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Optimizing Digital Radiography Exams

1Optimizing Digital Radiography Exams

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After completing this article, the reader should be able to:Explain the difference between digital and conventional radiography.List the factors that affect radiation exposure in digital radiography.List image quality factors and explain how they relate to under- or overexposure.Describe the methods used to reduce radiation exposure and maintain quality imaging.Provide ways to optimize general computed and digital radiography procedures.

As hospital and imag-ing departments replace conventional screen-film imaging with computed radiography or digital radi-ography, a greater emphasis is placed on balancing radi-ation exposure and quality imaging.

This course covers why many believe that this digi-tal revolution is contribut-ing to a “dose creep,” and it offers insight into how radiographers can help reduce radiation exposure and ensure quality medical images.

Robin L. Anderson, B.A.

Optimizing Digital Radiography Exams

A patient asks a radiographer how much radiation exposure he will be subjected to during a chest x-ray. The radiologic

technologist replies, “You don’t have anything to worry about. It’s all digital. The computer takes care of it.”

This common perception of an all-knowing and powerful computer dimin-ishes the fact that radiographers — not software — ensure a diagnostically sound exam through proper positioning, shield-ing and collimation. It is the human touch and knowledge that result in images that satisfy radiologists and keep radiation exposure as low as reasonably achievable (ALARA).

“Experience and skill entitle the radiographer to a technical opinion — to be able to decide whether an anatomical variant,a pathological influence, or a posi-tioning or technical error exists,” accord-ing to Angeline M. Cullinan, R.T.(R), FASRT, in “Optimizing Radiographic Positioning.”1

As computed radiography (CR) evolves and digital radiography (DR) starts to take hold, research is validating

digital technology’s ability to create great-er efficiency and productivity, along with its potential to decrease radiation doses. However, radiographers must understand their role in optimizing exams. The same high-speed “film,” greater contrast and improved spatial resolution that mean fewer retakes, also can increase radia-tion exposure and obscure the fact that a patient has been overexposed.

A “dose creep” occurs when radiation exposures increase to achieve the sharper, more defined digital images required by radiologists to assess the smallest details. Granted, the radiation used in routine x-ray exams is less than that employed in other imaging techniques, such as CT or f luoroscopy. However, the recorded increases that occur when radiology departments switch to digital imaging are raising concerns — especially for pediat-ric patients.

It is necessary to optimize medical imaging exams for all patients. This includes those exams that involve expo-sures viewed as minimal, such as chest, skull and extremities. Radiographers can use what they already know, combined

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can prove or disprove that radiation exposure below 5 rem causes harm, adhering to the ALARA principle protects patients.

Table 1 illustrates the way radiation dose and risk is assessed. The U.S. Food and Drug Administration compares the number of chest x-rays to the time it would take for an individual in the United States to receive the equivalent radiation from natural back-ground sources. For example, a CT abdomen exam is approximately equivalent to 500 chest x-rays, and it would take 3.3 years to receive the equivalent effec-tive dose from background radiation. (Excerpted from “Whole Body Scanning Using Computed Tomography (CT): What are the Radiation Risks from CT?” at www .fda.gov/cdrh/ct/risks.html.)

Sensitivity to Stochastic EffectsTo date, it is understood that even low doses of radia-

tion from medical imaging may cause cancer in any given population, depending on age and the types and frequency of x-ray exams.4-6

Sensitivity to the stochastic effects of radiation, those that cause cellular mutation in cells that lead to cancer or other genetic illnesses, varies with age. A newborn’s risk of developing cancer from radiation exposure is consid-ered the greatest, while a 55-year-old adult may receive up to 300 times more radiation.4,6 However, from the age of 1 year to 50 years, the risk varies by only about 8 times more radiation, with a jump to more than 200 times the

with the additional factors associated with CR and DR, to make a difference for patients.

This course offers an overview of digital imaging technology and explains why the radiographer’s role is so important to the ALARA principle. It also provides ways to decrease radiation exposure as you maintain image quality.

Radiographers aren’t in this alone. It takes a team of equipment manufacturers, radiologic technologists, quality control personnel and radiologists to ensure a diagnostically productive exam with the lowest radiation dose achievable.

In screen-film radiography (SFR), you already know to repeat an exam when an image is under- or overex-posed. In practice, radiographers learn quickly to avoid repeats by optimizing techniques, as Don points out.2

It is equally possible to achieve the same level of expertise in CR and DR exams in order to take advan-tage of all aspects of digital medical imaging. Those advantages include a more dynamic range, fewer repeats and lower radiation exposure due to sensitive receptors. Although SFR proved more sensitive in the past than CR, advances in the technology mean that digital imag-ing is fast displacing film. And as radiography depart-ments increasingly depend on digital systems, the results often mean an increase in radiation exposure.

Ultimately, optimization requires:3 A radiographic technique that applies kV, mAs and

beam filtration efficiently. Detector efficiency, which covers composition,

x-ray absorption and signal conversion. Image pre- and postprocessing, which encom-

passes f lat-fielding, dynamic range compression, digitization, contrast enhancement and spatial fre-quency enhancement.

An image display that optimizes film, soft copy, calibration and viewing conditions.

As with screen-film, a radiographer can optimize the first two, which in turn aids processing.

Radiation EffectsThe average chest x-ray dose is .02 mSv and a plain

film skull exam is only .07 mSv, which may seem insig-nificant compared with a range of 1 mSv to 10 mSv for CT procedures. Some researchers even argue that low radiation doses are beneficial. However, until research

Image courtesy of Philips Medical Systems.