Stroke imaging data lag behind technology

April 1, 2007

More than a decade ago, a National Institutes of Health study found that noncontrast head CT was the best way to image acute stroke. Studies are slowly emerging validating MR perfusion/diffusion imaging for emergent stroke.

More than a decade ago, a National Institutes of Health study found that noncontrast head CT was the best way to image acute stroke. Studies are slowly emerging validating MR perfusion/diffusion imaging for emergent stroke. But the quiet revolution taking place ahead of peer-reviewed data is the use of multimodal CT to evaluate the vascular pathology of strokes and perfusion of the brain.

"CT angiography and CT perfusion are robust techniques for evaluating stroke patients. They help us stratify patients into those who are candidates for thrombolytic therapy, as well as help us determine which patients are better served intra-arterially or intravenously," said Dr. Kevin Abrams, medical director of neuroradiology and MRI at Baptist Hospital in Miami. "Unfortunately, our experience is anecdotal."

The majority of strokes result from a blocked blood vessel. A noncontrast CT scan can help rule out hemorrhage, thereby opening the way for potential thrombolytic treatment. CTA and CT perfusion scans direct interventionalists to the blood clot and help establish the amount, if any, of salvageable tissue. The window for thrombolytic use is very narrow-three hours from stroke symptom onset for intravenous delivery and six hours for the intra-arterial approach. In addition, CTA can show whether a stroke is a large occlusive thrombus or a small distal thrombus. In general, the larger thrombi are treated intra-arterially, while the smaller ones are treated intravenously, Abrams said.

In about 13% of stroke cases, the clinical diagnosis of stroke is wrong, he said. CT, and particularly CTA, can help find other etiologies for patient symptoms. In addition, facilities that do not perform a CTA run the risk of performing invasive angiograms on patients without thrombus.

CT perfusion data are broken down into three color-coded parametric maps: mean transit time (MTT), relative cerebral blood volume (rCBV), and relative cerebral blood flow (rCBF). The MTT map is the most sensitive indicator for ischemia, the rCBV map is the most specific indicator of ischemia, and the rCBF map is somewhere in between, Abrams said. In general, MTT minus rCBV represents the ischemic penumbra, an area of mild to moderately ischemic tissue that may remain viable for several hours. The penumbra is where thrombolytic intervention is most likely to be effective.

The standard of care for stroke diagnosis and treatment has already begun to change, Abrams said. Guidelines have been developed defining what it means to be a stroke center. As far as imaging aspects for the standard of care, the world awaits more data on CTA and CT perfusion.

"What we can do is so much greater than what the data available indicate. Hopefully, in the next couple of years, more and more people will be doing multimodal CT and/or MRI and publishing their results," Abrams said.