Harmonic imaging

In this review, methodological aspects of cerebral perfusion imaging with u ltrasound contrast agents are described. The various experimental concepts contributing to an understanding of the phenomena are summarised and the re sulting investigation techniques on patients are illustrated. By means of h armonic imaging, human cerebral perfusion can be depicted as a two-dimensio nal scan. The two major principles of contrast measurement are analysis of the bolus kinetics and analysis of the refill kinetics. Using the bolus met hod, hypoperfused areas in stroke patients can be visualised and parameter images of wash-in and wash-out curves can be generated off-line. The recent ly developed theory on the refill kinetics of UCA enables us to calculate p arameters for describing the cerebral microcirculation, as they are less af fected by the depth dependence of the contrast effect. These parameters can also be visualised as images. The ultrasound methods described in this art icle represent new minimally invasive bedside techniques for analysing brai n perfusion. Although their development is still in an early stage,the pote ntial of these ultrasound technologies to compete with perfusion CT, perfus ion MRI,and SPECT in the diagnostic arsenal of brain imaging techniques is becoming evident.