Hyperpolarized He-3 MRI flourishes in unique role

Radiologists have learned a lot from hyperpolarized helium-3 imaging despite regulatory restrictions limiting its use.

An absence of FDA clearance for clinical workups and the scarcity of the gas, a nonradioactive byproduct of nuclear weapons manufacturing, means He-3 MRI will not be coming to a community healthcare facility near you any time soon.

Still, the technology has worked wonders for academic radiologists, who secured an FDA investigational new drug application for the agent in 1999. Its use has led to many discoveries concerning the pathophysiology of asthma, chronic obstructive pulmonary disease, emphysema, cystic fibrosis, and other diffuse, restrictive pulmonary disorders. More than 1000 He-3 MR studies have been performed on humans at the University of Virginia alone, according to Dr. Eduard E. de Lange, professor of radiology.

The airspaces of the lungs, which usually appear as dark smudges on conventional MRI, come into sharp focus after a patient inhales He-3. A patented process for hyperpolarizing He-3, held by GE Healthcare, boosts magnetization of the gas by an awe-inspiring factor of more than 100,000 to create superb conditions for performing pulmonary MRI.

Lung MRI enhanced with hyperpolarized He-3 is at least as efficacious as high-resolution CT for assessing pathological changes of lung parenchyma but doesn't involve the ionizing radiation that disqualifies CT for longitudinal studies or serial imaging of children, according to Dr. Mitchell Albert, director of the Hyperpolarized Noble Gas MRI Laboratory at Brigham and Women's Hospital. He described the status of helium imaging in May at the 2008 International Society for Magnetic Resonance in Medicine meeting.

He-3 MRI outperforms spirometry for pulmonary functional testing and global measures of lung performance such as total lung capacity. By using low flip angles and short echo times, He-3 MRI reveals global lung function and relevant regional deficits, said Dr. Yves Berthezène of the Hôpital Neurologique in Lyon, France. He also spoke at the categorical ISMRM session.

Apparent diffusion coefficient mapping with He-3 MRI measures the size and integrity of lung alveoli, making it a promising technique for monitoring the progression of emphysema and its response to investigational drugs. It also detects airway obstruction and parenchymal destruction from COPD.

The University of Virginia group hopes to detect different forms of asthma with He-3 MRI, de Lange said.

"Better classification will lead to better treatment,- he said in an interview with Diagnostic Imaging.

ADC mapping with He-3 MRI can be used to observe the early physiological changes of smoking on the lungs before they can be detected with other modalities, such as CT. In a paper presented at the 2007 RSNA, Chengbo Wang, Ph.D., of the University of Virginia, used the technique to compile the first definitive evidence showing the destructive effects of secondhand cigarette smoke.

The methods used to acquire ADC maps of the lung with hyperpolarized He-3 MRI continue to improve. Wang received an ISMRM Young Investigator Award in May for demonstrating the increased sensitivity of a long-time-scale data acquisition. Wang's technique acquires lung diffusion data for 1.5 seconds, 1500 times longer than the conventional 1 msec, short-time-scale technique.

Wang's experience with 52 subjects found that long-time-scale mapping produces more conspicuous patterns of focal ADC elevations among asthmatic and COPD patients than the short-time-scale technique.

With long-time-scale data acquisition, the mean change in the areas of restriction was increased among asthmatic patients compared with healthy subjects, a finding that challenges traditional assumptions about the disease, Wang said.

"Everyone thinks that the area of restriction-and the mean ADC values measuring it-should become smaller among asthma patients,- he said. "But our results are totally different.

This suggests the possibility of new discoveries from hyperpolarized He-3 MRI. As some researchers have predicted, asthma may be associated with microstructural remodeling that explains why it is hard to treat, he said. The finding begs for the kind of groundbreaking research that hyperpolarized He-3 imaging is particularly able to provide.

-By James Brice