Stroke protocol (CT)

A CT stroke protocol, often referred to as a code stroke CT, has become a fairly widespread and standardized approach to imaging patients presenting with acute neurological symptoms that may represent cerebral infarction or cerebral hemorrhage (together grouped under the vague term stroke). 

Indications

A CT stroke protocol is obtained in the emergency setting to rapidly diagnose and quantify patients presenting with probable ischemic strokes and to enable appropriate urgent management (e.g. endovascular clot retrieval or intravenous thrombolysis).

In most centers, CT is favored over MRI in the ultra-acute setting due to time and access constraints, despite acknowledging that MRI, and particularly diffusion-weighted imaging, is superior in identifying small infarcts and defining infarct core  ^1-3. 

Purpose

The purpose of this protocol is three-fold: 

1. to assess the brain for established infracts or alternative diagnoses
2. to identify the location and physiological effects of arterial blockage
3. to assess vascular anatomy that may impact endovascular access

To achieve this, stroke protocol CT usually includes 3 concatenated scans ²: 

1. non-contrast CT (brain)
2. CT perfusion (brain)
3. CT angiography (aortic arch to the vertex of the skull)

It should be noted that this is not uniformly accepted and some centers do not perform perfusion routinely ³. 

There is an increasing trend to perform multi-pass CTA of the brain to perform multiphase CT angiography collateral score in acute stroke.

Contraindications

As is the case with other contrast studies, contraindications, such as chronic renal failure and allergy may be important. It is important to note, however, that in the hyperacute setting of evolving stroke, this information is not always known. Furthermore, even if known, with the exception of severe life-threatening allergy, complications of contrast administration may be deemed less important than the appropriate assessment of the stroke. 

Non-contrast CT

A non-contrast CT of the brain, usually obtained volumetrically and reformated in three planes (sagittal, axial and coronal), is obtained first. In addition to a rapid overview of the brain (see an approach to CT head) that may demonstrate unexpected non-stroke findings (e.g. tumors) it specifically allows for the following stroke-related features to be sought:

• intracerebral hemorrhages
• hyperdense artery sign
• established acute cerebral infarction
• calculation of ASPECT score

See: CT head (technique)

CT perfusion

Intravenous contrast is then administered and various parameters of cerebral perfusion calculated. Typically these include

• cerebral blood volume (CBV)
• cerebral blood flow (CBF)
• mean transit time (MTT)
• time-to-maximum (Tmax) or time to peak (TTP)

These allow not only the diagnosis and quantification of areas of impaired perfusion but also the identification of infarct core and penumbra that are important in selecting patients for thrombolysis/endovascular clot retrieval. 

See: CT cerebral perfusion (technique)

CT angiography

The last component is CT angiography usually performed from the arch or the aorta to the vertex of the skull. It is performed using the arterial phase of intravascular contrast. It not only allows for the visualization numerous intracranial features relevant to the stroke setting but also anatomy that may be relevant to the endovascular intervention. 

• occlusive thromboembolism
• arterial dissection
• aneurysms and arteriovenous malformations
• spot sign in cerebral hemorrhage
• bovine arch

It should be noted that there is increased interest in the use of multiphase CTA particularly to accurately assess the degree of collateral circulation ³. 

See: CT angiography of the cerebral arteries (technique)

Practical points

• the code stroke, although not the most technically demanding radiological protocol, can be a high-stress situation with a variety of extrinsic factors weighing on the radiographer and surrounding healthcare teams
• workflow will differ between institutions and be based around what works best in that environment 
• some centers will perform the CT angiogram before the perfusion study in order to give the interventional team additional minutes to inspect and plan for a potential clot retrieval as the perfusion study is being performed. This requires advanced trained radiographers, customized pressure injection protocols, and custom CT protocols