treatment is unlikely: the diagnostic information can often be obtained now by cross-sectional imaging modalities.14

MRI: A growing role

For many years, the gastrointestinal tract was largely ignored by MRI practitioners because its motion degraded the images. Although MRI studies generally are too slow for routine use, applications are growing, particularly to solve problems raised by other modalities. In some medical centers, MR angiography is considered the technique of choice for evaluating suspected mesenteric ischemia, assessing the resectability of pancreatic cancer, and evaluating the hepatic vasculature in potential living related liver donors and liver transplant recipients.15 Cholangiography with MR has demonstrated utility in patients with suspected obstruction, primary sclerosing cholangitis, and inadequate endoscopic retrograde cholangiograms.1 Indeed, its absence of ionizing radiation and the need for anesthesia, its superior depiction of the biliary ducts proximal to an obstruction, and its noninvasiveness make MR preferable to the classic endoscopic study in many situations.1 Magnetic resonance studies have a sensitivity and specificity exceeding 90% in the identification of common bile duct stones, which are depicted regardless of their chemical composition.1 Magnetic resonance cholangiography can depict the dimensions of choledochal cysts and is valuable for assessment and interventional planning in patients with biliaryenteric anastomoses, in whom endoscopic studies are difficult or impossible.1 Finally, MR cholangiography may be useful in patients in whom endoscopic studies have failed or produced poor results. In the view of D. Bradley Koslin, MD, of the Oregon Health & Science University in Portland, MR cholangiography may well become "the first-line examination of the biliary tree when a therapeutic procedure is of low likelihood."1

The introduction of contrast agents is increasing the value of MRI for the gastrointestinal tract. Oral agents are classified as positive (ie, the lumen of the bowel appears bright), negative (dark lumen), or biphasic (bright on T1-weighted images and dark on T2-weighted images or vice versa).16 Availability of intravascular contrast agents would enable MRI to be used for pinpointing the source of gastrointestinal bleeding.14 A technique with reportedly great promise for the assessment of inflammatory bowel disease is hydro-MRI, in which a mannitol solution is used as an oral contrast agent.17 The study provides "superior"17 images of the bowel wall, permitting distinction of active inflammation from scarring and examination of the extramural tissues. Hydro-MRI is more helpful in Crohn's disease than in ulcerative colitis, which involves the mucosa rather than the entire thickness of the wall.

Nuclear Medicine: Role for Pet?

The ability of positron emission tomography with fluorine 18-labeled deoxyglucose (FDG)-PET to identify tissues with high metabolic rates (which often are malignant) permits identification of many primary tumors and metastases. Approximately 20% of patients with colorectal cancer have metastases at diagnosis, and the recurrence rate after apparently curative resection is 30% to 40%, suggesting a role for FDG PET. Although the study is not the preferred means of diagnosis of the primary tumor, it is more sensitive than helical CT in finding hepatic (89% to 95% versus 71% to 84%) and extrahepatic (87% to 92% versus 71% to 86%) metastases.1 PET may also have a role in patients with a high serum carcinoembryonic antigen concentration and an occult primary tumor and has been used in assessing operability; in one series of 249 patients, PET revealed unexpected lesions that changed the treatment plan in 26%.1 Whether PET is superior to multidetector-array CT is not yet clear. Two drawbacks of FDG-PET in evaluating colorectal cancer are difficulty in identifying lesions smaller than 1 cm and the high false-positive rate in the presence of inflammation, such as in patients with diverticulitis.1

The Future

Some roles remain for traditional methods of gastrointestinal imaging. The double-contrast barium study, with its ability to illuminate fine mucosal detail, is valuable for the initial diagnosis of ulcerative colitis or Crohn's disease.18 In this situation, the cross-sectional imaging modalities have a complementary role, depicting such pathology as intra-abdominal abscesses. Double-contrast barium studies also are important at some centers for the diagnosis of colorectal cancers,19 although they are being displaced by virtual colonoscopy (reviewed elsewhere in this issue). Yet another application of a classic technique is the use of fluoroscopy to confirm and classify gastrointestinal fistulas.20 Here again, cross-sectional studies are making significant inroads.

The gastrointestinal tract appears to be an excellent subject for molecular imaging. An interdisciplinary team at the University of Vienna took advantage of the propensity of human tumors and their blood vessels to overexpress the vascular endothelial growth factor receptor.21 Those investigators labeled recombinant human VEGF with iodine 123, administered it to 18 patients with known gastrointestinal tumors, and acquired SPECT images 1.5 hours later. Primary pancreatic cancers were imaged in seven of nine patients, with lymph node, liver, and lung metastases being seen in several. Half of the cholangiocarcinomas and hepatocellular carcinomas were likewise depicted. Further work will be necessary to determine if this technique will be useful for diagnosis or staging of gastrointestinal cancers. Nuclear medicine studies using specific labeled peptides,22 antibodies,22and oligonucleotides23 are being tested clinically.

Conclusion

Changes in the protocols for gastrointestinal imaging seem likely to continue. A new noninvasive option whose role is being defined is capsule endoscopy. Its advantages are the short time the patient remains at the hospital and its painlessness. The study may prove especially useful for evaluating small bowel disease and identifying the source of chronic or intermittent bleeding, although it generally is not given to evaluate polyps or tumors because of the risk of intestinal obstruction.24 Where this device fits in and the relative roles of multidetector CT and PET or MRI will not be known until more data become available.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Imaging Economics.

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