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Edwards et al. Vessel Plus 2021;5:42 https://dx.doi.org/10.20517/2574-1209.2021.18 Page 7 of 14

algorithms that estimate perfusion [Figure 3]. Inaccurate vessel selection negatively impacts the integrity of
the perfusion estimates [46,47] .

Patient positioning and movement
Software packages use the contralateral hemisphere as a reference to detect asymmetries in contrast flow
indicative of core or penumbra . Head tilt distorts the axial comparison of anatomical structures leading to
[26]
spurious results. Patient movement can be corrected to a degree via the software vendors’ automated
motion adjustment [48,49] . However, significant patient motion can result in distorted perfusion estimates and,
at times, artefactual perfusion “lesions”.

Radiation exposure
A standard CTP acquires at 80 kilovoltage peak and 100 milliamperes-second. Utilising optimised
[50]
[51]
protocols and multidetector scanners , the radiation dose is approximately 2 mSv, which is similar to a
single head CT or approximately 1 year of natural background radiation. Additional imaging must always
[52]
balance the improved diagnostic information vs. the risk of extra radiation.
Z-axis coverage
The number of data channels used in the craniocaudal plane (z-axis) and effective detector thickness
influence the z-axis coverage of a CT scanner . For example, a 64 row detector scanner with a detector
[53]
thickness of 0.5 mm has 32 mm of simultaneous z-axis coverage. Detector configuration and z-axis coverage
[54]
vary considerably by manufacturer . In recent years, there has been growing availability of 320-detector
[17]
row scanners able to acquire up to 160 mm of brain in a single acquisition .
CTP IN THE POSTERIOR CIRCULATION
Studies have routinely demonstrated the utility of CTP perfusion in ACS for both diagnosis and patient
[55]
selection for acute therapies . In contrast, there have been few studies examining the role of CTP in the
[3-7]
PCS. To date, studies have been limited to small retrospective populations [56,57] and case reports . We
[58]
summarise the existing evidence.
Specific challenges
Until recently, restricted axial slice coverage has limited the application of CTP to PCS. Older 16 and 64
detector row scanners have a limited scan range of 20-40 mm in the axial plane. As such, scans have been
generally focused on capturing regions supplied by the anterior circulation with limited coverage of
posterior circulation structures . This has resulted in missed infarcts and underestimates of the ischaemic
[59]
[60]
core and penumbra . Beam hardening artifact and radiation dose have also been considerations affecting
quality and safety of PCS CTP . Ultimately, these technical considerations have contributed to limited
[13]
uptake. A substudy of the Basilar Artery International Co-operation Study registry, the largest prospective
dataset of PCS due to acute basilar artery occlusion, demonstrated that only 4.6% of patients underwent
CTP . Advances in both software and hardware have enabled whole brain coverage, thereby overcoming
[61]
many of these challenges. One such advance includes the “toggling-technique” which expands coverage of
limited range scanners by jogging between two location acquiring axial images in a to-and-fro manner [62,63] .
Similarly, there is greater availability of multi detector scanners with wide z-axis coverage [17,60] able to image
the entire brain in a single acquisition slab.

Applications in posterior circulation stroke
Several studies have demonstrated an improvement in diagnostic accuracy when CTP is combined with
NCCT and CT angiography source images (CTA-SI) compared to NCCT and CTA-SI alone for the
diagnosis of PCS [15,16] . Furthermore, it may hold additional prognostic information to guide acute treatment