US Clinicians Initiate Efforts to Optimize CT Safety

Clinicians and physicists in the United States have endorsed new strategies to make computed tomography (CT) imaging safer, including implementation of a new metric for dose measurement, specific approaches to reduce exposure during needle biopsies, and ways to manage exposure protocols that differ by CT manufacturer.

The recommendations were published in March 2014 issue of the Journal of the American College of Radiology (JACR). In response to worries about the radiation dose in CT, JACR focused the entire issue to CT safety. The US government estimates that more than 80 million CT scans are performed in the United States each year. The University of California (UC) Davis (USA) specialists contributed three studies to the special issue.

CT is an effective diagnostic modality that can both identify life-threatening disorders and rule them out, providing clinicians with detailed data to develop a care plan; however, the technology comes with risks. Ionizing radiation can injure cells and has been shown to be a weak carcinogen. The challenge for radiologists and medical physicists is to find the optimal dosage for each CT exam. “The higher the dose, the better the images,” said Dr. John Boone, vice chair of research in the department of radiology. “But you also want the lowest possible doses. So you need to find that balance between acceptable image quality and acceptable dosage.”

In the study, Dr. Anthony Seibert, a UC Davis professor of radiology, and colleagues discussed the need to improve dose calculations. They mentioned to an actual case, in which a pediatric patient, who had scans before and after surgery, appeared to receive an overdose on the second CT. Further study revealed that the problem was a misleading metric—the volume CT dose index (CTDIvol), a standard measure of radiation output in CT. Manufacturers use phantoms along with instruments that measure radiation to assess CTDIvol. However, different companies use phantoms of different sizes. Moreover, many dose estimates are based on adult sizes, making pediatric estimates more difficult. “In this case, the difference was the way the manufacturers handled the metrics,” said Dr. Seibert. “A large phantom tends to underestimate dose, while a small phantom tends to overestimate it. When we compensated for the different-sized phantoms, it turned out the patient received almost the same dose in both scans.”

To help prevent future difficulties, the investigators recommended a new metric, size-specific dose estimate (SSDE), which provides a better way to measure patient doses and can also help compare scanners from different companies. It also addresses the urgent need to more effectively estimate dose from CT across a range of patient sizes, from newborn to a football linebacker. “I think SSDE is one step closer to a dose metric that will be more accurate in depicting the actual risk to patients,” said Dr. Seibert.

This is not the only area where differences between scanners can obscure dosage. In another study, Drs. Boone, Seibert and colleagues addressed the challenges of optimizing different CT systems. Physicists and radiologists must set up automatic exposure protocols, which vary doses based on tissue thickness. However, since manufacturers use different methods to control doses, transferring these settings between machines can be difficult and time-consuming. “It takes a lot of effort to optimize any given scanner,” said Dr. Boone. “Going through the procedures for every possible exam could take two years.”

To achieve this, the UC Davis team developed equations to translate settings among three machines, two made by GE Healthcare (Chalfont St. Giles, UK) and one by Siemens Healthcare (Erlangen, Germany). The planned strategy offers a more efficient way to manage CT protocols between different CT scanners.

A third study in the same issue of JACR, described how practitioners can lower radiation doses while performing interventional procedures under CT guidance. CT is frequently used to guide these procedures, ensuring the needle is precisely located. Dr. Ramit Lamba, UC Davis director of CT, described a number of ways to reduce radiation doses for both patients and doctors performing the procedures. He recommends using ultrasound, instead of CT, to guide some biopsies. He also recommends reducing scan lengths, lowering the tube current using dose-efficient scanning modes and limiting the number of guidance scans.

The special JACR issue is one part of an ongoing effort to reduce CT risks. As part of the University of California Dose Optimization and Standardization Endeavor (UCDOSE), all five UC medical centers are collaborating to improve CT protocols and education. “We’re trying to educate radiologists and medical physicists because the practice of CT is not consistent,” said Dr. Boone. “If someone is using higher doses to get results they could obtain with less radiation, we need to help them find the safer alternative.”

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University of California, Davis