CTP (using automated post-processing software algorithmic analysis) is
medically reasonable and necessary in patients with small acute ischemic
stroke (AIS) caused by a unilateral large vessel occlusion (LVO) in the
proximal anterior circulation evaluated at stroke centers; CTP can be used to
aid in selection for endovascular mechanical thrombectomy (EVT) if one of the
following other conditions is fulfilled:

  1. Treatment (femoral puncture) can be started within **6-24 hours** of the time last known to be at neurologic baseline and who meet the pre-CTP inclusion/exclusion criteria* as defined by the DAWN trial (1), or
  2. Treatment (femoral puncture) can be started within **6-16 hours** of the time last known to be at neurologic baseline and who meet the pre-CTP inclusion/exclusion criteria* as defined by the DEFUSE 3 trial (2)

*excluding criteria purely related to study mechanics (e.g., able to return for protocol required follow-up visits, etc.).

Noncontrast CT (NCCT) is the mainstay for initial AIS imaging due to
widespread availability, rapid scan times, and detection of intracranial
hemorrhage (which leads to very different management from infarction).
Multimodal CT includes NCCT, CT angiography (CTA) (to assess site of vascular
occlusion), and CT Perfusion Imaging (CTP). CTP consists of a temporal
sequence of head CT scans obtained during the wash-in and wash-out of an IV
bolus of iodinated contrast agent. Post-acquisition data analysis by dedicated
software allows creation of multiple hemodynamic parametric maps (based on
contrast time-density curves) for clinical interpretation. Hemodynamic
parameters include time to maximum contrast intensity (Tmax), mean transit
time (MTT), cerebral blood flow (CBF) and cerebral blood volume (CBV),
mathematically related by the equation CBF = CBV/MTT (3). These maps are able
to estimate brain regions with high probability of irreversible infarction
(ischemic core) versus areas of potentially reversible ischemia (penumbra).
Both core and penumbra are estimates of probabilistic tissue fate. While CTP
has been studied in acute ischemic stroke for decades (4), only recently was
it found likely to influence treatment decision (5). Subsequently, two level I
randomized controlled trials (RCTs) (DAWN and DEFUSE 3) found CTP helped
determine eligibility for EVT in the late time period (6-24 hr.) of an acute
(<24 hr.) ischemic stroke (AIS) (1,2), a paradigm shift away from confinement
to the early window (< 6 hr.).

The DAWN trial (Clinical Mismatch in the Triage of Wake Up and Late Presenting
Strokes Undergoing Neurointervention With Trevo) studied whether patients with
a clinical deficit that is disproportionately severe relative to the infarct
volume may benefit from late EVT (1). See table for key inclusion criteria.
Patients were randomly assigned to EVT plus standard medical management (MM)
(N=107, mean age 69.4 yr.) or to MM alone (N=99, mean age 70.7 yr.). Median
National Institutes of Health Stroke Scale (NIHSS) score was 17 (moderate to
severe stroke) for both groups. The trial was stopped for efficacy at the
first interim analysis. At 90 days, the rate of functional independence, as
defined by a score of 0-2 on the modified Rankin scale (mRS) of 0-6, was
greater for EVT than MM (49% versus 13%; adjusted difference, 33%; 95% CI,
21-44; posterior probability of superiority >0.999). The rate of symptomatic
intracranial hemorrhage did not differ significantly between the two groups
(6% in the EVT group and 3% in the MM group, P=0.50), nor did 90-day mortality
(19% and 18%, respectively; P=1.00).

**Key Inclusion Criteria for DAWN and DIFUSE 3**

**Parameter**

|

**DAWN**

|

**DEFUSE 3**  
  
---|---|---  
  
Prestroke baseline mRS (0-6)

|

mRS 1 (no significant disability)

|

mRS 2 (slight disability)  
  
Last known well to treatment time

|

6-24 h

|

6-16 h  
  
Minimum NIHSS score (0-42)

|

10 (moderate stroke)

|

6 (moderate stroke)  
  
LVO by MR or CT angiography

|

ICA (intracranial) and/or M1 segment of MCA

|

ICA (cervical or intracranial) and/or M1 segment of MCA  
  
Clinical-core mismatch  
(core volume based on a relative CBF threshold of <30% of normally perfused
tissue)

|

20 mL if age 80  
30 mL if age <80 and NIHSS 10-20  
50 mL if age <80 and NIHSS >20

|

70 mL and NIHSS 6  
  
Penumbra-core volume mismatch

|

not required

|

penumbra volume 15 mL (Tmax >6 s volume minus core volume) and a penumbra to
core ratio of 1.8  
  
mRS, modified Rankin score; NIHSS, National Institutes of Health Stroke Scale;
LVO, large anterior vessel occlusion; ICA, internal carotid artery; M1/MCA,
first segment of the middle cerebral artery; CBF, cerebral blood flow; Tmax,
time from contrast injection to maximum intensity.

The DEFUSE 3 trial (Diffusion and Perfusion Imaging Evaluation for
Understanding Stroke Evolution) was a multicenter, randomized, open-label
trial that used perfusion-core mismatch and maximum core size as imaging
criteria to select patients for late EVT (2). See table for key inclusion
criteria. Patients were randomly assigned to EVT plus standard MM or standard
MM alone. The trial was conducted at 38 U.S. centers and terminated early for
efficacy after 182 patients had undergone randomization (EVT N=92, median age
70; MM N=90, median age 71). Median NIHSS score was 16 (moderate to severe
stroke) for both groups. The EVT group showed a benefit in functional outcome
at 90 days (mRS score 0-2, 44.6% versus 16.7%; RR, 2.67; 95% CI, 1.60-4.48;
P<0.0001). The 90-day mortality rate trended in favor of EVT (14% vs. 26%
(P=0.05)), and there was no significant difference between groups in the rate
of symptomatic intracranial hemorrhage (7% and 4%) or of serious adverse
events (43% and 53%). In a subgroup analysis, both the favorable outcome rate
and treatment effect did not decline in transfer patients compared to direct-
admission patients (6).

The DAWN and DEFUSE 3 trials differed in their approach in identifying
salvageable brain (table). The DAWN trial selected patients based on a
clinical-core mismatch, whereas the DEFUSE 3 trial focused on a penumbra-core
mismatch. Both target the same conceptual goal, identifying patients with
enough salvageable, at risk, tissue to warrant EVT risks. In both cases, brain
tissue was designated as having irreversible injury if CBF was less than 30%
of that seen in contralateral perfused tissue by CTP (or less commonly,
magnetic resonance MR (MR) perfusion-weighted imaging (MR-PWI)) as detected by
RAPID automated perfusion post-processing software. Both trials demonstrated a
large clinical benefit, with numbers needed to treat (NNT) of 3-4 to prevent
functional dependence. There were differences in protocol as well. DAWN
excluded patients with an infarct involving more than one third of the
territory of the MCA at baseline. DEFUSE-3 enrolled patients with lower NIHSS
score (less clinical severity), larger core infarct, and slightly higher
baseline mRS disability score. One third of DAWN-eligible patients are
DEFUSE-3 ineligible. Epidemiologic data suggest that about one-third of AIS
patients present between 6-24 hours, and only 9.2% of these (or 2.7% overall)
meet DAWN or DIFFUSE 3 inclusion criteria (7,8). Of all patients with acute
ischemic stroke presenting to a single comprehensive stroke center, 1.7% of
patients qualified for DAWN clinical trial enrollment with an additional
0.6-1% qualifying for the DEFUSE-3 trial (7).

Both studies employed CTP or MR-PWI to select patients for EVT, with CTP
predominating. DEFUSE 3 subgroup analysis showed no statistical difference in
treatment effect between "patients selected on the basis of
diffusion/perfusion MRI and those selected on the basis of CT perfusion
imaging," though the authors admit statistical power was limited by the lower
number of patients enrolled as a result of early termination. DAWN subgroup
analysis did not include comparison by qualifying image method. One criticism
of both studies was the large number of "wake-up strokes" (~50%) vs. 14-28% in
the general population, perhaps contributing to overestimation of stroke age,
and therefore, better outcomes (9,10). The authors note that a higher
proportion of unwitnessed strokes are expected in trials enrolling patients
late after onset, as witnessed strokes are typically treated early, and that
the benefit persisted even after stratification into witnessed and unwitnessed
stroke groups (11,12). They also explain that outcomes were "paradoxically"
superior to early window (< 6 hr.) treatment trials, probably due to selection
for a large volume of penumbral (i.e., salvageable) tissue.

A subsequent prospective review (13) and retrospective registry (14) analysis
also support the value of CTP in late period EVT eligibility assessment, while
also emphasizing the need to correlate perfusion abnormalities with other
imaging (NCCT, CTA) and clinical information; they may be more sensitive than
CTP for detecting irreversibly damaged tissue as time progresses. For example,
CTP may show increased CBF over time due to collaterals to tissue already
irreversibly injured (15). Also stressed is the need for more standardization
of CTP vendor software and post-processing techniques to decrease variation in
calculated core and mismatch (3,15). However, even with fully automated
software, a significant clinician interpretation learning curve remains (16).

_Non-AIS indications_

One non-ischemic stroke CTP potential use is in determining delayed cerebral
ischemia (DCI), occurring in approximately 30% of patients within two weeks
after aneurysmal subarachnoid hemorrhage (SAH) (17). The most common etiology
of DCI is thought to be vasospasm produced by spasmogenic substances generated
during lysis of subarachnoid blood. Monitoring for DCI can be done with CTA to
confirm vasospasm in patients with elevated velocities on transcranial Doppler
(TCD) ultrasound (17). However, brain perfusion asymmetry on CTP has been
studied for this purpose as well. A 2014 systematic review and meta-analysis
(18), included four small observational studies of 188 patients (19-22). The
weighted averages and ranges of the pooled sensitivity and specificity of CTP
in the determination of DCI were 0.84 (0.7-0.95) and 0.77 (0.66-0.82),
respectively. The pooled odds ratio was 23.14 (95% CI, 5.87-91.19). The
authors conclude that "perfusion deficits on CTP may be helpful in identifying
patients with delayed DCI before development of infarction and neurologic
deficits." However, they also cite many definitional and methodology
limitations of the underlying studies (nonuniform DCI definition as an outcome
measure, CTP protocol and postprocessing software differences, lack of
consistency of what constitutes an abnormal CTP test result, optimal time to
perform CTP, and nonstandard hemodynamic parameter thresholds). In addition,
accurate quantification is dependent on an intact blood-brain barrier, which
may not be functioning in DCI.

Small cohort studies have explored the potential role of CTP for patients with
traumatic brain injury (TBI). A small study of 48 patients reported NCCT had
sensitivity of 39.6% compared to improved sensitivity of CTP of 87.5% for
cerebral contusions diagnosed on delayed follow-up imaging (23). Additional
studies suggest reductions in blood flow and volumes determined by CTP are
associated with worse outcome (24). A subset analysis of 30 patients from an
observational study reported the information obtained from CTP is useful in
decision making (25).

CTP has been proposed as a possible modality for non-invasive assessment of
brain tumors. Several small (<20 patient), retrospective reports showed
promising early results in diagnosing malignant verses normal tissue,
evaluating for metastatic disease, and differentiating tissue type in brain
tumors (26,27). A small prospective trial of 49 consecutive patients with
brain tumors or tumor-like lesions were evaluated with CT and CTP. The results
suggest CTP can aid in distinguishing glioma and lymphomas based on
quantitative measurements of CBV and permeability (28). The reports indicate
need for further investigation of cut-off values, accuracy and patient
selection criteria, to determine if clinically useful.

The 2019 update to the 2018 American Heart Association (AHA)/American Stroke
Association (ASA) guidelines for the early management of patients with AIS
state: "When selecting patients with AIS within 6 to 24 hours of last known
normal who have LVO in the anterior circulation, obtaining CTP, DW-MRI, with
or without MRI perfusion, is recommended to aid in patient selection for
mechanical thrombectomy, but only when patients meet other eligibility
criteria from one of the RCTs that showed benefit from mechanical thrombectomy
in this extended time window" (Class of recommendation I-strong; level
(quality) of evidence A) (29). Since only the DAWN and DEFUSE 3 RCTs show a
benefit of late period EVT, they further warn: "DAWN or DEFUSE 3 eligibility
should be strictly adhered to in clinical practice." These guidelines do not
recommend CTP to determine eligibility for either thrombolytic therapy or EVT
in the early (<6 hour) period (Class of recommendation I-strong; level
(quality) of evidence B-Nonrandomized), or for any other indication (e.g.,
prediction of hemorrhagic transformation (HT) in acute ischemic injury). Other
guidelines have similar recommendations (30-32). National Institute for Health
and Care Excellence (NICE) recommends: "Add CT perfusion imaging (or MR
equivalent) if thrombectomy might be indicated beyond 6 hours of symptom
onset" (33). This recommendation uniquely also includes posterior circulation
stroke. However, there is minimal evidence regarding the appropriate threshold
and utility of CTP in the posterior circulation, and MRI is a better
alternative (3).

The 2017 American College of Radiology (ACR)-American Society of
Neuroradiology (ASNR)-Society of Pediatric Radiology (SPR) guidelines lists
primary and secondary indications for CTP in neuroradiology, including
evaluation of AIS, neoplasia, trauma, cerebral hemorrhage, and vasospasm
following subarachnoid hemorrhage (34). The 2012 AHA/ASA guidelines for the
management of aneurysmal SAH state that "perfusion imaging with CT or magnetic
resonance can be useful to identify regions of potential brain ischemia"
(Class IIa; Level of Evidence B) (35). However, current UpToDate guidance
concludes that CTP "clinical utility.....remains to be established" and that
"the use of this technique as a monitoring tool may be limited by risks of
recurrent dye loads and radiation exposure" (17), as well as by lack of
standardized methodology (18). The 2015 AHA/ASA guidelines for the management
of spontaneous intracerebral hemorrhage has no mention of CTP (36), nor does
UpToDate (37). Regarding trauma and oncologic indications, there appears to be
insufficient evidence to support routine use of CTP.

In summary, we consider the concordant level I evidence of a large clinical
benefit after CTP imaging (using automated post-processing software
algorithmic analysis) in AIS secondary to LVO, to assist in late EVT
eligibility determination per AHA/ASA guidelines, medically reasonable and
necessary. Other stroke or non-stroke indications lack level I evidence and
are not considered medically reasonable and necessary at this time.