Understanding Imaging: Introduction to Contrast

Types of contrast

IV contrast

IV contrast is widely used in CT and MRI in contrast to oral.

IV contrast solutions today consist of low osmolar iodine solutions. Iodine contrast has a reputation to cause or worsen renal injury, although it is important to remember that this is an overstated risk. Check out our slide deck on renal injury on the wards for more information!

When to use IV contrast

• Perforation, abscesses, fistulae, aneurysms
• Can highlight vasculature (angiogram), visceral organs, and urinary collecting system based on the timing of the IV contrast

Oral contrast

Types of oral contrast

• Barium sulfate

The most widely used oral contrast solution used, commonly used in GI fluoroscopy studies (e.g. barium enema, UGI, SBFT)

• Gastrografin (water soluble)

If there is risk for the contrast to exit from the lumen of the GI tract, then a water soluble oral contrast agent such as gastrografin can be used instead

• Dilute iodinated IV contrast can be used instead of gastrografin as oral contrast if the patient is at risk for aspiration since it has less lung toxicity if aspirated

When to use oral contrast?

With the technological advancement and increased availability of CT and MRI, the use of oral agents is progressively shrinking.

It may still be used in

• IBD (especially in CT enterography which uses both oral and IV contrast)
• Abscesses
• Extravasation post-surgery/trauma
• Fistula mapping

Double vs Single contrast oral (also can used with rectal contrast)

Double contrast GI study = gas + oral contrast

Gas is used to distend lumen and barium coats the inner mucosa. It is superior to single contrast GI studies.

Single contrast GI study = oral contrast only

No gas to distend lumen, instead the oral contrast itself distends the lumen and coats the inner mucosa.

Positive vs Negative contrast

Positive oral contrast = bright

• Highlights the bowel lumen
• Pros
  • Distinguishes bowel from surrounding organs, surrounding organs, soft tissue masses, and extraluminal fluid
• Cons
  • Obscure the bowel mucosa
  • Hinder evaluation for pancreatic lesions
  • Obscure active IV contrast extravasation in GI bleeding
• Examples
  • Barium sulfate
  • Gastrografin

Negative oral contrast = dark grey

• Highlights the bowel wall
• Used in CT enterography (see below)
• Water can actually be used as a neutral contrast agent

Rectal or bladder contrast

See barium enema in our article on different modalities of imaging. The types of rectal or bladder contrast are the same as the oral contrast.

When to use rectal or bladder contrast?

• Penetrating trauma to pelvis
• If the abnormalities is distal then providing the contrast rectally may be faster than waiting for oral contrast to reach the colon (fistula, surgical anastomotic leak)
• You can also insert a foley catheter in order to fill the bladder with contrast in order visualize structural or functional abnormalities in the bladder, ureters or urethra