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As with any radiologic imaging test, there are a number of potential interpretive pitfalls at CT colonography CTC that need to be recognized and handled appropriately. Perhaps the single most important step in learning to avoid most of these diagnostic traps is simply to be aware of their existence. With a little experience, most of these potential pitfalls will be easily recognized. This review will systematically cover the key pitfalls confronting the radiologist at CTC interpretation, primarily dividing them into those related to technique and those related to underlying anatomy.
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: pitfalls related to technical factors, and pitfalls related to specific anatomic features. 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. Note that the adherent stool is nondependent on this prone 2D view, which could simulate a true lesion if untagged. Philadelphia: 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. From reference 5 , with permission. 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. 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.
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. 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.
CT Colonography: Pitfalls in Interpretation
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