Preparing for a CT scan is simpler than most people expect, but the instructions depend on the area being scanned and whether your exam uses contrast. This checklist covers what applies to most patients at King City Diagnostic Imaging. Always follow the instructions on your requisition and the ones our staff give you when you book.
Quick Facts: CT Scan Preparation
Without contrast, a CT scan usually needs no fasting, and you continue your regular medications unless told otherwise. With contrast, you may be asked not to eat for a few hours beforehand and to drink water as instructed. Arrive 15 minutes early, and bring your requisition and Ontario Health Card.
CT scan preparation: the day before and the morning of
Good CT scan preparation mostly comes down to asking the right questions when you book. If you have not booked yet, our step-by-step guide to booking a CT scan in Ontario walks through the process. In the day or two before your appointment:
- Confirm your appointment time, and plan to arrive 15 minutes early
- Gather your requisition, Ontario Health Card, medication list and prior imaging reports
- Check whether your exam uses contrast, and whether you have fasting instructions or need a blood test
- Set out comfortable clothing without metal zippers, buttons or snaps
- Plan your route to the clinic and check live traffic before you leave
What to bring to your CT scan
- Your requisition form from your referring doctor or nurse practitioner
- Your valid Ontario Health Card or identification
- A list of your current medications
- Any prior imaging reports related to the area being scanned
- Mobility aids such as a cane or walker, if you use them
What to wear to a CT scan
Wear comfortable clothing without metal zippers, buttons or snaps. You may be asked to change into a gown. Metal in the scanned area can affect image quality, so you will be asked to remove it, including:
- Jewelry such as necklaces, earrings and rings
- Watches
- Belts and belt buckles
- Hairpins and metal accessories
- Glasses, hearing aids and removable dental work, for head and neck scans
Preparation depends on the area being scanned and whether contrast is required.
CT scan without contrast
Most scans without contrast need no fasting. Continue taking your regular medications unless your doctor or our staff tell you otherwise, and remove metal from the area being scanned.
CT scan with contrast
Contrast is a special liquid that helps blood vessels, organs and other tissues stand out on the images. It may be given by mouth or through an IV. For a contrast scan:
- You may be asked not to eat for a few hours before the exam
- Drink water as instructed to stay hydrated
- Tell staff about any previous reaction to contrast dye or iodine
The IV injection is generally quick, and a temporary warm sensation is common. Our team confirms that all conditions are met before contrast is given. For more on contrast, kidney function and common myths, see our guide to CT scans in Ontario.
What contrast feels like
Many CT scans use contrast, an iodine-based liquid injected into a vein that makes blood vessels, organs and areas of inflammation stand out. As it goes in, you may feel a warm flush spreading through your body, a metallic taste in your mouth, and a sensation that you have wet yourself. You have not. It passes in under a minute and happens to nearly everyone, and knowing it is coming makes it far easier.
Some abdominal scans also use oral contrast, a flavoured drink that outlines the bowel. It takes about an hour to travel far enough, which is why those appointments run longer.
Contrast, allergies and kidney function
Two common beliefs are worth retiring. A shellfish allergy does not rule out contrast: a 2025 joint consensus statement from the American College of Radiology and the American Academy of Allergy, Asthma and Immunology states that patients with seafood or shellfish allergy are not at elevated risk from iodinated contrast. And iodine itself is not an allergen, since reactions relate to the specific contrast agent. What matters is a previous reaction to a contrast agent, so tell staff if you have had one.
Canadian guidance on kidney function and metformin has relaxed in recent years, but individual facilities and ordering physicians still apply their own protocols. Follow the instructions you are given, and never stop or change a prescribed medication on your own. Our guide to CT scans in Ontario explains the current thinking in more detail.
What to tell the staff before your scan
Let our team know if any of these apply to you:
- You are pregnant or may be pregnant
- You have kidney disease, diabetes, or take metformin
- You have asthma, severe allergies or a thyroid condition
- You are breastfeeding and are having a contrast exam
- You have pain or limited mobility that may affect positioning
A recent blood test (creatinine and eGFR) may be required before a contrast CT. Do not stop or change any prescribed medication on your own. Follow the instructions from our staff and your doctor.
On the day of your appointment
Check in at reception 15 minutes before your appointment time, and the CT technologist will call you when it is your turn. They will confirm your instructions, and you may change into a gown. During the scan you lie on a table that slides through a ring-shaped scanner and stay still, and you may be asked to hold your breath for a few seconds. The scanner takes images in just a few minutes, and you can return to your regular activities afterwards. Our guide to booking a CT scan in the GTA explains what happens at each stage.
Check in at reception 15 minutes before your appointment time.
If you feel nervous about your scan
A CT scanner is a large, ring- or doughnut-shaped machine, and you lie on a table that slides through the opening while the scanner takes images in just a few minutes. Tell the technologist if you are nervous, in pain or have limited mobility that may affect positioning, and bring a mobility aid such as a cane or walker if you use one. During the scan you may be given simple instructions, such as holding your breath for a few seconds.
Radiation: the short, honest answer
CT scans use ionizing radiation. The dose is kept as low as reasonably achievable while maintaining diagnostic image quality, using dose-reduction protocols tailored to each patient and exam. For perspective, a chest X-ray is about ten days of natural background radiation and a chest CT is about two years’ worth. There is no conclusive evidence of cancer risk at CT dose levels, and the risk of a missed or delayed diagnosis is usually the larger one. Patients who are pregnant or may be pregnant must tell staff before the exam, and alternative imaging such as ultrasound may be considered when appropriate.
After your scan
You can return to your regular activities. A radiologist analyzes your images within 24 to 48 hours, and the report is faxed to your referring physician, so contact your referring provider about your results. If your provider has also requested an X-ray or ultrasound, our guide to preparing for ultrasound, X-ray, mammogram, BMD and FibroScan exams covers those. Ready to book? Start with our guide to CT scans in Toronto and the GTA or call 905-539-0555.
Frequently asked questions
Can I eat before a CT scan?
For most scans without contrast, yes. If your exam uses contrast, you may be asked not to eat for a few hours beforehand, and the technologist will confirm your instructions.
Should I take my regular medications?
Usually yes, unless your doctor or our staff tell you otherwise. Tell staff about every medication you take, especially metformin if you have diabetes.
Is a CT scan painful?
No. A CT exam is painless. If contrast is given through an IV, the injection is quick and you may feel a temporary warm sensation.
Do I need a blood test before my CT scan?
Sometimes. A recent blood test may be required before a contrast scan to check kidney function. Ask our staff when you book.
Where can I find the official preparation form?
Download our exam preparation form, or call 905-539-0555 with any questions.
Can I drink water before a CT scan?
Usually yes. For a contrast scan you may be asked to drink water as instructed to stay hydrated, so follow the instructions staff give you when you book.
Do I have to take off my glasses or jewelry?
You will be asked to remove metal from the area being scanned, including jewelry, watches, belts and hairpins. Glasses, hearing aids and removable dental work may need to come off for head and neck scans.
What if I am pregnant or breastfeeding?
Tell staff before your exam. Patients who are pregnant or may be pregnant must inform staff before the examination, and alternative imaging such as ultrasound may be considered when appropriate. If you are breastfeeding and having a contrast exam, let our team know.
Book your CT scan
Call 905-539-0555 or book your appointment online. Learn more about CT scans at King City Diagnostic Imaging.
