Researchers at Washington University School of Medicine in St. Louis have shown for the first time that an experimental imaging technique can show changes caused by emphysema even in the smallest airways of the lung.
The technique, known as helium-3 diffusion magnetic resonance imaging (3He diffusion MRI) is said to be more sensitive than computed tomography (CT) or any other imaging method currently available for examining the lung.
Joel D. Cooper, M.D., Evarts A. Graham Professor of Surgery and head of the Division of Cardiothoracic Surgery said, ‘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 non-radioactive and highly polarised – hyperpolarised – form of helium gas. Hyperpolarising 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 travelled both along and across the airways.
These distances are recorded as colours 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 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,’ said 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. ‘That’s why this technique is sensitive; it tells us the radius of the airways.’