Pharmacology: Contrast Agents (Midterms)

Contrast Agents

● Substances administered to temporarily alter tissue radiodensity or signal characteristics.
● Enhance visualization of organs, vessels, or pathology during imaging (X-ray, CT, MRI, fluoroscopy).
● Act by interacting with X-rays (for iodinated/barium/metallic), sound (rare), or magnetic fields (MRI gadolinium).
● Pharmacologically considered diagnostic drugs with specific ADME properties, toxicity profiles, and contraindications.

Metal-Based Contrast Agents

Barium sulfate (BaSO₄)
Gadolinium (Gd³⁺)

Barium Sulfate: Routes of Delivery

○ Oral
○ Rectal
○ Via stoma

Barium Sulfate: Barium Sulfate

○ Esophagus, stomach, duodenum

○ Small bowel follow-through

○ Barium enema (colon)

○ Structural abnormalities, strictures, tumors, ulcers

Barium Sulfate: Contraindications

○ Suspected perforation of GI tract

○ Suspected fistula to mediastinum or peritoneum

○ Severe dysphagia at risk for aspiration

○ High-risk aspiration patients

○ Recent surgery involving GI tract anastomosis

Barium Sulfate

● No absorption means no metabolism

● GI transit only

● Completely excreted fecally

● No systemic distribution

Barium Sulfate: Adverse Effects

○ Constipation

○ Impaction if inadequate hydration

○ Aspiration pneumonitis

○ Extravasation into the peritoneum leads to barium

peritonitis

Gadolinium

● A lanthanide rare-earth metal with strong

paramagnetic properties.

● Toxic when free, so it must be administered as a

chelate, where a ligand “cages” the metal.

● Alter the relaxation times (T1/T2) of nearby hydrogen

protons leads to brighter signals on MRI.

● Water-soluble; ionic or nonionic depending on ligand.

● Introduced intravenously

Gadolinium: Indications

○ CNS imaging: brain tumors, MS plaques, infection, stroke

○ Body MRI: liver, kidneys, pancreas, breast

○ Vascular MRI: MR angiography (MRA)

○ Cardiac MRI

○ Joint MRI: arthrography

○ Detection of blood–brain barrier breakdown

Gadolinium: Contraindications

○ Severe kidney impairment

○ High risk of Nephrogenic Systemic Fibrosis (NSF) with

some agents

○ Prior hypersensitivity reaction to GBCAs

○ Pregnancy (crosses placenta; use only if essential)

○ Breastfeeding (minimal risk, but some guidelines advise

24-hour milk discard)

○ Unstable cardiac disease (due to rare risk of arrhythmias)
● Minimal protein binding.
● Crosses: ○ Damaged blood–brain barrier ○ Placenta (small amounts)
● Not metabolized; excreted unchanged.

Gadolinium: Route of Delivery

● Given intravenously → 100% bioavailability.

● Distributes rapidly into extracellular fluid space.

Gadolinium: Adverse effects

○ Warm sensation

○ Metallic taste

○ Mild nausea

○ Hypersensitivity reactions

○ Injection-site discomfort

○ Progressive fibrotic disease: skin thickening, organ

fibrosis

○ Gadolinium Retention

Ionic Iodinated Contrast Media

● Iodine bound to tri-iodinated benzene ring.

● Ionic = dissociates into cation + anion in solution.

● High osmolality (5–7x plasma).

Ionic Iodinated Contrast Media: Indication

○ CT imaging

○ IV urography

○ Some angiography

○ Oral/rectal alternative when barium is

contraindicated

○ ER use for perforation cases (water-soluble)

Ionic Iodinated Contrast Media: Contraindication

○ History of severe anaphylactoid reactions

○ Uncontrolled hyperthyroidism or thyrotoxicosis

○ Renal impairment

○ Dehydration

○ Multiple myeloma (risk of tubular precipitation)

○ Pregnancy (minimal but possible fetal thyroid effects)
○ History of severe anaphylactoid reactions

○ Uncontrolled hyperthyroidism or thyrotoxicosis

○ Renal impairment

○ Dehydration

○ Multiple myeloma (risk of tubular precipitation)

○ Pregnancy (minimal but possible fetal thyroid effects)

Nonionic Iodinated Contrast Media

● Tri-iodinated benzene ring without ionizing groups.

● Do not dissociate in solution means lower osmolality

● Lower chemotoxicity and better tolerance.

Nonionic Iodinated Contrast Media: Indications

○ CT scans (body, neuro, trauma)

○ Angiography (cardiac, cerebral, peripheral)

○ Venography

○ IV pyelography

○ Pediatric contrast studies

Nonionic Iodinated Contrast Media: Contraindication

● Prior severe contrast reaction

● Severe renal impairment

● Active hyperthyroidism

● Decompensated heart failure (due to volume load)

● Pregnancy

Nonionic Iodinated Contrast Media: Contraindication: After effects and possible long terms impairment (Adverse effect)

● Much lower risk of adverse reactions vs ionic

● Warm sensation

● Mild nausea

● Anaphylactoid shock

● Laryngeal edema

● Hypotension

● CIN risk still exists, but lower than ionic

● Can trigger thyrotoxicosis

● Can interfere with radioactive iodine uptake for weeks