PWI maps are derived from the signal intensity change caused by the passage of contrast agent through the capillary bed and reflect several aspects of cerebral perfusion. Dynamic susceptibility contrast (DSC)-PWI is based on repetitive T2*-weighted acquisitions typically performed every 1.5 seconds.
Figure 3.B.1. Technical aspects of DWI. Two diffusion sensitizing magnetic field gradients are added to a T2-weighted echo-planar imaging sequence (a). On DWI source images the signal is reduced if diffusion is measured in the direction of a tract (corpus callosum;
b) and increased if measured perpendicular (c). On postprocessed isotropic DWI (d), healthy tissue such as the right hemisphere shows low contrast between gray and white matter. Acute infarction is hyperintense on DWI (d; left caudate and lentiform nucleus and the insular cortex). The apparent diffusion coefficient (ADC) is reduced in the infarcted tissue (e). On DWI there is a much stronger contrast between affected and healthy tissue compared to conventional FLAIR images (f).
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The signal drop during contrast passage can be post-processed to maps of cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), time to bolus arrival or bolus peak, Tmax using delay corrected or uncorrected algorithms.
Results can be expressed as relative (to unaffected hemisphere) or absolute values and different software provide different results for the same parameter maps [72]. Both the volume and the severity of the initial perfusion deficit are associated with the growth of the initial DWI lesion at follow-up imaging.
The penumbra in acute stroke patients has been defined as brain tissue with loss of electric activity and potential recovery after timely recanalization of the occluded artery. It is widely accepted that extension of
hypoperfusion on PWI beyond the corresponding DWI boundary represents penumbra [73, 74]. This PWI > DWI mismatch has been used, validated, and refined in several studies [74–76]. Such baseline MRI findings can identify patients that are likely to benefit from reperfusion therapies and can potentially iden-tify subgroups that are unlikely to benefit or may be harmed [77, 78]. This concept is now being used for patient selection into clinical trials, both within and beyond established time windows.
Systematic analysis of DWI and PWI patterns in the non-randomized DEFUSE study [77] and post-hoc analyses [65] have allowed the identification of patients presenting at high risk for bleeding and poor outcome despite thrombolysis (“malignant” profile): presence of
Figure 3.B.2. Multimodal MR-based imaging in a patient with a right MCA stroke. SWI (a) shows a mild cortical hemorrhagic transformation which is a contraindication for intravenous thrombolysis. A typical mismatch pattern can be seen with scattered DWI lesions on DWI (b) and ADC (c) and perfusion deficit on MTT map (e). MRA (f) shows MCA occlusion on the right. Thrombolysis was withheld with respect to the hemorrhagic transformation of one cortical infarction (a) and the different ADC values (c) and FLAIR (d) signal intensities indicating a timely distribution.
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>100 ml DWI lesion and/or Tmax 8 seconds delay perfusion. Similarly, the initial mismatch definition of a perfusion deficit of 10 ml or more and 120% or more of the DWI lesion was then refined to “target mismatch,” i.e. a core <70 ml, a significant hypoperfu-sion of <100 ml, and a mismatch ratio of 1.8.
In patients who had reperfusion after thrombolysis, only the ones with an initial “target mismatch” had a more favorable clinical outcome.
In the post-hoc analysis of the randomized EPI-THET thrombolysis trial, target mismatch patients who were thrombolyzed had less infarct growth [79]. In a similar post-hoc analysis of the phase II and III trials with the fibrinolytic substance desmote-plase a perfusion deficit exceeding DWI lesion of at least 60 ml identified treatment responders [80]. Fur-thermore, fully automated assessments of perfusion deficits failed to reliably differentiate between critical ischemia and oligemia [81]. The RAPID software used in DEFUSE 2 enabled investigators to identify those patients who responded to early endovascular recanalization therapy [82].
The PWI > DWI mismatch concept has also been challenged [62, 83] on the grounds that the PWI lesion cannot discriminate reliably between benign oligemia and true penumbra, and because of noted
overestimation of the extent of infarction seen at follow-up [84].
Arterial spin labeling (ASL) has recently been introduced as new perfusion imaging technology not requiring contrast agent. Blood labeled with a radiofrequency pulse can be used as an endogenous contrast agent. Zaharchuk et al. [85] and Bokkers et al. [86] compared ASL-DWI mismatch to DSL perfusion-DWI mismatch. They found moderate agreement between the two methods, and called for further studies. Niibo et al. [87] found good correl-ations of ASL values with traditional MRI core and penumbra thresholds.
Based on these very recently published studies, ASL should be focused on patients with contraindi-cations to gadolinium contrast agents. Ongoing clin-ical development of ASL technology and further improvement concerning robustness and accuracy in stroke imaging can be anticipated.
The extension of hypoperfusion on perfusion imaging (PWI) beyond the corresponding DWI boundary represents penumbra. This PWI>DWI mismatch has been used to identify patients that are likely to benefit from reperfusion therapies.
Arterial spin labeling (ASL) enables imaging of perfusion without contrast agent. Blood labeled with a Figure 3.B.3. Value of FLAIR and T2* images in vessel pathology. In a patient with distal MCA occlusion there is a hyperintense vessel sign of the MCA and its branches on FLAIR (white circles on two left images) indicating slow flow. There is a hypointense thrombus sign on a T2*-weighted image in the distal MCA. The thrombus causes a blooming artifact that is larger than the diameter of the affected vessel (white circle on the right).
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radiofrequency pulse can be used as an endogenous contrast agent. ASL should be focused on patients with contraindications to gadolinium contrast agents.