Session: P1G

ity in medicine. The measurement of vessel diameters can be done with conventional B-mode imaging systems, which work at frame rates up to 100 Hz. Furthermore, there are special systems available, which can track vessel walls very precisely using the phase of signals that are sent at frame rates up to several thousand Hertz. Though, such systems are usually not able to provide the examiner with 2D images of the object. With respect to brachial artery flow-mediated vasodilatation (FMD), which is frequently used as a measure of endothelial function, it is necessary to observe diameter changes of small arterial vessels non-invasively for several minutes at a high resolution. In the past, the diameter had to be measured manually in tedious postprocessing of ECGgated image sequences. We developed a system composed of a Siemens Omnia ultrasound system with a VF13-5 transducer (9 MHz center frequency) and a personal computer, that is capable of calculating vessel diameter changes with an accuracy below the wavelength of the ultrasound system in real-time at a frame rate of 27 Hz. We implemented a two-dimensional active contour model using the Viterbi-algorithm and a phase-sensitive vessel wall tracking algorithm, in order to guarantee both, geometric information and accuracy. We could verify experimentally that the accuracy of the measurements is below 1 mm. Results from clinical measurements of carotid and brachial arteries show that arterial pulsations below 0.1 mm can be visualized reliably over several minutes. Furthermore, we can present examples from clinical FMD measurements, where the pulsation of the brachial artery is visible as well as an increase in arterial diameter after the deflation of a forearm cuff, which was used to produce an ischaemia for 5 minutes. With this system we want to find out, if FMD is suitable for an individual assessment of the risk for cardiovascular diseases.