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Perfusion CT Ischemic Stroke Protocols

Strokes are one of the big three killers in America, sitting only behind heart disease and cancer. Quickly and accurately identifying hemorrhagic and ischemic strokes is vitally important in caring for the patient. CT perfusion scans have been particularly useful in this area. These studies document blood flow to the brain, When performed appropriately they can be life-saving for stroke victims.

  1. Establish IV Access and Start Fluids

    • The process begins with placing an 18-gauge IV needle in the antecubital fossa of the right arm. If the right arm cannot be accessed, the left arm is used. Using the right arm avoids filling the left brachiocephalic vein with a concentrated contrast agent. A 1.5 mL/kg/hr normal saline infusion is then started, according to the protocol set forth by Dr. Wade Smith and his team from the Department of Neurology at the University of California in San Francisco.

    Routine Noncontrast Head CT

    • Prior to placing the patient in a 64-slice CT scanner, her head is tilted forward. Then a noncontrast head CT with contiguous 5 mm thick slices is obtained. Noncontrast CT is particularly helpful for identifying hemorrhagic strokes.

    Computed Tomographic Angiography of Brain and Neck

    • 110mL of an iodinated contrast agent is given through the peripheral IV at a rate of 4mL/s. After 20 seconds, the patient is then rescanned with settings at "3:1 pitch, section thickness 1.25 mm, 120 kV,170 mAs" according to the protocol of Dr. Smith and his team.

    CT Perfusion Image Acquisition

    • After the noncontrast head CT and CTA are performed, a dynamic CT perfusion study is obtained. 40mL of a nonionic contrast agent is administered at a rate of 4mL/s, followed by a 9 second delay. The patient is then rescanned for a total of 40-60 seconds. The scanner settings for this protocol are "field of view, 25cm; 120kV; 30-60mA; matrix, 512X512;" as set forth by Dr. Heidi Roberts, a radiation oncologist at the University of California San Francisco, and her team. The toggling table technique can be used for this particular scan. Two separate anatomic locations 2 cm thick, each divided into 1-cm areas, can be simultaneously imaged, with the scanner "toggling" back and forth between the two areas. This technique allows for two views typically in the middle cerebral artery territory. Preferred locations include the insula/basal ganglia area and the centrum semiovale and deep white matter.

    Image Analysis at Workstation

    • All images are transferred to a workstation. CTA maximum intensity projections also known as MIP images and brain perfusion maps are generated. These images are then added to the picture archiving and communication system (PACS) for analysis by a radiologist. The perfusion studies are carefully examined for cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT), or the time necessary for blood to flow through tissue. An abnormal CBF is helpful for identifying the central part of an infarct that is usually permanently damaged, whereas the CBV and MTT help locate damaged but potentially viable tissue.

    Special Use for Perfusion Studies

    • A penumbra is the term given when the CBV or MTT abnormality is greater than the CBF abnormality. This mismatch indicates that there might be tissue in the infarct worth saving. CT perfusion studies have thus been used for medical intervention (i.e., thrombolytic drug therapy) even outside the recommended time interval for stroke treatment.

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