Four out of five strokes are ischemic infarcts. An artery is constricted and the cerebral perfusion is reduced. In the worst case, cerebral areas will die off. To assess cerebral perfusion at a stroke unit, imaging procedures, e.g. perfusion magnetic resonance imaging or computer tomography, are used. These procedures provide first-class pictures which are used by neurologists and neuroradiologists to decide on suitable therapies. However, the instruments are large, the examinations are expensive and the patients must be taken to special rooms, so that often only few measurements can be made for each patient.
Therefore, PTB together with the Center for Stroke Research Berlin and the Institute of Biocybernetics and Biomedical Engineering of the Polish Academy of Sciences in Warsaw have developed a method for the continuous monitoring of cerebral perfusion directly at the patient‘s bedside. The method is based on a near-infrared reflectometer with a time resolution in the picosecond range. Thus, the individual measurements can be taken comparatively often, e.g. half-hourly, and not only at an interval of several days, as has been the case so far.
For the measurement, short laser pulses with an average power of a few milliwatts are irradiated into the head of the patient by means of optical fibres via a special cap. The optical contrast agent indocyanine green (ICG) is applied intravenously as a bolus. The contrast agent bolus modifies the optical properties of the illuminated tissue and, subsequently, the reflected light pulses. The data analysis exploits the high time resolution of the instrument and is, therefore, especially sensitive to cortical changes. In addition, events on the diseased and on the healthy hemisphere are compared with each other. In this way, information on the bolus migration at the cortex – and therefore on the cerebral perfusion – can be derived.
The method was successfully tested at the Department of Neurology of the Charité with several patients suffering from an acute unilateral ischemic stroke. For this purpose, a functional model of the PTB measuring device, which was certified by a notified body according to the Medical Devices Act, was used. There is hope that the positive results achieved by the stroke units can be further improved due to the quasi-continuous monitoring of the cerebral perfusion of stroke patients.
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