Radiation FAQ

Radiation FAQ

What is ionizing radiation?

How is radiation measured?

Where can we find radiation?

What are the risks of radiation exposure?

How much radiation is too much?

Can I get radiation just from being in your building?

Are certain people more sensitive to radiation?

Do you have special pediatric exams that lowers dose?

Is the radiation I receive similar to that of workers at the Fukushima plant in Japan?

Should I be concerned about having this exam in CT as I have had CT exams in the past?

Does CT imaging deliver more radiation than x-ray? 

How is CT radiation dose measured?

What is the risk of cancer from a CT scan?

Why isn’t low dose imaging the only goal?

Another practice says they have the lowest radiation dose from their CT scanner.  Is that true?

Does the oral and/or IV contrast given in CT have radiation in it?



What is ionizing radiation?

Radiation is energy that is emitted from a source, and includes light, microwaves, radio waves, and X-rays.

Ionizing radiation has enough energy that it can remove electrons from an atom, which causes the atom to become charged, or ionized. This includes alpha particles, beta particles, neutrons, gamma rays and X-rays.

How is radiation measured?

One of the reasons why radiation dose values can be confusing is because of the wide array of measurements that are reported.  While the energy of radiation can be measured, when we look at health effects, we look at how much energy is absorbed as the absorbed dose. This is measured in grays (Gy), defined as 1 joule of energy absorbed by 1 kilogram of mass.

We further refine this by looking at the equivalent dose, which takes into account the intensity of ionization by different type of radiation. This is measured in sieverts (Sv); this is the most important relevant measure of radiation dose for most purposes (1 Gray = 1 Sievert).
 

Where can we find radiation?

Radiation is all around us, and all of us are exposed to natural or background radiation. Major sources include cosmic radiation, radiation from the soil, and radon. 

What are the risks of radiation exposure?

What we know of the effect of radiation comes from data largely from atomic bomb survivors from Nagasaki and Hiroshima and the disaster at Chernobyl in 1986. In these settings, those studied received high doses of radiation.  It will take some time to understand the effects from the recent nuclear disaster at Fukushima.

Very high doses of radiation kill cells, and reliably result in specific side effects, which are called deterministic, which is to say that these are expected at certain high doses of radiation.  Typical doses from most medical imaging are well below the threshold for these effects.

What is not studied well is the effect of low doses of radiation, such as dose encountered through the environment and through medical imaging. There are arguments that any dose may be dangerous, and others that low doses may even be protective.  When dealing with radiation protection, we conservatively assume any amount of radiation may pose some random or stochastic risk for causing cancer, and that this risk increases with greater dose.

How much radiation is too much?

This is a difficult question.  But instead of just saying, “It depends,” let’s look at this in two ways.  First, let’s look at both estimates of what normal individuals receive annually, government-specified limitations on doses for radiation workers, and other doses.  It is important to understand there is a vast scale of radiation doses spanning several orders than can be accurately measured.

Natural background radiation is approximately 3mSv a year (Sv = Sievert)

The total dose allowed per year is 150mSv to workers such as radiographers.

Second, our practice operates under the principle of ALARA, or

As Low As Reasonably Achievable

With respect to medical imaging, this principle states that radiation dose must be enough to deliver sufficient image quality, but low enough to minimize risk to the patient.  For example, there is an easy way to avoid radiation exposure - not having the study.  At the same time, information is not obtained.  There are also scenarios in which the radiation used is too low to get an answer; this actually often contributes to a higher dose than if a properly dosed study was done, due to the potential need for repeat studies.

Can I get radiation just from being in your building?

Yes, some background radiation is present in all construction materials in all buildings.  But because of the shielding we use, you will not be receiving an additional dose of radiation just by being in our building.

Are certain people more sensitive to radiation?

When we look at stochastic or random effects of radiation (e.g., cancer), cells that grow more rapidly are more likely affected, as there are more opportunities for DNA damage to occur that can result in cancer.  Consequently, children are known to be more susceptible to radiation.

In addition, our bodies do have mechanisms by which DNA repair can take place.  Patients with rare disorders that limit this ability (such as xeroderma pigmentosum) are at increased risk for radiation-induced cancer.

Complicating all of this is the fact that different organs in the body have different sensitivities to radiation exposure.

Do you have special pediatric exams that lowers dose?

As the first practice in the Salem area to participate in the nationwide Image Gently campaign we take pediatric imaging radion safety seriously.  Some of the things we do to reduce dose include:

● Always asking if a study not using ionizing radiation would be clinically appropriate (ultrasound or MRI).
● Reducing dosages for CT examinations to within standards set by the American College of Radiology
● Decreasing the pulse rate on our fluoroscopy devices

Is the radiation I receive similar to that of workers at the Fukushima plant in Japan?

The radiation released at Fukushima Daiichi is different because it primarily arose from the release of radioactive isotopes, which constantly emit radiation based on a half-life measurement.  For example, cesium-137 has a half-life of 40 years, which means it will take about 280 years for radiation emission to decrease to about 1%.

This is in contrast to the carefully controlled doses of radiation that are used for medical imaging.  When an x-ray machine or CT scanner is acquiring images, radiation and exposure are present.  However, when these machines are off, radiation is not present.

Should I be concerned about having this exam in CT as I have had CT exams in the past?

Your doctor will order studies based on their clinical judgement regarding the need to answer a clinical medical question.  In some cases, however, monitoring a known disease process can involve multiple CT studies, and consequently, multiple doses of radiation.

However, risk does accrue with more studies, and so some questions you can ask your physician include the following:

Can other types of studies without radiation provide this information?
If we are following a known process, is there a way we can we can limit the parts of the body we are imaging?

Does CT imaging deliver more radiation than x-ray? 

Yes, typically.

How is CT radiation dose measured?

There are three ways to commonly measure CT dose.  These are CTDI (CT Dose Index), DLP (Dose Length Product), and Effective Dose.

The first two actually measure CT radiation dose to a cylinder made of acrylic, called a phantom.  This is done to improve uniformity of measurements.
The last one, effective dose, allows providers to estimate relative patient risk.

CTDI and DLP are NOT equivalent to patient dose.  So when you see these measurements in ads, note that these are measurements of radiation given to a piece of plastic and not a person.

Effective Dose is estimated by multiplying DLP by a dose conversion coefficient or k-factor.  This is measured in milliSieverts (mSv).

What is the risk of cancer from a CT scan?

We work backward from an estimated risk of radiation-induced fatal cancer of 4% per Sv.  (Again, this is an assumption, and nothing that has actually been proven with low-dose radiation as is delivered with a CT scanner.)

The dose from a CT scan will vary on the part of the body, the size of the patient, and a number of other factors.  That is why the effective dose measurement is helpful.  If we take an estimate of 3 mSv, then the associated risk is 0.012% or approximately 1 in 8,300.

One issue is that this small risk is difficult to separate from the much higher overall lifetime risk of developing fatal cancer, 1 in 4 for all males, and 1 in 5 for all females.

Additionally, the risk assumed needs to be set against the risk of possible disease or injury that needs to be evaluated.  For example, the risk of a life-threating injury following high-speed motor vehicle trauma would be anticipated to be higher than these risks.  By ordering your study, your physician has decided that in his or her own mind, that the risks of not doing the study outweigh those of doing the study.

The risks of everyday life should also be considered when reviewing these risks.  For example, the 0.012% risk stated above would be similar to the risk of death from smoking 850 cigarettes, or driving approximately 2,000 highway miles.

Why isn’t low dose imaging the only goal?

Because if images are of such poor quality that they do not answer the question at hand, then the patient is given a radiation dose for little reason, and there is a high risk that another study will need to be performed.

Another practice says they have the lowest radiation dose from their CT scanner.  Is that true?

Not necessarily.  To make this claim, multiple practices would have to obtain images of the exactly the same quality on the same person or object.

Comparisons between CT scanner radiation doses are impossible without equalizing image quality.  What is not being said is that the operator of the CT machine or machine itself can set the level of radiation given.  More radiation often results in a clearer image, but this needs to be weighed against the risk involved.

Even with a machine that produces less radiation, more radiation dose can be given than from a higher radiation machine if a higher quality image is sought.

So when ads say 50% less dose, it is really hard to know what that means.  The truth is, all current generation CT scanners have continued technological refinements that reduce dose substantially.

In the end the low-dose goal is to achieve diagnostic image quality.

Does the oral and/or IV contrast given in CT have radiation in it?


No, the contrast agents given in CT imaging are not radioactive.

 

I have more questions.  How can I get answers?
Please contact us at info@diaradiology.org.  We would be glad to try and answer your questions.  An additional helpful resource is www.radiologyinfo.org.

For information about the Sievert measurement and average doses click this link. Wikipedia Sievert