Magnetic Resonance Imaging (MRI) is a medical imaging technique commonly used to image inside the human body. Generally a strong magnetic field (e.g., 1.5 T) is applied to the patient to align the nuclear magnetization vectors of hydrogen atoms in the water of the tissue of the patient under investigation. Simultaneously a RF (e.g., 63.86 MHz) magnetic field (˜0.14 μT) is applied to perturb the aligned magnetization, causing the hydrogen nuclei to emit energy signatures that are detectable by a scanner in the MRI system.
The MRI signal is used to construct an image. Different tissues are detected because the protons in various tissues return to their equilibrium nuclear magnetization states at different rates. This effect is used to create contrast among different types of tissues. Diseased tissues, such as tumors, can be detected in this manner as well.
A time-varying magnetic field creates an electric field within any electrically conductive material and the electric field, in turn, induces an electric current, referred to as the “eddy current”. Generally the electromagnetic field penetrates into metals up to a certain depth, commonly referenced to a “skin depth”. An electromagnetic wave entering a conducting surface is damped so that the current density is largest near the surface of the conductor and reduces in amplitude by a factor 1/e at a distance referred to as the skin depth (δ) of the electrical conductor from its surface given by:δ=[2/(ωμ0σ)]1/2 where ω is the angular frequency of the radiation, μ0 is the permeability of the radiation in a vacuum, and σ is the electrical conductivity of the metal. δ can be seen to decrease if the electrical conductivity (σ) increases. For example, at ω=60 Hz in copper, δ is about 8.5 mm. At high frequencies δ may be much smaller (shallower).
The eddy current can heat up the electrically conductive material by the Joule effect and this process is called induction heating. Induction heating can cause thermal damage to tissues while conducting MRI scans for patients wearing implants that have leads, such as in spinal fusion stimulators, cardiac pacemakers and neurostimulation systems. A maximum temperature change of 25.38° C. has been reported for a deep brain stimulation implant shortly after initiating MRI. The heating can be more severe at metal tips. A temperature elevation of 63.18° C. has been reported to have occurred at the tip of a pacing electrode (unattached to a cardiac pacing pulse generator) within 90 s of initiating MRI.
Although selection of a conventional high electrical conductivity metal material can reduce MRI-induced induction heating somewhat by reducing the amount of energy that is actually introduced into the material via the skin effect, the resulting MRI-induced induction heating may still be too high for certain applications. There is thus a need for new materials and enabling processing systems and methods for forming such materials which provide modified surfaces which generate reduced MRI-induced induction heating.