CT Brain Perfusion Technique

Thursday, November 17, 2016

CT Brain Perfusion Scans

Various attempts have been made to establish a CT method to allow qualitative and quantitative evaluation of cerebral perfusion. CT perfusion provides this information by calculating regional blood flow rCBF and regional blood volume rCBV and mean transit time MTT. Perfusion studies are obtained by monitoring the passage of iodinated contrast through the cerebral vasculature. Attractive characteristics of this approach are the:

Widespread availability of CT scanners
High Image Quality
Relative low cost

In addition, simply extending the routine CT examination eliminates time consuming transport of patients between CT and Magnetic Resonance scanners that serves to further delay treatment.

How Perfusion Scan are done?

Perfusion studies are obtained by monitoring the passage of iodinated contrast through the cerebral vasculature.
CT perfusion software that interfaces with helical scanners can meet the clinical needs for evaluation of acute and subacute stoke. Such applications procedure a quantitative measure of regional hemodynamics by demonstrating blood flow in each pixel of the cerebral parenchyma that is imaged. The technique is based on the central volume principle, which state that cerebral blood volume can be calculated as the product of the total cerebral blood flow and the time needed for the cerebral blood passage: CBF = CBV / MMT. A workstation equipment with commercially available perfusion software can perform these complex calculations quickly. The goal in performing perfusion studies for patients with acute stroke is to distinguish infarcted tissue from the penumbra.

Technical Factors Brain Perfusion Scans 

The most common technique used associated with CT perfusion scanning is based on the first pass of a contrast bolus through the brain tissue. Brain Perfusion scans use a 50 ml IV bolus of a nonionic low osmolality contrast and injected at 4 to 5 ml / seconds. At a single location a helical scanner is used to produce a dynamic set of images. Duration of scan can take up to 40 to 45 seconds.

The slices are produced by repeatedly scanning the same region at the same table position, a technique some manufacturer refer to as the cine mode. Multislice scanners allow several z positon slices to be scanned simultaneously. Scans are typically acquired at 5 mm sections to lessen beam hardening artifacts, and then reformatted into 10 mm thick sections for viewing and filming to improve the signal to noise ratio.

Brain Perfusion Technique

The brain perfusion protocol begins with an unenhanced scan of the whole brain. Althrough the level for scanning the enhanced portion of the study may be selected at the time of examination based on the unenhanced CT findings, a transverse slice through the level of the basal ganglia contains territories supplied by the anterior, middle and posterior cerebral arteries, thus offering the opportunity to interrogate each of the major vascular regions.

The level of the basal ganglia contains territories supplied by the anterior (ACA), middle (MCA) and posterior cerebral arteries (PCA). Therefore, this is the level most frequently scanned for brain perfusion studies. The outlined areas on the upper two rows of images are the basal ganglia. Images courtesy of the university of Michigan Health System.

ct brain perfusion
CT Brain Perfusion Technique

Analysis of Contrast enhancement Curve

Analysis of the contrast enhancement curve guides the selection of preenhancement and post enhancement images. The preenhancement image is that last image before contrast arrives. The post enhancement image is the point immediately after the first pass of the contrast bolus when the time attenuation graph begins to flatten.

The perfusion software then generates color coded maps demonstration: 

  • Regional cerebral blood volume or rCBV
  • Blood mean transit time, or MTT, through cerebral capillaries.
  • Regional cerebral blood flow or rCBR
brain perfusion
Contrast Enhancement Curve

CT Perfusion is most frequently ordered in the evaluation of acute stroke, but is also ordered for vasospasm or tumor grading. With some modification CT perfusion can also be used to determine cerebrovascular reserve during temporary balloon occlusion protocols.

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