The present study describes a method for imaging the early stages of CRC in a mouse inflammation-carcinoma model using a 3.0-T clinical MRI scanner equipped with a dedicated mouse solenoid receiver coil. The use of the Fluorinert enema allows dark lumen imaging by T1W imaging with and without intravenous contrast as well as by T2W imaging. Using T2W MRI in combination with a contrast-enhanced T1W dark lumen MRI, the early stages of tumorigenesis, starting with inflammation, through the advanced tumor growth was followed over time in the same mouse. In the early stages, T2W MRI was used to detect acute inflammation 2 to 3 days after DSS exposure. During this period, the bowel wall was relatively stiff because of the inflammation and was already distended with secretions so that the Fluorinert enema was not necessary. By 7 to 9 days, the inflammation had subsided and the Fluorinert enema became necessary to render the lumen totally dark while sufficiently distended to allow direct depiction of the mucosal wall. Intravenous injection of a gadolinium contrast agent and T1W imaging highlighted the changes in the epithelial lining due to chronic inflammation, hyperplasia, and destruction of the mucosal layer. Tumors as small as 1.2 mm3 and as early as 29 days were detected after initiation. Individual tumor growth was followed during the course of the study, and volume measurements from MRC in vivo correlated with caliper measurements of the tumors ex vivo.
Early detection remains the best method of reducing mortality from CRC. The use of animal models allows the design of new and safer methods to screen for early signs of colon cancer. In addition, these models will expand the understanding of the molecular events leading to tumor formation and could help identify new response indicators that correlate with the early stages of tumorigenesis. Finally, these animal models allow preclinical testing of new strategies for prevention and intervention.
The ability of MRI to detect the early stages of colon cancer in mice provides a safe, less-invasive method to monitor the effects of new therapeutics, to determine the optimal time for collection of samples for molecular analysis, and to correlate response indicators to a realtime response in the animal. Magnetic resonance imaging is more appropriate in an animal model than CT because the high radiation doses required for serial mouse imaging during microCT could lead to unwanted perturbations such as increased DNA damage and tumor initiation in these models. Moreover, the use of MRI in animal studies could readily be translated to the clinic. The 3.0-T MRI scanner used in this study is a clinical scanner that was adapted for mouse studies.
Magnetic resonance colonography for detecting and staging colon cancer is being explored in the clinic as an alternative to optical colonoscopy and CT colonography, which are commonly performed in humans [8,13–15
]. For either MR or CT colonography, the colon needs to be distended to allow reliable imaging of the colon wall for the assessment of the bowel wall thickness, inflammation, and reliable depiction of tumors. Presently, water-based enemas, air, or carbon dioxide are used to distend the bowels [8,12,16–18
]. Recent studies comparing water-based dark lumen MRC with colonoscopy found that the dark lumen MRC was highly accurate in the detection of colorectal masses. In these studies, colonic lesions exceeding 5 mm in size were accurately detected with specificity greater than 95%. In addition, alterations of the colonic wall associated with diverticulitis and inflammatory bowel disease were also detected [12,16
]. Lumen distention with CO2
, which may offer less distortion and dephasing artifacts, was used to detect colonic lesions with dark lumen MRC [18
]. In addition to distending the colon, a high contrast between the mucosal lining and the lumen is critical for an accurate measurement of tumors. Water enemas are also used for bright lumen MRI where T2W images produce a dark-appearing mucosal wall in a bright lumen background. A gadolinium-based enema combined with T1W images will also produce the bright lumen background. Whereas a gadolinium enema bright lumen MRC allowed a high sensitivity rate, this method has difficulty differentiating between the mucosal lining, residual fecal material, and air bubbles from colonic masses [16
]. In the present study, a bright or signal-intense wall with a dark lumen background was obtained with both T1W and T2W imaging after distention with Fluorinert, which, unlike water enemas, produced no signal to noise background. Additional contrast was provided by intravenously injected gadolinium chelates, which enhance the colon wall and tumors.
Previous studies demonstrated the value of MRC in the ApcMIN mouse model using a 7-T MRI with a Bruker DRX300 console to image tumors in the colon. A Gd-DTPA-filled silicone tube was used to aid in the identification of the tumors. Whereas tumors as small at 1.5 mm3
were detected, repeated imaging to follow progression was not reported [19
]. In the present study, a Fluorinert enema was used to distend the mouse colon without trauma to the intestinal mucosa. Fluorinert is ideal for dark lumen MRI because it is dark on both T1W and T2W images. Perfluorochemical such as Fluorinert have a high safety profile; they have been safely used for partial liquid ventilation of critically ill premature infants with severe respiratory distress syndrome, and clinical trials to assess the use of perfluorochemical for liquid ventilation have been reported [20,21
]. The repeated use of the Fluorinert enema in mice resulted in no detectable toxicity or injury to the bowel. Mice, which were treated with AOM alone and therefore not expected to produce tumors, did not produce tumors even after receiving one or more Fluorinert enemas, suggesting that no additional inflammation was induced by Fluorinert. Although some of the mice with acute inflammation up to 3 days after DSS exposure did not tolerate the enema, most likely due to the pressure of the enema on the compromised mucosal wall, repeated use of the Fluorinert enema at all other times resulted in no detectable toxicity or injury to the bowel. Whereas it is not known whether Fluorinert can cross the epithelial barrier of the colon, there were no visible indications that the enema was creating an inflammatory response in the epithelial tissue.
To efficiently translate information obtained from mouse models of human cancer, it is crucial to incorporate technologies that can rapidly monitor disease progression without requiring the sacrifice of animals at each step. By using minimally invasive methods for detecting and monitoring tumor growth that could be used in the clinic, information derived from the mouse model can more rapidly be translated to the clinic. Magnetic resonance colonography provides a safe and reproducible method for early depiction of premalignant lesions in the mouse colon. The information and efficiencies gained by studying mouse models of colon carcinogenesis with noninvasive imaging techniques such as MRC could accelerate the development of interventions that interrupt the sequence of cancer development in the colon. These interventions might find their way earlier into clinical trials, using imaging techniques similar to those described here.