The technique, known as helium-3 diffusion magnetic resonance imaging (3He diffusion MRI) is more sensitive than computed tomography (CT) or any other imaging method currently available for examining the lung. The findings were published in the Proceedings of the National Academy of Sciences. The lead author is Dmitriy A. Yablonskiy, Ph.D., a professor of physics and an assistant professor of radiology at the School of Medicine’s Mallinckrodt Institute of Radiology.
“Our findings suggest this may be a new means for the early detection of emphysema by demonstrating the enlargement of the air spaces in the lung,” says Stephen S. Lefrak, M.D., professor of medicine and a co-author of the paper. “I suspect it also will help in understanding the development, evolution, progression and physiological effects of many lung diseases including emphysema, asthma and perhaps pulmonary fibrosis.”
Joel D. Cooper, M.D., Evarts A. Graham Professor of Surgery and head of the Division of Cardiothoracic Surgery added, “This technique may well help us refine our selection criteria and better predict the outcome of emphysema patients undergoing lung-volume reduction surgery.” Lung reduction surgery involves the removal of the most diseased areas of the lung in select patients.
3He diffusion MRI uses a nonradioactive and highly polarized – hyperpolarized – form of helium gas. Hyperpolarizing the gas, which is done using lasers, makes the helium detectable by MRI.
To perform the technique, a patient in an MRI machine inhales the gas and holds his or her breath for ten seconds. The resulting image shows how far the atoms of helium travel, or diffuse, within the lungs during a period of two thousandths of a second. The method reveals the distance traveled both along and across the airways.
These distances are recorded as colors ranging from red (the smallest distances) to violet (the largest distances). This information also indicates the physical diameter of the airways and of the alveoli.
If a large space is available, the helium molecules can move freely and travel relatively far. This is the case in the trachea, the relatively large tube that carries air from the mouth and nose into the chest and shows up as violet when imaged. In small airways within healthy lungs, the bronchioles and alveoli, the helium atoms have little room to move. These areas show up in the image as red or deep orange.
Emphysema progressively destroys the walls of the alveoli, the smallest spaces of the lung and the area where the blood releases its load of carbon dioxide and takes up a fresh supply of oxygen. The disease results in a loss of lung elasticity and an enlargement of alveolar spaces. The larger space gives the helium atoms more room for movement.
“Diffusion in emphysemic lungs can be five to six times greater than in normal lungs because of the enlargement of the airways,” says Yablonskiy. “That’s why this technique is sensitive; it tells us the radius of the airways.”
“It is a powerful method, telling us about lung structure on the 0.1 to 0.5-mm scale, too small for us just to take a picture of them,” added Mark S. Conradi, Ph.D., professor of physics and another co-author of the paper.
The study reports the use of the technique in two healthy patients and four with severe emphysema.
The findings demonstrate the method’s ability to follow the dynamics of lung destruction by examining the variations in damage within each patient as well as between patients. For example, the diffusion pattern in one patient showed an area of moderate emphysema, with a mean airway radius of 0.52 mm (compared to a mean radius of about 0.37 mm in normal lungs). Here, the transverse diffusion of gas was nearly 100 percent greater than in normal lungs. An area of severe emphysema in the same patient showed a mean airway radius of more than 1 mm and transverse movement of helium four times that of normal lungs.
Yablonskiy, DA, Sukstanskii AL, Leawoods JC, Gierada DS, Bretthorst GL, Lefrak SS, Cooper JD, Conradi MS. Quantitative in vivo assessment of lung microstructure at the alveolar level with hyperpolarized 3He diffusion MRI. Proceedings of the National Academy of Sciences, 99(5), 3111-3116. Mar. 5, 2002.
This research was supported by a Scholar Award from the Radiology Society of North America.
Journal
Proceedings of the National Academy of Sciences