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The principle of optimal patient selection by imaging confirmation of the relevant pathology was first demonstrated by the results of the intra arterial pro-urokinase stroke study, PROACT II. Four major factors are hypothesized to predict tissue response and clinical efficacy in stroke trials: time to treatment, the salvageable tissue-at-risk, the relevance of the patient sample to the treatment and the intrinsic effectiveness of the therapeutic strategy. In using MRI as a selection criterion, the goal would be a sample selection based upon a positive imaging diagnosis of a pathology rationally linked to the drug's mechanisms of action. MRI measurements of lesion volume acutely and chronically have proven to be a marker of clinical severity. MRI is increasingly used as a selection tool and an outcome measure in stroke trials, reflecting recognition that direct pathophysiological imaging may provide a more rational approach to stroke therapeutics.
Stroke is a leading cause of death in Western countries, with a mortality rate higher than most forms of cancer and now the commonest cause of long-term adult disability. Stroke diagnosis and management were revolutionized by the widespread introduction of computed tomographic (CT) scanning in the 1970s. CT scanning sensitively excludes cerebral hemorrhage, but early ischemic changes can be subtle. In the first few hours after stroke onset, when acute therapies such as thrombolysis are being considered, CT is often normal, although acute ischemic changes have become better recognized in recent years. Conventional magnetic resonance imaging (MRI) became widely available in most countries a decade after the advent of CT scanning, but has had a limited role in stroke diagnosis and management. Although MRI provides far better imaging of posterior fossa structures and facilitated non-invasive angiography (MRA), its sensitivity in acute stroke is not much better than CT. Other functional imaging techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) have been valuable research tools, but have not been of routine clinical use in the management of stroke.
Since the 1990s, the increasingly widespread availability of echoplanar MRI technology facilitated the introduction of diffusion-weighted imaging (DWI), perfusion imaging (PWI) and magnetic resonance spectroscopy (MRS). Diffusion-weighted imaging allows the hyperacute evaluation of the ischemic core within minutes of stroke onset and the distinction between acute and chronic ischemic lesions.
Magnetic resonance imaging provides non-invasive information about the brain's blood flow, water movement and biochemical abnormalities following stroke, and advances in magnetic resonance imaging (MRI) are transforming the investigation and treatment of cerebrovascular disease. Echoplanar techniques with diffusion and perfusion weighted imaging, together with developments in magnetic resonance spectroscopy and angiography, are replacing CT scanning as the diagnostic modality of choice. In this profusely illustrated book world leaders in these technologies review the scientific basis and clinical applications of MRI in stroke. It will appeal to a broad readership including stroke physicians, neurologists, neurosurgeons, rehabilitation specialists, and others with a clinical or research interest in cerebrovascular disease.
The disappointingly slow progress in developing effective therapies for ischemic stroke has led to a re-evaluation of the strategies for stroke drug development and the methods used in clinical trials. Magnetic resonance imaging (MRI) techniques have been proposed and have begun to be used in stroke trials as a means of optimizing patient selection and as a direct measure of the effect of treatments on the brain.
One objective in all clinical trials is the selection of a sample sufficiently homogeneous to reduce the statistical variance of the data, thereby optimizing the sensitivity of the design to detecting a therapeutic response, while remaining representative of the population of interest. Ischemic stroke trials have traditionally sought to limit the range of disease studied according to one or more of several dimensions, such as clinical severity at the time of enrollment, exclusion of non-ischemic causes for the clinical syndrome, lesion location and vascular territory, stroke mechanism and comorbidities. In the modern era of stroke clinical trials these dimensions have been assessed by clinical criteria at the bedside usually aided by the exclusion of cerebral hemorrhage or other non-ischemic pathology by non-contrast computed tomography (CT) scan as the only imaging tool required.
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