CT radiation dose, contrast and MRI safety: how often you can be scanned

Before a scan, most people worry about two things: how harmful it is and how often it can be repeated. The answer depends on the method. CT uses X-rays and involves a radiation dose. MRI uses a magnetic field and no radiation, but it has its own limits.

Below we give the doses in numbers, explain contrast and when it's safe, who shouldn't have an MRI and what to do about implants. We compared which scan suits which body area in a separate article.

Dose is measured in millisieverts (mSv). Natural background radiation is a handy comparison. According to RadiologyInfo, the average person gets about 3 mSv a year from natural sources, and a chest X-ray gives 0.1 mSv, about the same as 10 days of everyday life.

These are rough figures: the real dose depends on the machine, the protocol and your build. Modern scanners cut it considerably. At Oberig clinic, computed tomography is done on Siemens Somatom go.Top and Somatom Confidence machines with reduced doses.

The risk from one CT is very small but not zero. Radiation slightly raises long-term cancer risk, and these risks add up. So there's a simple rule: CT is done when the benefit of the result outweighs the risk. Extra care is taken with children and in pregnancy. Their tissues are more sensitive, and the benefit has to be clear.

How often can CT be repeated?

There's no strict limit. For the head, your doctor decides: after an injury or stroke, CT may be repeated the same day if your condition calls for it. For the lungs, it depends on the task: follow-up of a nodule on your doctor's schedule, or a yearly low-dose CT for lung cancer screening in smokers. CT isn't done "just to check" without a reason.

In cancer care, CT is repeated more often: to check how treatment works and to look for recurrence, on a schedule set by the oncologist. Here, a timely answer clearly outweighs the radiation risk, so skipping follow-up scans "to avoid radiation" isn't wise. If MRI also works, your doctor may alternate the two.

How to keep your dose down

• bring previous CT scans: they often make a repeat unnecessary
• ask your doctor if ultrasound or MRI could answer the question
• don't repeat CT at different centers "for a second opinion" without need: share your images instead
• keep a list of your scans with dates

CT contrast contains iodine. It makes blood vessels and lesions brighter and is often key to the diagnosis. In our department, up to 70% of CT scans use contrast. Side effects occur in about 3% of cases, and severe ones in about 0.04%.

The main limits are a past allergy to iodine contrast and reduced kidney function. That's why a creatinine blood test is done beforehand. Your doctor sets the interval between contrast scans based on your kidneys and how urgent the scan is. Drink more water after contrast, and wait 24 hours before breastfeeding.

What does a reaction look like? Mild ones include nausea, a rash or itching and usually pass quickly. Severe reactions are very rare, and staff are trained and equipped to handle them on the spot. Delayed rashes or itching can appear up to a week later. If that happens, tell your doctor.

Preparing for a contrast CT

Get a creatinine test if your doctor asks, and bring the result. Mention allergies, especially to contrast, diabetes and any glucose-lowering medicines you take, and kidney or thyroid disease. You usually don't eat for a few hours before the scan, but you can drink water. Afterwards, drink extra fluids: your kidneys clear the contrast within a day.

MRI uses no ionizing radiation, so there's no radiation dose. It can be repeated as often as medically needed. But the strong magnetic field attracts metal and can disrupt electronic devices. According to the NHS, before an MRI you remove all metal and fill in a form about implants, a pacemaker, past surgery, pregnancy and kidney problems.

Contraindications are the same for the head and other areas:

• a pacemaker, defibrillator or neurostimulator that isn't MRI-compatible
• metal fragments in the body, especially in the eyes
• some older metal clips on brain blood vessels
• cochlear implants and some insulin pumps
• the first trimester of pregnancy, a relative contraindication

Can you have an MRI with implants?

Many modern implants are MRI-compatible: joint replacements, plates, stents and some pacemakers are labeled as MRI-safe. RadiologyInfo stresses that you must tell staff about any implant before entering the MRI room. Bring your implant documents so the radiologist can check the model.

Titanium dental implants, crowns and braces usually don't prevent an MRI, but they can distort images of the face and jaw. Mention them too. We cover gadolinium contrast in detail on our contrast MRI page.

Can I have CT and MRI on the same day?

Often yes. If both use contrast, your doctor will check your kidneys and the gap between injections.

Is CT harmful for children?

Children are more sensitive to radiation, so they get CT only for clear reasons and at reduced doses.

Is one CT dangerous?

No. The dose from a single scan is small. The concern is many scans done without a clear reason.

Can I have an MRI with a tattoo?

Usually yes. Rarely, inks containing metal can heat up, so mention your tattoo.

Is MRI safe in pregnancy?

MRI without contrast can be done if needed, more often after the first trimester. Contrast isn't given in pregnancy unless absolutely necessary.

Which is safer for frequent follow-up?

If both work, MRI is chosen for frequent repeats because it involves no radiation.

At Oberig clinic, CT is done on reduced-dose machines and MRI on Siemens 1.5 and 3 Tesla scanners. Before your scan, the radiologist checks contraindications, implants and, if contrast is needed, kidney function, and recommends the method that gives an answer with the lowest risk.

Book by phone at +38 044 521 30 03 or online.