DOS 512 - Week 3 Discussion
Initial Post: CT Contrast Agents
Contrast agents are chemical formulations that can be administered orally, rectally, intravenously, or through catheters into internal body cavities. Their purpose is to alter the x-ray attenuation coefficient for particular tissues or organs by either increasing them (positive contrast) or decreasing them (negative contrast).1 The most common contrast agents in use for positive contrast are based on chemistries containing the high-Z atoms iodine (Z=53) or barium (Z=56). These two atoms have K-edges at 33 and 37 KeV respectively, so they stand out well at typical imaging energies. Negative contrast can be achieved by introducing a gas to a cavity either through direct insufflation or through the use of a gas-generating chemistries such as bicarbonate mixing with stomach acid to produce carbon dioxide.
At SCCA Proton Therapy, we use three different commercial preparations for positive contrast:
- Omnipaque is an iodine-based agent is used for intravenous injections. This allows us to visualize circulatory structures such as arteries, veins, and chambers of the heart.
- Optiray is an iodine-based agent that we dilute with saline and then use for retrograde urethrograms. This allows physicians to accurately localize the external sphincter of the urethra, which is at the inferior end of the prostate. This helps in defining target volumes in prostate treatments.
- VoLumen is a barium sulphate-based agent that fills the digestive tract without significantly enhancing the overall attenuation of the tissue. Even though the attenuation doesn't change much, it still makes the loops of bowel stand out better from adjacent structures such as blood vessels and fat. The change is also small enough that we don't need to override the density, even for proton treatment.
We do not typically use negative contrast procedures at our center.
In researching this assignment, I learned that regular 4% whole milk can be an effective and much cheaper replacement for low-attenuation agents like VoLumen, with similar bowel distension and wall visualization metrics.2,3 In addition to its lower cost, patients preferred milk over VoLumen, and it may be possible to add flavoring such as chocolate to make it even more palatable.4
I do not think that the image-enhancement utility achieved with contrast agents will be easily replaced any time soon. Other modalities such as MRI can offer visualization that is complementary to CT, and MRI can also be enhanced with the usage of contrast agents as well, but one will not replace the other.
- Fleckenstein P, Tranum-Jensen J. Anatomy in Diagnostic Imaging. 2nd ed. W B Saunders Company; 2001.
- Koo CW, Shah-patel LR, Baer JW, Frager DH. Cost-effectiveness and patient tolerance of low-attenuation oral contrast material: milk versus VoLumen. Am J Roentgenol. 2008;190(5):1307-13.
- Meyer SA, Gawde S. Utility of negative oral contrast (milk with 4% fat) in PET-CT studies. Indian J Nucl Med. 2012;27(3):151-5.
- Susman E. Milk proves to be an alternative contrast agent to VoLumen. AuntMinnie Website. http://www.auntminnie.com/index.aspx?sec=ser&sub=def&pag=dis&ItemID=73812. Writen November 30, 2006. Access December 10, 2014.