Trends in the number of radiology studies and a decline in radiation dose

A special report just published in Radiology offers a rare overview of the number of radiology exams performed each year world-wide and focuses on the associated radiation doses patients receive [1]. The research also found overall radiation exposure from medical imaging has declined over the past decade.

The report includes data from the U.S. National Council on Radiation Protection and Measurements (NCRP) retrospective assessment of the U.S. medical imaging data, and a similar study from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) worldwide assessment for 2009 to 2018. Using the data from those reports, researchers from Johns Hopkins University School of Medicine, the U.S. Centers for Disease Control and Prevention (CDC) and the University of New Mexico looked at the frequency of radiologic and nuclear medicine studies, annual collective and per capita effective dose in the United States for 2016 were compared with worldwide estimates from 2009 to 2018. 

"The information provided in this report has many potential uses, including following and possibly predicting trends, observing the effects of health planning policies, and comparing radiation doses from various practices," explained Mahadevappa Mahesh, PhD, the report's primary author and a professor of radiology and medical physicist at Johns Hopkins University School of Medicine. 

How many radiology exams are performed each year?

Mahesh et al. found there were an estimated 691 million radiologic, computed tomography (CT), dental and nuclear medicine studies performed in the United States in 2016. This is 16.5% of the 4.2 billion medical imaging exams performed worldwide, according to their assessment. The report gave the following estimates for the number of specific types of radiation emitting exams performed in the United States:

   • 74 million CT procedures (about 18% of the world’s estimated total)
   • 275 million conventional radiology procedures (11% of the world total)
   • 8.1 million interventional radiologic procedures (34% of the world total)
   • 320 million dental radiographic examinations (29% of the world total)
   • 13.5 million nuclear medicine procedures (34% of the world’s total)

Average patient radiation exposure from medical imaging declines

The report also gives estimates on the overall patient radiation exposure from these exams. Interestingly, the researchers found radiation doses have declined over the past decade, for several reasons.

"From 2006 to 2016, the annual average effective dose per person decreased worldwide from 0.65 milli-Sieverts (mSv) to 0.56 mSv, and decreased from 3 mSv to 2.2 mSv in the United States in the same period," Mahesh explained. "Reasons for the decline are multifactorial, such as awareness of radiation dose, education, attempts to optimize doses, newer technologies, and changes in practices," Mahesh explained. 

The report also found the annual collective effective dose in the U.S. was 717,000 person-Sieverts, which is about 17.6% of the world total. Overall, the annual average individual effective dose in the United States was 2.2 mSv compared with 0.56 mSv worldwide. 

The data show the United States accounts for a large and disproportionate share of global medical radiation procedures and collective effective dose, but use of CT increased more in other countries than in the United States.

Mahesh said population growth alone in both U.S. and worldwide do not account for all the trends observed in the report. He said the key trends include:

1. Estimated number of CT examinations in the U.S. increased about 20% between 2006 and 2016, whereas the global number almost doubled. In spite of that, the effective dose per capita for CT decreased by nearly 20% in the U.S.
2. Mammography substantially increased both in the U.S. and globally.
3. The number of conventional radiography (excluding dental) procedures decreased globally and in the United States.
4. The number of nuclear medicine (excluding dental) procedures showed a small increase globally, but decreased markedly in the United States.

"The information provided in this report has many potential uses, including following and possibly predicting trends, observing the effects of health planning policies, and comparing radiation doses from various practices," Mahesh explained. 

Many types of imaging studies are shifting to CT, MRI and newer technologies

The NCRP data revealed major shifts in the frequency of some conventional radiographic examinations in the United States in a growing shift toward CT imaging as a replacement. For example, since the 2010s, radiographic intravenous urography has been almost completely replaced by CT and MRI urography. Fluoroscopic examinations of the gastrointestinal tract declined substantially, likely because of replacement with fiberoptic imaging procedures.

In 2006, there were an estimated 281 million radiographic and diagnostic fluoroscopic procedures in the United States. This decreased to approximately 275 million in 2016, even though the population increased from approximately 300 to approximately 323 million, the report authors wrote. There was also an incremental decrease in chest, abdomen and pelvis, and urologic radiography, and an increase in hip and extremity radiographic and mammographic examinations.

In the U.S. and worldwide since 2006, some imaging modalities have been almost completely replaced, the researchers found. One of the biggest changes was with replacement of screen-film radiography units by digital radiography (DR) detectors.

Despite this, the authors found the effective dose per procedure appears to have changed little. The decrease in procedures (particularly abdomen and pelvis) resulted in a reduction in collective effective dose calculated with ICRP 60 weighting factors, or S60, from radiography and diagnostic fluoroscopy.

The UNSCEAR 2022 global estimate of conventional projection radiography use for the years 2009 to 2018 found about 17% fewer than the 3.143 billion estimated for 2006. The worldwide use of conventional projection radiography reflects the same changes observed in the U.S. over the last decade. The total number of procedures has decreased slightly despite an increasing population, mammography has increased, and gastrointestinal fluoroscopy and intravenous urography have declined.