CT perfusion (brain)

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CT perfusion of the brain is a dynamic, contrast-enhanced study utilized in patients with suspected strokes to differentiate salvageable ischemic brain tissue from the damaged infarcted brain.

NB: This article is intended to outline some general principles of protocol design. The specifics will vary depending on CT hardware and software, radiologists' and referrers' preference, institutional protocols, patient factors (e.g. allergy) and time constraints.

Indications

CT perfusions studies are performed in the context of patients presenting with acute neurological symptoms that may represent cerebral infarction or cerebral hemorrhage. It is often the penultimate study in the code stroke CT pathway.

Contraindications

Technique

  • patient position
    • supine with their arms by their side
    • the head is placed in a 'chin tuck position' with he chin tucked toward the chest (via a sponge behind) 
    • sponges can be placed either wise to ensure the head is straight
  • scout
    • base of the skull to the top of the vertex 
      • if this is part of the code stroke pathway the scout may be from the carina to the tip of the skull as a CT angiogram of the neck and head will be performed before or after
  • scan extent
    • most perfusion scans begin at the base of the pituitary fossa focusing on the basal ganglia and supra-ganglionic level with a shuttle range of around 10cm
  • scan direction
    • ​this is a functional scan, the scan will 'shuttle' up and down as contrast is injected
  • scan duration
    • scan duration is often 45-60 seconds, but to be more precise it would be as long as it takes for the contrast to fully dissipate
  • contrast injection considerations
    • cannula size 
      • this exam requires an injection of contrast at a considerable rate (ranging from 6-8ml/s) therefore a large bore (at least 18 gauge) cannula is required
    • injection rate
      • 6-8ml/s 
    • injection volumes
      • 35-40 ml of high concentrate iodinated contrast (300-350mg/cc) followed by 30-40ml of saline
  • scan delay
    • minimal scan delay 

Practical points

  • perfusion scans focus on the pituitary fossa focusing on the basal ganglia and supra-ganglionic level however patients with symptoms of cerebellar infarction will require an adjustment of the scan extent
  • thick image widths are often used to minimize noise
  • two of the most important factors of this exam is patient positioning and a quality IV access 
  • poor cardiac output can lead to inaccurate perfusion maps, performing the carotid angiogram before the perfusion is a good way to get an idea of cardiac output and if the scan duration needs to be adjusted  

Post-processing

The aim of this scan is to measure/observe contrast enhancement. The scan will calculate the enhance (if any) over time. Hence the functional nature of the scan.

Depending on the vendor of the machine there will be differing post-processing methods, however, the primary focus of the post-processing will be the measure of: