Radiography Exam Contrast Media and Pharmacology 1 — Questions and Answers
Question 1: Which class of iodinated contrast media has the lowest osmolality and is considered safest for intravascular administration, especially in high-risk patients?
- Nonionic iso-osmolar contrast media (Correct answer)
- Ionic high-osmolar contrast media
- Nonionic low-osmolar contrast media
- Ionic low-osmolar contrast media
Correct answer: Nonionic iso-osmolar contrast media
Nonionic iso-osmolar contrast agents (e.g., iodixanol) have an osmolality equal to blood (~290 mOsm/kg) and are associated with the lowest risk of adverse reactions.
Contrast media are classified by osmolality: high-osmolar (ionic, ~2000 mOsm/kg, e.g., diatrizoate), low-osmolar (nonionic monomers, ~600-900 mOsm/kg, e.g., iohexol, iopamidol), and iso-osmolar (nonionic dimers, ~290 mOsm/kg, e.g., iodixanol). Lower osmolality correlates with fewer adverse effects including nausea, heat/pain, and cardiovascular effects. Iso-osmolar agents are preferred for patients with impaired renal function and high cardiac risk.
Question 2: A patient receiving iodinated IV contrast suddenly develops generalized urticaria, bronchospasm, and hypotension. This reaction is classified as:
- Anaphylactoid (anaphylaxis-like) reaction (Correct answer)
- Vasovagal reaction
- Contrast-induced nephropathy
- Chemotoxic reaction
Correct answer: Anaphylactoid (anaphylaxis-like) reaction
Generalized urticaria combined with bronchospasm and hypotension constitutes a severe anaphylactoid reaction, which requires immediate emergency treatment with epinephrine.
Anaphylactoid reactions to contrast media are not true IgE-mediated allergic reactions but mimic anaphylaxis clinically. They are classified: mild (limited urticaria, nausea), moderate (widespread urticaria, bronchospasm, hypotension), severe (anaphylaxis, laryngeal edema, cardiac arrest). Severe reactions require immediate epinephrine (0.1–0.3 mL of 1:1000 IM in thigh), oxygen, IV fluids, and resuscitation. Premedication with corticosteroids and antihistamines is used for high-risk patients.
Question 3: The most important risk factor for contrast-induced acute kidney injury (CI-AKI) following IV iodinated contrast administration is:
- Pre-existing renal insufficiency (elevated serum creatinine/reduced eGFR) (Correct answer)
- Advanced patient age
- History of allergic reactions to shellfish
- Use of oral metformin
Correct answer: Pre-existing renal insufficiency (elevated serum creatinine/reduced eGFR)
Pre-existing renal insufficiency (eGFR <30–45 mL/min/1.73m²) is the most significant risk factor for contrast-induced acute kidney injury.
CI-AKI risk factors include: pre-existing chronic kidney disease (most important, especially eGFR <30), diabetes mellitus with nephropathy, dehydration, heart failure, large contrast volume, nephrotoxic medications, and multiple contrast doses in 24 hours. Shellfish allergy does NOT reliably predict contrast reactions (a common myth). Metformin is held 48 hours post-contrast if eGFR <60 due to risk of lactic acidosis if renal function worsens.
Question 4: Barium sulfate is the preferred contrast agent for gastrointestinal studies when aspiration is NOT a concern because:
- It provides superior mucosal coating and image quality compared to water-soluble agents (Correct answer)
- It is absorbed if it leaks into the peritoneum
- It has lower radiation attenuation than iodinated agents
- It can be given intravenously
Correct answer: It provides superior mucosal coating and image quality compared to water-soluble agents
Barium sulfate provides excellent mucosal coating for double-contrast GI studies, giving superior visualization of the mucosal surface compared to water-soluble contrast.
Barium sulfate is an insoluble, high-density compound used for oral/rectal GI contrast studies. It provides excellent mucosal coating in double-contrast (with air) studies. However, if aspirated, it causes pneumoconiosis; if it leaks into the peritoneum (perforation), it causes barium peritonitis (very serious). In cases of suspected perforation or aspiration risk, water-soluble iodinated contrast (e.g., Gastrografin) is substituted, though it provides inferior mucosal detail.
Question 5: Which route of contrast administration requires the highest iodine concentration for adequate opacification in CT imaging?
- Intravenous (IV) injection (Correct answer)
- Oral administration
- Intrathecal (myelography)
- Rectal administration
Correct answer: Intravenous (IV) injection
IV contrast requires high iodine concentration (e.g., 300–370 mgI/mL) because it is diluted by blood volume before reaching target tissues; oral and intrathecal contrast use much lower concentrations.
IV contrast (e.g., ioversol 350 mgI/mL) must be concentrated enough to provide adequate attenuation after dilution in the vascular system. Oral contrast can be highly diluted (30–60 mgI/mL or dilute barium) since it directly fills the GI lumen. Intrathecal contrast (e.g., iohexol for myelography) uses low concentrations (180–300 mgI/mL) because CSF volume is small and high concentrations cause neurotoxicity. Route determines required concentration.
Question 6: A patient reports a previous moderate contrast reaction. What is the standard premedication protocol to reduce risk of a repeat reaction?
- Prednisone 50 mg orally at 13 hours, 7 hours, and 1 hour before contrast plus diphenhydramine 50 mg IM/IV 1 hour before (Correct answer)
- Aspirin 325 mg one hour before contrast only
- Epinephrine IM immediately before injection
- No premedication is needed for moderate reactions
Correct answer: Prednisone 50 mg orally at 13 hours, 7 hours, and 1 hour before contrast plus diphenhydramine 50 mg IM/IV 1 hour before
The standard ACR premedication protocol for prior contrast reaction uses oral prednisone at 13h, 7h, and 1h before contrast, plus diphenhydramine 1h before — corticosteroids reduce reaction severity.
The ACR Manual on Contrast Media recommends: prednisone 50 mg PO at 13 hours, 7 hours, and 1 hour before contrast, plus diphenhydramine 50 mg IV/IM/PO 1 hour before. For urgent cases when oral premedication is not possible, a 4–5 hour IV methylprednisolone protocol can be used. Premedication reduces (but does not eliminate) breakthrough reaction risk. Using iso-osmolar nonionic contrast in addition to premedication is recommended for high-risk patients.
Radiography Exam Contrast Media and Pharmacology 1
Which class of iodinated contrast media has the lowest osmolality and is considered safest for intravascular administration, especially in high-risk patients?