Report from ISET: Stroke imaging data lag behind technology

February 12, 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. Today, anecdotal evidence suggests otherwise, according to Dr. Kevin Abrams, medical director of neuroradiology and MRI at Baptist Hospital in Miami.

More than a decade ago, a National Institutes of Health study found that noncontrast head CT was the best way to image acute stroke. Today, anecdotal evidence suggests otherwise, according to Dr. Kevin Abrams, medical director of neuroradiology and MRI at Baptist Hospital in Miami.

"We've been doing multimodal brain CT for several years and have a great deal of experience evaluating the vascular pathology of strokes and perfusion of the brain. Unfortunately, our experience is anecdotal. It's a case where the literature lags behind the technology," said Abrams, who spoke at the recent International Symposium on Endovascular Therapy in Miami.

Studies are slowly emerging that validate MR perfusion/diffusion imaging as an excellent technique for evaluating acute stroke. A Jan. 27 NIH study in The Lancet found diffusion-weighted MRI superior to noncontrast CT for the initial evaluation of acute stroke.

The problem is that MR scanners are not as readily available as CT scanners. In addition, MR imaging carries safety concerns for patients with pacemakers, stents, and other metallic implants, Abrams said.

"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," Abrams told Diagnostic Imaging.

At Miami Baptist, stroke patients are categorized as priority I or priority II. Priority I patients, who are candidates for thrombolysis, receive CTA and CT perfusion imaging. Priority II patients are not candidates for thrombolytic therapy and do not receive angiography or perfusion imaging.

Neuroradiologists also perform CTA on patients with an acute subarachnoid hemorrhage, especially if it is associated with a subdural or parenchymal hematoma. The CTA helps because these patients may not have time to undergo conventional arteriography. They may have to go directly from CT scanner to operating room to have the subdural hematoma evacuated and the aneurysm clipped at the same time, Abrams said.

He noted that in about 13% of cases, the clinical diagnosis of stroke is wrong. CT, and particularly the CTA study, can help find other etiologies for patient symptoms.

CTA can also show whether a stroke is a large occlusive thrombus or a small distal thrombus. In general, the larger thrombus is treated intra-arterially, while the smaller distal clot is treated intravenously, Abrams said.

Facilities that do not perform a CTA generally will send patients directly to the angiography lab for an arteriogram. If there is no clot, the patient has received an invasive procedure for no reason, Abrams said.

The perfusion study shows the size of the ischemia, including potentially infarcted tissue and potentially salvageable tissue. It can also detect a mismatch between the two.

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, thw CBV map is the most specific indicator of ischemia, and the CBF map is somewhere in between, Abrams said. In general, MTT minus CBV represents the ischemic penumbra.

The time frame is three hours from onset of stroke symptoms for intravenous thrombolytics and six hours for intra-arterial thrombolysis. Abrams noted that community and hospital educational programs have heightened stroke awareness. Not only are patients getting to the hospital sooner than they did five years ago, but patients and caregivers are better able to give a more accurate history of the onset of stroke symptoms.

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, including primary and comprehensive stroke centers. 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 next couple of years, more and more people will be doing multimodal CT and/or MRI and publishing their results," Abrams said.

For more information from the Diagnostic Imaging archives:

Study shows MRI superiority over CT for diagnosis of ischemic stroke

Stenting gives uneven aid to patients at risk of stroke

Diffusion-weighted MR finds new niche in stroke therapy

The window expands for more effective stroke treatment