MARVIS DEVICE PROTOTYPES
MaRVis Technologies GmbH has developed a series of guidewire prototypes exemplifying the huge potential of the MaRVis platform technology. The portfolio comprises standard and stiff (0.032 and 0.035 inch) as well as micro-guidewire (0.012 inch) prototypes. Further, a catheter sample has been produced by newly developed manufacturing methods.
A selected guidewire prototype has been described in more detail in the patent application WO 2009/141165. The geometry of this MaRVis guidewire prototype is designed with one central MaRVis rod and six radially arranged peripheral MaRVis rods. In this case the central MaRVis rod has a larger diameter than the peripheral MaRVis rods. This type of arrangement of the MaRVis rods allows an appropriate bending stiffness in combination with the required flexibility for efficient steerability of the guidewire in the blood vessels. At the same time, this guidewire has a diameter of 0.81 to 0.88 mm (0.032 to 0.035 inch), equal to common commercial guidewires comprising a metallic core. Embedding of the MaRVis rods in the outer envelope polymer leads to high stability of the guidewire and good transmission of forces across the medical device.
Variation in the geometry of the guidewire as well as the MaRVis rods and polymers used creates a multitude of guidewires with mechanical properties specifically adaptable by controlled and target-oriented design. MaRVis has validated this concept by development and characterization of a number of prototype guidewires exhibiting different combinations of axial stiffness, torsion capability, general stability and flexibility. These contain modified MaRVis rods or a different number of MaRVis rods whereby the properties of the guidewire are a result of the interaction between raw materials used (glass fibers and epoxy resins), the number and geometric arrangement of the individual rods and the mechanical properties of the envelope polymer.
MaRVis guidewires doped with ferromagnetic and paramagnetic micro- and nanoparticles (amongst others iron, iron oxide and tungsten) were successfully tested in magnetic resonance and X-ray imaging. Flexibility in the choice of various metallic marker particles allows target-oriented design of the intended MRI artifacts and adaptation to the selected procedures.
Variation of the size and concentration of marker particles leads to significantly different signals in MRI. MaRVis has identified the most suitable types of particles for obtaining stronger or weaker signals. It is well known that ferromagnetic metal particles create strong artifacts, i.e. the signals are much wider than the device itself is. Respective know how is available which particles and doping arrangements deliver which signal and/or artifact. Several examples are shown below.
Fig. 1-6: Comparison of different MaRVis rods containing different MRI marker particles (P1, P2, P3) in magnetic resonance imaging (in a water phantom) using a T2 weighted sequence. The samples were imaged in orthogonal direction (left) and in parallel (right) to the main magnetic field B0.
Fig. 1-7: Comparison of commercial Terumo guidewire with nitinol core (T1) and two MaRVis guidewire prototypes (M1 and M2) with different MRI marker particles in magnetic resonance imaging (in a water phantom) using a T2 weighted sequence. The samples were imaged in orthogonal direction (left) and in parallel (right) to the main magnetic field B0.
MaRVis has developed a doping concept based on passive negative MRI markers which strictly reduces the size of artifacts. Thereby, medical devices can be imaged with signals representing approximately their actual dimensions and not a large multiple of their diameters.
Fig. 1-8: Directed generation of more localized (L) and of broader (B) MRI signals and artifacts in test samples containing the same metal particles at the same concentration in magnetic resonance imaging (in a water phantom) using a T2 weighted sequence. The samples were imaged in orthogonal direction (left) and in parallel (right) to the main magnetic field B0.
MaRVis guidewire visualization has been tested in MRI in steady-state and flow phantoms and animal trials. Final in vivo proof-of-concept for interventional procedures has been achieved by demonstrating catheterization of the renal artery in a pig under MRI.
MRI visualization of different prototypes providing varying signal intensity has been shown in an arterial flow phantom using a basic real-time sequence (see Fig. 1-9).
Fig. 1-9: MRI visualization of guidewire prototypes in arterial flow phantom with a basic real-time sequence.
For optimal imaging of the MaRVis guidewires in MRI, the medical physics research group of Dr. Michael Bock at the German Cancer Research Center has developed a powerful new visualization technique ("MR sequence"). This combines a conventional MR image - which displays the guidewire and body tissue in grey shades - with an overlaid image, based on the specific magnetic properties of the embedded metal particles, only representing the MR guidewire in white or in coloured. Thereby a strongly improved visualization of the various guidewire prototypes has become possible. These results constitute the basis for further integrated development of marker particle doping and MR sequences.
Fig. 1-10: Imaging with optimized MR diagnostic sequences using two different parameter settings ("1 and "2") developed by Dr. Michael Bock, Dr. Axel Krafft and Alexander Brunner (German Cancer Research Center) compared to standard diagnostic settings ("standard") in a gradient echo sequence.
Fig. 1-11: Imaging of MaRVis guidewires with double echo MR sequence specifically optimized for the continuous MR marker particle doping of MaRVis rods by overlay of dephased image onto FLASH image. Developed by Dr. Michael Bock, Dr. Axel Krafft and Alexander Brunner (German Cancer Research Center) (published at ISMRM 2011 Meeting). The guidewire is represented in red.
Custom-adapted manufacturing processes have been established for production of small diameter MaRVis rods (~ 0.17 to ~ 0.27 mm diameter) and for the different MaRVis guidewire prototypes (0.31 to 0.88 mm [0.012 to 0.035 inch] diameter).