Stroke Overview

5 Stages of Intraparenchymal Hemorrhage

Stage Time (Range) Blood Products T1 T2

Hyperacute < 24 hours Oxyhemoglobin Isointense Bright

Acute 1-3 days (hours to days) Deoxyhemoglobin Isointense Dark

Early subacute > 3 days (days to 1 week) Intracellular methemoglobin Bright Dark

Late subacute > 7 days (1 week to months) Extracellular methemoglobin Bright Bright

Chronic > 14 days (≥ months) Hemosiderin Dark Dark

The effective therapeutic window for the posterior circulation

is thought to be longer than the 3-6-hour window, but the

exact time is variable and depends on collateral circulation.

Therefore, patients with vertebrobasilar thrombosis are

evaluated individually for risk vs. benefit of IA thrombolysis or

thrombectomy.

Ischemic Penumbra

Ischemic stroke results in a core of tissue that has undergone

irreversible injury. The ischemic penumbra is the area of brain

that may be salvageable with appropriate therapy. The

penumbra typically surrounds the ischemic core and is

supported by collateral circulation.

The ischemic penumbra can be identified by a combination of

MR diffusion (DWI) and perfusion (PWI). DWI is the most

reliable estimate of the ischemic core and generally correlates

with irreversible injury. However, with early reperfusion

following thrombolysis, some reversal of DWI can be

observed. PWI evaluates the presence of a penumbra. With

MR, the mismatch between the DWI and PWI defines the

penumbra. This model provides a practical means to estimate

the ischemic penumbra. In general, if there is no

diffusion/perfusion mismatch, therapy may be ineffective.

With the newer CT perfusion techniques, an ischemic

penumbra may also be measured with CT.

With the urgency of acute stroke, MR may be impractical.

However, with new faster MR protocols and the superiority of

MR to CT in detecting small vessel ischemia and brainstem

ischemia, MR may be a preferred technique.

CT Perfusion

Cerebral perfusion refers to the tissue-level blood flow in the

brain. This flow is evaluated by 3 main parameters at CT

perfusion (pCT): Cerebral blood flow (CBF), cerebral blood

volume (CBV), and mean transit time (MTT).

CBF is defined as the volume of blood moving through a given

unit of volume of brain per unit time. CBF uses units of

milliliters of blood per 100 g of brain tissue per minute.

Studies suggest that CBF is a reasonable marker for the

ischemic penumbra.

CBV is defined as the total volume of blood in a given unit of

volume of brain. This includes blood in the tissues as well as

blood in the large-capacitance vessels, such as arteries,

arterioles, capillaries, venules, and veins. CBV uses units of

milliliters of blood per 100 g of brain tissue. Some studies

suggest that pCT-acquired CBV is a reasonably reliable marker

of the ischemic core.

MTT is defined as the average of the transit time of blood