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 and MR perfusion.
Mean transit time is calculated by dividing cerebral blood volume (CBV) by cerebral blood flow (CBF)
Normal values of MTT in the brain are:
- gray matter: 4 s
- white matter: 4.8 s
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 only mildly reduced CBF, the red blood cells have longer contact time with the oxygen-permeable capillaries and allow for an increased oxygen extracted fraction. This may be sufficient to maintain the cerebral oxygen metabolism (benign oligemia) . However, evntually this will not be sufficient and the tissue will become ischemic.
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