Motivation

Vessel phantoms are essential for investigating and improving new imaging techniques such as Magnetic Particle Imaging (MPI) and for comparing them with established imaging modalities such as Magnetic Resonance Imaging (MRI). Therefore, the phantoms should ideally provide realistic geometry and dimensions, surfaces properties as well as the correct mechanical response to pulsatile blood flow conditions compared to human vessels. In this work a description of how to obtain realistic and flexible vessel phantoms is given. The flow dynamics in an aneurysm phantom are measured and visualized using MPI, MRI and optical transmission.

Methods

A realistic vessel phantom of an aneurysm was extracted from CT data. The resulting voxel-model was converted into a mesh surface and then prepared for 3D printing by hollowing and slicing. The model was 3D printed with an SLA printer using flexible resin to emulate the flexibility of human blood vessels¹. Following the proposed description an aneurysm phantom was fabricated. The flow dynamics in the phantom were visualized and analyzed with three imaging modalities: MPI, MRI and optical transmission.

MPI is a tracer-based radiation-free imaging technique exploiting the nonlinear response of magnetic nanoparticles to dynamic magnetic fields and enables real-time visualization of tracer distribution in 3D^2,3.

For MRI, a 4D flow Fast Low Angle SHot (FLASH) sequence was used which combines time-resolved 3D spatial with 3D velocity encoding⁴. The measurements were performed with a Siemens Magnetom 3T Vida.

For optical transmission, a CCD camera in slow-motion mode with 100fps was used.

Results and discussion

All imaging modalities show good agreement and hence provide complimentary information on the flow field and tracer distribution. MPI and the optical visualization show that a large part of the bolus initially passes the aneurysm, partly swirls into the phantom, and stays longer in the aneurysm. The MRI-measured flow dynamics are visualized with streamlines that show vortices in agreement with the optical visualization.

When comparing different duct angles an increase in the inflow from vessel into the aneurysm with decreasing angle was observed which is in agreement with literature⁵ and emphasizes the importance of correct geometries for realistic flow conditions.

Conclusion

A description of how to obtain realistic vessel phantoms is presented. The flow dynamics in an aneurysm were measured and visualized with MRI, MPI and quantitatively compared to optical visualization. MPI showed that it is possible to visualize flow in real-time. The measured flow dynamics are quantitatively in good agreement.

Image/Figure:

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Image/Figure Caption:

(1) Time series of real-time MPI bolus tracking in an aneurysm phantom. (2) Streamline representation of flow in an aneurysm acquired with MRI. (3) Time series of CCD camera images of pulsatile flow (0.6s pause, 0.2s flow) in an aneurysm and reference phantoms to convert measured intensities to bolus concentration.
(5) shows bolus concentration in dependency of time for four different positions in the aneurysm phantom as indicated in subfigure (4).

Author