Contrast Agents

OVERVIEW

Contrast agents are substances used in medical imaging as to enhance the contrast or the distinction between different tissues.

• aka contrast medium

RADIOGRAPHIC CONTRAST AGENTS

Composition

• Currently available agents contain a tri-iodinated benzene ring (either monomeric or dimeric; ionic or non-ionic)
• Ionic agents disperse into negative and positive ions — and have a higher toxicity — the non-ionic agents do not
• Additional components of contrast media allow large volumes of contrast to be administered quickly and with minimal toxicity
• Low osmolar contrast agents are typically used as they decrease osmolality, which is associated with adverse reactions

Use

• Iodine-based contrast agents are typically used for CT contrast scans and for urological studies such as IV pyelograms
• The allow for angiography and the demarcation of tumours
• Volume injected typically ranges from 50 ml to 150 ml, depending on the type of scan
• Oral contrast agents are also used to delineate gastrointestinal structures as they coat the luminal walls of the intestines

Adverse reactions to the injection of iodine-based contrast agents

• they include
  • nausea, vomiting
  • pruritis
  • vasovagal episodes
  • laryngeal edema
  • anaphylactic shock
  • nephrotoxicity (see Contrast-induced nephrotoxicity)
  • extravasation injury
• most occur <15 min after administration of the agent
• frequency
  • occur in ~ 5% of patients
  • 0.05% are severe
  • mortality rate is 1/75,000 injections
• high-risk patients include:
  • history of a severe-adverse contrast reaction
  • active asthma
  • multiple drug or environmental allergies
• risk of an adverse reaction to a repeat administration of intravenous contrast agent is ~ 15% (i.e. 3 x baseline risk)
  • premedication with steroids is recommended in patients with prior adverse reactions if the study is absolutely necessary
  • e.g. 50 mg prednisone PO, 12 and 2 hours before the procedure

GADOLINIUM-BASED CONTRAST AGENTS

These agents used to have the same effect as radiographic contrast agents, but for MRI

• Gadolinium is a toxic, rare earth metal that is safe to administer intravenously when attached to a chelate

Use

• Gadolinium chelates are used in nearly examinations of the brain and soft tissues
• no usually needed for imaging the bones and joints
• lower volumes required compared to riodine-based agents, usually <15 mL (dose is based on weight)
• oral contrast agents are not usually administered, although Barium and milk can be used

Adverse effects

• Minimal risk in patients with norma renal function
  • occasional episodes of vomiting and hives reported
  • anaphylaxis is very rare
• Nephrogenic systemic fibrosis
  • may affect patients with renal failure (serum Cr >3.0 mg/dl and GFR <30 ml/min)
  • >200 cases described, all associated with IV administration of a gadolinium chelate within 2 or 3 months before onset of symptoms
  • features include skin thickening, joint contracture, and severe pain
  • risk increases with repeat exposure
  • gadolinium chelates should be avoided in patients with moderate to severe renal failure

CONTRAST MEDIA EXTRAVASATION

Potential complication of CT contrast scans

• occurs in about 1 in 200 CT contrast scans
• rare with MRI, due to the low volume of contrast injected
• due to automated power injection, large volumes can extravasate in a short period of time
• risk factors
  • patient factors: elderly, emaciated, oedematous, confused
  • site of venous access: back of the hand, wrist, foot and ankle (more fragile veins with less subcutaneous tissue)
  • gauge of IV catheter: higher with small gauge cannula (22G or smaller)
  • high-osmolar contrast medium (less risk if pre-warmed to reduce viscosity)
  • pre-warming of high-osmolar (370 mg/ml) lowers the viscosity and has been proven to lower the probability of extravasation

Management (according to local protocol)

• stop contrast infusion and notify the radiologist immediately
• complete the acquisition of images
• apply an ice pack to the affected area
• elevate the affected extremity to reduce swelling (attempted aspiration of the extravasated media is not effective)
• observe the patient for at least 2h
• consider plastic surgery consultation if extravasations involves >100 ml of contrast agent
• notify the treating doctor requesting the study
• follow up the patient in the next few days until the resolution of local oedema
• patients should seek medical attention if:
  • increasing swelling or pain over time
  • blistering, ulceration, induration or other skin changes
  • altered tissue perfusion and/or changes in sensation

References and links

LITFL

• CCC — Contrast-induced nephropathy

Journal articles

• Boehm, I (2008). Seafood Allergy and Radiocontrast Media: Are Physicians Propagating a Myth? The American Journal of Medicine. 121 (8): e19. doi:10.1016/j.amjmed.2008.03.035. PMID: 18691465
• Brockow, K.; et al. (2005). Management of hypersensitivity reactions to iodinated contrast media. Allergy. European Journal of Allergy & Clinical Immunology. 60 (2): 150–158. doi:10.1111/j.1398-9995.2005.00745.x. PMID: 15647034
• Morcos SK, et al, and members of the Contrast Media Safety Committee of the European Society of Urogenital Radiology (ESUR). Dialysis and Contrast Media. Eur Radiol 2002;12(12):3026-30. PMID: 12439587
• Thomson, K; Varma, D (2010). “Safe use of radiographic contrast media”. Australian Prescriber, 33:19-22. Available at http://www.australianprescriber.com/magazine/33/1/19/22/
• Younathan CM, et al. Dialysis is not indicated immediately after administration of nonionic contrast agents in patients with end-stage renal disease treated by maintenance dialysis. AJR Am J Roentgenol 1994;163(4):969-71. PMID: 8092045

FOAM and web resources

• Renal Failure Network — Prophylactic hemodialysis for iv contrast exposure? (2009)