CHICAGO—Using an investigational magnetic resonance imaging technique, researchers have produced what they say is the first hard evidence that long-term exposure to secondhand smoke can cause structural damage in the lungs. The study was reported here at the Radiological Society of North America Annual Meeting by Chengbo Wang, PhD, a magnetic resonance physicist in the Department of Radiology at Children's Hospital of Philadelphia.
Hyperpolarized helium-3 diffusion MRI detected alveoli changes similar to those seen in the lungs of long-time smokers in more than 25% of nonsmokers exposed to secondhand smoke for at least 10 years, he said, noting that up to 30 million Americans have chronic obstructive pulmonary disease (COPD), which places them at increased risk for lung cancer.
How It Works
The method described at the meeting involves having the patient inhale hyperpolarized helium while lying in a conventional MRI. Dr. Wang's co-investigator, Talissa A. Altes, MD, Assistant Professor of Radiology at the University of Pennsylvania and Director of Clinical Research at Children's Hospital of Philadelphia, explained that the hyperpolarization process doesn't change helium's chemical structure, but simply makes it more visible on MRI.
In just six seconds, the scanner collects images showing how the helium gas diffuses in the alveoli. The method identifies lung damage by measuring how far the helium atoms move in a specific time period—1.5 seconds in this study, said Dr. Wang, who developed the software used in the technology.
Healthy lungs have small alveoli, so the helium atoms travel a shorter distance in a given amount of time than in damaged lungs with enlarged alveoli, he explained.
In order to compare travel times, apparent diffusion coefficient (ADC) values were calculated for each participant. The lower the value, the better the lung function. Normal lungs were defined as having an ADC of 0.024 cm2/sec, which was the mean value in nonsmokers plus two times the standard deviation.
The study involved 60 volunteers with normal spirometry measures, forced expiratory volumes in one second ranging between 58% and 92%, and without symptoms of lung disease.
Of the total, 15 were current or past smokers, and 45 had never smoked. Twenty-two of the nonsmokers had heavy exposure to secondhand smoke, meaning they lived or worked in an environment where they were exposed to smoke for at least a decade.
A total of 67% of smokers and 27% of nonsmokers with heavy exposure to secondhand smoke had ADC values above the mean, reflecting depressed lung function suggestive of preclinical COPD, Dr. Wang said.
“This finding may reflect a response to smoke exposure,” he said.
In contrast, only 4% of nonsmokers who had never smoked and had fewer than 10 years of exposure had ADC scores above the mean, he said.
Compared with the mean ADC in the nonsmokers, the differences were statistically significant in both the high-exposure and the smoker groups.
Since none of the people studied had any symptoms of lung disease, Dr. Wang said the findings suggest that hyperpolarized helium-3 diffusion MRI may eventually be useful for diagnosing COPD before symptoms occur.
Dr. Altes said that in many cases, the patients were exposed to secondhand smoke during childhood, “so there's not much we can do except to limit their exposure….But one patient who saw the changes in her lung [on the MRI images] said she was going to go home and tell her husband to stop smoking because he wasn't damaging just his own lungs, but also hers as well.”
There are no effective treatments for COPD, so “the clear message for these people is that they should avoid smoke exposure,” she said.
Intriguing, But Only Hypothesis-Generating
RSNA spokesperson Katarzyna J. Macura, MD, PhD, Assistant Professor of Radiology at Johns Hopkins Medical Institutions, said that the results should be considered hypothesis-generating but not much more: “This is an intriguing hypothesis. But the study population is not large enough to draw any population-wide conclusions beyond the hypothesis they tested.”
Dr. Macura added that she would like to see proof that what they are seeing indeed reflects secondhand smoke-induced lung damage.
To that end, she said that the next step should be animal studies to offer histologic proof that passive smoke can induce emphysematous changes.
Dr. Altes said that there have been studies in which researchers induced emphysematous changes in animal lungs and showed a correlation between ADC scores and destructive changes in the lungs suggestive of COPD. However, she acknowledged, the animal studies did not offer proof that tobacco smoke caused emphysema.
The study was funded primarily by the Flight Attendant Medical Research Institute, with additional funding from the National Heart, Lung, and Blood Institute, the Commonwealth of Virginia Technology Research Fund, and Siemens Medical Solutions, which provided the scanner.
© 2008 Lippincott Williams & Wilkins, Inc.