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Page 6 of 14 Edwards et al. Vessel Plus 2021;5:42 https://dx.doi.org/10.20517/2574-1209.2021.18

Time to drain

This is a deconvolution-based parameter which measures the time taken for contrast medium washout. It is
[35]
based on the time from arterial enhancement to tissue enhancement (Time to start) and MTT . It is
sensitive to multiple haemodynamic disturbance not necessarily captured by Tmax, delay time or Time to
peak

CTP THRESHOLDS
Core
Accurate measurement of ischemic core has been fundamental for patient selection in clinical trials of acute
reperfusion therapy. Large core volume is a predictor of poor outcome and complication following
[37]
[36]
acute reperfusion therapies. Conversely, overestimation of core may unnecessarily preclude patients from
receiving beneficial treatment .
[30]
Both CBV [22,38] and CBF have been validated to estimate ischaemic core volume. While early studies and
guidelines utilised CBV, more recently, CBF thresholds have become the standard for defining core
estimates. Multiple CBF thresholds have been correlated with final infarct size [20,39,40] . A CBF threshold of <
3 0 % h a s b e e n m o s t e x t e n s i v e l y v a l i d a t e d , b u t o n l y i n t h e a n t e r i o r circulation [17,32,34]
[Figure 1I-K, Figure 2I-L].

It is important to note that studies defining the optimal CTP thresholds for core have been typically based
on final infarct size on follow up MRI. CTP measures the transit of contrast through brain tissue rather than
cellular injury (which Diffusion-weighted imaging/DWI lesions reflect) . As such, these thresholds do not
[41]
represent real infarct tissue but are projected representations of tissue fate. Several factors have been shown
to result in CTP overestimates of core estimates including: a shorter time from stroke onset to imaging
[42]
and rapid reperfusion . Conversely, in conditions where blood flow and volume are restored to infarcted
[43]
brain tissue, such as late recanalisation or improved collateral flow, CTP may appear normal, despite a
relatively large infarct. Wherever possible, clinicians should validate CTP core representations by cross
referencing these maps with the “raw” CTP perfusion maps as well as non-contrast CT (NCCT) images to
corroborate actual infarct volume.

Penumbra
In stroke, penumbra represents the hypoxic cerebral tissue at risk of irreversible damage which is potentially
salvageable with restoration of blood flow. Penumbral volume is equal to the area of critically hypoperfused
brain tissue minus the ischaemic core. Early imaging studies exploring penumbra were based on final infarct
volume of hemispheric strokes with persistent large artery occlusion [38,41,44] .

Several parameters have been validated as successfully approximating penumbra including MTT, Time to
drain (TTD), Tmax and DT. Of these markers, DT and Tmax have been shown to be the most accurate
markers of penumbra . Delayed time greater than 3 s and Tmax greater than 6 s have become the
[17]
[32]
[20]
generally accepted thresholds for quantifying penumbra. Again, however, these thresholds are not validated
in the posterior circulation.
Technical factors
Vessel selection
Arterial and venous selection is automated in several perfusion software packages [20,45] . The selection of these
landmarks underpins the arterial input function and venous output function used in the deconvolution