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Monday, August 11, 2025

Iodinated contrast media


1. Introduction

  • Iodinated contrast media (ICM) are radiopaque agents used to enhance visibility of vascular structures, organs, and tissues in X-ray and computed tomography (CT) imaging.

  • Contain iodine atoms with a high atomic number, providing strong X-ray attenuation.

  • Can be classified by ionicity, osmolality, and structure.

  • Choice of contrast depends on patient factors (e.g., renal function, allergy history) and the diagnostic purpose.

2. Classification

A. By Ionicity

  • Ionic: dissociate into charged particles in solution; older generation; higher osmolality; higher risk of adverse reactions.

  • Non-ionic: remain as intact molecules; lower osmolality; better tolerability.

B. By Osmolality

  • High-osmolar contrast media (HOCM) – ~1500–2100 mOsm/kg; mostly ionic monomers (e.g., diatrizoate).

  • Low-osmolar contrast media (LOCM) – ~600–850 mOsm/kg; mostly non-ionic monomers (e.g., iohexol, iopamidol).

  • Iso-osmolar contrast media (IOCM) – ~290 mOsm/kg (similar to plasma); non-ionic dimers (e.g., iodixanol).

C. By Structure

  • Monomers – single benzene ring with three iodine atoms; can be ionic or non-ionic.

  • Dimers – two benzene rings linked; usually non-ionic; lower osmolality.

3. Mechanism of Action

  • Iodine atoms absorb X-rays due to high atomic number → appear white/bright on radiographic images.

  • Improves contrast between blood vessels or organs and surrounding tissues.

  • Does not interact chemically with tissues; purely physical attenuation effect.

4. Pharmacokinetics

  • Absorption: rapid following IV injection; oral agents absorbed minimally if gut mucosa intact (mainly used for GI opacification).

  • Distribution: distributes in extracellular fluid; minimal protein binding.

  • Metabolism: not metabolized; excreted unchanged.

  • Elimination: primarily renal excretion via glomerular filtration; half-life ~1.5–2 hours in patients with normal renal function; prolonged in renal impairment.

5. Routes of Administration

  • Intravenous (IV) – most common for CT angiography, venography, and contrast-enhanced CT.

  • Intra-arterial – for angiography, cardiac catheterization.

  • Oral – for gastrointestinal tract opacification.

  • Rectal – for lower GI studies.

  • Intrathecal – for myelography (only specific low-osmolar, non-ionic agents approved).

  • Intra-articular – for arthrography.

6. Common Agents

High-osmolar ionic agents (older)

  • Diatrizoate (Hypaque, Urografin)

Low-osmolar non-ionic agents

  • Iohexol (Omnipaque)

  • Iopamidol (Isovue)

  • Ioversol (Optiray)

Iso-osmolar non-ionic dimers

  • Iodixanol (Visipaque)

7. Therapeutic and Diagnostic Uses

  • CT imaging – enhancement of organ and vessel visualization.

  • Angiography – coronary, cerebral, peripheral vessels.

  • Urography – evaluation of kidneys, ureters, bladder.

  • Gastrointestinal studies – oral/rectal contrast for bowel imaging.

  • Myelography – spinal cord and nerve root visualization.

  • Hysterosalpingography – uterine cavity and fallopian tube imaging.

8. Contraindications

  • Documented severe hypersensitivity reaction to iodinated contrast.

  • Untreated hyperthyroidism or thyroid storm risk.

  • Severe renal impairment (relative contraindication unless benefit outweighs risk).

  • Known history of contrast-induced anaphylaxis despite premedication.

9. Adverse Effects

Mild (common, usually self-limiting)

  • Warm sensation, metallic taste.

  • Nausea, vomiting.

  • Mild urticaria, pruritus.

Moderate

  • More pronounced urticaria, bronchospasm, hypotension requiring treatment.

Severe (rare but potentially life-threatening)

  • Anaphylactoid reactions – laryngeal edema, cardiovascular collapse.

  • Contrast-induced nephropathy (CIN) – acute kidney injury within 48–72 hours.

  • Thyroid dysfunction – contrast-induced hyperthyroidism or hypothyroidism.

  • Neurotoxicity – seizures (especially after intrathecal use with inappropriate agents).

10. Drug Interactions

  • Metformin: increased risk of lactic acidosis if acute kidney injury occurs; metformin often withheld on day of contrast and for 48 hours post-procedure until renal function confirmed normal.

  • Nephrotoxic drugs (e.g., aminoglycosides, NSAIDs): increased risk of CIN.

  • Beta-blockers: may mask signs of anaphylaxis and reduce responsiveness to treatment.

  • Interleukin-2 therapy: increased risk of delayed hypersensitivity reactions.

11. Prevention of Adverse Effects

  • Hydration before and after procedure to reduce CIN risk.

  • Use of lowest effective dose of contrast medium.

  • Premedication with corticosteroids and antihistamines for patients with prior contrast allergy.

  • Iso-osmolar or low-osmolar agents preferred in high-risk patients.

  • Avoidance in unstable hyperthyroid patients.

12. Monitoring

  • Renal function (serum creatinine, eGFR) pre- and post-procedure in at-risk patients.

  • Observation for hypersensitivity reactions immediately after injection.

  • Thyroid function in susceptible individuals post-exposure.

13. Advantages and Limitations

Advantages

  • Excellent X-ray attenuation → high-quality imaging.

  • Rapid onset of effect.

  • Wide variety of administration routes.

Limitations

  • Risk of allergic-like and nephrotoxic effects.

  • Requires intravenous/intra-arterial access for many applications.

  • Short intravascular half-life limits imaging time window.




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