The radiation dose for x-ray radiography and CT is compared with the background radiation dose levels (average in United States of 3mSv). The different exams Abdomen, Bone, Head and Neck, Chest, Cardiac, and Mammography are presented. For reference the sources of background radiation of cosmic radiation and Radon are presented.
Background Radiation Overview
To provide perspective on the radiation dose that is given during diagnostic imaging exams we will discuss the fact that everyone receives radiation throughout the course of their lives. Some of this radiation occurs from natural sources and from within our houses.
This radiation source is often referred to as background radiation and everyone gets this background radiation.
The purpose of comparing with background radiation is not to say that the potential risk of cancer induction is the same as a given duration of exposure to background radiation, but rather to given context as dose values on their own may be rather abstract quantities.
In addition to background radiation the most common other sources of radiation exposure are: at work (i.e. occupational exposure), and radiation exposure from medical exams.
Cosmic Radiation
One source of background radiation that we often don’t appreciate is the radiation coming from cosmic rays. In this section we will describe cosmic rays.
Cosmic rays are typically either protons or atomic nuclei. Most of this radiation comes from outside our solar system, such as when stars near the end of their life and dissipate their energy as a supernova explosion.
The actual source of the cosmic rays isn’t that important for this discussion but just be familiar with the fact that there are fast moving particles moving toward earth all the time.
Something that many of us learned in elementary school is that the earth is a great big magnet. This is due to the molten metal core of the earth. Hence as the earth spins a magnetic field is generated.
The fact that the earth has magnetic field is why compasses works. This is just a small magnet within the compass interacting with the earth’s magnetic field.
Now we can come to the important part and the reason that we reminded you about the earth being a giant magnet.
Remember the cosmic rays are primarily charged particles (atomic nuclei or protons). All charged particles will experience a magnetic force from the magnetic field lines of the earth.
The magnetic field lines point straight up and down at the north and south poles and point to the side around the equator. This causes charged particles near the equator to be forced away from the earth’s surface. However, cosmic rays coming down directly toward the poles will enter earth’s atmosphere at a much higher rate.
Due to this effect at the poles there is a higher rate of cosmic rays. This phenomena is call auroras. One particularly well known aurora is the aurora borealis. These auroras are very impressive, colorful phenomenon in the northern and southern poles.
The gas that is present in the atmosphere determines the colors we see in the sky. The primary interactions are with oxygen (creates yellows) and nitrogen (creates purples and reds).
Since these cosmic rays do interact with molecules in the atmosphere the cosmic radiation is higher at higher altitudes as there is not as much atmosphere above those portions of the earth.
Finally, we point out that the cosmic radiation is higher in airplane flights. The radiation that you are exposed to on an airplane is dependent upon:
- The length of time of the flight.
- The altitude of the flight (because of the reduced atmosphere above you)
- How close you fly to the north or south poles.
Flight crews repeatedly travel in airplanes and thus will have a higher radiation exposure. In the US flight crews are not tracked as radiation workers with formal limits, but in the EU work schedules are modified such that each crew member receives less than 6mSv/year.
Rad Take-home Point: Background radiation is radiation that everyone is exposed to and one of the sources is cosmic radiation. Cosmic radiation is higher near the north and south poles. Air travel has increased radiation exposure due to cosmic radiation.
Radon
Radon is the largest source of background radiation in US.
Radon is a gas that is clear and does not have a smell. Radon is a naturally occurring element, but is very short lived (about three days). It is produced in the slow process of the elements thorium and uranium decaying, which eventually decay to the stable element lead.
Thus, the radon level that you exposed to can vary significantly based on the local geology and the building material used in construction of the buildings where you spend a lot of time such as home and work.
Since Radon is a gas is it easily inhaled and taken into the lungs. However, the elements that Radon decays into (ie. the daughters in the decay of Radon) are solid at room temperature.
The primary pathway for exposure is that Radon daughters stick to dust particles which then can settle in the lungs and deposit radiation directly to the lung tissue. This is why Radon exposure is the second largest cause of lung cancer behind cigarette smoking (according to epidemiological studies).
Radon mitigation via ventilation is now a common practice in the United States and frequently Radon testing is performed as part of the home buying process.
This gas inhaled by population and it causes 1.26 mSv of exposure on average person in the world while US citizen gets higher dose – 2.2 mSv. This dose is a bit lower in Japan.
Rad Take-home Point: Radon is the largest source of naturally occurring background radiation and the primary risk is lung cancer as its radioactive decay deposits dose locally in the lung tissue.
Medical Imaging and Background Radiation
We first present the total breakdown on sources of background radation globally and then we have some information specific to the background radiation in United States. The pie chart in this graphic annotates each component of the background radiation in terms of Low LET or High LET, where the high LET radiation is more damaging (definition of LET).
The takeaways here are that Radon is the highest source of background radiation globally. There is more high LET radiation in the background globally. As the cosmic radiation that we discussed above has both a high LET and low LET component.
Apart from radon, which is largest source of radiation, medical imaging doses (such as CT, nuclear medicine, fluoroscopy, and standard radiography) are the second highest source of radiation exposure.
If you add up all of the background radiation sources, on average, in the U.S. it’s about 3 mSv. The largest contributors to the background radiation being Radon and Cosmic rays discussed above.
This background radiation dose of 3 mSv is simply an average in the United States and can vary. If you live in Colorado or New Mexico, it may be 4-4.5 mSv.
In the next section we have some example radiation doses of diagnostic procedures and how many long that is equal to in terms of background radiation. This is just a simple calculation, where the value of 3 mSv is used as an average background radiation for the population in the US.
Rad Take-Home Points
Diagnostic Medical Imaging Radiation Doses
Just as the background radiation dose varies throughout the US there is also significant variance in the radiation dose for different diagnostic exams.
Below are some examples of the representative radiation doses of different types of exams and a comparable time of background radiation.
These radiation doses are from the RSNA (Radiologic Society of North America). We first summarize the results with two infographics, one for x-ray and one for CT. In these infographics we list only the approximate equivalent time of background exposure to be exposed to the same radiation dose. Then we provide the specific tables with the dose values and the approximate time equivalence for background radiation broken down by anatomical region.
Abdominal Imaging
CT of abdomen and pelvis corresponds to about 10 mSv of radiation dose which corresponds to about three years of background radiation (10/3=3.333). If there is a need for a repeated scan with and without contrast, this received dose corresponds to about four years of background dose (20/3=6.667)
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| CT – Abdomen and Pelvis | 10 mSv | 3 years |
| CT – Abdomen and Pelvis (w/ and w/out contrast) | 20 mSv | 7 years |
| CT- Colonography | 6 mSv | 2 years |
| Intravenous Pyelogram (IVP) | 3 mSv | 1 year |
| Barium Enema (Lower GI X-ray) | 8 mSv | 3 years |
| Upper GI Study with Barium | 6 mSv | 2 years |
On average CT colonography exposes patient to about 6 mSv of radiation, which is approximately two years of background radiation. In abdominal imaging, the representative averages dose of each procedure varies between one and three years of equivalent background radiation.
Bone Imaging
In standard x-ray radiography procedures of bone imaging, the dose is quite low, around 0.001 mSv. This corresponds to about three hours of background radiation in the United States.
Bone densitometry (which uses dual energy x-ray scanning) corresponds to about three hours of background radiation and spine x-rays correspond to about six months of background radiation.
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| Extremities X-ray (hand, foot, etc) | 0.001 mSv | 3 hours |
| Bone Densitometry (DEXA) | 0.001 mSv | 3 hours |
| Spine X-ray | 1.5 mSv | 6 months |
Head and Neck
In case of a standard non-contrast head CT scan it is about 2 mSv or eight months of background radiation. A head CT, if you do it with and without a contrast is about 4 mSv. That’s about 16 months of background radiation. Spine CT, corresponds to 6 mSv or about two years of background radiation.
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| Head CT | 2 mSv | 8 months |
| Head CT (w/ and w/out contrast) | 4 mSv | 16 months |
| Spine CT | 6 mSv | 2 years |
Chest
Then in the chest, for chest CT, about 7 mSv, so that’s a little bit more than two years. A lung CT for screening, 1.5 mSv or lower, which corresponds to about six months of background radiation.
Chest x-ray gives patients an equivalent of about ten days of background radiation dose.
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| Chest CT | 7 mSv | 2 years |
| Lung CT (Screening) | 1.5 mSv | 6 months |
| Chest X-ray | 0.1 mSv | 10 days |
Cardiac CT Imaging Doses
The doses for cardiac CT depend strongly on the type of equipment which is used for imaging. The doses reported in this table (12 mSv) correspond primarily to the low helical pitch method of acquiring cardiac CT data.
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| Coronary CTA | 12 mSv | 4 years |
| Ca Scoring CT | 3 mSv | 1 year |
In modern scanners the achievable doses are much lower for cardiac CT. For someone of medium body habitus, it is not uncommon to have below 1 mSv cardiac CT scans on state-of-the-art scanners. For instance I was able to visit USZ which routinely images well below 1 mSv for cardiac angiography exams on state of the art scanners.
Then a calcium scoring exam would be about 3 mSv. Again this can often be lower on a modern scanner. This corresponds to about one year of background radiation.
Mammography Imaging Doses
Mammography, has quite low radiation dose, about 0.01 mSv or about three hours of background radiation dose.
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| Mammography | 0.01 mSv | 3 hours |
PET/CT Imaging Doses
Then finally, we list the approximate dose of a PET/CT scan (Positron Emission Tomography). The radiation dose there is a little bit higher than the other standard diagnostic scans, about 25 mSv, which corresponds to about eight years of background radiation.
| Procedure | Approximate Effective Dose | Comparable Time of Background Radiation |
| PET/CT | 25 mSv | 8 years |
Relative Health Risks
The risk of negative health outcomes from receiving radiation should also be kept in context where there are other health and lifestyle choices which can be much more impactful on ones lifespan. The relative impacts of these different risks are given here as reported by the NRC.
The conclusion from the NRC data is that controlling smoking, weight and alcohol consumption are the most important choices that you can make and that the average overall life loss from medical radiation is on the order of the same as natural hazards (about one week).
| Health Risk | Estimated Life Lost |
| Smoking 20 cigarettes a day | 6 years |
| Overweight by 15% | 2 years |
| Alcohol consumption (U.S. average) | 2 years |
| All accidents combined | 1 year |
| Motor vehicle | 207 days |
| Home accidents | 74 days |
| Drowning | 24 days |
| Natural Hazards (earthquake, lighting, flood, etc) | 7 days |
| Medical Radiation | 6 days |
| Occupational Exposure 0.3 rem/year (age 18-65) | 15 days |
| Occupational Exposure 1 rem/year (age 18-65) | 51 days |
Summary
We introduced the primary sources of background radiation. Those different sources of background radiation add up to 3 mSv of background radiation in the United States on average.
Then we took some basic publicly available data for the doses. If you go to RadiologyInfo.org, you can see the source data from the RSNA.
We presented this dose data for representative clinical studies to compare the values with the average background dose in the United States.
This is just to provide a frame of reference and is not meant to imply equivalence in the effects of the background radiation dose with a given medical radiation dose.
The standard diagnostic exams ranged from an equivalent background time of a few hours to multiple years with increasing dose from x-ray radiography to CT to PET/CT.