Harmonic imaging--a new method for the sonographic assessment of cerebral perfusion.

In this review, methodological aspects of cerebral perfusion imaging with ultrasound signal enhancing agents are described. The various experimental bases, contributing to the understanding of the phenomena are summarised and the resulting human investigation techniques are illustrated. By means of harmonic imaging technology, human cerebral perfusion can be depicted as a two-dimensional scan. The two major principles of contrast measurement are analysis of the bolus kinetics and analysis of the refill kinetics. Using the bolus method, hypoperfused areas in stroke patients can be visualised and parameter images of wash-in and wash-out curves can be generated off-line. The recently developed theory on the refill kinetics of UCA enables us to calculate quantitative parameters for the description of the cerebral microcirculation, being less affected by the depth dependence of the contrast effect. These parameters, too, can be visualised as parameter images. The ultrasound methods described in this article represent new minimal-invasive bedside techniques for analysing brain perfusion. Although their development is still in an early state, the potential of these ultrasound technologies to compete with perfusion-CT, perfusion-MRI or single-photon emission computed tomography in the diagnostic arsenal of brain imaging techniques is becoming evident.