MR Angiography
Fig. 1: Arterial (left) and venous (center) vascular system of the lung of a volunteer. The surface rendering (right) shows both vascular systems simultaneously.
© dkfz.de
To position catheters in blood vessels it is necessary to visualize the vascular system. We therefore developed different MR angiography techniques that allow to depict the spatial structure of vascular trees.
With the help of fast 3D MR acquisition methods angiograms of the human blood vessels could be successfully acquired during the passage of a contrast agent. Using dedicated post-processing (correlation analysis, maximum intensity projection) a separate visualization e.g. of the arterial and venous blood vessels in the lung could be achieved.
Instrument Tracking with MR Imaging
Fig. 2: Movement of the catheter tip (green cross) from the aorta into the left renal artery of an animal during the embolisation procedure of the left kidney.
© dkfz.de
Conventional instruments for minimal invasive procedures such as catheters or biopsy needles are not directly visible in an MR image. To compensate for this disadvantage we use the method of active visualization where small radio-frequency coils are attached to the instruments (here: angiography catheters).
The MR signal of these coils is encoded to determine the coil position (and thus the location of the instrument) in only a few milliseconds. We have implemented the position measurement in quick succession with the acquisition of an MR image.
To give the operator the opportunity to concentrate on the manipulation of the instruments rather than manipulating the MR scanner controls we modified the MR imaging software (pulse sequence) so that the imaging slice automatically follows the interventional device.
Functional MR Measurements
Fig. 3: At the position of the localizer coil (red cross) in the aorta the blood flow velocity is measured as a function of time in 10.4 ms intervals (right).
© dkfz.de
During minimally invasive procedures often vessel constrictions are dilated or vessel segments are closed. To assess the success of such a procedure it is necessary to measure the change in blood flow after the intervention.
For this purpose we developed a method that uses the signal of the localizer coils at the catheter tip. Compared to conventional MR flow measurements this method is considerably faster - within one second nearly 100 measurements of the local flow velocity near the catheter tip are performed.
Instrument Localisation with a Faraday Sensor
Abb. 4: Schematic drawing of a prototype Faraday effect sensor. Through an optical fibre (not shown) and a polarisation foil light is coupled into an optically active TGG crystal. The rotation of the polarisation plane is converted into an intensity modulation of the light with the help of a second polarisation foil and detected outside the MR scanner.
© dkfz.de
If electrically conducting structures such as guide wires or active catheters are used in an MR scanner, coupling with the electric field of the transmitting resonator can lead to severe warming of the structures. To avoid this potentially dangerous heating for the patient, we have developed an alternative localisation method that utilizes only non-conducting components within the MR scanner.
Here, the location of a small sensor is measured using the Faraday effect: When an optically active medium (e.g. a Terbium Gallium garnet crystal) is placed in a magnetic field, the plane of polarisation of linearly polarised light is rotated in the crystal. The magnetic field strength can be determined through a measurement of this rotation angle. By switching additional, position-dependent magnetic field gradients the location of the sensor can be reconstructed.
Radiofrequency Coils
Fig. 5: (a) Volume resonator (birdcage) for sodium MR imaging. (b) Micro solenoid coil at the tip of an angiography catheter. (c) Prototype of an inductively coupled coil for localization.
© dkfz.de
In this research project radiofrequency coils are developed, manufactured and optimized not only for interventional MRI, but also for MR spectroscopy and MR imaging with other nuclei.
For interventional MRI small coils were attached to commercial angiography catheters and connected to the receiver of the MR scanner using micro-coaxial cables. The signal behaviour of the coils was assessed with simplified models and tested in imaging experiments. To minimize potential coil heating we also use inductively coupled coils without direct connection to the MR scanner.