Contrast
Formal Definition
Radiographic contrast agents: iodinated (CT) or gadolinium-based (MRI) substances administered intravenously or orally to enhance tissue differentiation, vascular anatomy, or pathological processes on imaging — enabling visualization of structures not distinguishable on non-contrast studies.
How It's Used on the Ward
"CT with contrast" or "can they get contrast?" — the question about contrast eligibility that precedes almost every advanced imaging order.
Example
""Ordering CT abdomen with contrast for suspected abscess: patient has AKI with creatinine 2.4. Discussed with radiology — risk-benefit favors contrast given the clinical urgency; will hydrate aggressively pre- and post-scan. If contrast-indicated study isn't possible, ultrasound is next.""
Clinical Context
Contrast eligibility requires: (1) adequate renal function (eGFR threshold varies by institution, typically >30–45 for iodinated CT), (2) no severe iodine allergy without premedication, (3) metformin held 48h post-contrast if eGFR borderline (risk of lactic acidosis). Gadolinium (MRI) carries nephrogenic systemic fibrosis risk with eGFR <30. IV contrast enables: vascular anatomy, enhancement patterns (abscess ring, tumor vascularity), perfusion. Non-contrast CT: hemorrhage, calcifications, bone — no contrast needed.