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CT colonography: Current trends and future developments

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Philips CT Marketing Philips Healthcare

Introduction

Virtual endoscopy was first described by Vining et al. in 19941. The virtual colonoscopy procedure that he introduced has since become a basic examination technique. It is a noninvasive procedure that simulates conventional colonoscopy using a combination of computer tomography (CT), three-dimensional computerized reconstruction and virtual reality.

 

A number of different terms are nowadays used in medical literature to describe the same technique: virtual colonoscopy, virtual endoscopy, 3D colonoscopy etc. Johnson et al.2 proposed a term in 1998 that is still in use today, and perhaps best describes the examination; this expression is composed correctly of the term "colonography" prefixed by the sectional imaging method used, giving us "CT colonography". At this point it should also be mentioned that colonography can also be performed, in theory, using MRI, but with the disadvantage that one has to accept poorer spatial resolution and motion artifacts.

CT colonography today

 

In recent years CT colonography has benefited greatly from developments in CT scanner technology and new methods of 3D image processing.

 

Since its invention by Hounsfield and Cormack in the early 1970s, the use of computer tomography in clinical practice has spread rapidly. The introduction of spiral CT by Kalender in the early 1990s represented a quantum leap in the technology, culminating in the development of multidetector CT systems (also known as multislice CT). The advantage of this equipment lies in its ability to scan greater volumes in thinner slices in less time. It is thus possible, for example, to examine an abdomen within a single breath hold and without disturbing respiration artifacts - an advantage that is particularly beneficial for CT colonography3,4. The thinner slices have made it possible to produce virtually artifactfree three-dimensional representations of the colon.

 

When evaluating CT data (up to 1,000 images per examination) both 2D and 3D procedures are used. The most important 2D procedures are the evaluation of the axial output images and the preparation of multiplanar reconstructions (MPR) that can be used to reconstruct any required scan orientation (e.g. coronary or sagittal). 3D image processing offers great potential for CT colonography. For example, in conventional colonoscopy it is not always possible to see behind intestinal folds, while there are now reconstruction algorithms for CT colonography that enable this to be done. The principal technique in this respect is the dissected representation of the colon (Philips "Filet View"). The colon is no longer shown in an endoluminal view as in conventional colonoscopy, but in a plan view similar to a pathological specimen (Figure 1). This enables the reader to see behind intestinal folds and, in addition, it provides a faster overview of the data set - a major advantage in the daily routine5.

 

CT colonography procedures not only differ in the hardware (CT system and workstation) and software used, but also in patient preparation and the examination technique. An examination in the prone and supine positions leads to a significant improvement in the sensitivity of polyp detection6. Fractionated oral administration of contrast agent ("fecal tagging") can improve discrimination between stool residues and genuine polyps, and thus potentially improve the specificity of CT colonography7- 8. Adequate inflation of the intestine with ambient air or CO² is also essential.

 

Previously published studies on CT colonography report widely differing results for the detection rate of colorectal lesions. Some publications report excellent results. The work of Pickhardt et al. published in the "New England Journal of Medicine" in late 20039 is outstanding. For the first time it was possible to demonstrate in a large group that CT colonography has the required sensitivity and specificity to be used for examination purposes in a low-risk population. The authors examined 1,233 patients on the same day in a multi-center study using CT colonography and optical colonoscopy. The sensitivity of CT for adenomatous polyps over 10 mm in size was 93.8%; for polyps over 8 mm it was 93.3%, and for polyps over 6 mm it was 88.7%. Macari10 and Chung11 have published similarly good results (at least for polyps > 10 mm). Both demonstrated a sensitivity of 100% for polyps > 10 mm in diameter.

 

However, there are published studies, some of earlier date, that report much poorer results. A large retrospective multi-center-population study (341 patients) was published in 2003 by Johnson et al. in Gastroenterology12. Here, using collimation of up to 5 mm, the authors reported a mean sensitivity of 75% and a mean specificity of 73% for polyps > 10 mm. In their study of 614 patients Rockey et al. achieved a sensitivity of 59 % for polyps > 10 mm, and 51% for lesions of 6-9 mm. Collimation of up to 5 mm was also used in this study.

 

From the above studies it can be seen that the method of examination depends, to a large extent, on the image quality. Moreover, the physician's experience in evaluating CT colonography data is an essential requirement.

Figure 1 Virtual colonography (Philips Extended Brilliance Workspace v3.0). The upper half of the image shows the dissection mode (“Filet view”).
Figure 1
Virtual colonography (Philips Extended Brilliance Workspace v3.0). The upper half of the image shows the dissection mode (“Filet view”).

 

The future of CT colonography

 

In the near future, standards should be set defining the minimum requirements for CT colonography. This will need an appropriate CT system and a physician with the appropriate training to perform and evaluate the examination.

 

If the examination is performed with these aspects in mind, high sensitivity and specificity can already be achieved in the detection of colorectal polyps. Nevertheless, the technique still has great potential for development, particularly in the post-processing procedures. One example of this is automatic bowel cleansing. In this procedure, an oral contrast medium is administered to patients during the preparation for the examination in order to label residues of stool or liquid (see "fecal tagging" above). Based on the differences in density between the labeled residues and the intestinal wall or the air in the colon, the computer subtracts the unwanted structures (Figure 2). In some cases this can render pre-examination bowel cleansing unnecessary, which would be an advantage, particularly in older and less cooperative patients. A series of published studies have already shown the great potential of this method13-16.

 

Many current studies17-20 deal with automatic polyp detection (Computer-Aided Detection - CAD) In this, suspect lesions are detected and marked by the computer. The physician can then decide whether these suspected lesions are actually polyps or, for example, stool residues (Figure 3). This algorithm could lead to a further reduction in examination time.

Figure 2 (Left image) Endoluminar view with extensive residual fluid. (Right image) Endoluminar view after
using the “automatic bowel cleansing” option. A 7-mm polyp, previously hidden below the level of the liquid, is now visible.Figure 3 Automatic polyp detection. The suspected polyp has been marked in green. The physician can now
assess the lesion further and decide whether or not it actually is a polyp.
Figure 2
Figure 3
(Left image) Endoluminar view with extensive residual fluid. (Right image) Endoluminar view after using the “automatic bowel cleansing” option. A 7-mm polyp, previously hidden below the level of the liquid, is now visible.
Automatic polyp detection. The suspected polyp has been marked in green. The physician can now assess the lesion further and decide whether or not it actually is a polyp.

 

Conclusion

 

CT colonography already offers high sensitivity and specificity and a high negative prediction value for polyps > 1 cm in size, given suitable equipment and experienced investigators. It is the method of choice for incomplete colonoscopies and colonoscopies in high-risk patients (hemophiliacs, patients treated with anticoagulants, etc.).

 

Nevertheless, its use in routine examinations should be subject to the introduction of quality controls with prescribed examination standards (preparation for examination, CT parameters, subsequent processing, qualified physician). Computer-assisted evaluation has the potential to further shorten the examination time

References

  1. Vining D, Shifrin R, Grishaw E. Virtual colonoscopy. Radiology 1994; 193: 446.
  2. Johnson CD, Hara AK, Reed JE. Virtual Endoscopy: What's in a Name? AJR 1998; 171:1201-1202
  3. Röttgen R, Schröder RJ, Lorenz M et al. CT-Kolonographie mit dem 16-Zeilen-Detektor zur Diagnostik kolorektaler Neoplasien und entzündlicher Kolonerkrankungen (CT-colonography with the 16-slice CT for the diagnostic evaluation of colorectal neoplasms and inflammatory colon diseases). Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr 2003; 175: 1384-1391
  4. Brambs H-J, Juchems MS. Virtual Endoscopy using CT Scan. Min Invas Ther and Allied Technol 2003; 12: 207-216.
  5. Juchems MS, Fleiter TR, Pauls S et al. CT colonography: Comparison of a colon dissection display versus 3D endoluminal view for the detection of polyps. Eur Radiol 2006; 16: 68-72.
  6. Fletcher JG, Johnson CD, Welch TJ et al. Optimization of CT colonography technique: Prospective trial in 180 patients. Radiology 2000; 216: 704-711.
  7. Bielen D, Thomeer M, Vanbeckevoort D et al. Dry preparation for virtual CT colonography with fecal tagging using water-soluble contrast medium: initial results. Eur Radiol 2003; 13: 453-458.
  8. Callstrom MR, Johnson CD, Fletcher JG et al. CT colonography without cathartic preparation: Feasibility study. Radiology 2001; 219: 693-698.
  9. Pickhardt PJ, Choi JR, Hwang I et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med 2003; 349: 2191-2200.
  10. Macari M, Bini EJ, Jacobs SL et al. Colorectal polyps and cancers in asymptomatic average-risk patients: evaluation with CT colonography. Radiology 2004; 230: 629-636.
  11. Chung DJ, Huh KC, Choi WJ et al. CT colonography using 16-MDCT in the evaluation of colorectal cancer. AJR 2005; 184: 98-103.
  12. Johnson CD, Toledano AY, Herman BA et al. Computerized tomographic colonography: Performance evaluation in a retrospective multicenter setting. Gastroenterology 2003; 125: 688-695.
  13. Pickhardt PJ, Choi JH. Electronic Cleansing and Stool Tagging in CT Colonography: Advantages and Pitfalls with Primary Three- Dimensional Evaluation. AJR 2003; 181: 799-805.
  14. Zalis ME, Hahn PF. Digital Subtraction Bowel Cleansing in CT Colonography. AJR 2001; 176:646-648.
  15. Zalis ME, Perumpillichira J, Del Frate C et al. CT colonography: Digital subtraction bowel cleansing with mucosal reconstruction initial observations. Radiology 2003; 226: 911-917.
  16. Zalis ME, Perumpillichira JJ, Kim JY et al. Polyp size at CT colonography after electronic subtraction cleansing in an anthropomorphic colon phantom. Radiology 2005; 236: 118-124.
  17. Luboldt W, Tryon C, Kroll M et al. Automated mass detection in contrast-enhanced CT colonography: An approach based on contrast and volume. Eur Radiol 2005; 15: 247-253.
  18. Summers RM, Jerebko AK, Franaszek M et al. Colonic polyps: Complementary role of computer-aided detection in CT colonography. Radiology 2002; 225: 391-399.
  19. Taylor SA, Halligan S, Burling D et al. Computer-assisted reader software versus expert reviewers for polyp detection on CT colonography. AJR 2006; 186:696-702.
  20. Taylor SA, Halligan S, Slater A et al. Polyp detection with CT colonography: Primary 3D endoluminal analysis versus primary 2D transverse analysis with computer-assisted reader software. Radiology 2006; 239: 759-767.

 

* Article reproduced from Medicamundi.


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