Acousto-optical RF field sensors for safer diagnostic and interventional MRI

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Although adverse events related to MRI are rare, safety concerns due to RF induced heating have a significant impact in clinical use especially for a growing number of patients with medical implants. Current regulations based on estimated whole body absorption rate (SAR) impose unnecessary restrictions on RF power levels, sometimes reducing the diagnostic efficacy, while even these low levels can create safety issues for specific patients. Miniature sensors with local E-field and temperature measurement capability on individual patients during the scans can alleviate these limitations while improving safety. These sensors can also have an impact on interventional MRI applications where active markers, essentially miniature coils that locally measure the RF signal, provide the best device visibility. These markers, however suffer from RF induced heating over conductive transmission lines, an issue that gets exacerbated in higher field MR systems and does not currently have a solution that satisfies all mechanical and electrical constraints. We recently developed a miniature acousto- optical (AO) RF field sensor which detects and transmits the RF signal over an optical fiber instead of conductive transmission lines, and therefore is totally free of RF induced heating and electromagnetic interference. The same device has the potential to measure temperature as well as the local E/H field for SAR evaluation when coupled with a properly designed antenna. In this application, we aim to optimize the design of the AO sensor using unique mechanical resonances of optical fibers at Larmor frequencies of 0.55T, 1.5T and 3T systems and use microfabrication techniques to make device with thin film piezoelectric transducers. We also propose a novel packaging strategy to embed these AO RF field sensors in sleeves that can conform to any commercially available MRI compatible catheter, to significantly increase the device options for interventional MRI. These active markers and multi-parameter E/H field sensors will be rigorously tested according to the latest MRI safety standards and evaluated in vivo for several challenging cardiovascular interventional MRI procedures. Successful demonstration of the AO sensor as an active MRI marker with a non-conducting transmission line will be a significant development for more effective, more efficient, safer, novel, and radiation-free interventional procedures. The combined quantitative RF field and temperature sensing capability for local RF dosimetry has the potential to impact clinical practice by broadening the MRI safety envelope as well as contributing to the fundamental understanding of coupling effects such as B-field coupling, standing waves and gradient coil effects on local SAR.