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1 Department of Diagnostic Radiology, National Institutes of Health, 10 Center Dr, MSC 1182, Bldg 10, Rm 1C660, Bethesda, MD 20892-1182. rms@nih.gov).
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While this work is potentially groundbreaking, there remain a number of items that will need to be addressed in future experiments. The catheter system used to simulate bleeding did not function at extravasation rates below 0.3 mL/min, so that slower rates of extravasation could not be investigated in vivo; however, mathematical modeling did allow estimation of detectability of rates below this level. A number of assumptions were made about the time-attenuation curve of aortic enhancement and the shape of the pool of contrast-enhanced blood. Precise determination of the contrast enhancement of the normal colonic wall was difficult.
The Practice
Clinical use.—Gastrointestinal hemorrhage is a common clinical problem. In the setting of intermittent bleeding, diagnosis and treatment can be delayed. In the proposed clinical use, the authors suggest performing "CT examination for bleeding" without oral contrast material and with glucagon. Colonic insufflation may improve the sensitivity of the examination, presumably by allowing hemorrhage to occur more freely or by separating colonic debris and allowing improved visualization of the bleeding site, although this was not specifically investigated by the authors. Bolus timing at the level of the superior mesenteric artery is required because of the importance of scanning at the appropriate time following administration of intravenous contrast material. Pre- and postcontrast images are compared side-by-side to identify subtle areas of colonic hemorrhage. As with 99Tc red blood cell scanning, repeat scanning at the same sitting may help detect intermittent bleeding if the initial scan is negative. Being noninvasive, a positive finding at a "CT examination for bleeding" might be followed by therapeutic surgery, endoscopy, or angiography. The authors predict that the CT examination for bleeding will not be useful for detecting small-bowel hemorrhage.
Future opportunities and challenges.—The report by Kuhle and Sheiman is the first step in the assessment of this potential new diagnostic test. Further refinement of their mathematical model is required. Timing of the intravenous bolus of contrast material was studied but will need to be optimized in the true clinical setting. Colonic wall enhancement must be assessed in the actual target population. The value of acquiring thinner sections, such as 1-mm collimation or reconstruction intervals, must be evaluated. Investigation is required of intra- and interobserver variability of interpretation, detectability of even slower bleeding rates, the duration over which scanning should be performed if initial scans are negative, and sensitivity and specificity for different types of colonic diseases. It is also possible that these techniques could be applied to other parts of the gastrointestinal tract, such as the upper tract or the small bowel.
Summary
Using an animal model of colonic hemorrhage, Kuhle and Sheiman have shown that CT angiography can likely depict colonic hemorrhage at rates lower than those visible at conventional angiography. Future work is necessary to determine if bleeding rates on the order of those detectable at 99Tc red blood cell scanning (about 0.1 mL/min) will be within the reach of helical CT angiography.
FOOTNOTES
See also the article by Kuhle et al in this issue.
REFERENCES
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