AN MRI CONTRAST AGENT FOR ULTRASONIC IMAGING? A BACKSCATTERING SIMULATION

Problemstellung: Superparamagnetic iron oxide (SPIO) nanoparticles coated by Carboxydextran (Ferucarbotran) are used as contrast agents for magnetic resonance imaging (MRI). These agents, like Resovist®, are used for the detection of focal liver lesions since they are only aggregated in Kupffer cells located in healthy liver tissue. Recently, it has been shown in vivo that these agents are detectable in liver sonography, too. We investigate their acoustical behaviour based on linear acoustic scattering theory for inhomogeneous media.

Methoden: We derived an acoustic backscattering model using a spatial autocorrelation for a two-component medium proposed by Yagi. The Carboxydextran coat of Ferucarbotran is degraded by the Kupffer cells and the SPIO cores are clustered in the lysosomes of the Kupffer cells. The mean size of the clustered SPIO cores was derived from laser microscopic studies, conducted by Tanimoto et al., of equivalent fluorescent latex beads aggregated in Kupffer cells. These SPIO filled lysosomes were modelled as randomly distributed scatters and the backscattering coefficient was simulated. Moreover, we simulated the acoustical backscatter coefficient of non-aggregated Ferucarbotran in liver tissue. The results were compared to the mean backscatter coefficient of healthy liver tissue.

Ergebnisse: For SPIO clusters in Kupffer cells of 7µm size the backscatter coefficient of liver tissue raises approximately by 1.7 dB compared to a typical backscatter coefficient of liver (approximately 1.61*10–3cm-1 Sr-1 at a 4MHz insonifying frequency). We also found that non-aggregated Ferucarbotran particles induce negligible backscattering.

Schlussfolgerungen: We find that the clustering of the SPIO cores of Ferucarbotran in the lysosomes of Kupffer cells might be the reason for the visibility of MRI contrast agents consisting of Ferucarbotran in clinical ultrasound images since the backscatter coefficient is increased by approximately 1.7 dB. We are currently investigating the usage Finite Element Methods for the backscattering simulation and carrying out in vitro experiments.