On the heating of inductively coupled resonators (stents) during MRI examinations

Abstract

Stents that have been implanted to preserve the results of vascular dilatation are frequently affected by in-stent restenosis, which ideally should be followed up by a noninvasive diagnostic modality. Active MRI stents can enable this kind of follow-up, while normal metallic stents can not. The prototype stents investigated in this study were designed as electric resonating circuits without a direct connection to the MR imager, and function as inductively coupled transmit coils. The model of a long solenoid coil is used to describe the additional power loss caused by such resonators. The theoretically estimated temperature increase is verified by measurements for different resonators and discussed for worstcase conditions. The RF power absorption of an active resonator is negligible compared to the total power absorbed during MRI. The local temperature increase observed for prototypes embedded in phantoms is in a range that excludes direct tissue damage. However, ruptures in the conducting structure of a resonator can cause hot spots, which may establish a high local temperature. This hazard can be reduced by designing resonators with a low quality (Q) factor or by setting the circuit slightly off resonance; however, this would lower the nominal amplification for which the resonator was designed. Magn Reson Med 54:775-782, 2005.

Sensitivity enhancement of remotely coupled NMR detectors using wirelessly powered parametric amplification

“…The pumping signal used to power the parametric circuit creates an additional heat source besides the NMR excitation pulse (14). For our prototype circuit, the output power level from the pumping loop was 13 dBm, which yielded an effective power received by the resonator of 0 dBm after attenuation was taken into account.…”

Wireless amplified NMR detector for improved visibility of image contrast in heterogeneous lesions

“…During RF excitation, the WAND was wirelessly decoupled by the excitation field, which can strongly modulate the capacitance of its constituting varactors. As a result, there would be negligible RF‐induced heating or excitation angle over‐flipping as are normally encountered in inductively coupled implantable resonators . During signal reception, less than 10 mW of power was required on the pumping loop to operate the WAND at 0.6 dBm below its oscillation threshold for efficient signal amplification.…”

A Review of Numerical Simulation and Analytical Modeling for Medical Devices Safety in MRI

“…Several medical implants have been studied through numerical simulations, like stents [15][16][17][18][19][20][21][22][23]. The real CAD model can sometimes be modeled [21], showing a good correspondence between experimental and numerical results; otherwise the geometry is simplified [15,18], which could lead to a different behavior regarding RF heating compared to reality.…”