[Harmonic imaging. A new method of ultrasound imaging of brain perfusion]

In this review, methodological aspects of cerebral perfusion imaging with ultrasound contrast agents are described. The various experimental concepts contributing to an understanding of the phenomena are summarised and the resulting investigation techniques on patients are illustrated. By means of harmonic imaging, 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 parameters for describing the cerebral microcirculation, as they are less affected by the depth dependence of the contrast effect. These parameters can also be visualised as images. The ultrasound methods described in this article represent new minimally invasive bedside techniques for analysing brain perfusion. Although their development is still in an early stage, the potential of these ultrasound technologies to compete with perfusion CT, perfusion MRI, and SPECT in the diagnostic arsenal of brain imaging techniques is becoming evident.