Thin-Slice Scans Maximize Resolution for Virtual Colonoscopy

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.

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.