BOSTON—Obtaining markedly better images of the internal architecture of the lungs may require nothing more than having patients inhale helium while the images are taken.

Hyperpolarized helium-3 magnetic resonance imaging (MRI) was introduced in the mid-1990s. Since then, it has been used primarily for static images, in which the lungs are filled with hyperpolarized helium-3 before the view is taken. Although this method provides good information about the gas distribution and structure of the lung periphery, it does not offer a view of the airways, explained Mitchell S. Albert, PhD, an Assistant Professor of Radiology at Brigham and Women’s Hospital and Harvard Medical School in Boston.

He and his colleagues adapted the technique in a recent study and found that the small airways could be visualized if the images were obtained dynamically while patients inhaled the hyperpolarized helium. With this approach, up to seventh-generation airway branching could be seen.[1]

AN IMPROVED IMAGING MODALITY

Previous attempts at dynamic imaging had captured airways only up to the fourth or fifth generation of branching. With these prior efforts, said Dr. Albert, “researchers had achieved the feat of visualizing the flow of gas into the lungs with high temporal resolution, but the airways were mainly visible during the initial passage of helium-3.” These protocols also require specialized pulsed sequences—not available with all MRI machines.

The new study appears to have overcome these limitations; it increased the number of branching generations that could be seen and it extended the time during which the airways stay visible. After two inspirations of room air, the six healthy adult volunteers in the study inhaled hyperpolarized helium-3 once over five to eight seconds. Imaging was begun before patients inhaled the helium-3 and continued for the duration of the inhalation.

A static image using hyperpolarized helium-3 MRI shows only the lung periphery.	Dynamic imaging with hyperpolarized helium-3 provides a detailed view of the airways.

DYNAMIC FOR AIRWAYS, STATIC FOR PERIPHERY

The dynamic imaging technique allows the airways to be distinguished from the periphery regardless of the imaging plane orientation. “As the patient inhales,” explained Dr. Albert, “the helium gas travels through the lung; the radiofrequency pulses that are delivered start to destroy the signal in the airways so that no signal reaches the lung periphery. Since the patient is still inhaling, the signal in the airways gets refreshed, so the airways remain bright.”

Are there instances when a static image would be preferable? “Static images give you a homogeneous signal intensity in healthy lungs,” said Dr. Albert. “You don’t see the airways, but static helium-3 images are proving useful for detecting chronic obstructive pulmonary disease. Dark regions in the image indicate areas of poor ventilation.”

Conversely, “a dynamic image would be used when you want to light up the airways,” Dr. Albert continued, such as to observe bronchoconstriction in an asthma patient. “Physicians would be able to see where in the airways problems are occurring, or if the airways are opening up again after treatment, and how long it takes.”

The technique used in the present study provides information that cannot be obtained using other modalities. Dynamic imaging does not require specialized pulse sequencing. Thus, it can be easily applied in a clinical setting to view both the airways and the lung periphery.

—Gale Jurasek

Reference
1. Tooker AC, Hong KS, McKinstry EL, et al. Distal airways in humans: dynamic hyperpolarized 3He MR imaging—feasibility. Radiology. 2003;227: 575-579.

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