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Technical Innovation |
1 All authors: Department of Diagnostic Radiology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433.
Received May 31, 2001;
accepted after revision November 15, 2001.
Address correspondence to J. S. K. Goh.
Introduction
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Methylcellulose is a compound familiar to radiologists because of its use for double-contrast barium studies of the small bowel. A search through MEDLINE [7] revealed the first report of its usage in MR imaging in 1992, albeit as an upper gastrointestinal contrast agent [8]. The compound we used was a pharmaceutical-grade sodium salt of carboxymethylcellulose (BDH Laboratory Supplies, Poole, England). The compound comes in the form of granules, so adjusting the viscosity of the solution for optimal distention was easily done.
We conducted a preliminary study to determine the optimal volume and concentration of methylcellulose. Exact viscosity measurements were not performed. Instead, concentrations by weight and volume were titrated using the guidelines given by the methylcellulose manufacturer. We found that the normal concentrations of 0.5-1.0% used in small-bowel enteroclysis provided inadequate viscosity. We decided that solutions of 2.5%, 5%, and 7.5% were most likely to be suitable for our purpose. Solutions over 7.5%, such as 10%, were found to be too viscous to be easily delivered via a syringe; solutions with higher concentrations also resulted in severe clumping and inadequate distention. Therefore, we did not evaluate them.
The carboxymethylcellulose was mixed in a sterile water solution and left overnight to dissolve. During introduction of the contrast agent, we placed the patient in the right lateral decubitus position on the MR imaging couch to discourage contrast flow into the descending colon. All our patients had received bowel preparation similar to that for a barium enema study. After an initial rectal examination to assess the tumor, we used two 60-mL syringes with tapered tips to deliver 80-120 mL of the methylcellulose directly into the rectum. The patient was then turned supinely for the positioning of the pelvic phased array coil and for MR imaging.
Initially, our patients were randomly selected to receive one of the three solution concentrations. We then instilled 60 mL of methylcellulose solution and had the images reviewed independently by two radiologists. The volume of contrast material instilled was determined by trial and error. An initial thick-section sagittal screening scan of the pelvis was obtained using a half-Fourier acquisition single-shot fast spin-echo sequence. Additional contrast material was then administered as needed, and the screening scan repeated. Bowel paralytic agents were not routinely used in our patients.
To assess image quality, we assigned an overall grade to each study that was based on a number of subjectively determined factors, including the uniformity of distribution of the contrast material in the rectum (i.e., absence of clumping and ability to remain stable, continuous column of contrast material within the rectum); ability of the reviewers to continue to visualize the haustral folds (as a mark of overdistention); degradation of image quality caused by susceptibility artifacts at the air—contrast interface (seen particularly when we initially used water, giving a bright signal band on T2-weighted images that affected the adjacent bowel wall visibility); and ability to reveal the tissue layers and especially the myenteric plexus layer in the wall of the rectum (this being one of the most important criteria because the distinction between TNM [9] stage T2 or T3 rests on whether the tumor has crossed this layer).
Each factor was subjectively graded as not visualized, poor, good, or optimally visualized and assigned a score of 1-4. A consensus was obtained by the two reporting radiologists, and the scores for each factor weve averaged to give a final overall quality value between 1 and 4. We decided that only those concentrations receiving a score of 3.0 or more would be considered acceptable.
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Our results showed that the 5% concentration of methylcellulose gave the best results (an average score of 3.35 compared with 2.75 for the 2.5% solution and 2.50 for the 7.5% solution). Images produced with the 5% solution received grades of 3 or 4 (i.e., good or optimal) for all factors. For the 2.5% methylcellulose solution, the consensus was that the solution had insufficient viscosity, resulting in poorer distention of the rectum and poor visualization of the myenteric plexus layer. The 7.5% solution was judged to be too viscous, with substantial clumping noted on the three studies and large pockets of intervening air, resulting in inadequate rectal distention.
All our patients tolerated the intrarectal contrast agent well. One patient receiving the 5% solution was unable to retain the contrast material because of poor anal tone, necessitating the use of an inflated 16-French Foley catheter placed in the distal rectum to act as a seal against leakage.
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The 5% methylcellulose solution was found to have sufficiently low signal on gadopentetate dimeglumine—enhanced T1-weighted images to provide good contrast to the enhancing rectal mucosa (Fig. 1A). Similarly, its high signal on T2-weighted images allowed identification of the low-signal mucosa (Fig. 1B). All the patients examined with this solution had sufficient rectal distention to allow the rectal layers to be satisfactorily visualized (as defined by the criteria described previously).
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From our observations, the 5% methylcellulose solution is a viable endorectal contrast agent for high-resolution MR imaging of the rectum. The solution is extremely inexpensive, with the cost of each 100 mL of 5% solution estimated at $0.30 (at current exchange rates). In addition, the solution is highly stable and easy to handle. It is well tolerated by patients and easily administered. Most important, bowel distention with the 5% solution is consistently good, and the solution has signal characteristics that allow optimal delineation of the rectal mucosa.
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