Note: inproceedings

Abstract: Purpose: Magnetic Particle Imaging is a tracer based imaging modality that is highly sensitive, quantitative, and allows for 3D imaging in real-time [1]. Several iron-oxid based tracer materials with different coatings are available, which – depending on their coating – distribute differently in the body after intravenous injection. Rahmer et al. [2] introduced a multi-spectral reconstruction approach making it possible to distinguish different tracers and colorize them differently in an image. Recently the first multi-colored in-vivo MPI images have been presented [3] using a field-free point scanner, where the lungs and the liver were measured simultaneously with two different tracers. In this work dynamic colored MPI in-vivo images are presented. We will show that the vascular system can be visualized with a blood-pool tracer while a second tracer allows imaging the accumulation in the liver, the kidney, and the spleen. Material and Methods: For the experiment a preclinical field-free point MPI scanner (Bruker, Ettlingen) was used. A 3D Lissajous-type imaging sequence with 12 mT drive-field strength and 2 T/m selection-field gradient was applied. A custom receive chain with a gradiometric receive coil with 72 mm bore size was used [4]. The experiments were carried out using a heathy mouse model. A tube was prepared with one bolus of 15 ?L Perimag (5.6 mg/ml) followed by a second bolus of 15 ?L LS-008 (5.14 mg/ml). The boli were applied within a single intravascular injection. The imaging area was focused on the heart and the liver to visualize the distinction of the tracers. Since the blood half-live of Perimag and LS-008 are strongly different, it is expected that Perimag will accumulate after few minutes in the liver while LS-008 will stay in the blood pool. The entire measurement consisted of 100000 frames with a repetition time of 21.54 ms. The bolus injection started after 2000 frames. For improved signal-to-noise ratio, 200-fold block-averaging was applied leading to an effective repetition time of 4.3 s. Results: The results show at the beginning the injection of both tracers individually. After mixing of both tracers in the heart the density of Perimag in the circulating blood is degraded since it is accumulated in the liver and further organs. Already after 7.5 min an essential part of the tracer was observed in the liver and the spleen. An accumulation in the kidney could be observed after 10.0 min. The long circulating tracer LS-008 stays in the bloodstream and in turn allows imaging the vascular tree and blood perfusion during the Perimag accumulation. Conclusion and Outlook: Multi-colored MPI enables the simultaneous visualization of different tracer in-vivo with high spectral resolution. In the presented work the arrival and saturation times could be derived from the reconstructed dynamic multi-spectral MPI data. It was shown that different organs could be differentiated spatially and also spectrally. However, further analysis on signal leakage between both tracer materials and the changing of particle behavior in-vivo is necessary. With functionalized tracers multi-colored MPI could potentially enable imaging of tracer binding to cancer tissue in real-time while a secondary tracer can monitor the organ perfusion.