How Much Radiation Am I Getting?

A common question from a lot of patients is "Isn't radiation bad for me?" or "How much radiation am I going to receive from this x-ray?" or "Is this exam going to give me cancer?" Those aren't dumb questions and honestly, they're very difficult questions to answer in terms that a patient can understand. Better yet, they're difficult questions to answer in terms that a physicist can understand. I'm going to try and attempt the impossible and give a "layman's" explanation to some of these issues.

Always remember that a licensed physician has calculated a "risk vs. diagnosis" with every examination that has been prescribed for you. A physician has determined that your risk of developing cancer or any other harmful effect is much lower than the importance that he or she finds out what is wrong with you and can diagnose you properly. If you are ever unsure about your physician's decision to administer a radiographic examination, you have every right to ask him or her if there is a better option available to assist in a proper diagnosis.

Knowing this, the first thing you need to know is, yes, ionizing radiation can cause cancer and other harmful effects. And yes, a lot of the imaging you receive from a medical facility uses radiation to acquire the pictures. Now, the good news is, hopefully a qualified professional that has extensive training and education on radiation is administering your exam (refer to my previous post on the "hopefully" part) and the radiation used for imaging purposes is at the lowest dose possible to decrease the potential for any damage to occur.

Now, let me explain how radiation damages your body. Ionizing radiation, specifically x-rays, are photons that pass through your body to create an image. Some of those photons do not make it all the way through your body to the film on the other side of you. These photons are either absorbed by your body or are "scattered" off the tissue into another direction. "Scattered" radiation may subsequently absorb into your body as well. When radiation is absorbed into the body, the radiation can either kill the cell that's absorbed it, damage the cell that's absorbed it, or simply have no effect at all. The latter is by far the most likely. However, with the previous explanation, every photon has the potential to cause damage to a cell. Every cell is different and has a different sensitivity to radiation, however, the faster a cell reproduces, the higher the sensitivity the cell has to radiation. Hence why radiation is more dangerous for fetus' and children, they are growing (cells are reproducing) faster than the average adult, and therefore their cells are ultra-sensitive to the radiation they are absorbing.

The radiation you receive is measured in "Sieverts" or "milliSieverts" (1/1,000 of a Sievert), The amount of radiation used in diagnostic imaging is extremely low and if any effect is generated from your exposure to radiation it will be either stochastic (random) or non-stochastic (with a threshold). Examples of stochastic effects include cancer and leukemia. There is no way currently to predict which individual will be affected. For any individual, therefore, the exposure to radiation can be thought of as increasing the probability that an effect could occur. The likelihood of developing cancer increases with increased exposure to ionizing radiation. Such effects are somewhat like buying lottery tickets too, the more tickets you buy, the higher chance of winning. Similarly the more dose one absorbs, the higher chance of contracting cancer and other stochastic effects. Hereditary effects are also stochastic. Examples of non-stochastic effects are cataracts and infertility. The severity of the effect varies with the dose and the effects are not seen below a certain threshold level of radiation. For these effects, there may be a threshold-that is, there may be a dose below which there is no effect.

The biggest concern with the general public seems to be sterility or infertility. Infertility is a non-stochastic effect, meaning there is a threshold before any changes can be determined. A single dose of only 300 milliSieverts to the testes results in temporary sterility among men. For women, a 3,000 milliSievert dose to the ovaries produces temporary sterility. Higher doses increase the period of temporary sterility.

The average person receives 2.4 milliSieverts per year in background radiation alone. Radiologic Technologists are allowed to receive the equivalent of 50 milliSieverts per year in order to continue working in and around radiation. The average annual dose to a Radiologic Technologist however, is only 3.2 milliSieverts.

Now that you know how radiation effects the body, the next thing you need to know is what kind of examination you're having. There are different amounts of radiation used in every examination. For instance, an x-ray will typically use less radiation than a CT scan, and some modalities use no radiation at all, like MRI and Ultrasound.

A CT Scan, otherwise known as a CAT Scan, uses quite a bit of radiation. A CT Scanner has an x-ray machine inside of a circle that spins and obtains images while you slide through it. The x-ray machine can take as many as 64 images (or x-rays) per rotation around the circle. The radiation dose that you receive is much higher than a conventional x-ray (up to 400 times). A typical CT of the head will typically give you 2 milliSieverts, that's nearly your annual dose of background radiation. A scan of your chest, abdomen, and pelvis will typically give you 18 milliSieverts (more than 6 times your average annual background radiation in less than 2 minutes...That's a lot of radiation).

X-Rays use quite a bit less radiation. A typical chest x-ray will produce a dose of about 0.1 milliSievert. Calculating that dose to the annual background radiation most people receive,that's equivalent to about 2 weeks of just living. Some examinations do involve a larger dose, for instance, an x-ray series of your lower back will produce a dose of about 1.5 milliSieverts. When compared to a CT Scan though, that's a pretty minimal dose.

A couple modalities that use zero ionizing radiation are MRI and Ultrasound. An MRI machine uses radio frequencies and a magnetic field to alter the alignment of atoms in the body. To make a long story short, the atoms in your body align to an exact polarization while under the magnet and a radio frequency measures how fast those atoms recover when "hit" with an electro-magnet. There is no ionizing radiation used in MRI and any harmful effect generated by having an MRI cannot be attributed to a radiation dose. Ultrasounds are similar in the fact that no radiation is used at all. An ultrasound uses ultrasonic sound waves to penetrate your body with a transducer that sends a signal back and forth to create an image for the sonographer. Once again, no ionizing radiation is used at all and any effect generated cannot be attributed to a radiation dose.

I hope that this either eases your concerns or makes you more aware of the dangers of radiation. There are still unknowns to account for but we can use what we DO know to our advantage. Diagnostic imaging with the use of radiation has saved millions of lives and the world is a better place with it's invention, however, overuse and negligence can regress the advances that this wonderful tool has provided us.


Curtis J. Carpenter R.T. (R)(CT)


Curtis Carpenter is the founder and President of Reliable Radiography, based in Vero Beach, Florida.

Visit the Blog at reliableradiography.blogspot.com