Imaging Collaterals in Acute Stroke (iCAS)

Intravenous tissue plasminogen activator (IV tPA) is the standard of care treatment for
stroke patients when treatment can be initiated within 3 hours after symptom onset.
Unfortunately, only a very small fraction of stroke patients is treated with tPA, and
benefits from tPA. This is the result of two main limitations of tPA therapy.

First, the proportion of patients that can be treated with iv tPA is small. Because of the
narrow three hour time-window during which tPA is effective, only patients who arrive to the
hospital early on can be treated with tPA. Most stroke patients, however, arrive at the
hospital after the three-hour time-window and are therefore excluded from tPA treatment.
Second, stroke patients who receive tPA do not always benefit because the treatment does not
restore blood flow in all patients.

Patients with persistent blood vessel occlusions and no improvement in their clinical
condition after receiving tPA or those arriving at the hospital outside the 3 hour time
window routinely undergo mechanical clot removal to open an occluded blood vessel in the
brain.

Mechanical clot removal increases the percentage of stroke patients who achieve
recanalization, and as a result may increase the proportion of patients who have good
clinical outcomes. However it is unclear for which stroke patients mechanical thrombectomy is
most suitable. Although effective at removing blood-clots, it appears that mechanical clot
retrieval is not beneficial for all patients. Whereas some patients benefit, others
experience no effect, and yet others are likely harmed by mechanical clot retrieval. In order
to avoid harm and maximize benefit it is important to know, prior to initiation of the
mechanical clot retrieval procedure, if the procedure is likely to result in a clinical
improvement. We hypothesize that the response to mechanical clot retrieval can be predicted
based on characteristics of an MRI scan obtained just prior to the retrieval procedure. In
particular, this study will assess whether a new non-contrast MR imaging method, arterial
spin labeling (ASL), which measures cerebral blood flow and can give insight into collateral
flow, is effective at predicting who will benefit most from treatment. The investigators hope
to learn if these new MRI techniques can help identify which patients are most likely to
benefit from mechanical clot removal after receiving tPA.

Inclusion Criteria

1. Age 18 and older

2. Clinical diagnosis of ischemic stroke and an associated score on the NIHSS of 5 or
more points

3. Planned to undergo (or has undergone) intra-arterial (IA) thrombectomy and/or
thrombolysis for acute hemispheric stroke. (Either as primary therapy or as adjuvant
therapy following intravenous tPA treatment)

4. Planned to undergo or has undergone an MR brain scan including MR perfusion imaging
(ASL & bolus PWI) and MR angiography of the circle of Willis prior to IA therapy

5. Intra-arterial thrombectomy can be started within 90 minutes of completion of the MR
perfusion scan and within 24 hours of symptom onset (Start of IA therapy is defined as
the time of insertion of the femoral artery sheath; Time of brain scan is defined as
the time that the scan is completed)

6. Able to obtain informed consent

Exclusion Criteria

1. Any pre-existing illness resulting in a modified Rankin Scale Score of 2 or higher
prior to the qualifying stroke

2. Creatinine clearance < 40 ml/min based on the NIDDK four-variable MDRD method
(non-weight based)

3. Documented allergy to MR contrast agent

4. MRI contraindications (pacemaker, etc.)

5. Pregnancy