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Page 2 of 14 Edwards et al. Vessel Plus 2021;5:42 https://dx.doi.org/10.20517/2574-1209.2021.18

[3]
acute therapy . Early studies established the role of thrombolysis and endovascular thrombectomy within
4.5 h and 6 h from symptom onset, respectively, of an eligible ischaemic stroke. More recent trials have used
[3-7]
advanced imaging to demonstrate a benefit of these treatments over an extended time window . These
studies have expanded the number of patients eligible for acute reperfusion therapies.

Multimodal imaging including CT perfusion (CTP) is recommended by most international guidelines as
part of the imaging work up for acute stroke. CTP follows the temporal course of an IV bolus of iodinated
contrast through the brain parenchyma to calculate multiple parameters. It has allowed clinicians to select
patients for therapy based on both time and tissue based characteristics. As such, this modality has
underpinned patient selection to trials demonstrating a benefit of acute therapy in the extended time
window.

[8,9]
Approximately 20% of strokes occur in the posterior circulation . Compared to anterior circulation stroke
(ACS), posterior circulation acute ischaemic strokes (PCS) are misdiagnosed three times more
frequently [10,11] . PCS frequently present with non-specific symptoms including headache, dizziness and gait
disturbance, making the diagnosis difficult on clinical grounds alone . Studies have reported that up to
[11]
[12]
90% of PCS do not meet the criteria for TIA at first medical contact . This often leads to a delayed or
inaccurate diagnosis.

To date, there have been few studies exploring the role of CTP in PCS. Given the limited evidence and
technical challenges of performing CTP for PCS, uptake has been limited. Advances in image acquisition,
[13]
scanner technology and software have overcome many of these barriers. Recent studies of CTP for PCS
have demonstrated promising utility including improved diagnostic accuracy and prediction of functional
[14]
outcomes [15,16] .

This review aims to: (1) provide an overview of the technical aspects involved in CTP acquisition and post
processing; (2) define common CTP parameters reported by existing software; (3) explore the evidence
regarding current ischaemic core and penumbra thresholds; (4) discuss common technical pitfalls
impacting CTP parameters; and (5) summarise existing evidence relating to CTP in PCS including specific
technical challenges and current applications.

OVERVIEW OF CTP
Acquisition of CTP
CTP protocol varies by institution and scanner. Brain coverage is dependent on the number and width of
[17]
CT detectors. Ideally there should be at least 80 mm of axial coverage . However, to fully capture the
posterior fossa and supratentorial brain, whole brain coverage (> 100 mm) is ideal. After administration of
iodinated contrast, a four dimensional sequence of scans are acquired over a designated period of time.
Ideally, to avoid bolus truncation, images should be captured over a minimum of 60 s [18,19] . The acquired
source images effectively track the course of contrast agent through the brain vasculature and tissue.

Post processing of CTP
The four-dimensional source images are converted into a range of perfusion maps which represent different
haemodynamic parameters. Maps are generated via deconvolutional algorithms based on the arterial input
function and passage of contrast through each voxel. Common outputs from CTP include mean transit time
(MTT), cerebral blood flow (CBF), and cerebral blood volume (CBV). Maps of contrast transit such as
Tmax or Delay Time are also produced, depending on software. These parameters are further processed to
provide maps that estimate the volume of tissue that is irreversibly damaged, or infarcted (ischemic core),
and ischemic tissue at risk of future infarction without reperfusion (penumbra).