CT Scan Radiation Explained: How Much, and Is It Safe?

At a Glance

How much radiation does a CT scan give?

Radiation dose is measured in millisieverts (mSv). The easiest way to picture it is to compare it with natural background radiation, which we all receive every day from the ground, the air, food and space: roughly 2 to 3 mSv a year for most people.

A CT scan (also called a CAT scan; NCCT for a plain scan, CECT with contrast) takes many X-ray views from different angles. Your actual dose depends on your body size, the length of body scanned, the number of phases and your doctor's question, so the figures below are ranges, not exact doses. As a rough guide for adults:

• Chest X-ray, for comparison: roughly a few days' worth of natural background radiation
• CT sinuses (CT PNS): a few weeks' to a few months' worth, one of the lower-dose CT scans
• CT brain (head CT): several months' to about a year's worth
• CT KUB for kidney stones, done here with a low-dose stone protocol: usually less than a routine abdominal CT
• Chest CT, including HRCT: several months' to about two years' worth, depending on the protocol
• CT abdomen and pelvis: a few years' worth; a triple-phase scan gives more, because each phase is a separate set of images

CT scan vs X-ray radiation

An ordinary X-ray is one flat picture, so its dose is small, but organs overlap and many problems stay hidden. A CT takes many views as the X-ray tube turns around you, so a chest CT gives many times the dose of a chest X-ray. The same thing lets it show a small lung nodule, a stone in the ureter or bleeding in the brain that an X-ray can miss. Often an X-ray or ultrasound comes first, and CT is used when they cannot answer the question.

Is a CT scan safe? Benefit and risk

The radiation from a routine diagnostic CT causes no immediate effects: no pain, burns, hair loss or sickness. The X-rays stop when the scan ends, and you are not radioactive afterwards.

The possible long-term concern is a small extra chance of cancer many years later. For an adult having a single scan, this risk is too small to measure directly; it is estimated mainly from people exposed to higher doses. Large studies of children and young people who had CT scans suggest a small increase in risk, which is one reason doctors are especially careful about scanning children. The estimated extra risk from one scan is small compared with the ordinary lifetime chance of cancer that everyone has; it is higher for children and young adults, and lower for older adults.

Against this, CT quickly finds problems such as bleeding in the brain, kidney stones, lung disease, injuries and tumours, and a timely answer often changes treatment. So the useful question is whether this scan is needed now, and whether another test could answer it. If a prescription is unclear, our team confirms the right test with you, and with your doctor if needed.

How we keep the dose low at AV Scan

Our CT protocols follow the ALARA principle (as low as reasonably achievable): enough dose to answer your doctor's question clearly, and no more. See also our quality and safety page.

• Fujifilm Supria CT with iterative reconstruction (Fujifilm Intelli IP). It reduces the grainy 'noise' in images, so scans can be done at lower dose settings while keeping the detail the radiologist needs
• Settings adjusted to your body size and the clinical question, so a slim adult or a child is not scanned with settings meant for a large adult
• Only the area that needs to be seen is scanned, and contrast phases are added only when they add information
• A dedicated low-dose stone protocol for CT KUB
• Ultrasound and MRI (no radiation) and digital X-ray (a much smaller dose) at the same centre, so one of these can be used instead of CT when it answers the question
• If you bring earlier reports and CDs, we compare them with the new scan, which can sometimes save you an extra follow-up scan
• Every CT is reported by our radiologists, Dr. Avinash Bansal and Dr. Vaishnavi Bansal

CT scans for children

Children are more sensitive to radiation than adults and have more years ahead of them, so a child has a CT only when the doctor feels it is needed. Ultrasound is often tried first, and MRI is used when it can answer the question. When a child does need CT, the settings are reduced to suit the child's size. Please tell us about any earlier scans your child has had.

CT scan in pregnancy and breastfeeding

If you are or could be pregnant, or your period is late, tell us before booking any CT or X-ray, and remind our staff on the day. Your doctor and our radiologist will decide whether the scan should go ahead, wait, or be replaced by ultrasound or MRI, which use no radiation. MRI in pregnancy is usually done without a contrast injection.

If you had a CT before you knew you were pregnant, the dose to the baby from a single standard diagnostic CT is generally below the level linked with harm to a developing baby. Please tell your obstetrician, who can look at which scan you had and put it in context.

Breastfeeding can continue normally after a CT. Current guidance also says there is no need to stop breastfeeding after a CT contrast injection; if you would still like to pause, ask us and we will explain how.

• CT of the head, neck, chest, arms or legs: the baby is outside the area scanned and receives very little radiation, so the scan is often still done when it is needed
• CT of the abdomen or pelvis: the X-rays pass through the womb, so it is usually avoided in pregnancy unless essential; ultrasound or MRI is often used instead
• Contrast for CT is used in pregnancy only when it is essential

How many CT scans are safe? Repeat scans and your records

There is no fixed safe number of CT scans per year or per lifetime. Instead, each scan should have a clear medical reason. Estimated risk adds up with each scan, so doctors take earlier scans into account, but past scans are not in themselves a reason to refuse one you need now. What helps:

• Keep your CT reports and CDs, with dates, in one folder or on your phone, and bring them to every appointment
• Tell your doctor and our team about any CT you have had, especially in the past year
• Ask whether the result will change your treatment, and whether ultrasound or MRI could answer the question, for example for follow-up of kidney stones
• Do not skip or delay a scan your doctor considers necessary

Contrast dye is a separate question

Contrast (dye) given into a vein makes blood vessels and organs show more clearly, and is used for many abdominal, blood vessel and tumour scans. It is not radioactive and does not itself add radiation; a contrast scan gives more dose mainly when it takes images in more than one phase. The safety questions about contrast are chiefly allergy-type reactions, the kidneys and the thyroid.

Before a contrast CT, a recent kidney function (creatinine) report may be requested; if you do not have one, the test can be done here before the scan. Tell us about any earlier reaction to contrast, asthma or allergies, kidney disease or dialysis, an overactive thyroid, planned radioactive iodine treatment or a thyroid scan (contrast can interfere with these for some weeks), and diabetes medicines: if you take metformin, we check your kidney function (eGFR) and advise you. Do not change any medicine on your own. Have nothing to eat for about 4 hours before a contrast CT; water is allowed. Our team confirms the exact preparation when you book on WhatsApp. More on our CT scan page.

Having a CT scan at AV Scan, Dadar West

A CT scan is painless. You lie on a table that moves through a short, open ring (not a closed tunnel) and, for chest and abdominal scans, hold your breath for a few seconds when asked. For a plain scan you are usually in the scan room for about 10-15 minutes; allow longer for a contrast scan. Most plain CT scans need no special preparation; wear loose clothes and leave jewellery and other metal at home, as metal blurs the images.

Tell us if you wear an insulin pump or a continuous glucose sensor, or have a pacemaker, nerve stimulator or other implanted electronic device. Some pumps and sensors need to be removed or kept out of the scanned area, following the maker's advice, so please bring any spare supplies you may need.

• WhatsApp a photo of your prescription to +91 98200 51511; our team confirms the test, the preparation and the exact price before your visit. You can also call us or book online
• Most reports the same day, sent to your WhatsApp
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• Open Monday to Saturday 8am-10pm and Sunday 8am-5pm
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