CT Perfusion

CT perfusion (CTP)

  • repeated scanning of the same area of the brain during passage of contrast from arteries through capillaries to the veins and into the venous sinuses
  • density curves are drawn for each pixel in the image, color-coded maps are derived from these curves

 

Parameters of CTP:

  • Time to peak (TTP)
    • shows the time to the apex of the time−density curve
    • reflects the time it takes until the contrast bolus reaches the tissue
    • the most sensitive marker for cerebral ischaemia
      • TTP increased – longer time for contrast to reach ischaemic area
  • Mean transit time (MTT)
    • time from wash in to wash out of the contrast medium
    • also prolonged in an ischaemic area
  • Time to maximum (Tmax)
    • flow-scaled residue function in the tissue
    • prolonged in ischaemic lesion
    • *widely used although physiological meaning is not well understood
    • reflect delay, dispersion and to a lesser degree also MTT
  • Cerebral blood volume (CBV)
    • volume of blood present at a given moment in a distinct brain area
    • calculated from area under the time−density curve
  • Cerebral blood flow (CBF)
    • blood supply to brain in a given time
    • derived from the gradient of the wash in of the time−density curve
  • relation between the parameters is described in this equation:
    • MTT = CBV/CBF

 

Tissue at risk:

  • goal is to assess ratio of infarct core (irreversibly damaged brain tissue) to penumbra, and identify ‘tissue at risk’
    • penumbra
      • critically hypoperfused, infarct core adjacent tissue
      • still viable with reperfusion
    • benign oligoaemia
      • outside penumbra
      • hypoperfused area to a lesser degree
      • remains viable even if reperfusion fails
  • approaches for estimating penumbra on CTP
    • visible mismatch comparing CBV and CBF = penumbra
      • presume that areas with significantly low CBV represent ischaemic core and areas with solely reduced CBF and/or TTP represent penumbra
    • reduction of the CBF to 30%–50% relative to mean CBF in contralateral hemisphere
      • most reliable predictor of infarct core

 

The CTP Time-Density Curve:

ene12849-fig-0003.png

 

Time−density curve on computed tomography perfusion.

  • Time to peak (TTP) = time to apex of curve
  • Mean transit time (MTT) = time from wash in to wash out of contrast agent
  • Area under the curve = volume of blood (CBV) present at a given moment in a distinct brain area
  • Cerebral blood flow (CBF) = gradient of wash in of time−density curve

 

Reference:

Kurz, K. D. et al. “Radiological Imaging In Acute Ischaemic Stroke”. Eur J Neurol 23 (2015): 8-17.

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