CT Contrast

• CT contrast contains iodine, a relatively dense element that appears bright on CT.
• What actually "enhances"?
• Anything that receives blood flow, with brightness corresponding to amount of blood flow.
• Vessels
• Tumors – hypervascularity
• Inflammation/infection – increased blood flow and leaky capillaries.

Timing of contrast

• Arterial vs venous imaging is based on timing of image acquisition after contrast administration.
• Contrast is typically administered via a peripheral vein, flows to heart, then follows circulation throughout the rest of the body.
• Peripheral vein
• Right heart (RA/RV)
• Pulmonary arteries (image at this time point for PE study).
• Left heart (LA/LV)
• Aorta (image at this time point for CT angiogram).
• Peripheral arteries
• Capillaries
• Veins (image at this time point for evaluation of most abdominal organs).
• Back to heart

Renal excretion

• Each time contrast passes through the kidneys, some is removed via renal excretion, decreasing amount of contrast present in circulation over time.
• Contrast becomes concentrated in renal collecting system and bladder.
• Ureters and bladder fill on delayed images (image at this time point for CT urogram).

Contrast limitations

• Evaluation for intracranial hemorrhage – brightness from contrast will obscure bright blood, so we use a non-contrast head to rule out hemorrhage.
• Kidney stones – renal calculi are already dense on CT, so contrast is generally not needed/helpful.
• Renal impairment
• Iodinated contrast is cleared by the renal system.
• Generally avoid IV contrast if GFR < 30 mL/min due to higher risk of contrast induced nephropathy.
• Not an absolute contraindication – use clinical judgement to evaluate risks vs benefit.
• Hydration may reduce risk of AKI if low/borderline GFR.
• Oral hydration preferred.
• Normal saline if unable to drink fluids.
• Chronic dialysis – generally okay to give contrast as there is no residual renal function to preserve.
• No risk of nephropathy with oral contrast.