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Science to Practice |
Department of Radiology,
University of Virginia Health Sciences System,
PO Box 800170,
Charlottesville, VA 22908-0001,
delange@virginia.edu
SUMMARY
Through the use of diffusion-weighted 3He MR imaging, Fain et al in this issue of Radiology demonstrated the potential for detection of early emphysema in the lungs of healthy smokers. The technique possibly could become an important tool for understanding the pulmonary processes that occur with emphysema, for implementing treatment before clinical symptoms occur, and for monitoring the effect of treatment.
THE SETTING
Smoking is by far the most common cause of emphysema, and findings in studies have shown that in asymptomatic smokers the decline in pulmonary function occurs at a slower rate in those who stopped smoking compared with those who continued the habit (1). Aside from smoking cessation, treatments are being developed (2) to modify the course of emphysema. Thus, accurate assessment of the severity and distribution of emphysema within the lung becomes essential for selecting patients to treat and for monitoring their response to treatment. Thin-section computed tomography (CT) is the most commonly used imaging method for assessing emphysema. Concerns in regard to radiation dose, however, may limit its use, particularly if repeat CT is required.
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In this issue of Radiology, Fain et al (6) investigated the potential of diffusion-weighted 3He MR imaging for the detection of emphysema at an early stage before it is clinically evident by investigating healthy smokers and nonsmokers and by comparing the findings with results of lung function tests and thin-section CT.
THE SCIENCE
In their study, Fain et al (6) obtained diffusion-weighted 3He MR images from 11 healthy smokers and eight age-matched nonsmokers. Lung function tests consisting of diffusing capacity of lung for carbon monoxide (DLCO), forced expiratory volume in 1 second, and forced vital capacity, as well as thin-section CT at selected anatomic levels, were performed. Mean ADC values correlated strongly with age in all subjects (P 
.001) and with the number of pack-years in the smokers (P = .007). Further, for both smokers and nonsmokers, mean ADC values in the upper and middle lung zones were significantly higher than those in the lower lung zones (P < .001). Differences in distribution of emphysematous changes were also noted with thin-section CT. Mean ADC values strongly correlated with DLCO (P < .001), with a significant difference between smokers and nonsmokers (P = .02). Mean ADC values also correlated strongly with the ratio of forced expiratory volume in 1 second to forced vital capacity (P < .001). Because of the small number of subjects and no histologic confirmation, the true sensitivity and specificity of the diffusion-weighted 3He MR technique for detection of early emphysema remain unknown. Further, the inhaled gas volume was the same for all subjects regardless of body size, and, consequently, there could have been variations in alveolar size on the basis of differences in inspiration. Also, because thin-section CT images included only part of the lungs, whereas 3He diffusion MR images included the entire chest, it could not be determined whether the MR images were superior to CT images for the depiction of emphysema.
THE PRACTICE
Clinical use:
Although 3He is not approved by the U.S. Food and Drug Administration for clinical use, the results are encouraging that regional emphysematous changes can be identified at an early stage when there are no clinical symptoms. An abnormal lung function test result, especially for DLCO, may also suggest emphysema; however, such an abnormal result also can be found in other diseases. Further, because a certain degree of alveolar damage must have occurred before changes in pulmonary function become noticeable, it is possible that the ensuing airspace enlargement may be detectable with diffusion-weighted 3He MR imaging before the lung function becomes abnormal. Large-scale clinical studies will be needed to fully determine the capability of this technique for detection of early emphysema.
Future opportunities and challenges:
Diffusion-weighted 3He MR imaging potentially could help identify those who are most likely to develop the clinical symptoms of emphysema. Recent developments in diffusion-weighted MR imaging by using hyperpolarized xenon 129 gas, which is substantially less expensive than 3He and is readily available, are encouraging (7). With consideration of these developments, hyperpolarized gas diffusion-weighted MR imaging potentially could be used to detect early emphysema and, thereby, help smokers to stop their smoking habit, particularly when they are shown the visible changes in the lung. Further, the technique might allow monitoring of the effects of newly developed drugs that are aimed at slowing the destructive process or restoring the damaged alveoli.
ACKNOWLEDGMENTS
The author thanks John P. Mugler III, PhD, and Talissa A. Altes, MD, for their valuable comments and suggestions.
FOOTNOTES
See also the article by Fain et al in this issue.
References
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