Mayo Clinic experts provide some insight about radiation dose from CT imaging

May 31 2013

Many misconceptions exist about the risks associated with radiation doses and CT scans. Experts at Mayo Clinic want patients to have the right information about radiation dose, including when to have procedures with radiation, such as a CT scan, versus when to have magnetic resonance imaging or other non-radiation tests.

Since 2001, a team of diagnostic radiologists and medical physicists at Mayo Clinic have been actively developing and implementing new techniques to lower doses from CT. They have installed new CT scanners and software and implemented new scanning instructions to lower the doses for more than 100 different CT protocols. Amy Hara, M.D., a diagnostic radiologist at Mayo Clinic Arizona says that doses have dropped more than 50 percent for several scan types. Cynthia McCollough, Ph.D., a medical physicist at Mayo Clinic Rochester, emphasized that their goal is to reduce the radiation dose without compromising the image quality — and advances in technology are making that possible.

In the following Q&A, Drs. Hara and McCollough provide some insight about the radiation dose from CT imaging and what it means for patients.

What are the risks of medical radiation exposure?

At the low radiation doses from medical imaging, the exact magnitude of risk is a controversial topic. This is because below 100 millisieverts of radiation, the risks are too low to be directly measured. In fact, some experts believe that there is no risk at these low doses. To be on the safe side of this debate, we assume that there is some small increase in cancer risk from even low doses of radiation and so try to keep the dose levels as low as reasonably possible, while at the same time making sure that the image quality is sufficient to provide an accurate diagnosis.

To be clear, there is minimal risk associated with a single or even multiple CT scans. Normally, people get about 3 millisieverts of radiation each year from background environmental radiation, such as radon and cosmic rays. In higher elevation areas such as Colorado, the background exposure can be up to 10 millisieverts per year, yet no increased incidence of cancer in higher background radiation locations has been observed. A CT scan can deliver anywhere from 2 to 10 millisieverts of radiation, depending on what type of scan a patient receives. For exams requiring multiple scans, this can add up to 20-30 milliSieverts, but this is still considered a low dose of radiation. For some, scans, such as a head CT, the exposure is very low, about 1 - 2 milliSieverts.

Because children have a longer life expectancy than adults and some of their tissues are more sensitive to radiation that for adults, we are particularly careful to use lower doses in children. For a CT scan of a baby, the amount of radiation that we use is about five times lower than for an adult. In some patients, such as young adults with a chronic condition that will require many CTs over a lifetime or pregnant women, patients may want to ask their doctor about alternative imaging options or low-dose scan techniques. All patients, however, should ask their doctor how the exam will be used to help diagnose or treat their symptoms. For a medically appropriate CT scan, the benefit is always much greater than the small potential risk. But like any medical test, if it's not needed it should not be performed.

If the risks are low, why the effort to lower doses?

Patients want peace of mind and the advances in technology are allowing us to lower the dose without compromising quality. Patients are more informed about their care and potential risks. The Internet has played a big role in this and patients are coming in with a lot more questions. Some people who have a lot of these tests are asking if there is a cumulative effect. That's why we are being extremely proactive in lowering the dose - so that patients can be reassured they are getting the lowest dose necessary.

Is there a preferred imaging test?

Every type of imaging test has advantages and disadvantages — cost, speed, availability, comfort, image quality and diagnostic accuracy all need to be considered. There are some exams that we consider to be the same because they have the same ability to make the right diagnosis. For instance, for a patient with Crohn's disease, MRI and CT have similar accuracy. Some patients will opt to have the MRI because it does not use ionizing radiation. However, MRI will be more expensive than a CT, is not available at every medical center, and the exam takes longer. Additionally, some patients can't undergo MRI due to having a pacemaker or other metallic object in their body and other patients are claustrophobic and do not want an MRI. So there are always multiple considerations in selecting the appropriate exam for a specific patient and specific medical question. We work with patients and their doctors to give them access to those exams that are most appropriate for their symptoms. Dose is just one of many considerations.

You mentioned that dose levels keep dropping. Where is this heading?

You can do more with less — that's where we're going. The dose is getting lower and lower and we are still getting the same - or better - image quality with the many recent improvements in scanner technology. The exciting thing now is that if you take a picture that doesn't have the quality that you need, you can often use computer power and sophisticated methods - some developed at Mayo - to sharpen the image appearance and bring out the needed detail. We didn't have this option 10 years ago.

There have been efforts to lower doses across the nation, and not just in CT imaging. Mayo Clinic is a world leader in this regard. Mayo Clinic in Arizona, Rochester and Jacksonville have been actively working on lowering dose levels for a long time, and have always invested in the most current scanner technology, which typically requires lower doses. We also track our dose levels and have lowered the overall CT doses used by 30 to 50 percent, depending on the type of exam. We continually replace our CT scanners so that we have the latest low-dose scanner technology. And, we work closely with CT scanner manufacturers to ensure access to the most cutting-edge dose reduction techniques. We recently completed a study showing that with some of the newest technology, we could reduce the dose by about 50 to 70 percent from the low doses we were already using. We found that we get similar diagnostic information and image quality, but without the added radiation. In other parts of our practice, such as perfusion imaging of the brain, we now use less than half of what was previously considered "low dose." In cardiac CT, for patients with stable heart rates we have reduced our doses to a mere 10% of the dose levels used five years ago. The entire imaging community has really rallied around these efforts. We all want to keep CT safe so that patients who need a CT can get the imaging they need without worrying.

What is your advice for patients?

The main message for patients is that radiation risks from CT are minimal. A CT scan can be a lifesaving procedure and you should not refuse a CT or any X-ray exam because of the fear of radiation exposure if it can provide meaningful information to help your doctor diagnose or treat your symptoms. If you are young (under 30) and not in a medical emergency, an MRI or ultrasound exam may be able to provide the needed information without using ionizing radiation, but it's important to make sure that the exam is equally as accurate and that issues such as cost, the need for sedation, timely access and patient comfort are also considered. Just because an exam does not require ionizing radiation doesn't make it the preferred exam. And, for non-emergency situations, have a needed CT exam performed at an accredited facility that uses low dose technology.

Talk with your physician. Ask questions like, "Do I really need to have this test? How will the information benefit me? Is there an imaging test that doesn't use radiation that is just as good?" Remember, the risk of developing a cancer from the low doses of radiation used in medical imaging is very small - so small that experts don't agree on whether or not it actually exists. And, if a cancer were to occur, it would not develop for many years. It's much more important to get the care that you need now then to worry about a small risk of something happening later. For adults, especially older patients, radiation shouldn't be something they worry about. If the exam is needed, get it.

What advice do you have for other physicians?

Physicians can play a huge role in reducing radiation doses by talking with a radiologist to answer questions. Also, any physicians ordering imaging procedures can help by answering some questions at the time of the order:
- What do I want to image and why?
- Can a previous imaging test answer the question?
- Is there an alternative?
- Will a targeted examination suffice?

SOURCE Mayo Clinic