Published online before print September 13, 2002, 10.1148/radiol.2251011664
(Radiology 2002;225:597-602.)
© RSNA, 2002
MR Colonography: Development of Optimized Method with ex Vivo and in Vivo Systems1
Diego R. Martin, MD, PhD,
Ming Yang, MD,
David Thomasson, PhD and
Craig Acheson, RT
1 From the Department of Abdominal Imaging, West Virginia University School of Medicine, Robert C. Byrd Health Sciences Center, PO Box 9235, Morgantown, WV 26505-9235. Received October 11, 2001; revision requested January 4, 2002; revision received February 20; accepted May 15. Address correspondence to D.R.M. (e-mail: dmartin@hsc.wvu.edu).
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ABSTRACT
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An ex vivo magnetic resonance (MR) colonographic system with a bovine colon with polyps of predetermined dimensions was developed for evaluation and optimization of different combinations of imaging sequences and intraluminal contrast agents. Findings were then applied during in vivo testing in human subjects. The results show that optimized contrast and lesion conspicuity and minimized motion artifacts can be obtained with true fast imaging with steady-state precession combined with water as an intraluminal contrast agent.
© RSNA, 2002
Index terms: Colon, MR, 75.121412, 75.121419, 75.12143 • Magnetic resonance (MR), experimental studies, 75.121412, 75.121419, 75.12143
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INTRODUCTION
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To date, imaging of the colon has been performed mostly with conventional radiography in combination with luminal distention with double-contrast barium and gas (1,2). Computed tomography (CT) and magnetic resonance (MR) imaging are useful for assessment of both neoplastic and nonneoplastic diseases that affect the colon (1,3–8), with the additional advantage of demonstrating serosal and extraenteric processes in the abdomen (5,6). Findings in recent studies suggest that colon screening would be similar to breast cancer screening in regard to cost per life saved by using a combination of limited colonoscopy or flexible sigmoidoscopy and double-contrast barium enema (5). CT colonography has shown promise but has not demonstrated adequate sensitivity with lesions smaller than 1 cm in diameter (9–12). MR colonography has theoretic advantages, including multiplanar capability and capacity for generating soft-tissue contrast 10–1,000 times greater than that on CT images.
Several combinations of sequences and contrast agents have the potential for use in MR imaging of the bowel, including T1-weighted fat-suppressed three-dimensional (13) volumetric interpolated breath-hold imaging (VIBE; Siemens Medical Systems, Erlangen, Germany) (14), T2-weighted half-Fourier rapid acquisition with relaxation enhancement (RARE) (HASTE; Siemens Medical Systems) (15), and true fast imaging with steady-state precession (FISP). Although true FISP has recently been proposed as a potential sequence for stomach and small-bowel imaging (16,17), the usefulness of this sequence for colon imaging has not been characterized, to our knowledge. Useful intraluminal contrast agents include gas or fluid, with fluid in the form of simple water or water with the addition of a paramagnetic agent such as dilute gadolinium chelate. The purpose of our study was to evaluate different combinations of imaging sequences and intraabdominal contrast agents to determine the optimal technique.
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Materials and Methods
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Ex Vivo Study
Ex vivo imaging was performed by using a bovine colon obtained from a slaughterhouse, with the bowel collected from one cow per experiment. Three animals were used to provide bowel samples, with the first two used for technical development and the third providing the bovine bowel segments used to generate the data presented herein. After removal, the colon was immediately washed and flushed with tap water followed by saline until all feculent material was removed. Total time elapsed from killing of the animal to completion of bowel washing was approximately 20 minutes. The bowel was placed in a plastic bag, which was then transported on ice in an ice chest. Colon specimens were refrigerated at 4°C overnight and used within 24 hours.
The bowel was cut into three contiguous 20-cm-long segments from the hepatic flexure to the splenic flexure, or the middle third of the large bowel. This was done to achieve uniformity between the selected segments and to avoid the larger thin-walled cecum and smaller diameter distal colon and sigmoid. The ends of the cut bowel segments were oversewn with silk surgical thread and further sealed with cyanoacrylate ester (Adhesive Systems, Frankfort, Ill), with assurance that the bowel was completely collapsed before final oversewing. The bowel segments were then distended fully by injecting air, water, or diluted gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) into the lumen through the bowel wall by using a syringe fitted with a 19-gauge needle. The concentration of gadopentetate dimeglumine was determined by making a series of sequential dilutions, which were placed in cups made of expanded rigid polystyrene plastic and imaged with volumetric interpolated breath-hold imaging. The minimum concentration required to achieve maximum signal-to-noise ratio was determined, and this optimal concentration was used subsequently for preparation of the gadolinium-enhanced intraluminal contrast agent.
Prepared bowel segments were placed in a sealable plastic bag, one segment per bag, and the bag was filled with vegetable oil. Oil was used to reproduce intraabdominal perienteric fat. The bags were sealed, displacing all trapped air, and then submerged in a 25 x 25 x 35-cm container filled with water. Three bowel segments were prepared and analyzed at the same time, with the three segments placed side by side, parallel to both each other and to the main magnetic field. The size of the outer container was selected to approximate the volume of a human torso. The outer container was closed and a quadrature surface torso coil (Siemens Medical Systems) was placed on top, with a posterior surface coil located directly under the container.
To simulate luminal polyps, lean bovine skeletal muscle was cut into cubes with volumes of 0.125, 1.0, or 4.0 cm3 (representing a diameter of 0.5, 1.0, and 2.0 cm, respectively) and sewn into the sidewall of an inverted bowel segment. To ensure separation and proper identification of the different polyps in a sealed bowel segment, the polyps were attached at equal 3–4-cm intervals from each other, in order from the smallest to the largest polyp. The first and last polyps, closest to the cut ends of the bowel segment, were placed approximately 5 cm from the cut ends. The bowel segment was then turned outside out, and the ends were closed.
Imaging was performed with a 1.5-T superconducting magnet (Quantum, with MR Ease, version A 1.0; Siemens Medical Systems). The following MR pulse sequences were performed: half-Fourier RARE (repetition time msec/echo time msec of 1,000/95, flip angle of 136°, matrix of 256 x 192, field of view of 380 mm, section thickness of 4 mm, 30 sections acquired, acquisition time of 30 seconds, bandwidth of 698 Hz/pixel, 5/8-phase partial Fourier transform, fat suppression off), true FISP (5.05/2.53, flip angle of 80°, matrix of 256 x 200, field of view of 380 mm, section thickness of 4 mm, 27 sections acquired, acquisition time of 29 seconds, bandwidth of 399 Hz/pixel, 7/8-phase partial Fourier transform, fat suppression off), volumetric interpolated breath-hold imaging (3.70/1.7, flip angle of 15°, matrix of 256 x 256, field of view of 380 mm, section thickness of 2.5 mm, 144 sections acquired, acquisition time of 28 seconds, bandwidth of 490 Hz/pixel, 6/8-phase partial Fourier transform, fat suppression on) (14). Images were acquired in transverse, coronal, and sagittal planes for all sequences.
In Vivo Study
In vivo imaging was performed with a series of three asymptomatic volunteers (two women and one man; mean age, 34 years; age range, 28–41 years) after they gave informed consent, according to guidelines approved by the institutional review board. These individuals were hospital staff volunteers at our facility who responded to a posted request for participation in this study. They subsequently underwent screening and were found to have good general health and no gastrointestinal or abdominal symptoms or high-risk family history for bowel malignancy. Each subject performed a bowel cleansing protocol (Tridrate Bowel Evacuation kit; Lafayette Pharmaceuticals, Lafayette, Ind), with orally administered clear fluids combined with magnesium citrate and bisacodyl the day before the procedure, followed by a bisacodyl suppository on the day of the procedure.
At the time of imaging, the subject was placed on the MR table in a left lateral decubitus position, and water (tap water prewarmed to room temperature) was infused into the rectum via a 14-F Foley catheter (Bard, Covington, Ga). Contrast material was introduced rectally in aliquots of 100–200 mL to a total volume determined on the basis of subject tolerance (average, 1,700 mL). The catheter balloon was inflated with 10 mL of water and taped in place throughout the procedure, with the catheter tubing closed during imaging. At the end of the procedure, the tubing was opened and the contrast material bag was placed on the floor below the patient, which allowed rapid drainage of the colon to maximize subject comfort at the end of the examination. The catheter balloon was then deflated and the catheter removed.
Imaging was performed with each sequence during suspension of respiration at end respiration. The entire colon was imaged in the transverse plane with volumetric interpolated breath-hold imaging. Imaging was also performed in transverse, coronal, and sagittal planes with the following sequences: one acquisition in coronal half-Fourier RARE and true FISP, two acquisitions in sagittal half-Fourier RARE and true FISP, two acquisitions in transverse half-Fourier RARE, and three acquisitions in transverse true FISP. Multiple acquisitions were overlapped by 1 cm. In three subjects given water contrast enemas, volumetric interpolated breath-hold imaging was performed before and after intravenous administration of gadopentetate dimeglumine. The contrast material was administered with a power injector (Medrad, Pittsburgh, Pa), with a dose of 0.1 mmol per kilogram of body weight at a rate of 2 mL/sec, followed by a 15-mL saline flush. The delay between the start of injection and initiation of imaging was 90 seconds (equilibrium phase).
Ex Vivo and in Vivo Image Analysis
Images were reviewed with a picture archiving and communication system, or PACS, workstation (Pathspeed version 7.1 or 8.1; GE Medical Systems, Mount Prospect, Ill). Three-dimensional volume rendering was performed with an Advantage Windows 4.0 system (GE Medical Systems). Images from multiple acquisitions were merged by using either an Advantage Workstation 4.0 or a RadWorks (GE Medical Systems) workstation.
Ex vivo images were assessed on the basis of contrast-to-noise ratios by comparing the SI from the lumen or polyp with that of the bowel wall ([lumen - polyp {or lumen - wall}]/background). The result is a relative value with no units. These measurements were repeated six times for each set of sequence–contrast material combinations. Two reviewers (D.R.M., M.Y.) placed with consensus the region of interest for SI measurement on a transverse image. The area of the region of interest corresponded to 50%–75% of the cross-sectional area of the polyp relative to the long axis of the bowel segment. The region of interest was placed at the center of the polyp in the section that showed the largest possible cross-sectional diameter. Each measurement was performed once for each polyp, and the entire process was then repeated six times to facilitate statistical analysis. Background noise was measured peripherally, outside the bowel-oil-water container but in the field of view on both sides of the image in the frequency direction, and results were averaged.
Subjective assessment of images on the basis of polyp conspicuity and edge sharpness of the bowel wall was determined on the basis of a four-point scale. With regard to polyp conspicuity, 0 represents inconspicuous and 1, 2, and 3 represent slightly, moderately, and clearly conspicuous, respectively. Conspicuity refers to the subjective impression of how readily visible or distinctive the polyp was in relation to the bowel wall and was believed to represent a cumulative contribution of factors, including relative SI, background noise, spatial resolution, and edge blurring or sharpness. With regard to edge sharpness, bovine bowel with thin mucosal ridges (approximately 1 mm thick) were assessed on transverse images on a four-point scale: 0, not visible; 1, blurred; 2, slightly blurred; and 3, sharply delineated.
Two reviewers (D.R.M., M.Y.), experienced in abdominal MR imaging, made determinations with consensus. The presence and size of the thin mucosal ridges used to evaluate edge sharpness were initially determined with direct visual examination of bovine bowel gross specimens during preparation. These ridges were found to extend longitudinally and around the circumference of the mucosa. For imaging evaluation, ridges were identified along a segment of bowel, and these same ridges were used for evaluation of each of the sequences tested. The section of bowel used for ridge evaluation was arbitrarily selected to correspond to the middle third of the respective ex vivo bowel segments.
In vivo images were assessed qualitatively with consensus of three reviewers (D.R.M., M.Y., D.T.) during one final review session to determine the presence of artifacts or image deterioration potentially related to patient or bowel motion or due to residual intraluminal gas pockets or stool debris. Contrast on images acquired in three test subjects, who were given both intravenous gadopentetate dimeglumine and water contrast enema, was determined for ex vivo specimens, with the exception that bowel wall SI was measured by placing six regions of interest with 2-mm diameter concentrically around the bowel wall. The middle descending colon was selected in each subject, as this reliably provided a round cross-sectional view of the bowel on transverse images in all cases. Imaging was performed with volumetric interpolated breath-hold nonenhanced and gadolinium-enhanced sequences. Results from all three subjects were averaged to provide one nonenhanced or gadolinium-enhanced bowel wall–lumen contrast value.
Statistical Analysis
The contrast-to-noise ratio data were analyzed with a two-factor analysis of variance, or ANOVA, and the comparisons between pairs (eg, air vs water) were performed with the Tukey multiple comparison method. The edge sharpness data were analyzed with cumulative logistic regression analysis. An institutional statistical consultant reviewed all results of statistical analysis. A P value less than .05 was considered to indicate a statistically significant difference.
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Results
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Ex Vivo Study
SI measurements between polyp and lumen for all combinations of imaging sequences and contrast agents are shown in the Table. Note that negative contrast indicates that the polyp SI is less than that from intraluminal contrast material. Statistically significant differences were determined between every pair of contrast agents (except air contrast) with a given sequence and between every pair of sequences with a given contrast agent (P < .01 in all cases). Differences between air and water contrast were determined with volumetric interpolated breath-hold imaging. The greatest contrast was achieved with true FISP with either intraluminal gadopentetate dimeglumine or water (P < .01). Of these combinations, maximal edge sharpness and lesion conspicuity (Figs 1, 2) were achieved with true FISP with either water or gadopentetate dimeglumine (P < .05) compared with volumetric interpolated breath-hold imaging or half-Fourier RARE. Clear depiction of the smallest polyp (0.5-cm diameter) was obtained with either water or gadopentetate dimeglumine on true FISP images (Figs 1, 2). True FISP images obtained with either water or gadopentetate dimeglumine provided the optimal data sets, and the observer could not subjectively discern the difference in lumen SI between water and gadopentetate dimeglumine. Thus, imaging in human subjects was focused on true FISP combined with water contrast, on the basis of lower cost with similar performance.
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Results of Comparison of Contrast and Edge Sharpness on MR Images of ex Vivo Bovine Bowel Obtained with Different Sequences and Contrast Agents
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Figure 2a. Transverse short-axis MR images through ex vivo bovine bowel segments were obtained at the same level with (a) half-Fourier RARE, (b) true FISP, and (c) fat-suppressed volumetric interpolated breath-hold imaging. Bowel contrast agents are (left) air, (middle) dilute gadopentetate dimeglumine, and (right) water. Thin mucosal folds, with blurring, are identified in a with water contrast (arrow). Thin mucosal folds can be seen most clearly in b with water contrast (right arrow) or gadopentetate dimeglumine (middle arrow) and on c with gadopentetate dimeglumine (arrow). A 1-cm-diameter polyp is seen with water contrast (a-c, right arrowheads) or air contrast (a-c, left arrowheads). With water contrast, the polyp (a-c, right arrowheads) is shown with clearly defined margins in b and with slightly blurred margins in a and c. With air contrast, the polyp (a-c, left arrowheads) is relatively most conspicuous in c and progressively less conspicuous in a and b. However, mucosal fold detail was relatively poor with air enema contrast, regardless of sequence used. See Figure 1 for summary of findings.
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Figure 2b. Transverse short-axis MR images through ex vivo bovine bowel segments were obtained at the same level with (a) half-Fourier RARE, (b) true FISP, and (c) fat-suppressed volumetric interpolated breath-hold imaging. Bowel contrast agents are (left) air, (middle) dilute gadopentetate dimeglumine, and (right) water. Thin mucosal folds, with blurring, are identified in a with water contrast (arrow). Thin mucosal folds can be seen most clearly in b with water contrast (right arrow) or gadopentetate dimeglumine (middle arrow) and on c with gadopentetate dimeglumine (arrow). A 1-cm-diameter polyp is seen with water contrast (a-c, right arrowheads) or air contrast (a-c, left arrowheads). With water contrast, the polyp (a-c, right arrowheads) is shown with clearly defined margins in b and with slightly blurred margins in a and c. With air contrast, the polyp (a-c, left arrowheads) is relatively most conspicuous in c and progressively less conspicuous in a and b. However, mucosal fold detail was relatively poor with air enema contrast, regardless of sequence used. See Figure 1 for summary of findings.
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Figure 2c. Transverse short-axis MR images through ex vivo bovine bowel segments were obtained at the same level with (a) half-Fourier RARE, (b) true FISP, and (c) fat-suppressed volumetric interpolated breath-hold imaging. Bowel contrast agents are (left) air, (middle) dilute gadopentetate dimeglumine, and (right) water. Thin mucosal folds, with blurring, are identified in a with water contrast (arrow). Thin mucosal folds can be seen most clearly in b with water contrast (right arrow) or gadopentetate dimeglumine (middle arrow) and on c with gadopentetate dimeglumine (arrow). A 1-cm-diameter polyp is seen with water contrast (a-c, right arrowheads) or air contrast (a-c, left arrowheads). With water contrast, the polyp (a-c, right arrowheads) is shown with clearly defined margins in b and with slightly blurred margins in a and c. With air contrast, the polyp (a-c, left arrowheads) is relatively most conspicuous in c and progressively less conspicuous in a and b. However, mucosal fold detail was relatively poor with air enema contrast, regardless of sequence used. See Figure 1 for summary of findings.
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In Vivo Study
With water contrast enema, bowel wall edge sharpness and bowel wall–lumen contrast on true FISP images were superior to those on volumetric interpolated breath-hold images and those on half-Fourier RARE images. On the latter with water contrast, large areas of signal void, which demonstrate a swirling pattern centrally in distended colon (Fig 3), were attributed to flow void. This phenomenon severely impaired in vivo imaging.
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Figure 3a. MR images in a test subject given water contrast enema show effects of bowel wall motion and intraluminal water flow on the following images: (a) transverse true FISP, (b) transverse half-Fourier RARE, (c) sagittal true FISP, (d) sagittal half-Fourier RARE, and (e) transverse volumetric interpolated breath-hold images. Transverse images of the hepatic flexure and proximal transverse colon (a and b, respectively) and sagittal images of the descending colon and splenic flexure c and d, respectively) show large areas of signal void centrally in the lumen ( in b and d) that are not seen in a and c. The pattern is consistent with flow void artifact in b and d that results from swirling fluid. This fluid motion could be a result of convection or of bowel wall contractions. SI in a and c is relatively resistant to this effect. (e) Transverse volumetric interpolated breath-hold image obtained at the same level as were a and b shows marked deterioration of colon wall definition and development of extra lines across the bowel image (arrowheads) as a result of bowel motion. Images a and b appear relatively unaffected by bowel wall motion (arrowheads), with bowel wall appearing sharply delineated. In a, residual stool debris is shown along the dependent aspect of the proximal transverse colon (arrows), but the margins of the adjacent bowel wall (arrowheads) remain distinct.
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Figure 3b. MR images in a test subject given water contrast enema show effects of bowel wall motion and intraluminal water flow on the following images: (a) transverse true FISP, (b) transverse half-Fourier RARE, (c) sagittal true FISP, (d) sagittal half-Fourier RARE, and (e) transverse volumetric interpolated breath-hold images. Transverse images of the hepatic flexure and proximal transverse colon (a and b, respectively) and sagittal images of the descending colon and splenic flexure c and d, respectively) show large areas of signal void centrally in the lumen ( in b and d) that are not seen in a and c. The pattern is consistent with flow void artifact in b and d that results from swirling fluid. This fluid motion could be a result of convection or of bowel wall contractions. SI in a and c is relatively resistant to this effect. (e) Transverse volumetric interpolated breath-hold image obtained at the same level as were a and b shows marked deterioration of colon wall definition and development of extra lines across the bowel image (arrowheads) as a result of bowel motion. Images a and b appear relatively unaffected by bowel wall motion (arrowheads), with bowel wall appearing sharply delineated. In a, residual stool debris is shown along the dependent aspect of the proximal transverse colon (arrows), but the margins of the adjacent bowel wall (arrowheads) remain distinct.
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Figure 3c. MR images in a test subject given water contrast enema show effects of bowel wall motion and intraluminal water flow on the following images: (a) transverse true FISP, (b) transverse half-Fourier RARE, (c) sagittal true FISP, (d) sagittal half-Fourier RARE, and (e) transverse volumetric interpolated breath-hold images. Transverse images of the hepatic flexure and proximal transverse colon (a and b, respectively) and sagittal images of the descending colon and splenic flexure c and d, respectively) show large areas of signal void centrally in the lumen ( in b and d) that are not seen in a and c. The pattern is consistent with flow void artifact in b and d that results from swirling fluid. This fluid motion could be a result of convection or of bowel wall contractions. SI in a and c is relatively resistant to this effect. (e) Transverse volumetric interpolated breath-hold image obtained at the same level as were a and b shows marked deterioration of colon wall definition and development of extra lines across the bowel image (arrowheads) as a result of bowel motion. Images a and b appear relatively unaffected by bowel wall motion (arrowheads), with bowel wall appearing sharply delineated. In a, residual stool debris is shown along the dependent aspect of the proximal transverse colon (arrows), but the margins of the adjacent bowel wall (arrowheads) remain distinct.
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Figure 3d. MR images in a test subject given water contrast enema show effects of bowel wall motion and intraluminal water flow on the following images: (a) transverse true FISP, (b) transverse half-Fourier RARE, (c) sagittal true FISP, (d) sagittal half-Fourier RARE, and (e) transverse volumetric interpolated breath-hold images. Transverse images of the hepatic flexure and proximal transverse colon (a and b, respectively) and sagittal images of the descending colon and splenic flexure c and d, respectively) show large areas of signal void centrally in the lumen ( in b and d) that are not seen in a and c. The pattern is consistent with flow void artifact in b and d that results from swirling fluid. This fluid motion could be a result of convection or of bowel wall contractions. SI in a and c is relatively resistant to this effect. (e) Transverse volumetric interpolated breath-hold image obtained at the same level as were a and b shows marked deterioration of colon wall definition and development of extra lines across the bowel image (arrowheads) as a result of bowel motion. Images a and b appear relatively unaffected by bowel wall motion (arrowheads), with bowel wall appearing sharply delineated. In a, residual stool debris is shown along the dependent aspect of the proximal transverse colon (arrows), but the margins of the adjacent bowel wall (arrowheads) remain distinct.
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Figure 3e. MR images in a test subject given water contrast enema show effects of bowel wall motion and intraluminal water flow on the following images: (a) transverse true FISP, (b) transverse half-Fourier RARE, (c) sagittal true FISP, (d) sagittal half-Fourier RARE, and (e) transverse volumetric interpolated breath-hold images. Transverse images of the hepatic flexure and proximal transverse colon (a and b, respectively) and sagittal images of the descending colon and splenic flexure c and d, respectively) show large areas of signal void centrally in the lumen ( in b and d) that are not seen in a and c. The pattern is consistent with flow void artifact in b and d that results from swirling fluid. This fluid motion could be a result of convection or of bowel wall contractions. SI in a and c is relatively resistant to this effect. (e) Transverse volumetric interpolated breath-hold image obtained at the same level as were a and b shows marked deterioration of colon wall definition and development of extra lines across the bowel image (arrowheads) as a result of bowel motion. Images a and b appear relatively unaffected by bowel wall motion (arrowheads), with bowel wall appearing sharply delineated. In a, residual stool debris is shown along the dependent aspect of the proximal transverse colon (arrows), but the margins of the adjacent bowel wall (arrowheads) remain distinct.
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Gross patient movement resulted in minimal deterioration of true FISP and half-Fourier RARE images but marked deterioration of volumetric interpolated breath-hold images (Fig 3). Residual gas pockets were problematic on true FISP images as they yielded poor contrast with bowel wall (Fig 4a); this finding is consistent with the results of ex vivo bowel imaging (Table). The poor contrast between gas pockets and bowel wall with true FISP necessitated imaging in both supine and prone positions. A small amount of residual stool was easily discernible from bowel wall margins (Fig 2).
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Figure 4a. Transverse MR images in a test subject given water contrast enema show the effect of residual gas on (a) true FISP and (b) volumetric interpolated breath-hold images through the splenic flexure and proximal descending colon. Fluid-gas level (arrows) is clearly visualized in a but is almost not discernible in b. Bowel wall remains visible adjacent to a gas interface (arrowheads) in b but is relatively difficult to visualize in a. Note excellent delineation of bowel wall in b, with no discernable motion artifact compared with the motion-deteriorated image in Figure 3e.
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Figure 4b. Transverse MR images in a test subject given water contrast enema show the effect of residual gas on (a) true FISP and (b) volumetric interpolated breath-hold images through the splenic flexure and proximal descending colon. Fluid-gas level (arrows) is clearly visualized in a but is almost not discernible in b. Bowel wall remains visible adjacent to a gas interface (arrowheads) in b but is relatively difficult to visualize in a. Note excellent delineation of bowel wall in b, with no discernable motion artifact compared with the motion-deteriorated image in Figure 3e.
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On volumetric interpolated breath-hold images in subjects given water contrast enema, intravenous administration of gadopentetate dimeglumine improved bowel wall–lumen contrast from 10 ± 3 to 19 ± 4, which represents a 1.9-fold increase (P < .05). Both gas and water develop low SI on volumetric interpolated breath-hold images, with bowel wall relatively higher in SI (Fig 4). Similarly, residual stool is low in SI on volumetric interpolated breath-hold images, comparable to that with water or residual gas, and does not interfere with bowel wall visualization.
In vivo imaging in volunteers who received water enemas revealed residual gas pockets. On true FISP images, bowel wall–lumen interface was not visualized in areas that were gas filled, while volumetric interpolated breath-hold images showed good wall delineation with either water or gas (Fig 4).
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Discussion
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Findings in this study show that true FISP in combination with water contrast enema provided the greatest polyp conspicuity compared with volumetric interpolated breath-hold or half-Fourier RARE imaging. Advantages of true FISP combined with water contrast include superb contrast and motion insensitivity and excellent image edge sharpness between lumen and bowel wall. Edge sharpness is enhanced owing to a phase cancellation effect that occurs at the interface of bowel wall and adjacent perienteric fat. Disadvantages include low contrast between the bowel wall and gas in the lumen. This feature requires the acquisition of both supine and prone images to contend with residual gas pockets. The presence of a small to moderate amount of residual stool was found not to impair visualization of bowel wall detail.
Two-dimensional imaging and the requirement for multiple acquisitions to cover the entire colon make true FISP volumetric reconstruction more time consuming. This problem can be diminished with improved acquisition speed, as might be achieved with multicoil coprocessing, that would allow acquisition of a greater number of sections in a breath hold. The considerable amount of SI generated with true FISP can facilitate use of acceleration techniques, which typically leads to reduction in image acquisition time at the expense of SI. Another potential advantage of using heavily T2-weighted true FISP of the bowel is the ability to depict lesions outside the bowel, including lesions with high water content, such as cysts and hemangiomas in the liver or other tumors in the pancreas (18).
Advantages of volumetric interpolated breath-hold imaging include the ability to image the entire colon in one breath hold with high spatial resolution. With water or gas enema, gadolinium enhancement is required to produce usable bowel wall contrast. Although the contrast is not as high as that with true FISP, one advantage of volumetric interpolated breath-hold imaging is that both gas and water produce similar low SI so that gas-fluid levels are almost imperceptible and do not cause a problem for bowel wall visualization. This advantage could potentially obviate both supine and prone imaging. Residual stool produces low SI comparable to that of water, which allows volumetric interpolated breath-hold images of even a poorly prepared colon to show contrast between the bowel wall and lumen.
Addition of intravenous gadopentetate dimeglumine also could potentially facilitate use of the same colon images for assessment of inflammatory bowel processes (6); extraenteric disease involving mesentery and retroperitoneum, such as lymphadenopathy (6); and solid organs including the liver, to look for focal lesions such as metastases (14). Disadvantages of volumetric interpolated breath-hold imaging include lower bowel wall–lumen contrast and increased motion sensitivity compared with those with true FISP. Postprocessing of three-dimensional reconstruction images and application of virtual colonoscopy software were possible, but the diminished bowel wall–lumen contrast made this evaluation less reliable than that with true FISP images.
Although true FISP and half-Fourier RARE imaging both produce relatively motion-insensitive T2-weighted images, half-Fourier RARE images are inferior in regard to edge sharpness. With water contrast enema, half-Fourier RARE images demonstrate a serious problem with flow void artifact, which is probably due to swirling of water in a distended large bowel lumen, that makes this technique unusable. Half-Fourier RARE imaging combined with gas enema produces less contrast and poor lesion conspicuity. Three-dimensional reconstruction postprocessing was unreliable. Half-Fourier RARE imaging has been found useful for identification and characterization of bowel disease, and it has been used for colonographic imaging (6,19–24). However, the data in this study indicate that half-Fourier RARE imaging combined with gas enema is suboptimal for colonography.
It has been common practice for the body coil to be used to both transmit and receive during imaging of the large bowel to achieve adequate field of view. However, the advantages in signal-to-noise ratio with surface coils are indisputable. In the current study, we used surface coils exclusively to provide optimal image quality, which is believed to be critical for optimization of MR colonography.
Limitations of this study include that polyps smaller than 0.5 cm in diameter were not manufactured to further challenge detection sensitivity with the ex vivo model, potentially useful variations in true FISP sequence design were not tested, and other combinations of contrast agents were not evaluated. The size of the smallest polyp we tested was based on what is believed to be clinically important: The risk of tumor in a polyp of this size would be very low (25,26). The decision to use lean bovine skeletal muscle for manufacturing polyps was based on the desire to have a soft-tissue analogue as opposed to a phantom polyp created from nonbiologic material. The SI characteristics were assumed to approximate those of an in vivo polyp, but this was not tested directly.
Other variables that were not examined include the use of true FISP combined with fat suppression or the use of a three-dimensional version of this sequence. However, fat-suppressed two-dimensional true FISP was not available at the time of this experiment. Furthermore, in earlier studies to assess three-dimensional volumetric true FISP (data not shown), we found less SI with greater noise and diminished contrast and edge sharpness compared with those with two-dimensional true FISP MR imaging. Attempts to further evaluate a three-dimensional sequence will be ongoing, as there are potential advantages inherent to volumetric acquisition with regard to facilitation of multiplanar reconstruction and volume-rendered image processing.
Another apparent limitation is that we elected to evaluate quantitatively the relative signal-to-noise ratios on only in vivo volumetric interpolated breath-hold images. We found measurement of bowel SI particularly difficult on true FISP images on which phase cancellation effects were noted adjacent to bowel wall. This difficulty was one of the reasons we emphasized an ex vivo phantom approach, as used in the current study, with which SI determinations were believed to be more reliable. Other contrast agents have been proposed, such as the addition of potassium and ferrous agents or fecal tagging with orally administered barium (15,24,27–30). Although not every conceivable combination was tested in this study, our approach was based on, first, presenting a method that is useful for efficient evaluation of different techniques, and second, comparing contrast agents and imaging sequences believed to have the greatest potential for optimal colon lesion depiction.
In summary, MR colonography with true FISP combined with water contrast enema provides optimal contrast and lesion conspicuity with minimal motion sensitivity compared with those with other sequence–contrast material combinations but requires imaging of the patient in both supine and prone positions to contend with residual gas pockets.
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ACKNOWLEDGMENTS
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We thank Gerry Hobbs, PhD, Department of Community Medicine, for invaluable assistance in statistical analysis review of the data.
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FOOTNOTES
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Abbreviations: FISP = fast imaging with steady-state precession,
RARE = rapid acquisition with relaxation enhancement,
SI = signal intensity
Author contributions: Guarantor of integrity of entire study, D.R.M.; study concepts, D.R.M.; study design, D.R.M., C.A., M.Y.; literature research, D.R.M., M.Y.; clinical studies, D.R.M., M.Y., C.A.; experimental studies, all authors; data acquisition and analysis/interpretation, all authors; statistical analysis, M.Y., D.R.M.; manuscript preparation, definition of intellectual content, editing, and revision/review, D.R.M., M.Y., D.T.; manuscript final version approval, all authors.
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TOP
ABSTRACT
INTRODUCTION
) (P < .05 [see Results]). 
= air. HASTE = Half-Fourier RARE, 3D-VIBE = volumetric interpolated breath-hold imaging.