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Original Research |
1 All authors: Department of Radiology, University College Hospital, Newcastle Rd., Galway, Ireland.
Received March 30, 2007;
accepted after revision February 23, 2008.
Address correspondence to C. G. Cronin
(carmelcronin2000{at}hotmail.com).
Abstract
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SUBJECTS AND METHODS. Forty consecutively enrolled clinically referred patients with known or suspected small-bowel abnormalities prospectively underwent 62 MRI small-bowel follow-through examinations in both the prone and the supine positions. Images were blindly and independently reviewed by two observers. Each small-bowel segment was assessed with a 3-point scoring system, and differences in bowel distention in the prone and supine positions were evaluated with a paired Wilcoxon's test. Differences between rates of lesion detection and characterization (e.g., ulceration, stricturing) were analyzed with a paired Student's t test. Interobserver agreement was estimated with the kappa coefficient.
RESULTS. In both normal and diseased small bowel, the prone position had statistically significantly higher distention scores than did the supine position (p < 0.05) with a high level of interobserver agreement. This finding, however, did not translate into improved lesion detection or characterization (p > 0.05).
CONCLUSION. Although use of the prone position results in superior small-bowel distention during MRI small-bowel follow-through, both the prone and supine positions are equal in terms of lesion detection and feature visualization.
Keywords: Crohn's disease • MRI small-bowel follow-through • patient position
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MRI small-bowel follow-through depends on sufficient bowel distention for optimal implementation [3, 4], the diagnostic accuracy of this approach declining in its absence [3, 5]. Previous investigators have used either prone [1, 6, 7] or supine [8–10] patient orientation during MRI examinations. Advocators of the former approach suggest that a prone position equates with bowel-loop separation, permitting improved mural evaluation [5]. Furthermore, it is believed [5] that this position decreases the volume of the peritoneal cavity and thus the number of imaging sections needed. Supine imaging may be of considerable benefit with regard to patient comfort, of particular importance when one considers that a single patient may need multiple sessions on the table during the course of a single MRI examination.
It remains unknown whether one position surpasses the other in terms of bowel distention and lesion detection. The prone position has been preferred [5], however, because it exerts mild pressure on the anterior abdominal wall, facilitating separation of small-bowel loops. The purpose of this study was to assess which patient position, prone or supine, is associated with better small-bowel distention and lesion detection. To the best of our knowledge, no such study has been reported in the medical literature.
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Patient Preparation
All patients fasted from midnight of the night before MRI. No bowel
preparation, medications to promote gastric emptying, or bowel relaxation or
paramagnetic contrast agents were administered before imaging. Bowel
distention was achieved with a single packet of polyethylene glycol solution
(Klean-Prep, Norgine) diluted in 1,000 mL of water and a small amount of
orange cordial, added for optimization of patient tolerance. Polyethylene
glycol (polyethylene glycol 3350, 59.0 g; anhydrous sodium sulfate, 5.685 g;
sodium bicarbonate, 1.685 g; sodium chloride, 1.465 g; potassium chloride,
0.7425 g) is a high-osmolarity non-absorbed contrast medium that provides
excellent intraluminal contrast and distention. Patients were instructed to
continuously ingest the solution over a 10- to 15-minute period at a constant
rate of approximately 75–100 mL/min monitored by an MRI technologist.
The patients ingested this solution while seated in the MRI preparation room
and were brought to the MRI unit only when images were being acquired.
Additional oral contrast medium was not administered between prone and supine
imaging acquisitions.
Initial images were acquired 10 minutes after the patient completed ingestion of 1,000 mL of the oral contrast agent. A second series of images was obtained 30 minutes after completion of ingestion and every 20 minutes thereafter until the contrast bolus was deemed to have adequately passed to the colon. For the first set of images, the MRI technologist randomly used the prone or the supine position. Patients were then repositioned and immediately imaged in the other orientation.
Imaging Technique
MRI was performed with a 1.5-T system (Magnetom Symphony, Siemens Medical
Solutions) equipped with high-performance gradient coils characterized by a
maximum gradient amplitude of 52 mT/m and a slew rate of 125 mT/m/ms. A
phased-array body coil also was used for optimization of signal reception.
Multiplanar rapid localization was followed by heavily T2-weighted 2D true
fast imaging with steady-state precession sequences in the coronal and axial
planes encompassing the diaphragmatic apex to the groin in craniocaudal
extent. These true fast imaging with steady-state precession acquisitions
incorporated the following parameters: TR/TE, 4.6/2.3; flip angle, 70°;
matrix size, 205 x 256; field of view, 400 x 400 mm. A slice
thickness of 5 mm enabled acquisition of 19 sections within a comfortable
single breath-hold of 21 seconds.
Image Analysis
Images were independently and qualitatively assessed by two consultant
radiologists with more than 6 and 10 years of experience (more than 250 and
350 examinations). Observers were blinded to symptoms and clinical data. A
minimum interval of 12 weeks passed between evaluation of the prone and supine
data sets from each patient. Studies were displayed and reviewed on a PACS
workstation without data compression. For image assessment, the small bowel
was considered to comprise five distinct segments: duodenum, jejunum, proximal
ileum (left of midline), distal ileum (right of midline), and terminal ileum.
Prone and supine images were qualitatively evaluated with regard to degree of
small-bowel distention with a visual assessment grading scale that ranged from
1 to 3. A score of 1 represented poor small-bowel distention; 2, moderate
distention; and 3, excellent distention (Figs.
1,
2, and
3).
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Statistical Analysis
Statistical analysis was performed with the SPSS program (version 14,
SPSS). Scores assigned to bowel distention in the diseased segments, all
small-bowel segments, and for each MRI small-bowel follow-through study as a
whole (by combining the scores for each segment of bowel for a given patient)
at supine and prone imaging were analyzed with a paired Wilcoxon's test for
ordinal data. Differences between rates of lesion detection in the prone and
supine positions were evaluated with a paired Student's t test. A
value of p < 0.05 was considered to represent the presence of a
statistically significant difference. Interobserver agreement with regard to
small-bowel distention was assessed with the kappa coefficient. A kappa value
of 0 indicated poor agreement; 0.01–0.20, slight agreement;
0.21–0.40, fair agreement; 0.41–0.60, moderate agreement;
0.61–0.80, good agreement; and 0.81–1.00, excellent agreement.
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All images were considered of diagnostic quality by both observers. Findings at 34 of the 62 examinations were deemed by both observers to be within normal limits, and 28 examinations showed pathologic changes, subsequently confirmed as Crohn's disease in 16 patients, bowel carcinoid in one patient, small-bowel lymphoma in four patients, polyps in two patients, and extraluminal lesions including a splenic mass in one patient, mesenteric nodes in two patients, and ovarian cysts in two patients.
For both observers, use of the prone position resulted in significantly
higher small-bowel distention scores in the diseased bowel segments than did
use of the supine position (n = 20; mean supine score observer 1, 2.2
± 0.8 [SD]); mean prone score observer 1, 2.6 ± 0.4; mean supine
score observer 2, 2.1 ± 0.6; mean prone score observer 2, 2.6 ±
0.4) (p < 0.05). Use of the prone position also resulted in higher
scores for all small-bowel segments (n = 310; mean supine score
observer 1, 2.2 ± 0.4; mean prone score observer 1, 2.5 ± 0.4;
mean supine score observer 2, 2.1 ± 0.5; mean prone score observer 2,
2.5 ± 0.5) (p < 0.05). Use of the prone position also
resulted in higher scores for each small-bowel study as a whole, comprising
the total scores assigned to each segment within a patient (n = 62;
mean supine score observer 1, 11.4 ± 1.9; mean prone score observer 1,
12.1 ± 2; mean supine score observer 2, 11.2 ± 1.9; mean prone
score observer 2, 12.2 ± 2.1) (p < 0.05). The distribution
of these scores for both observers is outlined in
Table 1. The degree of
interobserver agreement for scores assigned to bowel distention was considered
good (
= 0.79).
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All lesions were identified on both prone and supine images by each observer. No significant difference was detected between rates of identification of various mural characteristics for either position (Table 2). These characteristics included wall and fold thickening and fold attenuation, mural ulceration, pseudopolyps, bowel stenosis, strictured segments, bowel dilation, and separation of bowel loops. A number of extracolonic lesions were found, including two lymph node masses (6 cm and 4 cm in short-axis diameter), two polyps with a diameter less than 1 cm (9 mm and 8 mm), a splenic mass (2 cm), and two ovarian cysts (3 x 2 cm and 2.5 x 1.4 cm). There was no significant difference in visualization, detection, or lesion size between the two positions (p > 0.05).
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Optimization of bowel distention undoubtedly is key to prevention of false-positive [5, 12] and false-negative [4, 13] results during MRI small-bowel follow-through. However, the results of this study suggest that although use of the prone position results in significantly better small-bowel distention than does use of the supine position, this improved distention does not translate into improved lesion detection or morphologic evaluation. This finding may have positive implications in the imaging of patients unable to adopt one of the positions, be it as a result of pain or surgical incision, or in pediatric imaging, in which use of the prone position can cause increased levels of anxiety.
The importance of optimal small-bowel distention at MRI small-bowel follow-through has been repeatedly emphasized in the literature. Poor distention can lead to difficulties of interpretation, false-positive results occurring as the result of apparent thickening of the bowel wall and folds [5, 12], and false-negative results emanating from lesion obscuration due to suboptimal mural delineation [4, 13]. A number of approaches to ensuring reliable and reproducible small-bowel distention have been described [8, 13–23], including optimization of contrast agent, addition of osmotic and nonosmotic agents, refinement of volumes of contrast material administered [3], and the timing of image acquisition [3, 24]. The technique described is routinely used at our institution and has been validated in the literature [14, 25–27].
We investigated the effect of patient position on small-bowel distention, lesion detection, and lesion visualization. The hypothesis that distention seen in one position may surpass that in a different position stemmed from studies [28, 29] relating to colonic distention, during which improved observer confidence and polyp detection were found during CT colonography in both the supine and prone positions compared with supine acquisition alone. Fletcher et al. [29] determined that supplementary prone scanning increased sensitivity both by overcoming segmental collapse and by reducing perceptual errors due to poor polyp conspicuity in underdistended segments. The same may hold true for MRI small-bowel follow-through with regard to distribution of oral contrast material and may thus positively affect bowel distention. Other potential advantages of the prone position include compensation for inability to compress and separate bowel loops, as in conventional enterography, because the prone position exerts mild uniform pressure on the anterior abdominal wall, facilitating separation of small-bowel loops and decreasing the volume of the peritoneal cavity to be imaged.
MRI small-bowel follow-through has a number of advantages over other currently available techniques. In contrast to conventional enterography, MRI small-bowel follow-through provides detailed information about the small-bowel wall and has the potential to depict extraluminal pathologic manifestations of disease, such as abscesses and mesenteric phlegmon, fibrofatty proliferation, lymphadenopathy, and colonic skip lesions in Crohn's disease [2, 30, 31]. The cross-sectional nature of this approach renders evaluation of overlapping bowel loops straightforward, obviating the requirement for ionizing radiation exposure. The latter feature is a particular advantage because many patients undergoing this examination are young (mean age in this study, 35 years). In addition to these advantages, MRI small-bowel follow-through does not involve concessions in small-bowel functional information derived, as is the case with CT enterography [32]. Although the spatial resolution of conventional enterography may be better than that of MRI small-bowel follow-through, especially for early superficial mucosal lesions, the multiplanar imaging capabilities of this approach compensate for and perhaps negate this limitation [6]. Studies [10, 30, 33] have shown MRI small-bowel follow-through to be as effective as conventional enterography in the primary diagnosis of Crohn's disease, the main indication for this examination.
Limitations of this study included our inability to blind the observers to patient position. As the result of gravity, contrast solution conformed to the dependent position in the stomach and cecum. We believed, however, that the long interval between evaluation of prone and supine images would prevent recollection of the small-bowel appearances of particular importance in the evaluation of diseased bowel segments.
In conclusion, the prone imaging position offers statistically superior small-bowel distention of both normal and diseased loops during MRI small-bowel follow-through. The positions, however, are equal in terms of lesion detection and morphologic evaluation. Therefore, patients may be imaged in either position according to preference or necessity without compromise of diagnostic accuracy.
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