Quick answer
The CT scan (computed tomography) uses X-rays to produce very precise cross-sectional images of the body. Fast and painless (10 to 30 minutes), it can be performed with or without injection of iodinated contrast medium depending on the structures to be visualised.
🖥️ What is it?
The CT scan (computed tomography) uses X-rays to produce very precise cross-sectional images. Fast and painless, it can be performed with or without injection of iodinated contrast medium to better visualise certain structures.
📋 How to prepare
Fast for 4 hours if a contrast injection is planned. No fasting if there is no injection.
Bring your prescription, your previous examinations and your Carte Vitale (French health insurance card).
Tell us about any allergy to contrast medium and bring your kidney function results if available.
Tell us if you are pregnant or breastfeeding.
Diabetic patients on Metformin: stop 48 hours beforehand if an injection is planned (on medical advice).
▶️ How does it go?
You lie on the table, which moves through the ring of the machine.
The examination is completely painless and quiet.
If an injection is given: a sensation of passing warmth, which is normal and harmless.
Short breath-holds of 5 to 10 seconds may be requested for chest images.
The radiologist supervises from an adjacent room and stays in contact with you.
✅ After the examination
You can resume all activities immediately.
If an injection was given: drink plenty of fluids (1.5 L of water) to eliminate the product.
The report is sent to your doctor within 2 to 5 working days.
❓ Frequently asked questions
Is a CT scan dangerous because of X-rays?
What should I do if I have a known iodine allergy?
How long does a CT scan last?
✓ Why you can trust this page
Written by the Radiology team at Cochin Hospital AP-HP, Paris Cité University. Medical review by a radiologist of the department. Last updated: July 2026.