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Virtual endoscopy
European Journal of Radiology 42 (2002) 231– 239 www.elsevier.com/locate/ejrad
Virtual endoscopy Aytekin Oto * Department of Radiology, Hacettepe Uni6ersity, Kirkpinar Sokak 3 -9, Cankaya, Ankara, Turkey Received 24 January 2002; received in revised form 25 January 2002; accepted 28 January 2002
Abstract Virtual endoscopy is a new-generation technique which combines the features of endoscopic viewing and cross-sectional volumetric imaging. In the evaluation of gastrointestinal cancers, virtual endoscopy has been most commonly used in colorectal carcinomas and to a much lesser extent in gastric carcinomas. In this review, the current status of virtual colonoscopy was reviewed together with a brief discussion of virtual gastroscopy. © 2002 Published by Elsevier Science Ireland Ltd. Keywords: Colon; Stomach; Neoplasms; Computed tomography; Magnetic resonance; CT colonography
1. Introduction Virtual endoscopy is a type of interactive three-dimensional (3D) medical imaging tool which combines the features of endoscopic viewing and cross-sectional volumetric imaging. Improvements in high-resolution computed tomography (CT) and MR acquisition together with advances in 3D computer graphics provided a large clinical potential for virtual endoscopy in the evaluation of any hollow organ system and virtual imaging has also attracted attention from national cancer institutes [1]. One of the most common uses of virtual endoscopy in the gastrointestinal system has been in the field of virtual colonoscopy. Although very limited in number compared to colonoscopy, few studies were also carried out to investigate the use of virtual endoscopy in gastric cancers. In this review we will mainly discuss virtual colonoscopy and briefly mention the role of virtual gastroscopy in the evaluation of gastrointestinal cancers.
2. Virtual colonoscopy Virtual colonoscopy (VC) or CT colonography is a new-generation technique for colorectal evaluation by using volumetric CT data of the air distended, clean * Tel.: + 90-312-439-4939; fax: + 90-312-441-4263. E-mail address: [email protected] (A. Oto).
colon. Since its first description by Vining et al. in 1994, various terms have been used to describe this technique including virtual colonoscopy, virtual endoscopy, 3D endoscopy, CT colography and CT colonography [2]. For VC, bowel is prepared as for conventional colonoscopy and the colon is distended by air via a rectal tube. Then high resolution, thin section images are obtained in the supine and prone positions. This volumetric data is interpreted as two-dimensional axial images, multiplanar reformat images and also as computer-generated 3D reconstructed images simulating the endoluminal views at colonoscopy (Fig. 1).
3. Clinical validation Most of the data accumulated so far about VC is from studies including either symptomatic or high risk patients for colorectal cancer. In one of the pioneer prospective studies, Hara et al detected the polyps 1 cm or larger in diameter with 75% sensitivity and 91% specificity [3] (Fig. 2). Performance decreased as lesion size decreased. Later Fenlon et al. reported an overall polyp detection sensitivity of 71%, with a sensitivity of 91% for polyps larger than 1 cm, 82% for polyps between 6 and 9 mm, and 55% for polyps less than 5 mm [4]. They reported 19 false positive polyps and no false positive findings of cancer. In one of the largest series with 180 patients and over 250 polyps Fletcher et al. detected 85% of polyps larger than 1 cm and 88% of
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polyps larger than 0.5 mm [5] (Fig. 3). Similarly Kay et al. reported a sensitivity of 90% and specificity of 82.1% for lesions greater than 1 cm [6] (Fig. 4). Although there have been a few studies suggesting a lower performance for VC [7,8] (especially in asymptomatic patients), data in the literature generally suggests a sensitivity of over 85% and specificity of more than 90% for detection of large colorectal polyps (larger than 1 cm in diameter) and colorectal cancers by VC [9,10] (Fig. 5). Further multicentric trials especially on the asymptomatic screening population comparing the performance of VC with other full structural colorectal examinations are needed for better evaluation of performance characteristics of VC.
Fig. 2. Virtual endoscopic view of a 18 mm polyp in the sigmoid colon. A smaller (7 mm) second polyp (arrow) can also be visualized in the distal colon.
Several technical problems degrading the accuracy of the technique remain to be solved. The pitfalls of the technique include retained stool and fluid, underdistension of the colon, complex folds, diverticuli, pedunculated polyps, iliocecal valve, small bowel reflux, extrinsic defects and respiratory and streak artifacts (Figs. 6 and 7).
4. Clinical applications of VC in colorectal carcinoma
Fig. 1. (a) CT colography (three-dimensional image formed by volume rendering algorithm). Please note the small polyp in the sigmoid colon. (b) Virtual endoscopic view. Characteristic triangular appearance of normal transverse colon.
VC may potentially have several roles in the evaluation of colorectal carcinoma. VC may be the technique of choice in the patients with incomplete colonoscopy, distal occlusive carcinomas and in elderly patients whose clinical status does not allow an invasive procedure such as conventional colonoscopy. The prevalence of synchronous neoplasms in patients with colorectal cancer varies 1.5–9 and 27–55% for adenoma [11–15]. Examination of the entire colon is recommended to identify a synchronous cancer before surgery and thus to prevent a second redundant surgical procedure (Fig. 8). Colorectal cancer patients who had a preoperative full colonoscopy were shown to have fewer local recurrences, less likely develop distant metastases and have a longer disease-free survival time [16–18]. However, in patients with distal occlusive cancers it is not infrequently very difficult to evaluate the proximal colon by conventional colonoscopy and double contrast barium enema (DCBE). Fenlon et al. showed that VC is a feasible and useful method for evaluating the entire colon before surgery in patients with occlusive carcinoma [19]. They detected two cancers and 24 polyps in the proximal colon of the 29 patients with colorectal carcinoma. Conventional
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colonoscopy fails to visualize the entire colon in 5–10% of examinations depending on several factors including the endoscopist’s skill and the patient’s tolerance [20,21]. In patients with incomplete colonoscopy, VC was reported to be effective in evaluating the proximal colon and show the unrevealed colon in more than 90% of patients [22,23]. VC may play an important role in the diagnosis and staging of colorectal carcinoma (Fig. 9). In a study with 38 pathologically proven colorectal cancers, VC correctly localized all 38 cancers, compared with 32 using conventional colonoscopy [24]. In another prospective
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study in 34 patients, VC could accurately identify all colorectal masses but overcalled stool masses in poorly distended or poorly prepared colons [25]. In that study VC correctly staged 81% of colorectal cancers by overstaging two cases and understaging one case. Another advantage of VC is its ability to assess the extraintestinal spread of the tumor: particularly the liver metastasis (Fig. 9). Hara et al. reported 11% of highly clinically important extraintestinal findings in their series of 264 patients who underwent CT colonography [26]. The ability of VC to evaluate the whole abdomen including the liver may preclude the use of other redundant
Fig. 3. Virtual endoscopic appearance (curved arrow) (a) and conventional colonoscopic view (b) of the same polyp in the distal sigmoid colon. Same polyp can also be detected at CT colography image (arrow).
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Fig. 4. Virtual endoscopic (a) and conventional colonoscopic view (b) of the polyp in the ascending colon. The polyp can be detected on the axial image (arrow) (c). Density measurement from the lesion showed a negative value, pathognomonic of a benign lipoma. Diagnosis was confirmed by colonoscopic biopsy.
imaging studies. These results suggest that VC may be a feasible method for detecting and staging of colorectal neoplasms. Since its first introduction, VC has been discussed as a potential imaging modality for colorectal cancer screening. Screening for colorectal polyps and neoplasms has been accepted as most colorectal neoplasms arise from preexisting adenomatous polyps and detection and removal of these precursor adenomas results in a decrease in the incidence and mortality from colorectal cancer [27–29]. American Cancer Society
emphasized the importance of the examination of the full colon in their revised guidelines for screening for early detection of colorectal polyps and cancer in 1997 [30]. As a full structural colorectal imaging examination VC competes with barium enema and colonoscopy. The diagnostic potential of VC seems to be much greater than fecal occult blood testing (FOBT). FOBT by nature lacks accuracy and does not allow inspection of the mucosal surface. More than half of all colorectal cancers and large majority of polyps will be missed on a single screening FOBT [31,32]. VC competes most
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directly with screening barium enema and it was found to be more sensitive than single-contrast barium enema for the detection of polyps [33]. Unfortunately to our knowledge there is no published data comparing the efficacy of VC and DCBE in the screening population. DCBE has been shown to detect more than 70% of polyps larger than 7mm [34] and the detection rate for colorectal cancer has been variable from 70 to 96% [35–37]. However, DCBE is highly dependent on the skill of the operator and it has several disadvantages
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when compared to VC including requirement for coating the colon mucosa with barium, duration of the examination and the patient tolerance. When compared to conventional colonoscopy, VC is relatively simple, less invasive, does not require sedation. Although requires preparation of the colon, it takes less time to perform the procedure. However, despite several studies showing good performance characteristics of VC, there are still studies mentioning that the poor sensitivity of VC is not enough for a screening test and especially flat
Fig. 5. Adenocarcinoma located in the rectosigmoid region. Lesion is visualized on coronal two-dimensional image (a), zoomed CT colonography (b; arrow), and virtual endoscopic view (c; curved arrow). (d) Gross pathologic specimen of the lesion after resection.
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5. Technical considerations
Fig. 6. Axial image from the level of the sigmoid colon. Collapsed segment of the sigmoid colon does not allow its optimal evaluation on CT colonography.
lesions are impossible to see by this method [8]. Another major disadvantage of VC is the lack of its ability to obtain tissue for pathologic diagnosis or remove the polyps. Sonnenberg and colleagues concluded that for VC to replace colonoscopy, the cost would have to drop to less than 50% of the cost of colonoscopy, or compliance rates for VC would have to be 15–20% better than conventional colonoscopy [38]. In a recent interesting article by Angtuaco et al., which investigates the attitudes among potential patients and primary care physicians concerning VC and conventional colonoscopy for colorectal cancer screening, availability of VC was shown to encourage the potential patients to participate in colorectal cancer screening [39].
Fig. 7. Endoscopic view of the multiple diverticuli in the sigmoid colon (arrows).
A detailed explanation about the technical issues related to VC is beyond the scope of this article. However, some important technical points will be briefly touched in this review. A proper bowel preparation is accepted as a crucial part of the examination since residual stool presents several problems in the interpretation of the images. A polyethylene glycol electrolyte solution preparation or a phospho-soda preparation the day prior to VC have been used in most of the studies. Macari et al concluded that a phospho-soda preparation provided significantly less residual fluid [40]. Contrast material labeling of the unprepared colon for CT colonography was recently investigated and results were found to be comparable to sensitivity for polyp detection in prepared colons [41]. Commercially available nutrition packages are prepared containing food and beverages, barium suspension and colonic cleansing agents. Helical CT protocols described by most of the studies are similar and use a collimation of 3–5 mm, reconstruction intervals of 1–3 mm and a pitch of 1–2 [9]. Compared to single detector CT, specific advantages of using multislice CT for the purpose of VC were suggested. The 1 mm slice thickness acquisitions may be possible with multislice CT resulting in a better z-axis resolution and hopefully this is expected to translate to better detection of smaller polyps. However preliminary studies with multislice CT did not show any significant difference in depiction of polyps larger than 10 mm between single and multi-detector row CT for a small number of polyps [42]. Large-scale studies are needed for further evaluation of multi-detector imaging in VC. Additional prone scanning was shown to improve the accuracy of VC and most centers have chosen to collect data in both the supine and prone positions [43]. Polyps and colorectal cancers were shown to enhance significantly following contrast administration and Morrin et al concluded that the use of intravenously administered contrast material significantly improved to depict medium sized polyps in suboptimally prepared colons [44,45]. Administration of intravenous contrast also allows evaluation of parenchymal organs with the same examination. VC interpretation is time-consuming; interpretation times between 15 min and 1 h have been reported [46–49]. Most researchers agree that examination of 2D images (both axial and multiplanar) are adequate for primary evaluation of the colon. Three-dimensional images are helpful to increase the observer’s confidence and to differentiate small polyps from complex folds. New software development studies for better displaying the imaging data and more efficient ways of interpretation such as automated polyp detection or electronic bowel cleansing are under way [50]. These new developments are expected to improve the diagnostic perfor-
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Fig. 8. Large, obstructing carcinoma in the distal descending colon (arrow) was detected on both conventional and also virtual colonoscopy (a). Proximal colon could not be evaluated due to obstructing lesion by conventional colonoscopy. However CT colonography managed to show a second polyp in the region of the hepatic flexure (b). CT colonography additionally demonstrated the multiple metastases in the same patient allowing a more complete preoperative evaluation of the patient (c).
mance and reproducibility of VC as well as to cause a decrease in the cost of the examination.
are the new developments in MR colonography [54– 57].
6. MR colonography
7. Virtual gastroscopy
MR colonography has been following close behind CT colonography. Based on the principles of contrast enhanced 3D MR angiography, breath-hold MR colonography has become possible [51– 53]. MR colonography has the known advantages of MR technology such as not utilizing ionizing radiation and capability of multiplanar imaging. Preliminary studies particularly performed by Debatin’s group in Essen University showed competitive results with CT colonography [54]. Fecal tagging and using CO2 enema instead of gadolinium-based enema to distend the colon
Similar to VC, the stomach can be evaluated with thin-section, high resolution 2D, axial images, reformatted two-dimensional images or computer-rendered 3D images simulating endoluminal images. For this purpose stomach is distended with ingestion of effervescent granules to increase the contrast difference between the gastric wall and its lumen. Due to limited number of studies conducted so far, it is not possible to mention an optimal scanning protocol. Issues like necessity of additional prone scanning, use of smooth muscle relaxants or intravenous contrast material dur-
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Fig. 9. Adenocarcinoma in the ascending colon with stranding in the pericolonic fat suggesting serosal invasion. Pericolonic invasion was confirmed after surgical resection.
ing scanning needs to be further investigated. Preliminary results show that 3D CT used together with virtual CT endoscopy is helpful in identifying gastric lesions and virtual CT gastroscopic images is useful in identifying both intraluminal and submucosal components of the gastric lesions [58]. In their series of 60 patients with advanced gastric carcinoma, Lee and Ko could obtain good or excellent images in 80% of patients [58]. Most of the patients with poor images had tumors in the antrum region. Lee and Ko also stated that combining 3D images with axial CT slices improved the accuracy in classifying Borrmann type and tumor staging. Virtual gastroscopy displays a wider field of view than gastroscopy, allows a more accurate measurement of tumor size and less invasive compared to conventional endoscopy. However, virtual gastroscopy is limited in spatial resolution and thus will not depict flat or small gastric lesions and will allow a detailed evaluation of mucosa. Further evaluation of this new technique is warranted in the diagnosis and staging of gastric cancers.
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[43] Chen SC, Lu DS, Hecht JR, Kadell BM. CT colonography: value of scanning in both supine and prone positions. Am J Roentgenol 1999;54:126 – 32. [44] Oto A, Gelebek V, Sayan B, Sivri B, Akhan O, Besim A. CT density of colorectal polypoid lesions: evaluation of contrast enhancement in virtual colonoscopy. Radiology 1998;221:577. [45] Morrin MM, Farrell RJ, Kruskal JB, Reynolds K, McGee JB, Raptopoulos V. Utility of intravenously administered material at CT colonography. Radiology 2000;217:765 – 71. [46] Johnson CD, Hara AK, Reed JE. Computed tomographic colonography (virtual colonoscopy):a new method for detecting colorectal neoplasms. Endoscopy 1997;29:454 – 61. [47] American Society for Gastrointestinal Endoscopy. Technology status evaluation: virtual colonoscopy —November 1997 ASGE. Gastrointest Endosc 1998;48:708 – 10. [48] Fenlon HM, Ferrucci JT. Virtual colonoscopy: what will the issues be? Am J Roentgenol 1997;169:453 – 8. [49] Royster AP, Fenlon HM, Clarke PD, Nunes DP, Ferrucci JT. CT colonoscopy of colorectal neoplasms: two-dimensional and three-dimensional virtual –reality techniques with colonoscopic correlation. Am J Roentgenol 1997;169:1237 – 42. [50] Summers RM, Johnson CD, Pusanik LM, Malley JD, Youssef AM, Reed JE. Automated polyp detection at CT colonography: feasibility assessment in a human population. Radiology 2001;219:51 – 9. [51] Prince MR, Narasimham DL, Stanley JC, et al. Breath-hold gadolinium-enhanced MR angiography of the abdominal aorta and its major branches. Radiology 1995;197:785 –92. [52] Leung DA, McKinnon GC, Davis CP, Pfammatter T, Krestin GP, Debatin JF. Breath-hold, contrast-enhanced, three-dimensional MR angiography. Radiology 1996;200:569 – 71. [53] 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 – 91. [54] Luboldt W, Steiner P, Bauerfeind P, Pelkonen P, Debatin JF. Detection of mass lesions with MR colonography: preliminary report. Radiology 1998;207:59 – 65. [55] Luboldt W, Debatin JF. Virtual endoscopic colonography based on 3D MRI. Abdom Imag 1998;23:568 – 72. [56] Luboldt W, Frohlich JM, Schneider N, Weishaupt D, Landolt F, Debatin JF. MR colonography: optimized enema composition. Radiology 1999;212:265 – 9. [57] Ogata I, Komohara Y, Yamashita Y, Mitsuzaki K, Takahashi M, Ogawa M. CT evaluation of gastric lesions with three-dimensional display and interactive virtual endoscopy: comparison with conventional barium study and endoscopy. Am J Roentgenol 1999;172:1263 –70. [58] Lee DH, Ko YT. Gastric lesions: evaluation with three-dimensional images using helical CT. Am J Roentgenol 1997;169:787 – 9.
Virtual endoscopy Aytekin Oto * Department of Radiology, Hacettepe Uni6ersity, Kirkpinar Sokak 3 -9, Cankaya, Ankara, Turkey Received 24 January 2002; received in revised form 25 January 2002; accepted 28 January 2002
Abstract Virtual endoscopy is a new-generation technique which combines the features of endoscopic viewing and cross-sectional volumetric imaging. In the evaluation of gastrointestinal cancers, virtual endoscopy has been most commonly used in colorectal carcinomas and to a much lesser extent in gastric carcinomas. In this review, the current status of virtual colonoscopy was reviewed together with a brief discussion of virtual gastroscopy. © 2002 Published by Elsevier Science Ireland Ltd. Keywords: Colon; Stomach; Neoplasms; Computed tomography; Magnetic resonance; CT colonography
1. Introduction Virtual endoscopy is a type of interactive three-dimensional (3D) medical imaging tool which combines the features of endoscopic viewing and cross-sectional volumetric imaging. Improvements in high-resolution computed tomography (CT) and MR acquisition together with advances in 3D computer graphics provided a large clinical potential for virtual endoscopy in the evaluation of any hollow organ system and virtual imaging has also attracted attention from national cancer institutes [1]. One of the most common uses of virtual endoscopy in the gastrointestinal system has been in the field of virtual colonoscopy. Although very limited in number compared to colonoscopy, few studies were also carried out to investigate the use of virtual endoscopy in gastric cancers. In this review we will mainly discuss virtual colonoscopy and briefly mention the role of virtual gastroscopy in the evaluation of gastrointestinal cancers.
2. Virtual colonoscopy Virtual colonoscopy (VC) or CT colonography is a new-generation technique for colorectal evaluation by using volumetric CT data of the air distended, clean * Tel.: + 90-312-439-4939; fax: + 90-312-441-4263. E-mail address: [email protected] (A. Oto).
colon. Since its first description by Vining et al. in 1994, various terms have been used to describe this technique including virtual colonoscopy, virtual endoscopy, 3D endoscopy, CT colography and CT colonography [2]. For VC, bowel is prepared as for conventional colonoscopy and the colon is distended by air via a rectal tube. Then high resolution, thin section images are obtained in the supine and prone positions. This volumetric data is interpreted as two-dimensional axial images, multiplanar reformat images and also as computer-generated 3D reconstructed images simulating the endoluminal views at colonoscopy (Fig. 1).
3. Clinical validation Most of the data accumulated so far about VC is from studies including either symptomatic or high risk patients for colorectal cancer. In one of the pioneer prospective studies, Hara et al detected the polyps 1 cm or larger in diameter with 75% sensitivity and 91% specificity [3] (Fig. 2). Performance decreased as lesion size decreased. Later Fenlon et al. reported an overall polyp detection sensitivity of 71%, with a sensitivity of 91% for polyps larger than 1 cm, 82% for polyps between 6 and 9 mm, and 55% for polyps less than 5 mm [4]. They reported 19 false positive polyps and no false positive findings of cancer. In one of the largest series with 180 patients and over 250 polyps Fletcher et al. detected 85% of polyps larger than 1 cm and 88% of
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polyps larger than 0.5 mm [5] (Fig. 3). Similarly Kay et al. reported a sensitivity of 90% and specificity of 82.1% for lesions greater than 1 cm [6] (Fig. 4). Although there have been a few studies suggesting a lower performance for VC [7,8] (especially in asymptomatic patients), data in the literature generally suggests a sensitivity of over 85% and specificity of more than 90% for detection of large colorectal polyps (larger than 1 cm in diameter) and colorectal cancers by VC [9,10] (Fig. 5). Further multicentric trials especially on the asymptomatic screening population comparing the performance of VC with other full structural colorectal examinations are needed for better evaluation of performance characteristics of VC.
Fig. 2. Virtual endoscopic view of a 18 mm polyp in the sigmoid colon. A smaller (7 mm) second polyp (arrow) can also be visualized in the distal colon.
Several technical problems degrading the accuracy of the technique remain to be solved. The pitfalls of the technique include retained stool and fluid, underdistension of the colon, complex folds, diverticuli, pedunculated polyps, iliocecal valve, small bowel reflux, extrinsic defects and respiratory and streak artifacts (Figs. 6 and 7).
4. Clinical applications of VC in colorectal carcinoma
Fig. 1. (a) CT colography (three-dimensional image formed by volume rendering algorithm). Please note the small polyp in the sigmoid colon. (b) Virtual endoscopic view. Characteristic triangular appearance of normal transverse colon.
VC may potentially have several roles in the evaluation of colorectal carcinoma. VC may be the technique of choice in the patients with incomplete colonoscopy, distal occlusive carcinomas and in elderly patients whose clinical status does not allow an invasive procedure such as conventional colonoscopy. The prevalence of synchronous neoplasms in patients with colorectal cancer varies 1.5–9 and 27–55% for adenoma [11–15]. Examination of the entire colon is recommended to identify a synchronous cancer before surgery and thus to prevent a second redundant surgical procedure (Fig. 8). Colorectal cancer patients who had a preoperative full colonoscopy were shown to have fewer local recurrences, less likely develop distant metastases and have a longer disease-free survival time [16–18]. However, in patients with distal occlusive cancers it is not infrequently very difficult to evaluate the proximal colon by conventional colonoscopy and double contrast barium enema (DCBE). Fenlon et al. showed that VC is a feasible and useful method for evaluating the entire colon before surgery in patients with occlusive carcinoma [19]. They detected two cancers and 24 polyps in the proximal colon of the 29 patients with colorectal carcinoma. Conventional
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colonoscopy fails to visualize the entire colon in 5–10% of examinations depending on several factors including the endoscopist’s skill and the patient’s tolerance [20,21]. In patients with incomplete colonoscopy, VC was reported to be effective in evaluating the proximal colon and show the unrevealed colon in more than 90% of patients [22,23]. VC may play an important role in the diagnosis and staging of colorectal carcinoma (Fig. 9). In a study with 38 pathologically proven colorectal cancers, VC correctly localized all 38 cancers, compared with 32 using conventional colonoscopy [24]. In another prospective
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study in 34 patients, VC could accurately identify all colorectal masses but overcalled stool masses in poorly distended or poorly prepared colons [25]. In that study VC correctly staged 81% of colorectal cancers by overstaging two cases and understaging one case. Another advantage of VC is its ability to assess the extraintestinal spread of the tumor: particularly the liver metastasis (Fig. 9). Hara et al. reported 11% of highly clinically important extraintestinal findings in their series of 264 patients who underwent CT colonography [26]. The ability of VC to evaluate the whole abdomen including the liver may preclude the use of other redundant
Fig. 3. Virtual endoscopic appearance (curved arrow) (a) and conventional colonoscopic view (b) of the same polyp in the distal sigmoid colon. Same polyp can also be detected at CT colography image (arrow).
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Fig. 4. Virtual endoscopic (a) and conventional colonoscopic view (b) of the polyp in the ascending colon. The polyp can be detected on the axial image (arrow) (c). Density measurement from the lesion showed a negative value, pathognomonic of a benign lipoma. Diagnosis was confirmed by colonoscopic biopsy.
imaging studies. These results suggest that VC may be a feasible method for detecting and staging of colorectal neoplasms. Since its first introduction, VC has been discussed as a potential imaging modality for colorectal cancer screening. Screening for colorectal polyps and neoplasms has been accepted as most colorectal neoplasms arise from preexisting adenomatous polyps and detection and removal of these precursor adenomas results in a decrease in the incidence and mortality from colorectal cancer [27–29]. American Cancer Society
emphasized the importance of the examination of the full colon in their revised guidelines for screening for early detection of colorectal polyps and cancer in 1997 [30]. As a full structural colorectal imaging examination VC competes with barium enema and colonoscopy. The diagnostic potential of VC seems to be much greater than fecal occult blood testing (FOBT). FOBT by nature lacks accuracy and does not allow inspection of the mucosal surface. More than half of all colorectal cancers and large majority of polyps will be missed on a single screening FOBT [31,32]. VC competes most
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directly with screening barium enema and it was found to be more sensitive than single-contrast barium enema for the detection of polyps [33]. Unfortunately to our knowledge there is no published data comparing the efficacy of VC and DCBE in the screening population. DCBE has been shown to detect more than 70% of polyps larger than 7mm [34] and the detection rate for colorectal cancer has been variable from 70 to 96% [35–37]. However, DCBE is highly dependent on the skill of the operator and it has several disadvantages
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when compared to VC including requirement for coating the colon mucosa with barium, duration of the examination and the patient tolerance. When compared to conventional colonoscopy, VC is relatively simple, less invasive, does not require sedation. Although requires preparation of the colon, it takes less time to perform the procedure. However, despite several studies showing good performance characteristics of VC, there are still studies mentioning that the poor sensitivity of VC is not enough for a screening test and especially flat
Fig. 5. Adenocarcinoma located in the rectosigmoid region. Lesion is visualized on coronal two-dimensional image (a), zoomed CT colonography (b; arrow), and virtual endoscopic view (c; curved arrow). (d) Gross pathologic specimen of the lesion after resection.
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5. Technical considerations
Fig. 6. Axial image from the level of the sigmoid colon. Collapsed segment of the sigmoid colon does not allow its optimal evaluation on CT colonography.
lesions are impossible to see by this method [8]. Another major disadvantage of VC is the lack of its ability to obtain tissue for pathologic diagnosis or remove the polyps. Sonnenberg and colleagues concluded that for VC to replace colonoscopy, the cost would have to drop to less than 50% of the cost of colonoscopy, or compliance rates for VC would have to be 15–20% better than conventional colonoscopy [38]. In a recent interesting article by Angtuaco et al., which investigates the attitudes among potential patients and primary care physicians concerning VC and conventional colonoscopy for colorectal cancer screening, availability of VC was shown to encourage the potential patients to participate in colorectal cancer screening [39].
Fig. 7. Endoscopic view of the multiple diverticuli in the sigmoid colon (arrows).
A detailed explanation about the technical issues related to VC is beyond the scope of this article. However, some important technical points will be briefly touched in this review. A proper bowel preparation is accepted as a crucial part of the examination since residual stool presents several problems in the interpretation of the images. A polyethylene glycol electrolyte solution preparation or a phospho-soda preparation the day prior to VC have been used in most of the studies. Macari et al concluded that a phospho-soda preparation provided significantly less residual fluid [40]. Contrast material labeling of the unprepared colon for CT colonography was recently investigated and results were found to be comparable to sensitivity for polyp detection in prepared colons [41]. Commercially available nutrition packages are prepared containing food and beverages, barium suspension and colonic cleansing agents. Helical CT protocols described by most of the studies are similar and use a collimation of 3–5 mm, reconstruction intervals of 1–3 mm and a pitch of 1–2 [9]. Compared to single detector CT, specific advantages of using multislice CT for the purpose of VC were suggested. The 1 mm slice thickness acquisitions may be possible with multislice CT resulting in a better z-axis resolution and hopefully this is expected to translate to better detection of smaller polyps. However preliminary studies with multislice CT did not show any significant difference in depiction of polyps larger than 10 mm between single and multi-detector row CT for a small number of polyps [42]. Large-scale studies are needed for further evaluation of multi-detector imaging in VC. Additional prone scanning was shown to improve the accuracy of VC and most centers have chosen to collect data in both the supine and prone positions [43]. Polyps and colorectal cancers were shown to enhance significantly following contrast administration and Morrin et al concluded that the use of intravenously administered contrast material significantly improved to depict medium sized polyps in suboptimally prepared colons [44,45]. Administration of intravenous contrast also allows evaluation of parenchymal organs with the same examination. VC interpretation is time-consuming; interpretation times between 15 min and 1 h have been reported [46–49]. Most researchers agree that examination of 2D images (both axial and multiplanar) are adequate for primary evaluation of the colon. Three-dimensional images are helpful to increase the observer’s confidence and to differentiate small polyps from complex folds. New software development studies for better displaying the imaging data and more efficient ways of interpretation such as automated polyp detection or electronic bowel cleansing are under way [50]. These new developments are expected to improve the diagnostic perfor-
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Fig. 8. Large, obstructing carcinoma in the distal descending colon (arrow) was detected on both conventional and also virtual colonoscopy (a). Proximal colon could not be evaluated due to obstructing lesion by conventional colonoscopy. However CT colonography managed to show a second polyp in the region of the hepatic flexure (b). CT colonography additionally demonstrated the multiple metastases in the same patient allowing a more complete preoperative evaluation of the patient (c).
mance and reproducibility of VC as well as to cause a decrease in the cost of the examination.
are the new developments in MR colonography [54– 57].
6. MR colonography
7. Virtual gastroscopy
MR colonography has been following close behind CT colonography. Based on the principles of contrast enhanced 3D MR angiography, breath-hold MR colonography has become possible [51– 53]. MR colonography has the known advantages of MR technology such as not utilizing ionizing radiation and capability of multiplanar imaging. Preliminary studies particularly performed by Debatin’s group in Essen University showed competitive results with CT colonography [54]. Fecal tagging and using CO2 enema instead of gadolinium-based enema to distend the colon
Similar to VC, the stomach can be evaluated with thin-section, high resolution 2D, axial images, reformatted two-dimensional images or computer-rendered 3D images simulating endoluminal images. For this purpose stomach is distended with ingestion of effervescent granules to increase the contrast difference between the gastric wall and its lumen. Due to limited number of studies conducted so far, it is not possible to mention an optimal scanning protocol. Issues like necessity of additional prone scanning, use of smooth muscle relaxants or intravenous contrast material dur-
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Fig. 9. Adenocarcinoma in the ascending colon with stranding in the pericolonic fat suggesting serosal invasion. Pericolonic invasion was confirmed after surgical resection.
ing scanning needs to be further investigated. Preliminary results show that 3D CT used together with virtual CT endoscopy is helpful in identifying gastric lesions and virtual CT gastroscopic images is useful in identifying both intraluminal and submucosal components of the gastric lesions [58]. In their series of 60 patients with advanced gastric carcinoma, Lee and Ko could obtain good or excellent images in 80% of patients [58]. Most of the patients with poor images had tumors in the antrum region. Lee and Ko also stated that combining 3D images with axial CT slices improved the accuracy in classifying Borrmann type and tumor staging. Virtual gastroscopy displays a wider field of view than gastroscopy, allows a more accurate measurement of tumor size and less invasive compared to conventional endoscopy. However, virtual gastroscopy is limited in spatial resolution and thus will not depict flat or small gastric lesions and will allow a detailed evaluation of mucosa. Further evaluation of this new technique is warranted in the diagnosis and staging of gastric cancers.
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