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Everything you need to know about Computed Tomography (CT) & CT Scanning


Colon: Neoplastic Disease: CT Colonography: New Concepts in Image Display and Analysis

 

Karen M. Horton MD, and Elliot K. Fishman MD


 

Introduction

CT colonography (CTC) is considered by many to be a potential breakthrough in the screening for colorectal polyps and cancer. Accuracy for detection of polyps > 1cm in size is as high a 90% using 3mm collimation. The technique involves volume acquisition of CT data after colonic cleansing. Once the data is acquired, there are a variety of two-dimensional and three-dimensional visualization techniques which can be utilized to a obtain a detailed examination of the entire colon. The viewing/display technique which has gained widespread attention is the simulated endoluminal view of the colon (virtual colonoscopy). This method allows the colon to be viewed from the inside, similar to the view through an endoscope as used by the gastroenterologists. Although this method of display can be useful, it is time consuming and computer intensive. In addition, most radiologists are not familiar with this endoluminal perspective, and will thus require considerable training. In practice, it is often necessary to utilize a variety of imaging display techniques to best visualize the anatomy and pathology.

This exhibits illustrates the utility of 3D volume rendering of the and reviews various display techniques which can be utilized when performing CTC.. These display techniques and be useful in a variety of clinical applications including screening studies for colorectal polyps, evaluation and staging of patients with known colorectal cancer, and in surgical planning. In many cases, an optimized CT examination using different display parameters can obviate the need for additional studies (i.e. contrast enema or colonoscopy).


 

Technique

Patient Preparation

CTC requires bowel cleansing for optimal results. Most centers use 1 gallon of GoLytely the evening before the study. Immediately before the exam, an enema tip is inserted into the rectum and either room air or carbon dioxide is instilled to maximum patient tolerance (1-2L). Both prone and supine imaging is necessary. Bowel relaxants such as Glucagon can be administered when necessary to alleviate spasm. However, a recent study of 60 patients by Yee et al showed that the routine use of IV Glucagon dose not significantly improve colonic distention. Further investigation is necessary.

Although IV contrast is not typically necessary when screening for colorectal polyps, it is definitely helpful when performing CTC for cancer staging, follow-up, or surgical planning. In these circumstances, the patient receives 120cc of Onmipaque 350 at 3cc/sec, with a scan delay of 40-50 seconds.

Scanning Parameters

There are many different protocols being investigated for CT colonography. Most are dependent on the type of scanner and its capabilities. The optimal technique has not yet been determined. With helical scanners, investigators have utilized 3-5mm collimation with a pitch of 1.3-2.0. This technique usually requires a 40-50 second breath hold. Prone and supine imaging should be obtained, to allow visualization of the entire mucosa. In one study, by Chen et al, 59% of the scans have adequate distention for polyp detection using either the prone or supine acquisition, compared with 87% having adequate distention using a combination of the supine and prone acquisition together.

All of out CTC examinations are performed on a Siemens Somatom Plus 4 Volume Zoom scanner (Siemens Medical Systems, Iselin, NJ). Multidetector CT is ideal for CTC by combining high resolutionm rapid data acquisition and high performance throughput.

Our Technique

Scanner: Siemens Somatom Plus 4 Volume Zoom scanner (Siemens Medical Systems, Iselin , N.J.) with 8 detectors.

kVp 120

mA 180

collimation 4 X 2.5mm

slice thickness 3mm

reconstruction interval 1-2mm

breath hold approximately 25 seconds

IV contrast not routine used for polyp screening

All scan data is then transferred to a free standing Silicon Graphics Onyx Infinite Reality or O2 workstation (Silicon Graphics, Mountain View, California) running 3D Virtuoso software (Siemens Medical Systems, Iselin, N.J.).


 

Image Display

There has been an evolution in 3D rendering techniques for CTC over the last several years. Initially, CTC was performed using surface-rendering techniques. However, due to the inherent loss of source data with this method, volume rendering is now considered to be the rendering technique of choice. Although this method requires more computer processing capabilities, it has a definite advantages over surface rendering by allowing the entire range of attenuation values within a volume set to be displayed. We use volume-rendering exclusively for CTC and in most of our 3D work, volume rendering is the technique of choice.

With volume rendering, parameters can be applied to the volume set to affect the appearance of the data in order to best demonstrate anatomy and pathology. These parameters include widow width, level, opacity and brightness and can be adjusted interactively by the user. The window width and level functions are similar to the windowing settings on standard CT scanners . For instance, the window setting can be adjusted to display the colonic air or soft tissue. Opacity refers to the degree with which adjacent structures obscure structures behind them. This can be varied from 0-100%. When set at 100% opacity the appearance of the colon is similar to surface rendering. When low opacity setting are chosen, the colon appears "see through". Brightness affects the appearance of the image by scaling the value of every pixel without changing the apparent diameter of the viewed structures. It can be varied from 0-100%. Selection of brightness setting are largely subjective and are based on individual user preference.

When performing CTC using volume rendering, the user is able to adjust any of these parameters interactively in real time. This interactivity allows the user to customize the parameters for each individual case in order to optimize the display of anatomy and pathology. The following is a description of 4 image displays which we use routinely when performing CTC.

2D Multiplanar Reformation

Simple 2D multiplanar reconstructions of the CT data allows quick visualization of the colon in the axial, sagittal and coronal planes, simultaneously. Most radiologists are familiar and comfortable with 2D MPR. It is quick and available on all workstations. A possible abnormality detected in one plane can immediately be visualized in the other two planes. The ability to visualize an abnormality in multiple planes, increases confidence and helps to better characterize the morphology of the lesion.

Endoluminal View

3D endoscopic views of the colon have gained the most attention. With this method, the colon is viewed from the inside, simulating the gastroenterologist’s view through an endoscope. Although this technique was originally performed using surface rendering, volume rendering is now considered to be the rendering technique of choice. For endoluminal imaging, a central axis must be created to allow simply navigation through the colon. In addition, perspective volume rendering can be useful, by allowing differentiation of near and far objects. When utilizing endoluminal examination of the colon, both forward and retrograde viewing the colon must be performed, since small lesions may be obscured behind haustral folds.

Simulated Double Contrast Enema

Using volume rendering, display parameters can be manipulated to obtain an edge enhanced view of the colon, which simulates an air contrast barium enema. In order to create a double contrast effect, a percentage classifier is necessary. A trapezoid is created, and only the attenuation values under the trapezoid are included in the image. Attenuation values lying outside the trapezoid appear black . Small polyps can also be detected with this display method and appear as a filling defect or ring shadow, as on conventional double contrast enema.

Simulated Single Contrast Enema

The display parameters can be manipulated to obtain an image which simulates an single contrast enema. This is achieved by creating a negative window level to accentuate the air- filled colon (approximately —350) and a negative window width (approximately —500). The negative window width displays air as white. Therefore, the air-filled colon will appear similar to a single contrast enema. As with conventional single contrast enemas, this display method is especially valuable in patients with strictures or malignant narrowings. As with conventional single contrast enema, small polyps appear as filling defects on a CT simulated single contrast enema. Unlike a conventional single contrast enema where small polyps may not be detected if they are dependent and within the contrast pool, small polyps are still visible on the 3D images, because cut planes can scroll through the volume.

Computer Enhanced Image Display

This display technique accentuates the colonic wall and folds. In can be created by setting the window level at negative 200-300, with a negative window width of approximately 1000. The opacity is set at around 60% and the brightness can be adjusted to according to user preference. With this display method, extracolonic structures appear white, thus highlighting the colon. Polyps and masses are relatively easy to detect with these settings. However, regions of stricture or malignant luminal narrowing may be overlooked if this is the only display method utilized.

Simulated Soft Tissue Window

When a polyp or mass is detected, the display parameters can be manipulated in order to evaluate possible extraluminal extension or regional adenopathy. This is unique to volume rendering and is not possible with surface rendering techniques. To create this display, the window level and width are set similar to soft tissue windows used with routine abdominal/pelvic (i.e. window level = 400, window width = 10). This display method allows visualization of the pericolonic fat and vessels which is essential when evaluating for local tumor extension. In addition this display method is extremely helpful when a colon mass/polyp is detected. Unlike with surface rendering techniques in which a polyp and adherent stool appear identical, volume rendering with the simulated soft tissue window displays all of the attenuation values within the lesion, thus allowing differentiation between a true polyp and stool.


 

Conclusions

Although CT colonography is still a very new technique, initial reports and results for its use in colorectal polyp detection is encouraging. Its exact role in the widespread screening for colorectal cancer will not be determined for years. However, the basic technique of performing 3D CT after colonic cleansing and insufflation, can be routinely performed now for a better evaluation of the colon in a variety of patients including those with known colorectal cancer, for surgical planning, staging and follow-up. In many instances, by using volume rendering and optimizing the parameters, the display of anatomy and pathology can be improved. These display techniques described in this exhiibit are often very useful and easily adapted and accepted by radiologists who have been trained in conventional fluoroscopic examinations of the colon. This can often obviate the need for other examinations, such as the contrast enema. In most cases, several display algorithms may be necessary to optimize detection of pathology. This is similar to the common practice of viewing different "windows" (lung window, soft tissue window, bone window and liver window) on routine thoracic or abdominal scans in order to accentuate certain densities.


 

References

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© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.