Miniaturized Acousto-Optic Sensor for RF Field Measurement Under MRI
Author(s)
Bradley, Lee
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Abstract
RF field measurement under MRI presents a significant measurement challenge. Both
components of the RF field, the electric field and the magnetic field, are uniquely important
for real-time sensing during an MR-sequence: the electric field contributes to deposited RF
power in the tissue, leading to increased tissue temperature, while the magnetic field is used
for imaging and localization. Conventional RF field sensors use long electrical conductors
for signal transmission, which distort images and contribute to local RF power hot-spots,
potentially damaging tissue on or in the patient. Current MRI-compatible sensors employ
transduction from electrical to optical signals for RF immune signal transmission, which
subverts these issues. However, demonstrated sensors either utilize complex, actively pow-
ered components, or rely on large, rigid components for passive sensing, all of which may
constrain the device geometry, performance, and application.
The feasibility of an acousto-optic (AO) RF field sensor structure based on light inten-
sity modulation via an antenna-coupled piezoelectric transducer modulating a fiber-Bragg
grating (FBG) was demonstrated earlier. This work develops and validates an end-to-end
model of the AO sensor to enable design optimization, and a fabrication process based on
conformal piezo-composite polymer transducers over optical fibers. The use of coaxial
piezoelectric transducers increases the acoustic coupling efficiency and minimize the size
of the sensor for seamless integration on interventional MRI catheters as compared to pla-
nar and bulky piezo crystals glued to optical fibers. Sensors are designed and fabricated for
both E-field and B-field sensing capabilities under 3T MRI. These devices are also used to
validate the complete electromechanical model for the antenna-coupled coaxial transducer
to predict the sensor response to arbitrary incident RF fields. Experiments under MRI show
that the sensitivity of fabricated devices is comparable to the previous FBG-based sensor
and confirm the feasibility of fully RF-immune device tracking under MRI.
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Date
2025-05-13
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Text
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Dissertation