New tool for imaging stroke may give safer, more accurate picture of damage

A new magnetic resonance imaging (MRI) technique that uses a special form of xenon, an inert gas that dissolves readily in the blood, may allow better detection of strokes, the third-leading cause of death in the United States. Called hyperpolarized xenon MRI, the technique being perfected by Mitchell Albert, PhD, professor of radiology and director of the Hyperpolarized Gas MRI Laboratory, Marc Fisher, MD, professor of neurology, and Xin Zhou, PhD, has the potential for producing a more accurate picture of stroke damage by safely tracing where blood flows in the body.

Conventional MRI takes images of protons within the body, whereas hyperpolarized xenon MRI detects xenon atoms. Xenon dissolves in the blood when inhaled, and it can be detected by an MRI scanner once it reaches tissues or organs of interest.

As described in NMR in Biomedicine, Dr. Albert and colleagues used an experimental procedure to create reduced blood flow (ischemia) in an animal model and then imaged the blood flow using xenon MRI. "This is the first time that hyperpolarized xenon MRI has been used to distinguish normal from abnormal blood flow in the brain," said Dr. Zhou.

MRI is already used as a tool in the diagnosis of stroke. Approximately 87 percent of strokes are ischemic, and MRI can detect tissue damage from acute ischemic stroke early in the progression of a stroke. Currently, two traditional MRI methods are often used for assessment in patients suspected of having had a stroke, diffusion-weighted imaging and perfusion-weighted imaging, and the areas of damage identified by these two methods are compared. A mismatch between the two can identify two areas of the stroke-a core area and an outer area called a "penumbra."

"The target of acute stroke therapy is the penumbra because this ischemic region is still potentially salvageable," Zhou said. "When the penumbra is detected, physicians may decide to use clot-busting therapies to restore blood flow and re-establish neurobiological function."

"Xenon is important because it is an inert atom that dissolves readily in the blood, so it is ideal for safely tracing where blood flows in the body," said Albert. "Where blood flows, xenon flows."

Another advantage is that xenon is not found naturally in biological tissue, and thus does not have a background signal in the body. As a result, when xenon is introduced to the body via inhalation, the MRI signal has high sensitivity.

"With further improvement of hyperpolarization levels, this method has great potential as a complementary diagnostic tool that is fast and non-invasive," said Zhou. Ongoing research in Albert's laboratory is exploring this possibility.

Albert has been a leader in hyperpolarized xenon MR imaging since its inception in 1994, when he co-invented the technology as a doctoral student at the Stony Brook University. Albert was awarded a Presidential Early Career Award for Scientists and Engineers for his work. This study was funded by a National Aeronautics and Space Administration award to Albert; Zhou, who now is at the Wuhan Institute of Physics and Mathematics at the Chinese Academy of Sciences, is supported by the Hundred Talents Program of the Chinese Academy of Sciences.

Contributed by John P. Roche, UMass Medical School