How Many MRIs, X-Rays, And CT Scans Are Safe In A Lifetime?

One of the noteworthy accomplishments of the medical world is medical imaging. By passing rays through the human body we get exact images of the insides, helping accurately diagnose the severity of a disease. Among a large number of such imaging technologies, the most prominent and common ones are the magnetic resonance imaging (MRI) scan, X-ray, and computerized tomography (CT) scan.

MRIs Do Not Pose Any Radiation Risk

An MRI scan uses magnetic field and radio waves to produce images of the internal structure of the body. During this scan, a temporary magnetic field is created in the patient’s body by passing electric current through the coiled wires around the body. Radio waves are sent and received by a transmitter/receiver in the machine. These signals are used to make images of the scanned body. As there is no radiation involved in this procedure, it is a safe (and painless) method to scan almost any body part.


X-Rays Pose Radiation Risk

X-rays are passed through the body to get black-and-white images of the inside of the body. In a contrast X-ray, for better visibility of the tissues, a dye or a contrast medium (iodine or barium) is injected into the body. The X-rays are passed through the body, so a risk of radiation exposure is associated with these scans.

CT Scans Pose The Maximum Radiation Risk

Computed tomography or CT scan is a computer-aided X-ray technique. This technique too uses X-rays but here, unlike the normal X-ray image, you get to see cross-sectional images of the organs and body parts. Because this procedure also uses X-rays, radiation exposure is a threat.


Measuring Radiation In Terms Of Natural Radiation

CT scans and X-ray imaging use low levels of ionizing radiation to produce images. Ionizing radiation is considered to be more harmful than non-ionizing radiation as it can lead to cancer. So, how do we measure radiation dosage? The low level of radiation a patient is exposed to during these scans is measured in units called millisieverts (mSv).

For a better perspective, let’s compare the radiation dose received during these scans to naturally occurring radiation in the environment. According to the United States Nuclear Regulatory Committee, a person’s average annual radiation exposure from natural sources is about 3.1 mSv.1 If we correlate this:

  • Radiation exposure during a single chest X-ray (0.014 mSv) is equivalent to 3 days of natural radiation
  • An abdomen X-ray (0.7 mSv) is equivalent to 4 months of natural background radiation
  • A CT head scan (2 mSv) is equal to 1 year of natural radiation exposure2

About 100 CT Scans Pose A High Risk Of Cancer

Experts reckon the safety of scans more in terms of radiation dose and frequency than numbers. An MRI, by the very nature of the procedure, does not pose any radiation risk. The amount of radiation that a particular organ receives during a CT scan depends on the number of scans undergone, the size of the patient, the specific design of the scanner being used, and the rotation or exposure time.3 Around 100 CT scans lead to an effective dose of 600 mSv, which would pose a high risk of cancer.4 According to the Health Physics Society, with exposure below 50 to 100 mSv, risks to health are too negligible to be measured or nonexistent.5

Cancer Risk From Radiation Depends On The Patient’s Age, Sex, And Type Of Scan

A study by Smith-Bindman et al., however, concluded that radiation doses from commonly performed diagnostic CT examinations are higher and more variable than generally cited. They also pointed out that the number of CT scans that might lead to cancer depended on the patient’s age and sex, and the type of scan. Other interesting derivations from their study were: “An estimated 1 in 270 women who underwent CT coronary angiography at age 40 years will develop cancer from that CT scan (1 in 600 men), compared with an estimated 1 in 8100 women who had a routine head CT scan at the same age (1 in 11 080 men). For 20-year-old patients, the risks were approximately doubled, and for 60-year-old patients, they were approximately 50% lower.”6


So is there any cause for concern? A Harvard Teaching Hospital expert sums up the safety angle: “Radiation-induced soft tissue injury occurs at about 2000 mGy [equals 2000 mSv] and higher. A human CT scan produces far less radiation: the typical dose for a head CT scan is less than 70 mGy, and a typical body dose is less than 20 mGy. And since the body repairs itself, any damage (if it were to happen), would be repaired in less than one year.”7

The radiation dosage one is exposed to may usually be much below the safe parameters and these numbers shouldn’t unnerve us just yet. But it is definitely a good idea to understand the effects of radiation on the body and take precautions to avoid or minimize exposure wherever possible.