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Technical Innovation |
1 All authors: Department of Radiology, University of Rome "La Sapienza," Policlinico Umberto I, Viale Regina Elena 324, 00161 Rome, Italy.
Received April 27, 2001;
accepted after revision June 19, 2001.
Address correspondence to A. Laghi.
Introduction
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We evaluated an oral isosmotic polyethylene glycol solution as an oral contrast agent for MR imaging of the small bowel in a population of healthy volunteers. Polyethylene glycol solution simulates the properties of water as it relates to signal intensity, with the added advantage of nonabsorbability, thus providing good distention of all the small-bowel loops from the jejunum to the terminal ileum.
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MR Protocol
MR imaging was performed on a 1.5-T MR scanner (Magnetom Vision Plus;
Siemens Medical Systems, Erlangen, Germany) with a maximum gradient field
strength of 25 mT/m. We used a phased array body coil to examine the subjects,
who were in a supine position. After obtaining localizing images in three
axes, we acquired a coronal T2-weighted HASTE sequence using the following
parameters: TR/TE, infinite/90; echotrain length, 104; section thickness, 6
mm; intersection gap, 20%; field of view, 350-400 mm; effective matrix, 192
x 256; signal average, 1; and half-Fourier reconstruction. Fifteen
slices were obtained during a single breath-hold of 20 sec. Images were
acquired every 5 min until the cecum was observed to be filled by the oral
contrast agent for a maximum of 30 min.
Image Analysis
Images were reviewed by two radiologists who were experienced in both
gastrointestinal and MR imaging. Agreement was by consensus. Small-bowel loops
were divided into three segments including the jejunum, the proximal ileum,
and the distal ileum—ileocecal region. The time required for the
contrast agent to reach the ileocecal region was recorded.
Images were assessed for visibility of all three regions. Qualitative analysis consisted of luminal distention, performed as described by Minowa et al. [4], with small-bowel loops evaluated using a three-step scale (0 = poor, 1 = good, 2 = optimal).
The quantitative analysis included an evaluation of bowel caliber and wall thickness. A bowel caliber not exceeding 3 cm and wall thickness less than 3 mm were considered indicative of normal bowel.
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The jejunum showed optimal distention in all subjects in 5-10 min, whereas the proximal ileum showed good distention in four subjects and poor distention in the remaining six subjects in 5-10 min. Good to optimal distention was observed for the proximal ileum in all subjects and for the terminal ileum in seven subjects in 10-20 min (Fig. 1A,1B). Good to optimal evaluation of the ileocecal region was obtained in 25 min in all subjects (Fig. 2A,2B).
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Bowel caliber evaluated at the level of the jejunum, the proximal ileum, and the distal ileum measured an average of 22.5 mm (range, 19-26.8 mm), 18.6 mm (range, 16.4-22 mm), and 15.4 mm (range, 12-16 mm), respectively; wall thickness measured an average of 2.6 mm (range, 2-2.9 mm) at the level of the jejunum, 2.2 mm (range, 1.4-2.8 mm) at the level of the proximal ileum, and 2.2 mm (range, 1.7-2.9 mm) at the level of the distal ileum.
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The use of various oral contrast agents, both positive and negative, has been proposed; however, none of these have proved sufficiently successful to be widely used clinically [5].
Recently, water has been advocated as a contrast agent. Water is relatively safe, although water overload, vomiting, and related aspiration remain potential risks [5]. The major limitation of this method concerns early water reabsorption, which prevents the visualization of the ileocecal region in more than 30% of patients [4, 5].
A more invasive approach for obtaining optimal luminal distention is MR enteroclysis with methylcellulose, as reported by Umschaden et al. [6]. Methylcellulose presents the same signal properties as both water and polyethylene glycol solution, but methylcellulose cannot be ingested orally and therefore requires preliminary positioning of a nasoenteric tube. Patient discomfort is increased and vomiting inside the MR scanner, despite all precautions, may occur [6].
In this study, we used an isosmotic polyethylene glycol solution on the basis of a technique optimized for sonographic examination [7]. Once in the small bowel, the unabsorbable and unfermentable polyethylene glycol remains unmodified, linked with water molecules, fills the lumen, and distends the intestinal loops [7]. The relatively small amount of polyethylene glycol solution (600 mL) was easily ingested by each subject and did not cause substantial side effects; bowel distention was optimal in the jejunum and good to optimal in the proximal and distal ileum. Obtaining sequential images is mandatory to evaluate small-bowel loops when maximally distended and to characterize correctly any mobile filling defects in the lumen as either gas bubbles or food residue. The relatively rapid transit time of polyethylene glycol solution limited the examination to less than 30 min.
Our imaging technique included the acquisition of a HASTE sequence that was able to clearly show normal small-bowel loops, with a reduced amount of artifacts from physiologic (respiratory and cardiac) motion [8].
Our study has shown that MR imaging using polyethylene glycol solution as an oral contrast agent is feasible and provides a physiologic overview of the normal small bowel. Polyethylene glycol solution is easily prepared and administered and was well tolerated in all subjects. Additional studies in patients with small-bowel disease are necessary to evaluate the possible clinical value.
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