Biomedicine

CT Scanner Delivers Less Radiation

Faster, more sensitive scans and better image processing may reduce the risk of x-ray-related cancers.

Repeated CT scans can increase the risk of cancer in patients exposed to the radiation.

A new CT scanner exposes patients to less radiation while providing doctors with clearer images to help with diagnoses, according to researchers at the National Institutes of Health.

ct scanner
Ticktock: The heart of a 74-year-old female as imaged in less than half a second by an advanced CT scanner.

“CT” stands for Computerized Tomography, which involves combining lots of x-ray images taken from different angles into a three-dimensional view of what’s inside the body. The technology can be especially useful for diagnoses in emergency situations, and the number of CT scans in recent years has increased dramatically, says Marcus Chen, a cardiovascular imager at the National Heart, Lung and Blood Institute, in Bethesda, Maryland.  But the increase in the use of CT scans raises concerns about the amount of radiation to which patients are exposed, says Chen.

The risk of developing cancer from the radiation delivered by one CT scan is low, but the large number of scans performed each year—more than 70 million—translates to a significant risk. Researchers at the National Cancer Institute estimated that the 72 million CT scans performed in the U.S. in 2007 could lead to 29,000 new cancers. On average, the organ studied in a CT scan of an adult receives around 15 millisieverts of radiation, compared with roughly 3.1 millisieverts of radiation exposure from natural sources each year.

This concern has led researchers to seek ways to reduce the amount of radiation exposure a patient receives in a scan. They are working to improve both hardware, to make the scans go faster and need less repetition, and software, to process the x-ray data better (see “Clear CT Scans with Less Radiation”).

The new CT scanning system, from Toshiba Medical, combines several improvements to reduce radiation exposure. The overall body of a CT scanner is shaped like a large ring. An x-ray tube and a detector spin separately in the ring, opposite one another, and a patient lies in the center.  X-rays travel through the patient as they are delivered by the tube and captured by the detectors. The new Toshiba machine has five times as many detectors as most machines, which means that more of an organ can be captured at a time, decreasing the number of passes of the scanner required.

The x-ray components in the new system also spin faster—it takes only 275 milliseconds for them to complete a rotation, instead of 350 millisesconds—which means a patient gets irradiated for less time. In cases where doctors are looking at a moving organ such as the heart, the faster spinning also reduces the number of times a doctor may need to try to get a good image. “It’s like having faster film in your camera,” says Chen.  Changes to the way the system generates x-rays and computes the images also mean patients spend less time getting hit with radiation.

Chen and colleagues at the National Heart Lung and Blood Institute used the Toshiba system to examine 107 adult patients of different ages and sizes for plaque buildup and cardiovascular problems. Patient size matters because more x-rays are required to image a larger person. “A lot of imaging centers will use one setting for all patients,” says Chen. “You get beautiful image quality on everybody, but the downside is that some patients get more radiation than they probably should.” In his study, the system takes a quick preliminary scan that uses low-dose x-rays to figure out how big a patient is and how much radiation will be needed for the diagnostic image.

Most patients who got a scan in the new Toshiba machine received 0.93 millisieverts of radiation, and almost every patient received less than 4 millisieverts. Radiation exposure was decreased by as much as 95 percent relative to other CT scanners currently in use.

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