CHICAGO — A novel magnetic resonance imaging technique may have provided the proverbial smoking gun in the secondhand smoking debate.
Using helium-3 diffusion MRI, researchers at the University of Virginia in Charlottesville and the Children's Hospital of Philadelphia have for the first time identified structural damage in the lungs of individuals with high exposure to secondhand smoke.
The technique is similar to conventional MRI, except that the images are acquired after the patient inhales hyperpolarized helium. The scanner is a standard 1.5-tesla commercial scanner, but is modified by the addition of a multinuclear imaging package and a radiofrequency coil tuned to the helium frequency of 48.5 MHz. The intensity of the MR signal is enhanced by a factor of more than 100,000 on a 1.5-tesla scanner and by a factor of 1 million on a 0.15-tesla scanner. This allows for the evaluation of lung structures on a microscopic level, lead investigator Chengbo Wang, Ph.D., said at the annual meeting of the Radiological Society of North America.
The MR images show how far the helium atoms diffuse inside the lungs over 1.5 seconds. In some smokers, the alveoli become enlarged and develop holes, allowing the helium atoms to infiltrate the lung microstructure to a greater extent. This is reflected in higher apparent diffusion coefficient (ADC) values, which have been shown to be larger when measured in emphysematous lungs, compared with healthy lungs (Radiology 2006;239:875–83).
For the study, 60 individuals underwent helium diffusion MRI after inhalation of 50 cc of hyperpolarized helium diluted with nitrogen to a total volume of approximately one-third of their forced vital capacity, as measured by spirometry on the day of imaging. In all, 23 individuals had low exposure, defined as never having lived with a smoker nor worked in an occupation with high exposure to secondhand smoke; 22 had high exposure, defined as at least 10 years' exposure at home or work; and 15 were current or former smokers.
At baseline, their age ranged from 41 to 79 years, and the range of percent-predicted forced expiratory volume in 1 second values was 86%–112% (low-exposure group), 79%–120% (high-exposure group), and 49%–121% (current or former smokers). The threshold for an elevated ADC was set as 0.024 cm
Only 1 (4%) of 23 participants with low exposure had an elevated ADC value, compared with 6 (27%) of 22 participants with high exposure and 10 (67%) of 15 smokers. The difference in ADC values was significant between smokers and the low-exposure group and between the high- and low-exposure groups.
As for why some smokers had normal ADC values, Dr. Wang said that they may be genetically wired to be less sensitive to the harmful effects of smoking, while others are more sensitive and incur more lung damage. The study was funded by the National Institutes of Health, the Flight Attendant Medical Research Institute, and the Commonwealth of Virginia Technology Research Fund. Siemens Medical Solutions provided the scanner. The study researchers did not disclose any personal conflicts of interest.
Shown here from left to right are lungs with low exposure to secondhand smoke, high exposure to secondhand smoke, and in smokers. The red color indicates normal alveoli. Radiological Society of North America/Dr. Chengbo Wang