CT colonography

Virtual Colonoscopy: A Kinder Gentler Way to Prevent Colon Cancer

Colorectal cancer is the 3rd most common cancer but early screening and knowing your risk factors can make a big difference in your prognosis. Judy Yee, MD, FACR, Professor and Vice Chair, Department of Radiology and Biomedical Imaging, UCSF; Chief, Department of Radiology, Veterans Affairs Medical Center, San Francisco, CA discusses why getting a colonoscopy is important and dispels myths about the process. Recorded on 11/10/2015. Series: "UCSF Osher Center for Integrative Medicine presents Mini Medical School for the Public".

CT Colonography (CTC) - The Evolution

CT (Computed tomography): A method of body imaging in which a thin X-ray beam rotates around the patient. A computer analyzes the generated data to construct 2D or 3D images. 
 
Timeline
1994: CTC first introduced
1997-2007: Major Validation Trials
2008: ACRIN 6664: National CTC trial
2008: American Cancer Screening endorsement in Joint Guidelines
2008: USPSTF/ 2009: CMS inadequate evidence for national screening coverage

Why Should You Care CTC Screeening

 
Why should you care?
-YOU will need to get screened
-Someone you know has colorectal cancer
-Someone you know will develop colorectal cancer
-Someone you know will die from colorectal cancer
-Colorectal cancer is preventable in most individuals
-Many of colonrectrecal arise from adenomatous polyp: premalignant growth on the lining of the colon wall.
 

What Are The Advantages of CTC

  1. Less invasive—better patient acceptance. No six foot scope advanced to the cecum, translate to better patient acceptance.
  2. Safer: The perforation rate 10-20x lower than colonoscopy for a diagnostic, lower for therapeutic
  3. Very short procedural time. Scan in two positions: supine (lying on the back) and prone (lying on the front), which is less than 10 seconds.
  4. No need for sedation which is necessary for both colonoscopy and sigmoidoscopy (looking inside the sigmoid colon and rectum).
  5. Less costly than colonoscopy

Preparing The Colon for CT colonography

2 categories that is utilized

Dry Prep

-10 oz MG Citrate

-45 ml NaPhoshate-contraindicated in pts with renal or cardiac failure

Wet Prep

-4 liters Polyethylene Glycol

 

The reason CTC does not recommend wet preparation is that it leaves too much fluid in the colon which can obscure lesions.  

Use of Tagging Agents for CTC

At the VA they use ACRIN (American College of Radiology Imaging Network) Tagging Protocol since the VA is one of the sites for ACRIN trials. They use two different tagging agents and a cathartic  so patients have to remain on a clear liquid diet on the day before and during the morning of.   They will take aliquots (portion of the total amount) of tagitol, which is used to tag solid stool in the colon at 8:00 AM, 11:00 AM, and 2:00 PM. Then in the late afternoon, saline or cathartic or PEG (laxative) is given, which is followed by disacodyl tablets, another form of laxative.

Inflating the Colon for Better View

From colonic distention, we moved over to use CO2 in deflation, which is mechanical distention. Why use CO2? It has lipid solubility, high partial pressure gradient, which means there is net resorption making it easier and more comfortable for patient after the procedure is over. 

Validation Trials for CTC

National Validation Trials 

  • ACRIN 6664
  • Munich Trial
  • IMPACT
  • Japanese National Trial
  • SIGGAR 

 

Radiation Dose for CTC

Radiation Dose: A perspective

  • Average annual natural background dose: 3mSV

How We Reduce CTC dose

Interpreting CTC Images

 

Two ways to read CTC: primary 2D and primary 3D. Both skills are necessary as you can use both 2D and 3D complementary. 

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