Siemens receives FDA clearance for MRI stroke-detection technique

Diffusion MRI opens path to early stroke treatment

Real diagnostic progress is being made in this "Decade of the Brain," and MRI is the imaging modality out front. New functional MRI techniques will lead to radical improvements in the treatment of stroke similar in scope to past advances in treating cardiac injury. The latest technology to enter clinical use in this neurological application is diffusion MR imaging.

Siemens Medical Systems of Iselin, NJ, has received 510(k) certification from the Food and Drug Administration for its Diffusion Imaging package, a fast-MRI sequencing software package developed for use on the vendor's Magnetom Vision 1.5-tesla MR system. Diffusion imaging must be used with a high-speed gradient MR system, the standard being echo-planar imaging, said Siemens product manager Yuri Wedmid.

The breakthrough of diffusion imaging is that it enables rapid detection of acute ischemic stroke within the first 24 hours, which is increasingly vital as researchers determine the importance of early intervention in stroke.

"Proper treatment can make the difference between a person dying or being severely compromised by a heart attack and making an almost total recovery," said Philip Drew, a medical imaging technology consultant based in Concord, MA. "This has been largely realized. The hope has been that we could do something equivalent for stroke, with imaging techniques that allow physicians to recognize the nature, the extent, the severity, and the stage of a stroke so they can initiate proper treatment. But the brain is a much tougher problem than the heart. The heart is a comparatively simple organ."

As we near the end of the Decade of the Brain, a basic research initiative proclaimed by President Bush in 1990 and shepherded by the National Institute of Mental Health, that tough nut is cracking. Diffusion MR followed by perfusion MR, which is still under development, will provide vital tools in enhancing clinical treatment of the brain.

"It (diffusion MR) is an extremely useful indicator of an acute ischemic insult, allowing us to direct the management of patients with acute ischemia much more effectively. We use this in every patient admitted to the emergency room for whom an MRI for stroke is ordered," said Dr. Michael Brant-Zawadzki, director of MRI at Hoag Memorial Hospital in Newport Beach, CA (one of the clinical sites used in testing Siemens' diffusion MR product), and a clinical professor of radiology at Stanford.

Perfusion MRI requires a contrast agent, explained Siemens' Wedmid. This technique, when cleared for use by the FDA, will help determine the severity of a stroke by observing the flow of MR contrast in the blood into and out of an area of tissue. If the flow is rapid, the tissue would be relatively healthy and the ischemic stroke, if any, would be minor.

Diffusion MRI, on the other hand, is a completely noninvasive technique that tunes the high-speed gradient switching sequence to focus on diffusion in the brain tissue, he said. Diffusion is a physiological event involving the movement of water molecules from cell to cell. It is rapid in normal tissue and lower in tissue affected by stroke. Injured areas appear as bright spots on the MR image.

"This is an excellent way of screening for stroke or determining that there is stroke," Wedmid said. "Perfusion would probably be used in a later study once you identify an area of question and you want to determine more about it."

While diffusion MRI can detect both types of stroke-hemorrhagic and ischemic-the real breakthrough comes in the diagnosis of ischemia, or blood-starved tissue. This is the larger class of stroke, he said.

"Hemorrhagic stroke can be determined rather readily already," Wedmid said. "In order to determine that you in fact have ischemic stroke, there are no current techniques except for diffusion imaging."