Catheter-based magnetic resonance microcoils for microscale imaging and spectroscopy.

摘要

Implantable RE-coils have enabled sub-millimeter resolution magnetic resonance images (MRI) of deep structures. Scaling down the size of RF coils has similarly provided a gain in signal-to-noise ratio (SNR) in nuclear-magnetic-resonance spectroscopy. By combining both approaches we designed, fabricated, and imaged with an implantable microcoil catheter. The microcoil was designed with a diameter of 1 mm so that it could be used with intracranial and intraductal catheters for neuroimaging and breast oncology respectively. Experimental verification of the first-generation coil design was achieved through ex vivo imaging of neural tissue. While 3-T MRI typically provides 1 to 30 voxelsmm3 , we report that the improved SNR provided by the MRI microcoil can enable microimaging with tens, hundreds, and even can provide hundreds, and even thousands of voxels in the same volume while maintaining sufficient SNR.;The second-generation design makes multiple improvements to the previous design. While the first generation microcoil provided microscopic resolution MRI it was not capable of implantation at a clinically relevant depth. The second generation now has leads that are twice as long, 5 cm in length. Body-temperature soak experiments have shown that the microcoil is sufficiently stable for use, however further improvement is required if the microcoil is intended for use beyond 12 h. A comparison of the SNR of the microcoil to a conventional head coil was performed with voxel sizes of 0.121 x 0.121 x 0.8 mm3. The microcoil provided a maximum SNR of over 200 in contrast to the head coil maximum of 5. Proof-of-principle microliter spectroscopy was done comparing 1 microliter of sample using the microcoil in comparison to the milliliters of sample used by the head coil.;The risk of MR-related heating of depth electrodes with microwire arrays at 3 T(128 MHz), was assessed combining conventional in vitro methods with network-analyzer measurements. Heating in excess of 6°C corresponded to network-analyzer measurements indicating microwire resonance at the MRI operating frequency. Unsafe conditions were eliminated by removal of the implant resonant conditions. Our data strongly support the potential for safe use of depth electrodes at 3 T, but highlight the importance of considering self-resonance when assessing MRI compatibility of implanted structures.