CT Colonography: Principles and Practice of Virtual Colonoscopy

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Tips and pointers for how to effectively handle these potential pitfalls are included. CT colonography CTC has rapidly evolved into a highly effective minimally invasive test for detecting colorectal polyps and cancers. However, even when proven state-of-the-art techniques are consistently applied, there are a number of potential pitfalls that may be encountered at interpretation.

When suboptimal techniques are applied, the number and severity of interpretive pitfalls can rapidly multiply, underscoring the need for high quality practice standards. At first glance, the laundry list of potential pitfalls at CTC may seem rather daunting Table 1. These can largely be divided into two main categories: With proper awareness, these potential pitfalls can be effectively managed so as to minimize any negative impact on diagnostic performance.

Common interpretive pitfalls will be reviewed, with illustrations to demonstrate the typical appearances. A more extensive review with hundreds of illustrations can be found in our dedicated referenced textbook. Residual stool represents a fundamental diagnostic challenge for CTC interpretation, even when cathartic agents are employed. Although laxatives and lavages generally remove the major bulk of fecal volume, residual adherent debris can closely mimic the appearance of soft tissue polyps, especially if not tagged by oral contrast.

Unlike formed stool, which typically contains foci of near-air density, smaller particulate fecal matter can more closely approximate uniform soft tissue attenuation. This underscores the critical need for oral contrast tagging, which is highly effective for internally labeling otherwise nonspecific residual adherent stool Figs. Both 3D translucency rendering B and 2D correlation C show dense internal contrast tagging, easily excluding a polyp.


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Note that the adherent stool is nondependent on this prone 2D view, which could simulate a true lesion if untagged. Saunders; , with permission. However, both 3D with translucency rendering B and coronal 2D correlation C show dense internal contrast tagging, excluding a flat polyp. Care must be taken in such cases to ensure a true flat soft tissue polyp does not lie deep to the contrast. As such, we continue to employ both of these contrast agents in our CTC bowel preparation. At primary 3D evaluation, translucency rendering can rapidly demonstrate internal tagging of fecal material Figs.

Nonetheless, it is the 2D display that provides the most definitive assessment for equivocal findings seen on 3D. In our experience, a false positive interpretation due to residual stool is extremely rare when using our dedicated cathartic preparation with the dual contrast tagging regimen. Untagged stool, however, continues to be a major issue at same-day completion CTC following incomplete optical colonoscopy OC. Incomplete tagging of solid stool will likely be an important issue facing non-cathartic approaches as well. Untagged residual luminal fluid can obscure even large polyps and masses when they are submerged.

Prone transverse 2D CTC image with polyp windowing A shows a flat cecal polyp arrowhead , which is submerged under opacified fluid but is nonetheless detectable. On the soft tissue window setting B , however, the lesion is obscured by the dense surrounding fluid. This windowing phenomenon is also the reason why 2D lesion measurement must take place on the wider polyp window setting. The lesion was confirmed at subsequent OC D and proved to be a tubulovillous adenoma.

Although we initially performed electronic cleansing on the tagged fluid prior to interpretation very early on in our CTC experience, we discontinued this practice in due to the troublesome artifacts that were introduced discussed later. Due to the complementary shifting of luminal fluid between supine and prone positions, it is extremely rare to have a significant polyp completely submerged on both views with our standard CTC preparation. In this setting, we administer 30 ml of diatrizoate once the patient has adequately recovered from sedation, and wait up to 2—3 hours prior to scanning.

In the future, a better approach might be to give diatrizoate as part of the original OC preparation, which would allow for a reduction in the amount of cathartic needed and also provide fluid tagging for CTC in the event of an incomplete OC examination. The minimum requirement for a diagnostic CTC evaluation is to have all segments at least partially distended on at least one view. Cases with focal or segmental partial but incomplete collapse may be suboptimal but are often diagnostic.

Compared with excellent luminal distention, such cases generally require more scrutiny, as the luminal narrowing is compounded by dynamic thickening of the colonic folds, which makes interpretation more challenging. However, because relevant colorectal lesions require detection on just one view, confirmation on the lesser quality view is generally achievable even in cases of inadequate distention Fig. Supine 2D CTC images A and B show long-segment collapse of the sigmoid colon, largely obscuring a mm polyp arrows , which is easily identified on the alternate position C and D.

This proved to be a tubular adenoma after resection at OC E. For cases in which complete focal collapse persists at the same point on both supine and prone displays, a decubitus view will usually allow for diagnostic assessment. Because the sigmoid and descending colon account for the vast majority of such cases, typically related to underlying diverticular disease as discussed later on , a right lateral decubitus view is typically performed Fig. Online assessment of the 2D images for adequate left-sided distention during CTC examination should be made by the technologist at the CT console because the scout view alone can be unreliable or misleading.

Supine 2D CTC image A shows long-segment collapse of the sigmoid colon, related to diverticular disease.

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The prone view had a similar appearance not shown. Luminal distention on the decubitus view C , however, was excellent and allowed for a diagnostic examination. Additional reasons include that glucagon is generally not effective, Buscopan is not available in the U. For cases of truly non-diagnostic evaluation in the sigmoid or descending colon, unsedated same-day flexible sigmoidoscopy may be performed to complete the screening evaluation. Automated low-pressure CO 2 delivery provides for adequate distention on a more consistent basis than manual room air insufflations, and should be considered the standard of care.

This issue is exacerbated in morbidly obese patients, where the low-pressure CO 2 cannot overcome extracolonic pressures. In such cases, decubitus positioning or even conversion to manual room air may be necessary on occasion Fig. An equilibrium pressure of 20 mm Hg was utilized for the automated CO 2. Decubitus positioning B and increase to 25 mm Hg resulted in good luminal distention of this segment, as shown by frontal C and lateral D 3D colon maps. There are a wide variety of potential artifacts related to CT scanning, image reconstruction, and post-processing that can result in interpretive challenges.

Most radiologists with ample experience in body CT interpretation, including advanced visualization techniques, will be adept at handling most of these imaging artifacts. The larger concern stems from potential interpretation by non-radiologists with minimal CTC training and little or no familiarity with either general CT interpretation or the basic physics of medical imaging.

Although CAD has been advanced as a way to compensate for inadequate training, this notion ignores the fact that poor specificity would lead to an unacceptable false-positive rate. Artifacts related to respiratory and other patient motion are much less common with multi-detector CT MDCT scanners having 16 or more channels, due to shorter acquisition times. Dynamic spilling of the opacified fluid between differential air-fluid levels is almost always apparent.

Beam-hardening artifacts related to metallic objects such as spinal hardware or hip prostheses are accentuated by the low-dose CTC technique Fig. Polyp windowing reduces the impact of the beam-hardening artifact, and may allow for lesion detection.


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  3. CT Colonography: Pitfalls in Interpretation.
  4. Novel reconstruction algorithms should reduce the effect of beam hardening in the near future. Prone 2D CTC image A shows multiple arciform streaks off the air-fluid level in the descending colon, which is caused by the intra-scan flow of fluid. Note the lack of motion or artifacts elsewhere on the image.

    The artifacts are also evident on the 3D endoluminal view B. Streak artifact across the rectum is also apparent on the 3D endoluminal view C. A number of important artifacts result from post-processing of the MDCT source data. However, geometric distortion is the unavoidable trade-off, which can greatly compromise polyp recognition.

    By decreasing the number of fly-throughs from four down to two on well-distended cases, one can decrease interpretation time without introducing troublesome spatial distortion. Submerged semi-solid stool that approaches soft tissue density can appear polypoid as well.

    Introduction

    Interrogating these pseudopolyps will not only increase interpretation time, it also has the potential to decrease specificity if such lesions are mistaken for true pathology. With our current bowel preparation, the fluid level almost never overlaps between supine and prone positioning. Furthermore, true lesions are detectable on 2D within tagged fluid pools without subtraction. As digital fluid subtraction techniques continue to improve, the various artifacts will perhaps be minimized or eliminated.

    Concerns regarding potential harms from radiation dose exposure related to CT have increased recently in the U. Although any health risks related to CT-level doses in adults are too small to measure, 21 it nonetheless behooves radiology as a specialty to minimize dose to the lowest levels possibly that maintain diagnostic accuracy. Fortunately for CTC, there are a number of factors that further reduce the concern for radiation.

    The study is generally performed on older adults and excludes most of the chest. Furthermore, the inherent characteristics of the colon wall-air interface allow for substantial dose reduction at CTC compared with standard abdominal CT imaging. However, at extremely low dose levels, image noise can become an issue even for CTC, especially with the use of traditional filtered back projection for image reconstruction.

    Noise is especially problematic when reading thin images in obese patients, particularly when viewing soft tissue windows. It is critical not to mistake the image noise within a true lesion on 2D soft tissue windowing or 3D translucency rendering as low-attenuation heterogeneity from stool or fat. Image noise is much better tolerated on the wider 2D polyp windows.

    On the 3D endoluminal view, image noise from low-dose technique manifests as surface mottling when mild to luminal streaks when severe Fig. The rectal catheter is visible but the rectal polyp is largely obscured. When the same CT image data are reconstructed with a newer iterative reconstruction algorithm B , the polyp arrow becomes much more conspicuous.

    With a fixed mA low-dose technique, image noise can be accentuated inferiorly due to the bony pelvis but may be unnecessarily low for the upper abdomen. Tube current modulation can avoid this discordance by boosting the mA only as needed to maintain a static noise level. Some CTC protocols target more aggressive dose reduction on the prone view since much of the information is redundant to the supine view.

    Perhaps the most significant dose reduction will come from the implementation of the newer iterative reconstructions techniques, some of which allow for routine sub-mSv scanning for CTC Fig. One potential pitfall that has actually become a useful interpretive asset is the tendency for true soft tissue polyps, particularly flat and villous lesions, to demonstrate a thin surface coating of adherent positive oral contrast Fig.

    On 2D, contrast coating of polyps is easy to distinguish from internal tagging of stool, which is a critical distinction. In effect, this thin surface coating of contrast serves as a beacon for polyp detection. One notable pitfall is the fact that small coated polyps may mimic tagged stool on the 3D translucency view, since the underlying soft tissue signature may be obscured by the overlying shell of contrast. Transverse 2D CTC images with polyp A and soft tissue B window settings show a multi-lobulated mass occupying the expected location of the ileocecal valve.

    Note the distinct contrast etching that outlines the surfaces of the lesion. The search pattern for detecting polyps on 2D, especially small 6—9 mm lesions, is simply too onerous to maintain acceptable performance. In contrast, lesion conspicuity at 3D is greatly enhanced but requires adequate CTC software for execution. Subsequent CTC trials adding primary 3D detection alongside 2D review showed markedly improved sensitivity for polyps. Although CTC is the most accurate method available for polyp measurement, 30 inaccurate size assessment remains an important potential pitfall because it could lead to inappropriate patient management.

    It is important to recognize that 2D polyp measurement on the standard orthogonal views i. Polyp measurement on the 3D endoluminal view can also be problematic if care is not taken to optimize the vantage point. Incorrect caliper placement, partially submerged polyps, and thick contrast coating of polyps can also lead to erroneous 3D measurement.

    Over-sizing diminutive lesions on 3D is a common pitfall that can lead to over-aggressive management if not carefully correlated with the 2D polyp size.

    9781416061687 - CT Colonography: Principles and Practice of Virtual Colonoscopy, 1e by PICKHARDT

    Over time, one can generally learn to appreciate lesions that are clearly diminutive in size without the need to formally measure each one. Polyp measurement on soft tissue windows could lead to inappropriate management. On the prone 2D CTC images C and D , the polyp arrow is submerged under densely opacified fluid, which further decreases the apparent polyp size. Note how the lesion is barely perceptible on the soft tissue window setting D.

    For suspected polyps approaching 5—6 mm or greater detected at CTC, we recommend performing a careful combined 2D and 3D size assessment. For most polyps, the 2D and 3D measurements will be within 1 mm of each other. Unless the lesion is at or near a critical size threshold i. However, if the lesion is bordering one of these two important thresholds, or there is a relatively large discrepancy between the 2D and 3D assessment, then a judgment call is necessary to determine the most appropriate size to report.

    Polyp size need only be reported to the nearest mm. Beyond CTC, OC with use of a calibrated probe is probably the next best method, whereas visual estimation at OC is less accurate, and pathologic ex vivo measurement is least accurate of all. Fold thickening at CTC is largely due to inadequate luminal distention, underlying diverticular disease, or a combination of the two.

    At 2D evaluation, it can be challenging to distinguish an unimportant thickened fold from relevant pathology. However, as long as the lumen is at least partially distended, primary 3D evaluation can generally make this distinction, as the smooth, uniform, and often circumferential nature of incidental fold thickening is readily apparent Fig.

    - CT Colonography: Principles and Practice of Virtual Colonoscopy, 1e by PICKHARDT

    Atypical thickened folds will occasionally need confirmation at OC; biopsy is sometimes necessary, but often yields only normal mucosa. Other causes of apparent focal soft tissue thickening at 2D CTC include complex or convergent folds, redundant colonic segments with subclinical twisting, and dynamic spasm or collapse, all of which are best appreciated on the 3D endoluminal view. These pitfalls must be distinguished from true flat lesions, which are discussed below.

    The smooth thickening appears to be related to a point of slight twisting or torsion. Note the fat extending into the fold on B, which excludes an infiltrating cancer. From these pathologic features, and combined with its high prevalence, diverticular disease not surprisingly represents the leading cause of nondiagnostic segmental evaluation at CTC.

    At 2D evaluation, evaluating segments with advanced diverticular disease can be a daunting task Fig. However, as long as luminal narrowing does not impede 3D endoluminal evaluation Fig.

    This appearance can make it challenging to exclude superimposed neoplastic pathology. Exclusion of superimposed polyps is a much simpler task on the 3D endoluminal view. Supine A and B and decubitus C CTC images show an area of persistent wall thickening and luminal narrowing arrowheads in the setting of advanced sigmoid diverticular disease.


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    7. The prone images had a similar appearance not shown. The 3D colon map shows the site of this persistent stenosis arrow and red dot , which proved to be a diverticular stricture. Unlike barium enema examination, the diverticula themselves cause little problem at CTC interpretation. However, there are additional pitfalls related to diverticular disease beyond fold thickening and inadequate luminal distention. Frankly inverted diverticula are relatively rare but represent a related pitfall. Prolapsing mucosal polyps represent redundant colonic mucosa in the setting of sigmoid diverticular disease.

      These non-neoplastic lesions can be difficult to differentiate from neoplastic disease. Although the traditional definition was a lesion height that is less than half the width, a preferred definition for flat polyps less than 3 cm in size is an elevation above the mucosal surface that does not exceed 3 mm. Nonetheless, the sensitivity of combined 3D-2D polyp detection with contrast tagging appears to be satisfactory. In our clinical screening experience, more large flat advanced adenomas were detected at primary CTC screening compared with primary OC screening. In fact, the majority of flat lesions detected or missed at CTC are hyperplastic.

      The prevalence and clinical relevance of flat colonic lesions have been the source of great debate. A clear distinction must be made between relatively flat lesions, and completely flat or depressed lesions , which are quite rare. Supine transverse 2D A and B and 3D endoluminal C CTC images show a large flat soft tissue mass arrowheads opposite the ileocecal valve arrow that has a somewhat lobulated appearance and results in fold distortion on 3D.

      Note the contrast coating portions of the lesion on B. This carpet lesion was confirmed at same-day optical colonoscopy D and proved to be a tubulovillous adenoma. Most non-flat lesions of this large size would be malignant. Although a wide variety of neoplastic 54 and non-neoplastic 55 causes for a submucosal luminal impression exist at CTC, only a relatively small subset might potentially be confused for a mucosal-based soft tissue polyp or mass.

      The same cannot be said for luminal examinations like BE and OC. At CTC, specific definitive diagnosis can be made in the case of submucosal lipomas, pneumatosis, and extrinsic impression from extracolonic structures. The presence of inwardly displaced but uninterrupted folds at 3D endoluminal CTC strongly suggests extrinsic impression Fig. At subsequent OC B , however, the lesion proved to be a submucosal venous bleb. Transverse 2D image C , however, shows the small bowel loop arrow extending across the adjacent colon. At 3D D , the preservation of the overlying colonic fold is a sign that the lesion is caused by extrinsic impression.

      There are a number of findings specific to the anorectum that deserve attention because they are relatively common and can mimic neoplastic disease. The first key is awareness of anorectal-specific pathology. In many ways, the anorectal region represents the most important source of pitfalls at CTC, since common incidental findings may distract the reader from important underlying pathology, which may be relatively subtle due its specific location.

      The anorectal pitfalls that seem to cause the most trouble at CTC interpretation are hypertrophied anal papillae, internal hemorrhoids, the rectal balloon catheter, and low rectal tumors. A variety of other anorectal pathology is much less commonly encountered. Hypertrophied anal papillae represent focal fibrous protrusions at the dentate line that essentially represent internal skin tags.

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