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MR Colonography Using Colonic Distention with Air as the Contrast Material

Work in Progress

¹ Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave., Boston MA 02215.
² Department of Gastroenterology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston MA 02215.

Received April 19, 2000; accepted after revision June 20, 2000.

 
Address correspondence to M. M. Morrin.

Introduction

Top Introduction Subjects and Methods Results Discussion References

 
Virtual colonography, a potential minimally invasive colorectal cancer screening test, has become a reality with the advent of helical CT techniques that permit the rapid acquisition of thin two-dimensional sections covering the entire colon in a single breath-hold. On the basis of these two-dimensional images, computer software can now render three-dimensional endoluminal views of the colon insufflated with room air via a rectal tube [1, 2]. The availability of high-performance gradient systems that allow acquisition of three-dimensional MR data sets in less than 30 sec has facilitated the development of MR colonography [3]. Although, to our knowledge, there are no studies comparing the accuracy of MR and CT colonography, the potential advantages of MR colonography over CT colonography include better diagnostic accuracy for polyps of all sizes, superior contrast resolution, and lack of ionizing radiation [3, 4]. However, because of concerns regarding air-related magnetic susceptibility artifacts, current practice involves the administration of an enema of 1-2 L of diluted gadolinium to distend the colon and to generate a positive contrast with the colonic wall. To our knowledge, there are no studies comparing MR colonography using gadolinium enema with CT colonography using air insufflation; however, studies comparing barium enema with CT colonography suggest that air insufflation is better tolerated than fluid enema [5]. In this paper, we report the feasibility of colonic distention with room air as a contrast agent in MR colonography using a short TE half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequence to minimize susceptibility artifacts.

Subjects and Methods

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Seven patients (4 men, 3 women; age range, 30-75 years; mean age, 47.6 years) underwent MR colonography using colonic insufflation with room air. Five patients subsequently underwent elective colonoscopy 1-2 hr later; the remaining two patients had previously undergone MR colonography examinations using the standard gadolinium enema. The study protocol was approved by our institutional review board, and written informed consent was obtained from all patients before undergoing MR colonography. All patients underwent standard bowel preparation 24 hr before MR colonography (Fleet Prep Kit 1; Fleet Pharmaceutical, Lynchburg, VA). One milligram of IV glucagon was administered before air insufflation in all cases. Patients were placed in the right lateral decubitus position on the MR table, a 12-French soft-tipped rectal tube was inserted, and room air was gently insufflated into the colon to patient tolerance, ranging from 20 to 40 bulb compressions. All patients were asked to grade discomfort on a 10-point Likert scale of 1-10 (1 = least, 10 = worst) 24 hr after both MR colonography and colonoscopy. All patients were scanned in the supine and prone positions using a 1.5-T MR system (Quantum; Siemens, Erlangen, Germany) equipped with an ultrafast three-axis gradient system characterized by a maximum amplitude of 22 mT/m and a slew rate of 120 mT/m per millisecond. A standard body array multicoil receiver was used centered to achieve coverage of as much of the colon as possible. Scout images using T1-weighted gradient-echo sequences were obtained in the coronal, sagittal, and axial planes to confirm adequate colonic distention. Short TE multislice HASTE images were acquired in the axial and coronal planes from the diaphragm to the pubic symphysis. The HASTE technique uses single-slice selective excitation and multiple refocusing radiofrequency pulses [6]. Each echo is acquired after the application of a different phase-encoding gradient pulse. HASTE images were acquired using the following parameters: TR/effective TE, /32; echo train length, 104; rectangular field of view (247 x 330 mm) for both axial and coronal acquisitions; matrix, 192 x 256; slice thickness, 4 mm; no interslice gap. Four 18-sec breath-hold acquisitions were obtained in the axial plane to cover the entire colon. Four contiguous acquisitions to cover the colon in the supine position axially and a further four contiguous acquisitions in the prone position were obtained. Two 18-sec breath-hold acquisitions were obtained in the coronal plane to cover the entire colon. The acquisition of axial and coronal images in the prone and supine positions reduces the possibility of missing small lesions as a result of respiratory misregistration. Each slice was acquired in approximately 500 msec. Eighteen slices were obtained during each breath-hold resulting in 7.2-cm coverage per breath-hold.

MR data were transferred onto a workstation (Advantage Windows, General Electric Medical Systems, Milwaukee, WI) equipped with navigator software permitting the radiologist to obtain both multiplanar reformations of the air-distended colon and an endoluminal perspective through the entire distended colonic lumen. Magnified source images were viewed in rapid cine sequence, and three-dimensional shaded-surface endoluminal images were generated in areas of bowel such as compound folds that could not confidently be examined using the magnified axial sequences alone. Two MR radiologists, who were aware of the colonoscopic findings, evaluated the resultant source images by consensus reading. Assessments were made in five segments throughout each colon (rectum, sigmoid, descending colon, transverse colon, and ascending colon) in both the supine and prone positions, yielding a total of 70 segments. Overall image quality, degree of image degradation as a result of artifact from intraluminal air, adequacy of colonic distention by anatomic segment, and bowel-wall conspicuity were each assessed using a 5-point scale: 1, poor; 2, fair; 3, adequate; 4, good; and 5, excellent. A distention score of 3 or more represented adequate distention.

Results

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One patient had a 1.5-cm polyp identified in the proximal ascending colon at colonoscopy. This polyp was identified at MR colonography (Fig. 1A,1B,1C,1D). A second patient had a 6-mm sessile polyp in the proximal transverse colon at colonoscopy that was not identified at MR colonography. Two patients had diverticular disease that was correctly identified at MR colonography. For the 7 patients, mean discomfort during the exam was rated 3.5 out of 10. Of note, the two patients who had undergone an earlier fluid MR colonography before their air-distended MR colonography reported less discomfort during the air-distended examination (Mean ± standard deviation [SD], 3.2 ± 0.6 versus 5.9 ± 0.7, respectively).

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —59-year-old man who presented with rectal bleeding. Axial MR image of air-distended ascending colon acquired using short TE multislice half-Fourier acquisition single-shot turbo spin-echo technique shows 1.5-cm polyp (arrow).

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —59-year-old man who presented with rectal bleeding. Axial MR image viewed with inverted windows reveals polyp (arrow) seen in A.

View larger version (126K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —59-year-old man who presented with rectal bleeding. Axial inverted MR image one slice above A reveals small diverticulum (arrow).

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D. —59-year-old man who presented with rectal bleeding. Endoluminal MR image shows flat 1.5-cm polyp (arrow) adjacent to small diverticulum (arrowhead).

Overall image quality of axial images was rated as excellent in two cases and good in five cases. The image quality was rated good in the five cases because, although the short TE multislice HASTE imaging significantly reduced air-related susceptibility artifacts, it did not eliminate them. Overall image quality of endoluminal images was rated as good in seven cases (Fig. 1D). Overall image susceptibility artifacts caused by air in adequately distended segments was considered acceptable (Mean ± SD, 3.9 ± 0.9). Artifacts including ghost, bowel motion, susceptibility artifacts, and chemical shift did not significantly degrade image quality in any case. Overall colonic distention and bowel-wall conspicuity was more than adequate (Mean ± SD, 3.6 ± 0.7 and 3.8 ± 0.7, respectively). Adequate colonic distention was seen in 63 (90%) of 70 segments (Fig. 2A,2B). More adequately distended segments were seen in the prone position (33/35) than in the supine position (30/35). Segmental bowel-wall conspicuity (in the adequately distended segments) was good or excellent in 59 (93%) of 63 segments.

View larger version (100K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —55-year-old man who presented with abdominal pain. Coronal MR image of air-distended colon shows excellent image quality, bowel-wall conspicuity, and bowel distention and minimal air-related susceptibility artifact.

View larger version (59K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —55-year-old man who presented with abdominal pain. Coronal inverted MR image shows bowel wall conspicuity better than that seen in A.

Discussion

Top Introduction Subjects and Methods Results Discussion References

 
MR colonography is a promising imaging technique that can image the colon from cross-sectional and endoluminal perspectives in a minimally invasive fashion. Current MR colonography techniques require a 1.5-2 L water enema containing paramagnetic contrast material in the form of 10 mmol/L gadolinium concentration [7].

The potential advantages of MR colonography over CT colonography include better diagnostic accuracy for polyps of all sizes, superior contrast resolution, and lack of ionizing radiation [3, 4]. However, a gadolinium enema may limit widespread acceptance of MR colonography. Given that the optimal concentration of gadolinium is 10 mmol/L, a standard 2-L enema would require 40 mL of a 0.5 mol/L gadopentetate dimeglumine solution. Consequently, the cost associated with a dilute gadolinium enema would be expected to place a considerable burden on any colorectal cancer screening strategy using MR colonography. In addition, the current technique requires patients to retain a large-volume enema for at least 30 min. Of note, the two patients in our study who underwent MR colonography with enema and subsequently with air, preferred MR colonography performed with air distention. Furthermore, a significant proportion of elderly or debilitated patients may have difficulties retaining the enema, and its use may be further limited by discomfort, embarrassment, and, in our experience, soiling of the MR table.

MR colonography has not been used with air insufflation, largely because of concerns regarding susceptibility artifacts. To develop an enema-free MR colonography, we acquired virtual colonoscopic images using short TE multislice HASTE sequences. HASTE, which is a single-slice T2-weighted sequence that acquires images in less than 1 sec, combines subsecond temporal resolution, which effectively freezes bowel motion as well as narrow spacing of the radiofrequency refocusing pulses that minimize susceptibility artifact [8]. It is important to note that the selection of a 4-mm slice thickness may have contributed to our failure to identify a 6-mm polyp at MR colonography. The use of significantly thinner images has been shown to improve the detection of smaller polyps with standard MR colonography [3], and future studies using thinner slices and air contrast are required.

In conclusion, we have shown that MR colonography with HASTE imaging using room air rather than gadolinium enema as a colonic contrast agent is a feasible virtual colonoscopy technique. Air distention seems to be well tolerated by patients. Resultant images provide adequate luminal distention and wall conspicuity in the nondependent portion of the colon.

References

Top Introduction Subjects and Methods Results Discussion References

 

1. Vining DJ, Gelfand DW, Bechtold RE, Scharling ES, Grishaw EK, Shifrin RY. Technical feasibility of colon imaging with helical CT and virtual reality. (abstr) AJR 1994;162 [American Roentgen Ray Society 94th Annual Meeting Program Book suppl]:104
2. Hara AK, Johnson CD, Reed JE, et al. Detection of colorectal polyps with CT colography: initial assessment of sensitivity and specificity. Radiology 1997;205:59 -65[Abstract/Free Full Text]
3. Schoenenberger AW, Bauerfeind P, Krestin GP, Debatin JF. Virtual colonoscopy with magnetic resonance imaging: in vitro evaluation of a new concept. Gastroenterology 1997;112:1863 -1870[Medline]
4. Wildermuth S, Debatin JF. Virtual endoscopy in abdominal MR imaging. Magn Reson Imaging Clin N Am 1999;7:349 -364[Medline]
5. Nara AK, Johnson CD, Reed JE, Ahlquist DA, Nelson H, Harmsen WS. Computer tomographic colographic (virtual colonoscopy) for polyp detection: early comparison against barium enema. (abstr) Gastroenterology 1997;112 [American Gastroenterological Association 98th Annual Meeting Program Book suppl]:575[Medline]
6. Luboldt W, Bauerfeind P, Steiner P, Fried M, Krestin GP, Debatin JF. Preliminary assessment of three-dimensional magnetic resonance imaging for various colonic disorders. Lancet 1997;349:1288 -1291[Medline]
8. Semelka RC, Kelekis NL, Thomasson D, Brown MA, Laub GA. HASTE MR imaging: description of technique and preliminary results in the abdomen. J Magn Reson Imaging 1996;6:698 -699[Medline]

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