Magnetic Resonance (MR) Colonography: Another “Virtual” Tool for the Detection of Colorectal Cancer
During a traditional colonoscopy, a colonoscope is inserted into the colon through the rectum.
Image used with permission from the National Cancer Institute. Copyright 2006, Terese Winslow.
Magnetic resonance colonography, also called MR colonoscopy, is an emerging technique for detecting colorectal cancer.
Most colorectal cancers develop from pre-cancerous polyps or growths, which can be removed if detected early. The gold standard for screening in high-risk, asymptomatic individuals is colonoscopy, with the insertion of a colonoscope ― a long tube with a light ― through the rectum and into the colon to project images onto a video screen. Recently, computed tomography (CT) colonography, popularly called “virtual colonoscopy,” was introduced as a less invasive alternative. The use of MR imaging is another virtual option.
How It Works
Anti-spasmodic medication is administered prior to MR colonography to prevent colonic spasms, which would distort the image. A rectal tube is inserted and the colon is distended with a solution that contains a contrast medium. Images of the colon are displayed using a conventional magnetic resonance imaging (MRI) scanner.
During early MR colonography procedures, patients underwent bowel cleansing, as residual stool or fluid in the colon can be mistaken for lesions, or obscure them. More recently, “fecal tagging” has replaced bowel cleansing, whereby MR contrast media is used to render fecal masses “invisible”.[1]
Who Might Benefit
Colorectal cancer is the second-leading cause of death from cancer in Canada;[2] 17,600 new cases were diagnosed in 2005 and approximately 6,500 Canadians died from the disease.[3]
MR colonography may be useful for the evaluation of segments of the colon that cannot be visualized with conventional colonoscopy, or for symptomatic patients who cannot undergo conventional colonoscopy. Its value in screening asymptomatic individuals has not yet been determined.[1]
Advantages and Disadvantages
Like CT colonography, MR colonography has similar diagnostic accuracy to colonoscopy in detecting colorectal cancer.[4,5] However, MR colonography is unable to detect lesions smaller than 5 mm, although polyps of that size are usually benign.[6] About 20% of patients who undergo MR colonography scanning will also require conventional colonoscopy and biopsy to confirm the diagnosis and for polyp resection.[1]
Compared with colonoscopy, there is no need for sedation or analgesics during MR colonography. There is also a lower risk of perforation, and all colon segments can be evaluated. An advantage of MR over CT colonography is the absence of ionizing radiation. Disadvantages include the longer examination times, patient discomfort associated with liquid enema retention, and the increased costs and more limited availability of MRI units. More comfortable air-based colonic distention methods may eventually replace fluid-based methods.[1]
Regulatory Status
A number of MRI scanners are licensed in Canada for diagnostic imaging purposes.
Cost
A US estimate of per-patient charges for MR colonography is US$1,100 to US$1,400, compared with US$600 to US$1,100 for CT colonography, and US$700 to US$1,000 for conventional colonoscopy.[1]
References
[1] Magnetic resonance colonography for colorectal polyp and cancer detection. [database online]. Plymouth Meeting (PA): ECRI; 2004.
[2] Canadian Cancer Society, et al. Canadian cancer statistics 2006. Toronto: The Society; 2006. Available: http://www.ncic.cancer.ca/vgn/images/portal/cit_86751114/31/23/935505938cw_2006stats_en.pdf.pdf
[3] Screening for colorectal cancer. Ottawa: Health Canada; 2007. Available: http://www.hc-sc.gc.ca/iyh-vsv/alt_formats/cmcd-dcmc/pdf/rectcancer_e.pdf
[4] Purkayastha S, et al. Clin Radiol 2005;60(9):980-9.
[5] Haykir R, et al. World J Gastroenterol 2006;12(15):2345-50.