Mean transit time (MTT)

Mean transit time (MTT) corresponds to the average time, in seconds, that red blood cells spend within a determinate volume of capillary circulation. It is assessed as part of the CT perfusion protocol, which is basically used in the context of ischaemic stroke diagnosis and management, and MR perfusion, which has a broader applicability. 

It is expressed by the formula: MTT = cerebral blood volume (CBV) / cerebral blood flow (CBF)

Normal values of MTT in the brain are:

  • gray matter: 4s
  • white matter: 4.8s

When the cerebral perfusion pressure (CPP) drops beyond the threshold of the brain auto-regulation, the compensatory cerebral vasodilatation, aiming to keep this pressure, becomes overwhelmed (see Monro-Kellie theory), and then the CBF starts to decrease in correlation with the CPP reduction. As a result, MTT will be prolonged. In the beginning, with a mildly reduced CBF, the red blood cells having a long contact time with the oxygen-permeable capillaries will allow an increased oxygen extracted fraction, and this may be sufficient to maintain the cerebral oxygen metabolism ("benign oligemia") 1. However, after a certain point, this will not be sufficient and the brain cells will start to suffer (ischaemia). 

MRI physics
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Article Information

rID: 43698
Section: Physics
Synonyms or Alternate Spellings:
  • Mean transit time
  • MTT

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