Arterial input function measurements using radiotracer and microbubbles: preliminary results

The main objective of the study is to investigate the possible use of the Doppler signal from microbubbles (Levovist) to measure the arterial input function in dynamic positron emission tomography (PET). We have measured the dynamics in a fluid (water or plasma equivalent) of radioactive tracers and microbubbles contrast agents (MB) experimentally. The experimental system allows us to simulate the dispersion of these tracers in large arteries, and, using an appropriate phantom, the dispersion in the heart and lungs. Time-activity curves for Tc-99m were obtained using an NaI(Tl) probe connected to a multiscaler system. A Doppler probe was used to measure scattering from the microbubble. The data were processed using MATLAB. The measurement with the radioactive tracer shows a reasonable agreement with theoretical models. Noise present in the Doppler signal, probably caused by the pump, has been reduced using a denoising procedure in the wavelet domain based on a Daubechies wavelet function. Differences in tracer kinetics between Tc-99m and MB found in water are significantly reduced in plasma-equivalent fluid. We conclude that the two methods are comparable and that this method might provide a noninvasive way of measuring arterial input functions for PET.