RSNA: Users need more knowledge of dose reduction

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While current CT scanners come equipped with many dose reduction techniques, more operator training, education and automated tools are needed to facilitate dose reduction during imaging exams, Cynthia H. McCollough, PhD, of the Mayo Clinic in Rochester, Minn., said during a presentation Nov. 28 at the 96th annual meeting of the Radiological Society of North America (RSNA) in Chicago.

Three main techniques--tube current modulation, hardware advances such as beam-shaping filters and adjustment of collimators and use of software applications to reduce noise--are important and can help reduce radiation dose, McCollough offered.

Blocking collimators during spiral exams can prevent radiating parts of the body that don't require reconstruction. “These are good tools, and education is the key for the community to understand how to use them and how to use them correctly,” she urged.

While the aforementioned tools may be helpful to reduce dose, automatic exposure control may be the most successful method used to adapt the scanner output to individual patient size. The tube current can be adjusted up and down on the z-axis and can be modulated to move around the patient angularly—performing right-size dosing for patients.

The scanners ensure the correct radiation dose for a procedure in terms of the patient's size, but tools that can automatically estimate certain patient size during an exam still need improvement, McCollough continued.

CT scout views, which have the same issues as radiography, can often over- or underestimate patient size. “Automatic exposure dose can drive radiation dose to perhaps an erroneous estimate of patient size,” said McCollough. “The system can run into limits, particularly for the overweight patient population.”

Users of certain modalities are trained to employ multiple reduction tools; however, she said that they must make sure they understand the tradeoffs of using such dose reduction tools. McCollough cited two examples of tradeoffs:

  • If a technologist decreases the pitch of an exam, the exam will get longer and one must ask, 'can the patient hold his breath that long and what will happen to contrast?' 
  • If a technologist decides to adjust the collimation of an exam, it will produce a wider image width, but may require a thinner image for 3D image rendering.

“There are questions here that users need to be well educated on and they must understand these types of tradeoffs,” she said.

Because various manufacturers and modalities have different tools for measuring dose and adjusting for patient size, McCollough said that there is often a lack of knowledge with the proper use of these techniques.

McCollough offered that more and more literature has found that reducing the normal 120 kVp levels to 80 kVp or 100 kVp during exams can reduce dose. By using kVp levels of 80 to 100 Mayo has realized a 10 to 40 percent dose reduction during exams.

However, McCollough offered that at the lower kVp levels, “we want the contrast to noise ratio to be as good or better and we want our noise to be close to what we are normally used to.

“For the future of kVp imaging, ... we should be moving to 100 kVp on systems that have the ability to do so.”

The American College of Radiology (ACR), FDA and other agencies are working toward creating more standardized benchmarks and terminology for dose reduction techniques.

McCollough concluded: “Information to make informed decisions about dose reduction is needed. It’s all about being clear and transparent. We need to get this information and guidelines out there to guide performance and proper radiation and dose.”