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Thin-Slice Scans Maximize Resolution for Virtual Colonoscopy

June 8, 2009
Written by: , Filed in: Abdominal Imaging
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In patients undergoing virtual endoscopy to screen for colorectal cancer and polyps, the colon is gently distended with either room air or carbon dioxide via an insufflation tube immediately before and during the examination.

Once distended, a scout radiograph is taken to verify that the distention is adequate.

Often, the radiograph shows that the sigmoid colon is not distended as much as needed. If the patient can tolerate some more pressure inside the colon, then more air or carbon dioxide may be pumped into the colon.

Scanning
In our practice, we currently use a 16-slice scanner. Depending on the patient’s height, roughly 290 contiguous scans (1.25 mm) can be done through the abdomen in about 12 seconds (a single breathhold) in an average-sized patient.

First, the patient is scanned in the supine position. Then we flip the patient over and repeat the scan with the patient in the prone position.

The advantage of thin-slice scanning is that we get better spatial resolution on the resultant 3-dimensional or multiplanar reformations.

This provides better definition of folds, better definition of polyps sitting next to a fold, and better isotropic data (decrease volume averaging).

The scan should be performed in a single breathhold or else registration will be an issue and some parts of the colon may be missed.

Related CME:
NEW FOR 2009
Abdominal & Thoracic CT/MR/US: Optimizing Practice
The University of California, San Francisco School of Medicine
Evolving technology improves radiological practice through improved image quality for anatomical interpretation and generation of functional data. The use of MDCT and fast, multi-phase imaging sequences also translates to a tremendous increase in data that requires review and handling and greater need for contrast and radiation safety awareness. This activity targets the community radiologist involved in body and cardiovascular imaging with a focus on developing an efficient workflow practice in the modern imaging environment.
Read more or order: Abdominal & Thoracic CT/MR/US: Optimizing Practice

CT/MRI of the Abdomen and Pelvis
University of California San Francisco Department of Radiology and Biomedical Imaging
Provides an updated review on the use of helical (single and multidetector-row) CT and MRI for imaging of the abdomen and pelvis. Participants will learn the current approach and uses of CT/MRI in the examination of the abdomen and pelvis including advances in abdominal MRI techniques, genitourinary applications of CT/MRI, CT colonoscopy, CT/MR angiography, and CT cholangiography.
Earn up to 12 AMA PRA Category 1 Credits™.
Abdominal Imaging CME

Radiation Dose
With virtual colonoscopy, CT tube currents are operated at low levels (I use about 80 mAs), which is much lower than a conventional CT scanner. With that low dose, the results will have grainy images and thin slices.

Although detecting extracolonic abnormalities is not the primary purpose of virtual colonoscopy, grainy images make it harder to detect these abnormalities. However, grainy images really do not affect the screening examination.

I scan most patients at 70 mAs to 100 mAs. I am considering lowering these dosages much further. A recent study suggests going as low as 2.5 mAs.

With these parameters, the radiation dose is similar to that of a double-contrast barium enema, even doing supine and prone examinations, essentially 2 examinations.

With use of tagging, we may not need the second set of examinations, and may be able to do only 1 examination in either the supine or prone position.

Although we still need to perform 2 examinations, our goal is to some day reduce the radiation dose by half.

Conclusion
Virtual colonoscopy performed with thin-slice scanning helps maximize the spatial resolution on the resultant 3-dimensional or multiplanar reformations, and provides better definition of the folds and the polyps sitting next to the folds.

Author: Richard S. Breiman, MD

Excerpted from his paper: Low-Dose Multi-Slice Scans Improve Resolution and Decrease Radiation Exposure for Virtual Colonoscopy.

Dr. Breiman practices Diagnostic Radiology in Oakland and San Francisco, California.

Related CME:
NEW FOR 2009
Abdominal & Thoracic CT/MR/US: Optimizing Practice
The University of California, San Francisco School of Medicine
Evolving technology improves radiological practice through improved image quality for anatomical interpretation and generation of functional data. The use of MDCT and fast, multi-phase imaging sequences also translates to a tremendous increase in data that requires review and handling and greater need for contrast and radiation safety awareness. This activity targets the community radiologist involved in body and cardiovascular imaging with a focus on developing an efficient workflow practice in the modern imaging environment.
Read more or order: Abdominal & Thoracic CT/MR/US: Optimizing Practice

CT/MRI of the Abdomen and Pelvis
University of California San Francisco Department of Radiology and Biomedical Imaging
Provides an updated review on the use of helical (single and multidetector-row) CT and MRI for imaging of the abdomen and pelvis. Participants will learn the current approach and uses of CT/MRI in the examination of the abdomen and pelvis including advances in abdominal MRI techniques, genitourinary applications of CT/MRI, CT colonoscopy, CT/MR angiography, and CT cholangiography.
Earn up to 12 AMA PRA Category 1 Credits™.
Abdominal Imaging CME
.
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