1. Field of the Invention
The present invention concerns a device to reduce the electromagnetic field load caused by the presence of a medical intervention apparatus in magnetic resonance examinations.
2. Description of the Prior Art
Many medical procedures today are conducted in an interventional manner. Diagnostic or therapy methods that—in contrast to conservative procedures—conduct targeted procedures (interventions) on pathological tissue in order to positively affect the course of the illness are designated as interventional. Invasive or, respectively, minimally invasive procedures are also examples of such methods, wherein medical instruments or implants are brought to the treatment or examination location by various techniques.
Particularly in the treatment of vascular diseases, guide wires are used that are inserted by a physician into hollow vessels (lumens) of a patient. Later a catheter (for example) is inserted along this wire for diagnosis or therapy. The guide wire itself often has a diameter of less than 0.5 mm. Due to the high mechanical demands, such guide wires have been conventionally produced from metal and metal alloys. Magnetic resonance is a modality particularly suited for imaging of the human body with a high tissue contrast. It would be particularly suitable for observation of intervention apparatuses during their insertion and placement in the human body, but the electromagnetic fields in the two-digit or three-digit megahertz range that are required for imaging are problematical. Therefore, heretofore there have been high risks in the real-time tracking of guide wires and other electrically conductive intervention apparatuses given movement in the human body. This is highly risky because high-frequency currents can form on elongated electrically conductive apparatuses due to injection of the transmission fields. This leads to heating at corresponding powers and, in the disadvantageous case, to tissue burns inside the human body. The currents are even higher at such elongated intervention apparatuses if waveguide resonances occur along the entire length due to the intervention apparatus. If the lengths of the guide wires are also on the order of the wavelength of the magnetic resonance frequencies being used, the risk of endangering a patient is particularly high.
A prediction about the potential for danger by means of simulation calculations is very difficult since the actual danger is situation-dependent. The length and orientation of the intervention apparatus in the patient, the conductive paths outside the patient, and even properties of the patient, play a role. Finally, the position of the intervention apparatus in relation to the transmission antenna, and the transmission antenna type, are also very influential.
Due to the high risks from the induced radio-frequency currents, the real-time tracking of intervention apparatuses with the aid of magnetic resonance has previously been realized with the transmission power being drastically limited. Another solution is the construction of guide wires or catheters from non-conductive materials. However, all desired material properties cannot be ensured with these materials; such intervention apparatuses are additionally very expensive and complicated.