DWI in Clinical Practice

The detection of infarcted tissue and the delineation of hypoperfused tissue in a penumbral state that is at risk of infarction, is of paramount importance to guide the treatment of acute stroke. Computed tomography (CT) remains the most frequently applied image modality because of its broader availability. However, CT is mainly useful for the exclusion of intracerebral hemorrhage and rather insensitive for the detection of acute cerebral ischemia (Fiebach et al., 2002; Saur et al., 2003). CT detects net water increases, whereas DWI is sensitive to extra- to intracellular water shifts (Kucinski et al., 2002). Net water increases follow a slower time course (Schuier and Hossmann, 1980), while diffusion slowing is a process that occurs immediately after ischemic cell depolarization and is detected at about 60 s after cardiac arrest in a rat model (Huang et al., 1997; de Crespigny et al., 1999, 2001).

DWI in acute stroke has therefore become popular, since changes in proton self-diffusion are an early indicator of alterations in cellular homeostasis (Moseley et al., 1990a). Early experimental studies show, that DWI detects both the core and the penumbra of the evolving infarction, but is not able to differentiate between the two (Kohno et al., 1995). This is in line with further animal studies which showed that ischemic areas with ADC decreases may normalize, if the duration of the ischemia is not too long (Li et al., 2000), although ADC normalization does not necessarily indicate salvaged tissue (Neumann-Haefelin et al., 2000; Ringer et al., 2001).

Recent clinical studies have shown that ADC normalization occurs frequently in acute stroke patients with rapid recanalization, and that ischemic brain tissue with initially decreased ADC, especially within 3 h after stroke onset, may include ''tissue at risk'' (Fiehler et al., 2004). These observations of ADC normalizations have influenced the simplistic concept of the DWI/PWI mismatch as an indicator of irreversibly damaged tissue (DWI lesion) and the tissue-at-risk-of-infarction, the penumbral tissue. Additionally, the idea that the severity of the ADC decrease might be linearly linked to the likelihood of tissue recovery was refuted by studies which showed that even severely decreased ADC values may normalize in human stroke (Fiehler et al., 2002). Clearly, ADC normalization depends on the duration and severity of ischemia rather than the absolute value, as has been shown in previous experimental studies (Hoehn-Berlage et al., 1995; Kohno et al., 1995).

Although an ADC decrease in cerebral ischemia is not an absolute indicator of irreversible tissue damage, the DWI/PWI mismatch concept earns merit as an easy-to-apply method for the delineation of tissue-at-risk in acute ischemic stroke patients. Early detection of these alterations can dramatically impact treatment decisions and the therapeutic outcome for stroke victims (Lansberg et al., 2000; Thomalla et al., 2003). This is mainly because DWI is able to detect acute ischemic lesions within the ''window of opportunity'' for advanced stroke therapies, and promises to help reduce the extent and severity of ischemic damage in acute stroke victims (Rother, 2001; Rother et al., 2002). The clinical application has recently been described in a review of centers that routinely perform MRI screening of acute stroke patients before thrombolytic therapy (Hjort et al., 2005). An important

Stroke Mri Dwi

Fig. 3.52. Two acute stroke patients (upper and lower rows, respectively) with onset of symptoms as early as 1.5 and 2 h before MR imaging. Both patients show a normal T2-weighted image, a diffusion slowing in DWI and ADC, a perfusion deficit in the territory of the middle cerebral artery (MCA), and a vessel occlusion in magnetic resonance angiography (MRA). While the first patient has a huge mismatch territory (PWI-DWI) and responds favorably to thrombolytic recanalization of the occluded MCA, the second patient shows a large diffusion lesion with negligible mismatch volume due to a carotid artery occlusion. The first patient recovered completely and had only a small striato-capsular infarction on follow-up MRI after 24 h, but the second patient developed malignant MCA infarction.

Fig. 3.52. Two acute stroke patients (upper and lower rows, respectively) with onset of symptoms as early as 1.5 and 2 h before MR imaging. Both patients show a normal T2-weighted image, a diffusion slowing in DWI and ADC, a perfusion deficit in the territory of the middle cerebral artery (MCA), and a vessel occlusion in magnetic resonance angiography (MRA). While the first patient has a huge mismatch territory (PWI-DWI) and responds favorably to thrombolytic recanalization of the occluded MCA, the second patient shows a large diffusion lesion with negligible mismatch volume due to a carotid artery occlusion. The first patient recovered completely and had only a small striato-capsular infarction on follow-up MRI after 24 h, but the second patient developed malignant MCA infarction.

step towards the future application of MRI in acute stroke management is the use of the DWI/PWI mismatch concept to select patients in an extended time window at 3 to 9 h after stroke onset in order to apply a new thrombolytic agent (Hacke et al., 2005).

The selection of stroke patients eligible for thrombolysis in an extended time window, and in patients with unknown stroke onset, is an emerging application of stroke MRI (Schellinger et al., 2003). The combination of DWI, PWI and MR angiography (MRA) - termed ''stroke MRI'' - has developed towards a clinical tool that is utilized by many stroke centers to define ''tissue at risk of infarction'' where there is a high likelihood of surviving the ischemic insult in case of timely reperfusion. Two typical examples of stroke MRI in acute stroke patients are illustrated in Figure 3.52.

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