A vascular stent as an active component for locally enhanced magnetic resonance imaging: initial in vivo imaging results after catheter-guided placement in rabbits.

RATIONALE AND OBJECTIVE: A vascular stent constructed as a high frequency resonator improves the local signal-to-noise ratio at magnetic resonance (MR) imaging. After catheter placement and intravascular expansion, the stent can be used as an inductively coupled coil for MRI. The imaging properties of this balloon-expandable active MRI stent (AMRIS) were evaluated after x-ray fluoroscopy guided placement in the abdominal aorta of five rabbits using MR angiography (MRA) and flow measurements. METHODS: The AMRIS was implanted in the abdominal aorta of five rabbits using a balloon catheter inserted through the common carotid artery. The rabbits were examined by MRA (3D fast low-angle shot) at 1.5 tesla before and after intravenous injection of an iron-oxide-based blood pool contrast medium (dose 50 micro mol Fe/kg) and flow measurements (ECG-triggered phase contrast cine gradient-echo sequence). Signal-to-noise ratios (SNR) were calculated and flow volume curves were generated. The in-stent increase in temperature was measured in vitro using a fiberoptic thermometry system. RESULTS: The SNR was 5.0 +/- 0.6 outside the stent and 23.2 +/- 14.1 within the stent ( < 0.0 5) in plain MRA, 19.5 +/- 5.0 outside and 30.7 +/- 8.2 within the stent ( < 0.05) in contrast enhanced MRA, and 5.8 +/- 1.6 and 13.9 +/- 5.9, respectively ( < 0.05) in the magnitude images of the flow measurements. Flow volume curves within and distal to the stent were comparable. CONCLUSIONS: The expandable active MRI stent produces local signal enhancement in MRA and MR flow measurements after catheter placement and thus may improve assessment of the stented vessel segment by MR imaging.

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

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 worst-case 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.