Monitoring the Diagnostic Dose

The first patient in the UCMC's 256-slice CT scanner, the mummy Meresamun

The first patient in the UCMC's 256-slice CT scanner, the mummy Meresamun

Radiation had a bad reputation to overcome. Known for a long time for killing its discoverer and by frightening yellow-and-black warnings, the view of radiation has softened over the years as scientists and physicians corralled its powers for good. Whether used for screening or diagnosis in the form of X-rays and CT scans or therapeutically to kill tumors, radiation has become an essential tool for physicians.

But even with all these beneficial uses, radiation remains dangerous. A recent New York Times article discussed cases where patients received overdoses of radiation during medical procedures, usually due to computer programming errors undetected by the technicians. On the heels of that report, the Food & Drug Administration – which has oversight over medical devices – unveiled a new initiative to reduce unnecessary radiation exposure. Officials hope to improve patient safety by establishing tighter requirements for manufacturers of medical imaging devices, revising the accreditation process for those who use such devices, and conducting more research to find what level of radiation exposure is safe and appropriate for patients.

I asked Michael Vannier, professor of radiology at the University of Chicago Medical Center, to explain what these changes meant for the field and patients. Vannier said that he and his colleagues welcomed the attention being paid to these issues, even as radiologists and manufacturers were already seeking new ways of getting the maximal benefits from a minimum of radiation. At the Medical Center, a state-of-the-art 256-slice CT scanner acquired in 2008 provides higher quality scans than previously possible using 30 percent less radiation, Vannier said – and a computer upgrade scheduled for next week will reduce that radiation by a further 40 percent.

“What will happen, I think, is that the manufacturers will add capabilities to the instruments that make it possible to much more automatically and reliably monitor and minimize the dose,” Vannier said. The rest of our conversation is available below.

Why is the FDA initiative happening now, and why is it necessary?

Vannier: Well, you have several incidents that have attained a lot of notoriety. But what’s also happened is that CT scanning has become extremely popular. Your chances of having a CT scan in your lifetime are extremely high, because it is a very versatile and highly available technique. But it does use X-rays, and the dose you receive from a CT scan is higher than the dose received from X-ray techniques as a general rule. You don’t want to do them unnecessarily.

The second thing is that the CT scanners themselves have improved in their dose efficiency very significantly over the years. If a scan is done with an older scanner, it may very well require a higher dose than the state-of-the-art equipment, which means that getting the same exam in different places can mean different doses received. Even if you know what kind of exam you’re getting, the instrument doing it doesn’t necessarily tell you that the dose is low.

The third thing is the general facts of physics that govern how CT scanners work. If you give a minimal dose you can get an acceptable level of noise in images and the quality can be very high. But if you double the dose, you may see no improvement in image quality, so it’s deceptive in that way, and there’s a potential of overdosing or selecting the wrong dose setting. It takes special care to ensure that the dose is maintaining the standards that are held to that we call ALARA – As Low As Reasonably Applicable, which is the FDA-mandated standard.

In the past, for CT scans in general, it was very difficult to look at scans and tell what dose was used. In the latest scanners, which we use for exams here today, it actually puts a record of the dose in with the images, a printed-out diary or record, if you will. It’s possible to know with a high degree of confidence exactly what dose was received, whereas in past years it wasn’t possible, and people using older equipment may not be included in such a system.

What measures are currently in place to ensure minimal exposure to radiation?

There is often careful discussion about exactly what alternatives are available and planning for the examination: choosing the technical settings to absolutely minimize the exposure. And then there is shielding of different parts of body, lead shields so that only the area of concern is irradiated. Then the equipment that is used, such as multi-slice scanners. Older, single-slice scanners and 4-slice scanners require higher doses than the newest scanners. There have been some really impressive technical innovations that have dramatically reduced dose requirements.

This is work that’s not at the end, it’s being pursued very diligently, that’s why we welcome the attention. From using the highest quality equipment and state-of-the-art methods to having a computerized monitoring system, ensuring doses never exceed the minimum necessary and doing whatever we can to ensure exams are not repeated unnecessarily. All of that is underway, but it’s important to see those aspects of quality make it into the community and that everybody attends to these kinds of details so that we’re not going to have any unnecessary risks associated with overdosing.

How much is currently known about how to determine a “safe” level of radiation exposure?

If you look at the population risk in general, and increase the number of high-exposure exams that a population gets, there will be increased overall risk. But it’s much more complicated to say what the risks are in an individual. In an elderly individual who gets X-ray exams, the bad effects of that are often vanishingly small in comparison with the risks of not having an exam and missing something that could have been corrected. On the other hand, for a child who is genetically more vulnerable and has a much longer life expectancy and has the potential for many more exams, we want to make sure to use every effort to minimize or substitute something else whenever you can. One place where vigilance is extremely important is in CT scanning of children and it is used very sparingly, only in instances where it is medically essential.

For screening exams [i.e. mammography, CT colonography] it is just not justifiable to use any high doses; those often are done with a fraction of the dose used for a diagnostic exam, when there is a high degree of confidence that there’s a real abnormality there and you can’t afford to miss it. It really varies with parts of the body too. Extremity injuries in the ankle, foot, or hand – those are almost not counted, as it doesn’t expose any part of the body that’s radiation sensitive.

What do you think will happen as a result of the new FDA initiative?

What has happened in some of these incidents that have stimulated FDA action is that they did audits of records and found that, unbeknownst to people operating the scanners, they were selecting protocols that were inappropriate or used more dose than necessary. It was a problem of their technology management. Unfortunately, the scanner manufacturers had relied on users to be diligent about it, while the users were relying on manufacturers.

I think manufacturers need to be held to a high standard, that’s why we’re so happy about these changes. There’s not so much a cause for alarm, we just don’t want to see inconsistency, such that if you get the same exam in different places, you can’t be sure you’re getting the lowest dose possible. I think there will ultimately be new requirements for certification of instruments and more recordkeeping, which is good. It’s something that can guide the decision making and the people that benefit are really the patients themselves.

This has been slow in coming in the US, because the radiation concerns in Europe have been much greater. In the European Union there is a much higher degree of dose management. Because a lot of us have close friends practicing in Europe and several manufacturers have big branches there, we have basically been adopting their best practices over a period of years. That’s kept us able to be confident that we are keeping doses as low as possible.

About Rob Mitchum (526 Articles)

Rob Mitchum is communications manager at the Computation Institute, a joint initiative between The University of Chicago and Argonne National Laboratory.

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