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Department of Radiology - Medical Physics

Experimental Radiology

Interventional MRI

In this research group, novel imaging concepts for MR-guided interventions are being investigated. In interventional procedures, minimally invasive access to the target organ is achieved either through the skin (percutaneous access) or via the blood vessels (intravascular access). For both access pathways MR-compatible devices are developed, optimized imaging strategies are set up to visualize the tissue morphology and the functional changes achieved during therapy, and the new technologies are tested in phantom and animal experiments.


Research Projects

Thomas Lottner, Simon Reiss, Simon Stephan, Ali C. Özen

MR-guided interventions might be beneficial for the patient, as they offer functional imaging during intervention (e.g., quantitative perfusion after stent placement in the coronary arteries). In this project we develop devices (active catheters, guidewires) and imaging methods (radial MRI) for visualizing and tracking of interventional equipment in the MR environment.

 

 

 

Successful engagement of the coronary artery with an active catheter with a loop coil on the tip of the catheter.
(click to view video)

 

References

Cooperation Partners

Grant Support

Simon Reiss, Thomas Lottner, Ali C. Özen

A large variety of both active and passive medical implants consist of conductive materials that can be safety hazards during MRI due to RF heating. In addition, the growing field of MR-guided intervention necessitates the use of active instruments which also cause potential sources of RF heating. In this project we develop novel methods to assess and reduce RF-induced heating of active and passive medical devices.

a) Measurement setup and simulation of the heating close to a coil used for active catheter tracking. b) The maximum heating at the tip of an active catheter depends on input impedance of the tracking coil which is illustrated as a color coded overlay of the Smith chart.

a) Comparison of the theoretically determined and measured electric field in the vicinity of a vascular stent. The results compare well to the temperature increase during RF induced heating measured with MR thermometry. b) Time course of the temperature increase at the tip of the stent during MR imaging.


References

Cooperation Partners

Grant Support

Katharina Schleicher, Axel J. Krafft

In MRI, labeling of interventional instruments is often realized with iron oxides which are non-toxic and which can create large signal voids even at low concentrations. Recently, iron oxides have been used as markers in MR-safe intravascular guidewires – unfortunately, the markers appear as dipole artifacts in the MR image. To overcome the orientation-dependence of the dipole artifact, a dedicated radial MRI technique was developed which varies the echo time (TE) as a function of the orientation of the radial spoke against the direction of the static magnetic field B0.

Top: Fiber-based guidewire with iron oxide markers that cause field distortions with a characteristic dipole shape. Bottom: Conventional Cartesian or radial sampling leads to strong variation of the artifact size with the orientation angle, whereas the new TE-modulated radial acquisition (sin² TE) is less angle-dependent. From: Schleicher KE, et al. MAGMA 31(2):235-242 (2018)


References

Cooperation Partners

MaRVis Interventional GmbH

Grant Support

Andreas Reichert

Magnetic resonance imaging (MRI) as a noninvasive imaging technique offers unique advantages over computed tomography or ultrasound imaging for interventional procedures. MRI has an excellent soft tissue contrast, provides morphological imaging in arbitrary scan planes and enables functional imaging without the application of ionizing radiation. Unfortunately, closed-bore magnets severely limit the patient access and often do not offer a direct line of sight to the interventional instrument. To overcome these limitations, we combine small assistance systems with real-time instrument tracking sequences to facilitate interventional procedures in closed-bore MR systems, for example to perform MR-guided prostate biopsies.

Top: A small, patient-mounted assistance system (GantryMate) for MR-guided needle interventions [2]. Bottom: Automatic image plane alignment with a targeted region (yellow circle). The passive marker needle guide is automatically detected with a phase-only cross correlation (POCC) algorithm which enables real-time targeting maneuvers [1].


References

Grant Partners

  • Tiroler Innovationsförderung (Project “GantryMate”)
  • ZIM/IraSME


Past Projects

MR-guided Hyperthermia

Temperature measurement methods have been designed for MR-guided hyperthermia in tumor patients (Cooperation with J. Gellermann and O. Voigt/Univ. Tübingen)

Dadakova T, et al. Fast PRF-based MR Thermometry Using Double-Echo EPI: In Vivo Comparison in a Clinical Hyperthermia Setting. Magn Reson Mater Phy 2014, Nov 8. [Epub ahead of print]

Temperature map during a hyperthermia treatment in patient with a myxoid liposarcoma in the left leg – the local temperature increase in the tumor can be clearly identified.

MR-guided Intravascular Procedures

To accelerate the image acquisition during MR-guided intravascular procedures, novel image acquisition and reconstruction technologies are investigated, which utilize data from a limited number of projections to reconstruct vascular trees in real time (DFG project: BO 3025/2-1 and /2-2)

Brunner A, et al. An MR-compatible stereoscopic in-room 3D-display for MR-guided interventions. Magn Reson Mater Phy 2014;27(4):277-282

MR-guided Prostate Therapy

For MR-guided prostate therapies, temperature measurement technologies and radio-frequency coil systems have been developed to perform high intensity focused ultrasound (HIFU) therapies with novel therapeutic US transducers (EUROSTARS project PROFUS, closed)

Yiallouras C, et al. Three-axis MR-conditional robot for high-intensity focused ultrasound for treating prostate diseases transrectally. J Ther Ultrasound 2015;3:2

MR-safe Guidewires

To enable intravascular interventions, novel MR-safe guidewires are investigated, and optimized imaging strategies are presented in order to be able to visualize the guidewires with variable contrasts (Cooperation with MarvisTech)

Prof. Dr. Michael Bock
Director of Experimental Radiology

Tel. +49 761 270-94140
Fax +49 761 270-93790
E-Mail: michael.bock@uniklinik-freiburg.de

University Medical Center Freiburg
Dept. of Radiology · Medical Physics
Killianstr. 5a
79106 Freiburg, Germany
 

Tracey Webb-Kolbinger
Administrative Assistant
Tel. +49 761 270-93840
Fax +49 761 270-93790
E-Mail: tracey.webb-kolbinger@uniklinik-freiburg.de