Principles and Practical Application of Brain MRI in Acute Ischemic Stroke

Fig. 10.1

Diffusion-weighted image lesion pattern according to stroke mechanism. Intracranial atherosclerotic stenosis (a), extracranial atherosclerotic stenosis (b), cardioembolism (c), and aortic arch atherosclerosis (d). Reproduced by permission of Journal of Stroke (Kim BJ, et al. J Stroke. 2014;16:131–45)

Specific lesion patterns are prone for a specific mechanism of stroke. For example, aortic arch embolism has a higher propensity of causing left hemisphere stroke, whereas cardioembolic stroke demonstrates a higher propensity to be located at the right hemisphere. Embolic stroke associated with patent foramen ovale (PFO) is dominantly found from the posterior circulation territory, which reflects the increased blood flow through the posterior circulation provoked by Valsalva maneuver, which also increases the right-to-left shunt, simultaneously (paradoxical embolism) [4].

The volume of initial DWI lesion highly predicts the functional outcome of ischemic stroke. DWI volume less than 30–40 ml was associated with good outcome at 90 days after stroke. Benefit from intravenous thrombolysis was seen only with DWI lesions up to 25 ml. However, DWI >70 ml also demonstrated benefit from intravenous thrombolysis when the artery was recanalized. DWI lesion volume >100 ml is used as an indicator of malignant profile which predicts poor outcome.

10.2 Perfusion-Weighted Image: Imaging the Penumbra

Evaluation for the salvageable tissue is critical, selecting patients for reperfusion therapy in hyperacute stage of stroke. After a cerebral blood vessel is occluded, the core of the area develops infarction rapidly (ischemic core). However, though the neuronal function of the surrounding part is suspended, minimal blood flow supplied by collateral circulations maintains the viability of neurons. When the blood supply is restored, the neuronal activity of this area may be recovered. This conceptual area is named “penumbra,” and when the difference between ischemic core and penumbra is large, it is regarded that the salvageable tissue is large, and the benefit of treatment can be large.

The perfusion status of the brain can be evaluated semiquantitatively by the PWI. The passage of contrast alters the local magnetic field and the signal intensity decreases rapidly in the surrounding brain tissue, due to the paramagnetic effect of the contrast. Echo-planar image technique is used to measure the signal intensity every second and voxel by voxel during the first 1 min after the injection of contrast. Finally, the time-concentration curve can be obtained from each voxel and the curve can be deconvolved by the arterial input function. The deconvolved curve is used to measure various parameters such as cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), time to peak (TTP), and Tmax (Fig. 10.2):

CBF: Blood supply in a given time period to the brain tissue. CBF is usually taken as the curve height at Tmax of the deconvolved curve. CBF most directly represents the final viability of the infarcted tissue.
CBV: Whole blood quantity within the target area. CBV is presented as the area under the deconvolved curve. Initially, CBV increases by the dilation of blood vessels to maintain CBF in the infarcted tissue, but which decreases as the infarction evolves.
MTT: Average time required for the blood to enter the cerebral artery and maintain inside the tissue. MTT can be calculated by the following formula: [MTT = CBV/CBF]. MTT demonstrates the widest range of perfusion deficit and, therefore, may overestimate the true penumbra.
Tmax: Time needed for the tissue residue function to reach maximum. Tmax is regarded as the most appropriate perfusion parameter representing the penumbra.

Fig. 10.2

Time-concentration curve at tissue level and after deconvolution. Various perfusion parameter maps are shown (TTP time to peak, MTT mean transit time, CBF cerebral blood flow, CBV cerebral blood volume). Reproduced by permission of Journal of Stroke (Kim BJ, et al. J Stroke. 2014;16:131–45)

Though the optimal cutoff value of Tmax is still under debates, Tmax >6 s delay is most widely used to define the penumbra. PWI lesion visually exceeding 1.2–1.8 times of DWI lesion is considered as PWI-DWI mismatch positive. Patients with PWI-DWI mismatch demonstrated a more favorable outcome after thrombolysis than those without. However, using PWI-DWI mismatch solely in patient selection for thrombolysis failed to demonstrate a beneficial effect [5].

10.3 Gradient Echo or Susceptibility-Weighted Image: Microbleed and Clot Imaging

Cerebral microbleeds (CMBs) are small (2–10 mm) hypointense lesions observed from GRE or SWI, which are most often located at the deep structures (deep CMBs; Fig. 10.3a) or at the cortico-subcortical junctions (lobar CMBs; Fig. 10.3b). Deep CMBs have been closely linked to traditional vascular risk factors similar to small vessel disease (age and hypertension), whereas multiple lobar CMBs have been shown to be related to cerebral amyloid angiopathy or other degenerative diseases [6]. These conditions are bleeding prone, and large numbers of CMBs are associated with poor outcome of stroke. The risk of ICH after intravenous thrombolysis increases with the number of CMB in a dose-response relationship manner. The risk of ICH is higher in cases with lobar CMBs. But yet, it is not conclusive weather CMBs should influence the decision-making in thrombolysis.

Fig. 10.3

Image findings from gradient echo. Deep cerebral microbleeds (a), lobar cerebral microbleeds (b), long susceptible vessel sign (c), and tortuous susceptible vessel sign (d). Reproduced by permission of Journal of Stroke (Kim BJ, et al. J Stroke. 2014;16:131–45)

Ferromagnetic objects, such as red blood cells, distort the ambient field causing magnetic susceptibility artifacts. These artifacts enhance the detection of red blood cell-rich red thrombi clots, which are more common in cardioembolic strokes. Well-organized long-standing platelet-rich white thrombi are more resistant to thrombolytic therapy than fresh, fibrin-rich red thrombi. Therefore, susceptible vessel sign (SVS) itself may predict a higher possibility of recanalization. Several other factors of the SVS should be considered predicting the recanalization: