The clinical significance of diffusion-weighted MR imaging in stroke and TIA patients.

BACKGROUND AND
PURPOSE: Diffusion-weighted magnetic resonance imaging (DWI) is an advanced imaging technique that allows non-invasive evaluation of water diffusibility in brain tissue. The following report focuses on the clinical significance of DWI in stroke and TIA patients SUMMARY OF REVIEW: (1) TIA patients demonstrate DWI lesions at a rate of 1 in 6 to 2 in 3. Symptom duration, speech or motor symptoms and aetiology seem to correlate with the rate of DWI positivity. (2) In stroke patients, the DWI detection rate of ischaemic lesions is >95%. Small lesion size and location in the brainstem increase the risk of false-negative DW-images. A negative DW-image in a patient with stroke-like symptoms should stimulate the search for an alternative diagnosis. However, one half of such patients can be expected to have ischaemic stroke as the best final diagnosis. (3) Infarct age determination based on DWI characteristics is not possible in the first few hours. However, the combined interpretation of DWI-images and apparent diffusion coefficient (ADC) maps enables the distinction of infarcts ?5 day old from infarcts >10 days old. On average in DW-images alone, the hyperintense signal disappears after two months. Normalisation can occur as early as one month and as late as four and a half months. (4) DWI lesion size is a prognostic marker of stroke outcome. However, in a mixed stroke population, outcome prediction by DWI cannot replace clinical outcome scores. (5) The mismatch concept hypothesises that DWI lesions reflect irreversibly infarcted tissue that is surrounded by an area of reduced perfusion. The larger the perfusion-diffusion mismatch the more tissue is potentially salvageable, e.g., by early reperfusion. Although this concept is appealing, more recent data have shown that DWI lesions are not necessarily irreversibly damaged tissue and that perfusion abnormalities tend to overestimate the ischaemic penumbra. More recently, the mismatch between clinical stroke severity as measured with the NIH-stroke Scale Score (NIHSSS) and the volume of DWI lesions has been introduced. (6) In posterior circulation stroke, DWI lesion detection rate is significantly lower than in anterior circulation stroke. (7) DWI features provide important information about stroke aetiology. Multiple DWI lesions in more than one circulation suggest cardioembolism. However, this assignment should be restricted to DWI lesions showing the same appearance on ADC-maps. In patients with lacunar syndromes, every fourth to sixth patient can be expected to have >1 DWI lesion, indicating an embolic mechanism. Thus, DWI findings may be clinically useful to tailor the aetiological work-up, which may result in early implementation of specific treatment for secondary stroke prevention. (8) DWI may detect clinically silent ischaemic lesions after carotid interventions. A systematic review reported the rate of new DWI lesions as being significantly higher in carotid stenting patients (37%) compared to carotid endarterectomy patients (10%). As caveats, all studies included were non randomized trials. In addition, the clinical significance of these lesions is unclear. Studies, comparing the risk of silent ischaemia in carotid stenting versus endarterectomy patients and evaluating the value of DWI as surrogate marker in a randomised, prospective setting are currently under way.
CONCLUSION: DWI provides clinically useful information and has the means to improve the quality of diagnosis, treatment, and outcome prediction in stroke and TIA patients.