Report from NCRP: New CT technologies can reduce radiation dose, untenable fears

April 30, 2007

Attempting to separate fact from fiction, medical physicist Cynthia H. McCollough, Ph.D., gave context to a notorious newspaper report about the dangers of CT with news of innovative equipment performance features that help radiologists keep patient dose under control.

Attempting to separate fact from fiction, medical physicist Cynthia H. McCollough, Ph.D., gave context to a notorious newspaper report about the dangers of CT with news of innovative equipment performance features that help radiologists keep patient dose under control.

The U.S. public lacks basic education on medical-based radiation sources and risks, McCollough said in a lecture presented at the 2007 National Council on Radiation Protection and Measurements meeting. The media send mixed signals, she said. They either boast about the powers of fictional radiation-made heroes, such as Spiderman or the Incredible Hulk, or they exploit fears about radiation, she said. McCollough heads Mayo Clinic's CT Clinical Innovation Center in Rochester, MN.

Millions of Americans became aware of CT radiation exposure from an April 2001 article in the USA Today article by Dr. Richard Semelka, an editor with Medscape, an Internet website. Semelka criticized radiologists for underplaying the health risks of multidetector CT. He cited studies that estimate that the 600,000 abdominal and head CT exams performed annually on children under the age of 15 could result in 500 deaths from cancer attributable to CT radiation.

The article caused emotional and physical harm ranging from mild anxiety in some patients to those who refused exams or had abortions because they had undergone a CT procedure, McCollough said.

"The question shouldn't be 'is the CT safe' but 'is the CT needed to answer the clinical question,'" she said.

But the latest generation of CT scanners can display their CT dose index, a feature that allows radiologists to compare the radiation output from different imaging protocols, according to McCollough. Quantification of dose within a scan volume (CTDIvol) helps them calculate the dose-length product (DLP). The DLP in turn gives them a handle on energy levels applied to different organs. And they can use these parameters to estimate the effective dose, expressed in mSv, which points to potential biological risks.

Dose measurement will help radiologists administer a safe radiation dose to the right patient for the appropriate diagnostic task. The challenge clinicians now face is to keep doses low while keeping image quality high enough to be of diagnostic value, she said.

New CT scanners leave the assembly line with built-in features that allow more aggressive filtration of beam energy and image noise. But clinicians can also reduce dose and adjust image quality by and pushing the right knobs and turning the right dials.

Automated exposure control, for instance, allows for imaging protocols tailored to a patient's body size. Tube current (mAs) modulation is available to adjust dose exposure in the x, y, and z imaging planes. The technique can reduce dose by about 50% while keeping image noise levels steady. It can also save up to 40% of dose in cardiac CT by reducing tube current during portions of the cardiac cycle, McCollough said.

"We save, on average, 20% of dose, taking into account patient body size. Even in large, obese patients you can save some dose," she said.

Patients will be increasingly inclined to ask professionals about the type of radiation dose they will be exposed to when they undergo imaging exams, McCollough said. The concepts currently used by physicians could prove too esoteric. Effective dose, on the other hand, is a broad measure of risk, but it may be useful to compare risk between exams and for informed consent decisions, she said.

Special section: Exclusive coverage of the 2007 National Council on Radiation Protection and Measurements meeting

This year's historic NCRP meeting reveals the disturbing growth of patient exposure to ionizing radiation from medical imaging and proposes practical solutions to regulate its growth. Extended coverage from Diagnostic Imaging lays out the facts and recommendations to better protect patients, physicians, and medical staff.

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