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Edwards et al. Vessel Plus 2021;5:42 https://dx.doi.org/10.20517/2574-1209.2021.18 Page 3 of 14

Several software packages exist for determination of these parametric maps. Calculation of these maps is
vendor-specific with no standardised method for transformation of the raw data. Different mathematical
[20]
techniques underpin the output represented by these maps . Studies have shown significant differences in
[21]
perfusion parameters derived from identical source data depending on software package and post
processing method used [Figure 1A-H, Figure 2A-H].
[20]
Parameters
There are several considerations when interpreting perfusion maps. The normal ranges for perfusion
[22]
parameters vary by anatomical location , age and neurological state . Cerebral haemodynamics are
[23]
autoregulated via complex metabolic mechanisms including blood pressure, arterial partial pressure of
carbon dioxide and pH.

Perfusion parameters represent the temporal characteristics of contrast as it travels from the arterial system
through the brain parenchyma. The two principal determinants of this transit are arterial flow and intrinsic
tissue characteristics. Arterial flow is approximated by the arterial input function. Factors that cause delay
[24]
and dispersion of arterial flow include: reduced cardiac output, arterial stenosis, and injection rate . Tissue
perfusion is mathematically deducted via deconvolution using the arterial input function to account for
arterial flow.

Mean transit time
MTT represents the mean time for blood to pass through a voxel. It is correlated with mean arterial pressure
[25]
and cerebral perfusion pressure and is measured in seconds .
Cerebral blood volume
CBV is the fraction of blood vessels in a single voxel. It is measured in millilitres/100 g .
[26]
Cerebral blood flow
CBF is the flow rate of blood through a single voxel in a given time. It is mathematically related to CBV and
[26]
MTT via the central volume principle which states CBF = CBV/MTT . CBF is measured in
millilitres/100 g/min.

Time-density relationship measures (Time to peak, Time to drain, Tmax and Delay Time)
These parameters provide a visual interpretation of the time density curve. They reflect the time taken from
the start of the scan to the maximum recordable intensity of the contrast medium in brain tissue. Although
related, different mathematical methods underlie the calculation of these perfusion parameters [22,27] . An in-
depth discussion of these parameters lies outside the scope of this review. Time-density relationship
parameters are measured in seconds.

Time to peak

Time to peak (TTP) is a simple yet robust measure of blood flow [28,29] . This parameter represents the time
from contrast injection to the maximum tissue enhancement value. Importantly, this parameter does not
incorporate deconvolution, as such is sensitive to delay and dispersion of contrast bolus. Failure to correct
for delay and dispersion may lead to overestimation of core and underestimation of penumbral
volumes [30,31] .