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Virtual colonoscopy
Virtual Colonoscopy David J. Vining Virtual colonoscopy (VC) is an evolving technique that combines volume scanning and computer visualization methods to enable minimally invasive and effective colorectal cancer screening. Although VC offers significant clinical and public health advantages over conventional endoscopic screening, several issues confront VC's emergence into the medical marketplace.
Copyright© 1999by W.B. Saunders Company
IRTUAL COLONOSCOPY (VC) is a rapidly evolving technique that combines volume scanning and computer visualization methods to enable minimally invasive and effective colorectal cancer (CRC) screening. Although VC offers advantages such as better patient compliance, lower cost, and fewer risks compared to conventional diagnostic colonoscopy (DC), this new technique must first overcome several political, economic, and social hurdles before to its acceptance as a common radiologic procedure.
V
TECHNIQUE
The VC technique consists of three basic steps: (1) bowel preparation, (2) abdominal/pelvic scanning with helical CT or MRI, and (3) image analysis.
Bowel Preparation Optimal bowel preparation is essential for accurate VC results. Regardless of whether helical CT and MRI are used for the procedure, both modalities image the bowel at only one moment in time. The presence of residual feces may simulate polyps or masses, and retained fluid can obscure subtle lesions. In addition, collapsed segments of bowel during scanning may conceal or mimic neoplasms. Before scanning, a patient must undergo some form of cathartic bowel cleansing. 1 The addition of an oral contrast agent to the cleansing regimen has also been found useful for opacifying residual water and stool. 2 During scanning, the patient may require one or more of the following: pharmacologic bowel relaxation, intravenous contrast, contrast enema, and/or gas distention of the colon. The goal of bowel preparation is to achieve a clean and From the Department of Radiology, Wake Forest University School of Medicine. Winston-Salem. NC. Address reprint requests to David J. Vining, MD, Department of Radiology, Wake Forest University School of Medicine. Medical Center Blvd. Winston-Salem. NC 27157-1088. Copyright 0 1999 by W.B. Saunders Company 0887-2171/99/2001-0006510.00/0 56
well-distended colon that facilitates polyp detection and reduces the occurrence of false-positive findings. At the Wake Forest University School of Medicine (WFUSM), bowel preparation consists of the following: low-residue diet on the day before an examination, oral sodium phosphate (1.5 oz) during the night before and on the morning of an examination, and an oral iodinated contrast agent administered at least 2 hours before helical CT scanning. Intravenous Glucagon ® (Eli Lilly and Co, Indianapolis, IN) (1 mg) is administered after the patient has been positioned on the CT table, and the colon is distended with carbon dioxide gas. This method has been found to yield consistent VC results and to be tolerated by most patients. Refinements in colonic lavage preparations, contrast agents, and controlled bowel distention are expected to improve the accuracy of VC. Future development of mucosal contrast agents that are either absorbed or secreted differently between normal and abnormal areas of bowel wall remains a research challenge.
Abdominal~Pelvic Scanning Regardless of whether helical CT or MRI is used for imaging, it is important to acquire data volumes that are free of artifacts and yield the highest spatial resolution. At WFUSM, we use helical CT with a single breath-hold acquisition, 5-ram x-ray beam collimation. 2:1 pitch, l-mm reconstruction interval. 120 kilovolts (kVp), and 200 mAs. Although lower mAs techniques are capable of reducing radiation dose. we choose to use a higher milliamperage setting to provide better low-contrast resolution against the background of oral contrast. The use of MRI has certain advantages over CT, such as avoiding ionizing radiation and providing better soft-tissue contrast resolution3; however, its disadvantages include limited availability, greater cost, and lower patient acceptance. With the introduction in 1998 of a new generation of helical CT scanners that are capable of scanning faster and
Seminars in Ultrasound, CT, and MRI, Vo120, No 1 (February), 1999: pp 56-60
VIRTUAL COLONOSCOPY
57
Fig 1. Colon polyp. Helical CT images reveal a 1-cm polyp arising from the left rectal wall. (A) The polyp is not seen when abdominal window/level settings (440 W, 40 L) are used, but (B) it is easily detected (arrow) with lung settings (1900 W, - 6 0 0 L). (C) A surface-rendered image depicts the 3D appearance of this lesion.
improving z-axis spatial resolution, the debate as to whether CT or MRI is better suited for VC is far from over.
ImageAnalysis Several schemes exist for interpreting VC image data, but the ultimate goal of any method is to determine whether a colon contains polyps or masses (Fig 1). Various image analysis techniques exist, including the following: 1. Panning through two-dimensional (2D) multiplanar slices 2. Flying through three-dimensional (3D) surface-rendered or volume-rendered colon models 3. Navigating through computer simulations of the colon with the assistance of flight-path planning algorithms 4. Splitting or unfolding colon models 5. Computer-assisted polyp detection
In October 1998, at the First International Symposium on Virtual Colonoscopy held in Boston, MA, a general consensus among investigators was that inspection of 2D slices was sufficient for identifying potential lesions and that 3D imaging was useful for resolving whether a suspicions finding represented a true polyp or an anatomic variant, such as a hanstral fold. Inspection of the original image data is also important because incidental findings may be discovered that might account for a patient's abdominal symptoms. The illusion of traveling inside a patient's 3D colon is made possible by two computer techniques: perspective projection (objects closer to a user's viewpoint appear larger than objects of similar size that are farther from that viewpoint), and real-time rendering (generating 3D images at a rate of 15 to 30 frames per second). Depending on whether surface-rendering or volume-rendering is employed, the anatomy can resemble an opaque
58
Fig 2. Hybrid rendering. Hybrid rendering combines the imaging characteristics of surface-rendering and volumerendering simultaneously in a single image display. In this example, a surface-rendered colon is embedded inside a volume-rendered abdomen.
Fig 3. Computer-assisted polyp detection. Computer-assisted polyp detection algorithms correctly identified the location of the rectal polyp shown in Figure 1, and color-enhanced this lesion to increase its conspicuousness. The various colors reflect wallthickness measurements.
DAVID J. VINING
specimen or a semitransparent object, respectively. Hybrid rendering techniques are emerging which combine imaging features of surface-rendering with volume-rendering in the same interactive display (Fig 2). Virtual colonoscopy image analysis is often tedious, time-consuming, and prone to error of interpretation because a physician must navigate through the entire colon to visually search for lesions. The time required to analyze a typical VC case may be greater than an hour, but if VC is to be economically viable, interpretation time should be a matter of minutes. Image analysis is hindered by the fact that the normal visual cues (ie, mucosal color changes) that aid DC do not exist in VC. At WFUSM, we are developing computer-assisted polyp detection (CAPD) algorithms that estimate the colon's wall thickness at regular intervals and combine this information with polyp-shape features to identify potential lesions. Potential lesions found by CAPD are presented to a radiologist in an electronic list, and after a physician selects a lesion, the software automatically positions the physician's viewpoint toward the lesion for final inspection (Fig 3). 4
VIRTUAL COLONOSCOPY FUTURE CHALLENGES
Although VC has evolved into a feasible CRC screening procedure, several political, economic, and social challenges must first be addressed before its widespread implementation.
Accuracy VC must prove to be equal or superior to the current gold standard, DC, for the detection of colonic polyps and masses, an achievement that is already occurring at numerous institutions. Preliminary results from clinical trials comparing VC to DC indicate that the sensitivity and specificity of VC ranges from 75-100% and 86-90%, respectively, for the identification of polyps 1-cm or greater (those deemed significant lesions). 2,5-7 As VC comes under attack from the endoscopic community because of its inability to accurately detect diminutive polyps (5-mm or less), it is important to realize that even the DC gold standard has been shown to have a miss rate of approximately 24%. 8 In the future, radiologists and gastroenterologists will have to reach a consensus on what polyp size detected by VC will necessitate colonoscopy for polyp removal.
Reimbursement VC will require a competitive reimbursement rate to make it appealing for radiologists to perform and for insurers to support. In the case of Medicare reimbursement, VC might claim current procedural terminology (CPT) codes for a noncontrast CT scan of the abdomen (74150), CT scan of the pelvis (72192), and 3D reconstruction of these data (76375), for a total reimbursement of $577. 9 A colonoscopy procedure performed only for diagnostic purposes receives approximately $525, depending on local hospital charges. The establishment of a unique CPT code and relative value unit for VC might be considered, but such an initiative will be slow and complex; hence, the use of current CPT billing codes should be considered. Furthermore, VC will require acceptance by the Health Care Financing Administration as an effective CRC screening procedure. Until recently, Medicare reimbursement for screening did not exist. However, in the 1997 Balanced Budget Act, Congress authorized Medicare reimbursement for CRC screening by means of barium enema, sigmoidoscopy, or colonoscopy. Organized lobbying by radiologic organizations, industry, and health advocacy
59
groups will be necessary to make VC a reimbursable screening method.
Ease of Performance For VC to succeed, it will have to be easy to perform for all those involved. For patients, a "kinder and gentler" bowel preparation is necessary. For technologists, the design of equipment and protocols to streamline patient throughput is essential. For radiologists, efficient image interpretation and reporting need to be implemented.
Availability VC requires access to helical CT or MR1 scanners, devices that are emerging as standards of care in most medical centers. Several companies and academic institutions are actively developing VC systems, and although the ultimate success of any system will depend on its ability to efficiently and accurately process image data, unique software features and system costs will distinguish these evolving systems. Technologists trained in proper VC techniques and radiologists experienced in interpreting VC images are necessary to provide proficient services. Eventually, VC may evolve into a radiology subspecialty, such as mammography, thus requiring personnel training and quality-assurance monitoring.
Acceptance The greatest challenge that VC faces is acceptance by primary care physicians, insurers, and the public. The National Health Interview Survey revealed that only a small percentage of eligible individuals has been screened for colorectal cancer. In 1992, only 17.3% of people aged 50 years or older had undergone fecal occult blood testing in the previous year, and 9.4% had undergone flexible sigmoidoscopy in the previous 3 years. 1°,11Educational programs designed to change the pubfic's perception of CRC screening will need to be instituted. CONCLUSION
VC has captured the public's attention because of its potential for decreased discomfort and inconvenience, as well as considerably lower cost and risks, compared to routine CRC screening methods. Although VC is technically feasible, several political, economic, and social issues must first be solved before VC can take its place in the radiologic armamentarium.
60
DAVID J. VINING
REFERENCES
1. Golub RW, Kerner BA, Wise WE, et al: Colonoscopic bowel preparations-Which one? Dis Colon Rectum 38:594599, 1995 2. Pinean BC, Mikulaninec C, Vining DJ: Virtual colonoscopy using oral sodium phosphate colonic lavage and an oral iodinated contrast agent for the detection of colonic spaceoccupying lesions. Am J Gastroentero193:1697, 1998 (abstr) 3. Schoenenberger AW, Bauerfeind R Krestin GR et al: Virtual colonoscopy with magnetic resonance imaging: In vitro evaluation of a new concept. Gastroenterology 112:1863-1870, 1997 4. Vining DJ, Ge Y, Ahn D, Stelts D, et al: Enhanced virtual colonoscopy system employing automatic detection of colon polyps. Gastroenterology 114:698, 1998 (abstr) 5. Hara AK, Johnson CD, Reed JE, et al: Detection of colorectal polyps with CT colography: Initial assessment of sensitivity and specificity. Radiology 205:59-65, 1997 6. Nunes D: Virtual colonoscopy: A gastroenterologist's view. Presented at the First International Symposium on Virtual Colonoscopy, Boston, MA, October 1-2, 1998
7. Dachman AH: CT colography: The University of Chicago experience. Presented at the First International Symposium on Virtual Colonoscopy, Boston, MA, October 1-2, 1998 8. Rex DK, Cutler CS, Lemmel GT, et al: Colonoscopic miss rates of adenomas determined by back-to-back colonoscopies. Gastroenterology 112:24-28, 1997 9. Cigna CIGNA/Medicare Administration. CIGNA North Carolina 1998 fee schedule. Nashville, TN, CIGNA/Medicare Administration, U.S. Department of Health and Human Services, 1997, pp 34-38 10. Anderson LM, May DS: Has the use of cervical, breast, and colorectal cancer screening increased in the United States? Am J Public Health 55:840-842, 1995 11. Brown ML, Potosky AL, Thompson GB, et al: The knowledge and use of screening tests for colorectal and prostate cancer: Data from the 1987 National Health Interview Survey. Prey Med 19:562-574, 1990
Copyright© 1999by W.B. Saunders Company
IRTUAL COLONOSCOPY (VC) is a rapidly evolving technique that combines volume scanning and computer visualization methods to enable minimally invasive and effective colorectal cancer (CRC) screening. Although VC offers advantages such as better patient compliance, lower cost, and fewer risks compared to conventional diagnostic colonoscopy (DC), this new technique must first overcome several political, economic, and social hurdles before to its acceptance as a common radiologic procedure.
V
TECHNIQUE
The VC technique consists of three basic steps: (1) bowel preparation, (2) abdominal/pelvic scanning with helical CT or MRI, and (3) image analysis.
Bowel Preparation Optimal bowel preparation is essential for accurate VC results. Regardless of whether helical CT and MRI are used for the procedure, both modalities image the bowel at only one moment in time. The presence of residual feces may simulate polyps or masses, and retained fluid can obscure subtle lesions. In addition, collapsed segments of bowel during scanning may conceal or mimic neoplasms. Before scanning, a patient must undergo some form of cathartic bowel cleansing. 1 The addition of an oral contrast agent to the cleansing regimen has also been found useful for opacifying residual water and stool. 2 During scanning, the patient may require one or more of the following: pharmacologic bowel relaxation, intravenous contrast, contrast enema, and/or gas distention of the colon. The goal of bowel preparation is to achieve a clean and From the Department of Radiology, Wake Forest University School of Medicine. Winston-Salem. NC. Address reprint requests to David J. Vining, MD, Department of Radiology, Wake Forest University School of Medicine. Medical Center Blvd. Winston-Salem. NC 27157-1088. Copyright 0 1999 by W.B. Saunders Company 0887-2171/99/2001-0006510.00/0 56
well-distended colon that facilitates polyp detection and reduces the occurrence of false-positive findings. At the Wake Forest University School of Medicine (WFUSM), bowel preparation consists of the following: low-residue diet on the day before an examination, oral sodium phosphate (1.5 oz) during the night before and on the morning of an examination, and an oral iodinated contrast agent administered at least 2 hours before helical CT scanning. Intravenous Glucagon ® (Eli Lilly and Co, Indianapolis, IN) (1 mg) is administered after the patient has been positioned on the CT table, and the colon is distended with carbon dioxide gas. This method has been found to yield consistent VC results and to be tolerated by most patients. Refinements in colonic lavage preparations, contrast agents, and controlled bowel distention are expected to improve the accuracy of VC. Future development of mucosal contrast agents that are either absorbed or secreted differently between normal and abnormal areas of bowel wall remains a research challenge.
Abdominal~Pelvic Scanning Regardless of whether helical CT or MRI is used for imaging, it is important to acquire data volumes that are free of artifacts and yield the highest spatial resolution. At WFUSM, we use helical CT with a single breath-hold acquisition, 5-ram x-ray beam collimation. 2:1 pitch, l-mm reconstruction interval. 120 kilovolts (kVp), and 200 mAs. Although lower mAs techniques are capable of reducing radiation dose. we choose to use a higher milliamperage setting to provide better low-contrast resolution against the background of oral contrast. The use of MRI has certain advantages over CT, such as avoiding ionizing radiation and providing better soft-tissue contrast resolution3; however, its disadvantages include limited availability, greater cost, and lower patient acceptance. With the introduction in 1998 of a new generation of helical CT scanners that are capable of scanning faster and
Seminars in Ultrasound, CT, and MRI, Vo120, No 1 (February), 1999: pp 56-60
VIRTUAL COLONOSCOPY
57
Fig 1. Colon polyp. Helical CT images reveal a 1-cm polyp arising from the left rectal wall. (A) The polyp is not seen when abdominal window/level settings (440 W, 40 L) are used, but (B) it is easily detected (arrow) with lung settings (1900 W, - 6 0 0 L). (C) A surface-rendered image depicts the 3D appearance of this lesion.
improving z-axis spatial resolution, the debate as to whether CT or MRI is better suited for VC is far from over.
ImageAnalysis Several schemes exist for interpreting VC image data, but the ultimate goal of any method is to determine whether a colon contains polyps or masses (Fig 1). Various image analysis techniques exist, including the following: 1. Panning through two-dimensional (2D) multiplanar slices 2. Flying through three-dimensional (3D) surface-rendered or volume-rendered colon models 3. Navigating through computer simulations of the colon with the assistance of flight-path planning algorithms 4. Splitting or unfolding colon models 5. Computer-assisted polyp detection
In October 1998, at the First International Symposium on Virtual Colonoscopy held in Boston, MA, a general consensus among investigators was that inspection of 2D slices was sufficient for identifying potential lesions and that 3D imaging was useful for resolving whether a suspicions finding represented a true polyp or an anatomic variant, such as a hanstral fold. Inspection of the original image data is also important because incidental findings may be discovered that might account for a patient's abdominal symptoms. The illusion of traveling inside a patient's 3D colon is made possible by two computer techniques: perspective projection (objects closer to a user's viewpoint appear larger than objects of similar size that are farther from that viewpoint), and real-time rendering (generating 3D images at a rate of 15 to 30 frames per second). Depending on whether surface-rendering or volume-rendering is employed, the anatomy can resemble an opaque
58
Fig 2. Hybrid rendering. Hybrid rendering combines the imaging characteristics of surface-rendering and volumerendering simultaneously in a single image display. In this example, a surface-rendered colon is embedded inside a volume-rendered abdomen.
Fig 3. Computer-assisted polyp detection. Computer-assisted polyp detection algorithms correctly identified the location of the rectal polyp shown in Figure 1, and color-enhanced this lesion to increase its conspicuousness. The various colors reflect wallthickness measurements.
DAVID J. VINING
specimen or a semitransparent object, respectively. Hybrid rendering techniques are emerging which combine imaging features of surface-rendering with volume-rendering in the same interactive display (Fig 2). Virtual colonoscopy image analysis is often tedious, time-consuming, and prone to error of interpretation because a physician must navigate through the entire colon to visually search for lesions. The time required to analyze a typical VC case may be greater than an hour, but if VC is to be economically viable, interpretation time should be a matter of minutes. Image analysis is hindered by the fact that the normal visual cues (ie, mucosal color changes) that aid DC do not exist in VC. At WFUSM, we are developing computer-assisted polyp detection (CAPD) algorithms that estimate the colon's wall thickness at regular intervals and combine this information with polyp-shape features to identify potential lesions. Potential lesions found by CAPD are presented to a radiologist in an electronic list, and after a physician selects a lesion, the software automatically positions the physician's viewpoint toward the lesion for final inspection (Fig 3). 4
VIRTUAL COLONOSCOPY FUTURE CHALLENGES
Although VC has evolved into a feasible CRC screening procedure, several political, economic, and social challenges must first be addressed before its widespread implementation.
Accuracy VC must prove to be equal or superior to the current gold standard, DC, for the detection of colonic polyps and masses, an achievement that is already occurring at numerous institutions. Preliminary results from clinical trials comparing VC to DC indicate that the sensitivity and specificity of VC ranges from 75-100% and 86-90%, respectively, for the identification of polyps 1-cm or greater (those deemed significant lesions). 2,5-7 As VC comes under attack from the endoscopic community because of its inability to accurately detect diminutive polyps (5-mm or less), it is important to realize that even the DC gold standard has been shown to have a miss rate of approximately 24%. 8 In the future, radiologists and gastroenterologists will have to reach a consensus on what polyp size detected by VC will necessitate colonoscopy for polyp removal.
Reimbursement VC will require a competitive reimbursement rate to make it appealing for radiologists to perform and for insurers to support. In the case of Medicare reimbursement, VC might claim current procedural terminology (CPT) codes for a noncontrast CT scan of the abdomen (74150), CT scan of the pelvis (72192), and 3D reconstruction of these data (76375), for a total reimbursement of $577. 9 A colonoscopy procedure performed only for diagnostic purposes receives approximately $525, depending on local hospital charges. The establishment of a unique CPT code and relative value unit for VC might be considered, but such an initiative will be slow and complex; hence, the use of current CPT billing codes should be considered. Furthermore, VC will require acceptance by the Health Care Financing Administration as an effective CRC screening procedure. Until recently, Medicare reimbursement for screening did not exist. However, in the 1997 Balanced Budget Act, Congress authorized Medicare reimbursement for CRC screening by means of barium enema, sigmoidoscopy, or colonoscopy. Organized lobbying by radiologic organizations, industry, and health advocacy
59
groups will be necessary to make VC a reimbursable screening method.
Ease of Performance For VC to succeed, it will have to be easy to perform for all those involved. For patients, a "kinder and gentler" bowel preparation is necessary. For technologists, the design of equipment and protocols to streamline patient throughput is essential. For radiologists, efficient image interpretation and reporting need to be implemented.
Availability VC requires access to helical CT or MR1 scanners, devices that are emerging as standards of care in most medical centers. Several companies and academic institutions are actively developing VC systems, and although the ultimate success of any system will depend on its ability to efficiently and accurately process image data, unique software features and system costs will distinguish these evolving systems. Technologists trained in proper VC techniques and radiologists experienced in interpreting VC images are necessary to provide proficient services. Eventually, VC may evolve into a radiology subspecialty, such as mammography, thus requiring personnel training and quality-assurance monitoring.
Acceptance The greatest challenge that VC faces is acceptance by primary care physicians, insurers, and the public. The National Health Interview Survey revealed that only a small percentage of eligible individuals has been screened for colorectal cancer. In 1992, only 17.3% of people aged 50 years or older had undergone fecal occult blood testing in the previous year, and 9.4% had undergone flexible sigmoidoscopy in the previous 3 years. 1°,11Educational programs designed to change the pubfic's perception of CRC screening will need to be instituted. CONCLUSION
VC has captured the public's attention because of its potential for decreased discomfort and inconvenience, as well as considerably lower cost and risks, compared to routine CRC screening methods. Although VC is technically feasible, several political, economic, and social issues must first be solved before VC can take its place in the radiologic armamentarium.
60
DAVID J. VINING
REFERENCES
1. Golub RW, Kerner BA, Wise WE, et al: Colonoscopic bowel preparations-Which one? Dis Colon Rectum 38:594599, 1995 2. Pinean BC, Mikulaninec C, Vining DJ: Virtual colonoscopy using oral sodium phosphate colonic lavage and an oral iodinated contrast agent for the detection of colonic spaceoccupying lesions. Am J Gastroentero193:1697, 1998 (abstr) 3. Schoenenberger AW, Bauerfeind R Krestin GR et al: Virtual colonoscopy with magnetic resonance imaging: In vitro evaluation of a new concept. Gastroenterology 112:1863-1870, 1997 4. Vining DJ, Ge Y, Ahn D, Stelts D, et al: Enhanced virtual colonoscopy system employing automatic detection of colon polyps. Gastroenterology 114:698, 1998 (abstr) 5. Hara AK, Johnson CD, Reed JE, et al: Detection of colorectal polyps with CT colography: Initial assessment of sensitivity and specificity. Radiology 205:59-65, 1997 6. Nunes D: Virtual colonoscopy: A gastroenterologist's view. Presented at the First International Symposium on Virtual Colonoscopy, Boston, MA, October 1-2, 1998
7. Dachman AH: CT colography: The University of Chicago experience. Presented at the First International Symposium on Virtual Colonoscopy, Boston, MA, October 1-2, 1998 8. Rex DK, Cutler CS, Lemmel GT, et al: Colonoscopic miss rates of adenomas determined by back-to-back colonoscopies. Gastroenterology 112:24-28, 1997 9. Cigna CIGNA/Medicare Administration. CIGNA North Carolina 1998 fee schedule. Nashville, TN, CIGNA/Medicare Administration, U.S. Department of Health and Human Services, 1997, pp 34-38 10. Anderson LM, May DS: Has the use of cervical, breast, and colorectal cancer screening increased in the United States? Am J Public Health 55:840-842, 1995 11. Brown ML, Potosky AL, Thompson GB, et al: The knowledge and use of screening tests for colorectal and prostate cancer: Data from the 1987 National Health Interview Survey. Prey Med 19:562-574, 1990