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Digital Subtraction Bowel Cleansing in CT Colonography

¹ Both authors: Department of Radiology, Division of Abdominal Imaging and Intervention, White 270, Massachusetts General Hospital, 55 Fruit St., Boston, MA 02114.

Received June 13, 2000; accepted after revision July 31, 2000.

 
Address correspondence to M. E. Zalis.

Introduction

Top Introduction Subjects and Methods Results Discussion References

 
CT colonography is a rapidly evolving, minimally invasive method of evaluating the colon. The impetus driving the development of this technique is the need to improve compliance with and cost-effectiveness of colon cancer screening [1]. Results of several preliminary series indicate that CT colonography has a high sensitivity for detection of colorectal cancer and significant polyp lesions [2]. However, the widespread implementation of CT colonography remains limited in part by the persistent requirement for patients to undergo purgation cleansing of the large bowel. Qualitative interview data collected from potential candidates for colon screening indicate that the perceived discomfort and embarrassment associated with bowel cleansing are themselves independent barriers to screening (Weitzman ER, unpublished data).

We describe a paradigm for CT colonography that we call digital subtraction bowel cleansing. This technique greatly reduces the need for bowel purgation and results in CT colonography image data sets suitable for endoluminal navigation. In digital subtraction bowel cleansing, the patient ingests high-density contrast material before undergoing imaging. The oral contrast material is of sufficient attenuation and volume to opacify uniformly all the colon contents. The patient then undergoes CT with gas insufflation of the colon, as in standard CT colonography. After this procedure, we use software that we developed to subtract the ingested colonic contents from the CT source images as a postprocessing step, leaving soft-tissue elements such as polyps unaffected. The result is an image data set suitable for multiplanar and endoluminal examinations in which significant colonic lesions are easily recognizable. As an augmentation of CT colonography, digital subtraction bowel cleansing may contribute to a more easily tolerated method of examining the colon.

Subjects and Methods

Top Introduction Subjects and Methods Results Discussion References

 
After receiving institutional review board approval, we performed CT colonography on five subjects, including three healthy volunteers and two patients in whom colonic malignancy was suspected. Patient preparation for digital subtraction bowel cleansing began 48 hr before the CT examination. The subjects were asked to ingest six to seven doses of 5 mL of dilute contrast material with each meal or snack, but they otherwise were not asked to change their eating habits.

Opacification of the bowel was equally well achieved using both iodinated and barium-based contrast agents. In three patients, barium contrast material was administered at a concentration of 2.1% w/v (EZ-CAT; EZ-EM, Westbury, NY). For iodinated contrast material, we used a 1:40 dilution of diatrizoate meglumine and diatrizoate sodium (Gastroview; Mallinckrodt Medical, St. Louis, MO). Approximately 3 hr before being imaged, patients ingested approximately 800 mL of contrast material of the same concentration as they had on the preceding days. The final, larger bolus of contrast material guaranteed opacification of the cecum and its contents, a region that in earlier trials we had found difficult to opacify uniformly.

Image acquisition for CT colonography was performed after bowel opacification with the subjects in both the supine and prone positions after gentle air insufflation of the colon (to the point of patient tolerance, typically between 10 and 20 pumps of a hand-held bulb). Insufflation was carried out in stages over a 2-min period to minimize bowel cramping. No glucagon was administered. Helical CT technique (CTi scanner; General Electric Medical Systems, Milwaukee, WI) consisted of 120 kVp, 100 mA, a pitch of 1.5, a 5-mm collimation, and a 2-mm image reconstruction interval. Subjects were hyperventilated before image acquisition and asked to hold their breath for the part of the acquisition near the diaphragm. Patients were instructed to resume gentle respiration when they wished as image acquisition continued into the pelvis.

We performed the postprocessing steps of digital subtraction bowel cleansing on a software platform we developed using a scientific programming package (MATLAB; Mathworks, Natick, MA). Digital Imaging and COmmunications in Medicine (DICOM)-standard image files were captured from the commercially available picture archiving and communication system in use at our institution (Impax; Agfa, Ridgefield Park, NJ), and a pixel-level analysis algorithm was used to subtract the opacified bowel contents from the images. The output of the subtraction process was a set of images that remained within the DICOM standard, thereby decoupling the subtraction process from the display procedure. Consequently, the processed images could be displayed on any workstation capable of displaying the original data set. The subtracted "cleansed" axial images were sent to a commercially available workstation for complete evaluation of the CT colonography data set using coordinated multiplanar and volume-rendered endoluminal reconstructions (Vitrea; Vital Images, Minneapolis, MN).

We have implemented two different algorithms for subtraction: one that uses matrix convolution and a second that involves an extension of the roving "magic squares" computer graphics segmentation technique [3]. Both methods operate on the individual source CT images. In our experience, simple threshold functions are insufficient for subtraction because the volume-averaging artifacts present at the pixel level in all CT images result in air-fluid level artifacts that limit multiplanar and endoluminal examination. The matrix method uses mathematic gradient operators to identify regions of each image that show high regional pixel. These areas, which include salient regions of air-water interface volume averaging, were subtracted from the images as a separate step. In the second method, a raster, step-wise evaluation of neighborhood pixels was performed to identify relevant changes in pixel value. Both of these methods leave the native soft-tissue elements of the colon unaffected. Because a threshold function is a component of the matrix algorithm, bone elements present on the images with attenuation values that fall within the targeted range are also subtracted.

Results

Top Introduction Subjects and Methods Results Discussion References

 
Digital subtraction bowel cleansing results in images in which opacified bowel contents are removed while normal soft-tissue elements of the bowel, such as haustral folds, are left unaffected (Fig. 1A,1B,1C,1D). As currently implemented, the matrix-based algorithm offers a time savings because it uses a preoptimized machine code for performance of Fourier-based convolutions. In comparing the two methods, we found that when operated on a 400-MHz PC-based system, the matrix algorithm completes the subtraction process for a data set of 200 CT images (512 x 512 matrix, 16-bit depth) in 29.6 min of mean run time, whereas the raster method requires 4.2 hr.

View larger version (97K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —41-year-old healthy male volunteer. Axial image obtained during CT colonography before digital processing shows uniform opacification of colon contents.

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —41-year-old healthy male volunteer. Axial image obtained during CT colonography after digital processing reveals removal of opacified bowel contents. Native haustral folds are left unaffected.

View larger version (79K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —41-year-old healthy male volunteer. Magnified view of A.

View larger version (73K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D. —41-year-old healthy male volunteer. Magnified view of B.

The subtraction process results in data sets suitable for multiplanar and endoluminal examination. Significant soft-tissue lesions of the colon mucosa are left in place on the images and are easily recognized both on multiplanar and endoluminal views (Fig. 2A,2B,2C,2D). Because there were minor variations in technique used among the small cohort of patients in this study, detailed performance statistics were not generated.

View larger version (138K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —40-year-old male with colonoscopy-confirmed rectal mass. Axial source image obtained during CT colonography before digital processing shows uniform opacification of colon. Partially obscured mass (white arrow) and native haustral fold (black arrow) are indicated.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —40-year-old male with colonoscopy-confirmed rectal mass. Axial source image for CT colonography obtained at same level as in A after digital processing using matrix method reveals that opacified bowel contents have been subtracted, whereas mass (white arrow) and fold (black arrow) are left unaffected.

View larger version (100K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —40-year-old male with colonoscopy-confirmed rectal mass. Volume-rendered endoluminal view reconstruction of CT colonography obtained before digital processing shows partially obscured mass (white arrow) and haustral fold (black arrow).

View larger version (104K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2D. —40-year-old male with colonoscopy-confirmed rectal mass. Volume-rendered endoluminal view reconstruction of CT colonography obtained after digital subtraction using matrix method reveals that opacified colonic contents have been removed, whereas rectal mass (white arrow) and haustral fold (black arrow) remain visible.

Discussion

Top Introduction Subjects and Methods Results Discussion References

 
At least one study has been performed in an animal model to evaluate the concept of subtraction of tagged fecal contents in CT images [4]. However, to our knowledge, no imaging studies of bowel subtraction in humans have been previously reported.

Digital subtraction bowel cleansing offers the possibility of reducing a compliance barrier to colon examination by reducing patient stress during preexamination bowel cleansing while simultaneously permitting multiplanar and endoluminal examinations. Because the subtracted images remain within the DICOM standard, the process remains compatible with commercial display systems.

Image acquisition for CT colonography is currently performed with the patient in two positions—prone and supine. This method has been adopted in part to address the problem of submerged polyps. Although effective, the protocol increases the radiation exposure for the patient, a fact that has raised concern about CT colonography screening. Digital subtraction bowel cleansing offers a possibility for reduced radiation-dosage CT colonography in that all retained colon fluid and fecal material can be opacified and subtracted in a single acquisition.

Finally, although the preliminary data we present are encouraging, a study using a larger cohort is needed to provide rigorous validation of CT colonography with digital subtraction bowel cleansing. Ultimately, this validation should be performed in a group similar in composition to the population intended for colon cancer screening.

References

Top Introduction Subjects and Methods Results Discussion References

 

1. McFarland EG, Brink JA. Helical CT colonography (virtual colonoscopy): the challenge that exists between advancing technology and generalizability. AJR 1999;173:549 -559[Free Full Text]
2. Fenlon HM, Nunes DP, Schroy PC III, Barish MA, Clarke PD, Ferrucci JT. A comparison of virtual and conventional colonoscopy for the detection of colorectal polyps. N Engl J Med 1999;341:1496 -1502[Abstract/Free Full Text]
3. Jain AK. Fundamentals of digital image processing. Englewood Cliffs, NJ: Prentice Hall, 1989: 200-230
4. Sheppard DG, Iyer RB, Herron D, Charnsangavej C. Subtraction CT colonography: feasibility in an animal model. Clin Radiol 1999;54:126 -132[Medline]

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