Cerebral perfusion pressure and mean transit time
Cerebral perfusion pressure (CPP) is defined as the mean arterial (blood) pressure, MAP or MABP, minus intracranial pressure. Local CPP may be useful parameter in assessment and treatment of cerebrovascular diseases. Several studies have suggested that the regional ratio of cerebral blood flow (perfusion) and cerebral blood volume, measured by PET, could be used as an index of local CPP (Sette et al., 1989; Schumann et al., 1998; Watabe et al., 2014).
Cerebral vascular mean transit time (MTT) is the inverse of the proposed CPP index, that is, the ratio of cerebral blood volume to cerebral blood flow. MTT has also been measured by PET in several studies (Ito et al., 2003; Ibaraki et al., 2007).
MTT is a frequently reported parameter in contrast-enhanced MRI studies, because it does not have the scaling problems that are present in the cerebral blood flow and blood volume estimates.
Measurement by PET
Cerebral blood flow can be measured with PET using diffusible tracers, most commonly [15O]H2O. Limited extraction of [15O]H2O in the brain may potentially lead to 19% overestimation of MTT (Ibaraki et al., 2007), or underestimation of CPP.
Cerebral blood volume can be measured using [15O]CO as tracer. Correct calculation of blood volume from [15O]CO PET measurements requires that local cerebral small-to-large vessel hematocrit ratio is known. Different methods have provided slightly different measures for the small-to-large hematocrit ratio, and changes in cerebrovascular diseases have also been observed. This may lead to biases in estimates of MTT and CPP.
Ibaraki M, Ito H, Shimosegawa E, Toyoshima H, Ishigame K, Takahashi K, Kanno I, Miura S. Cerebral vascular mean transit time in healthy humans: a comparative study with PET and dynamic susceptibility contrast-enhanced MRI. J Cereb Blood Flow Metab. 2007; 27: 404–413. doi: 10.1038/sj.jcbfm.9600337.
Ito H, Kanno I, Takahashi K, Ibaraki M, Miura S. Regional distribution of human cerebral vascular mean transit time measured by positron emission tomography. Neuroimage 2003, 19: 1163–1169. doi: 10.1016/S1053-8119(03)00156-3.
Schumann P, Touzani O, Young AR, Baron J-C, Morello R, MacKenzie ET. Evaluation of the ratio of cerebral blood flow to cerebral blood volume as an index of local cerebral perfusion pressure. Brain 1998; 121: 1369-1379.
Sette G, Baron JC, Mazoyer B, Levasseur M, Pappata S, Crouzel C. Local brain haemodynamics and oxygen metabolism in cerebrovascular disease. Brain 1989; 112: 931-951.
Watabe T, Shimosegawa E, Kato H, Isohashi K, Ishibashi M, Hatazawa J. CBF/CBV maps in normal volunteers studied with 15O PET: a possible index of cerebral perfusion pressure. Neurosci Bull. 2014; 30(5): 857-862. doi: 10.1007/s12264-013-1458-0.
Updated at: 2018-11-21
Created at: 2014-11-03
Written by: Vesa Oikonen