Acousto-optic-based time domain electric field sensor for magnetic resonance imaging applications.

An acousto-optic (AO)-based electric field sensor is presented for time domain measurement under magnetic resonance imaging (MRI). A fully MR-compatible sensor is designed and fabricated using a phase-shifted fiber Bragg grating mechanically coupled to a piezoelectric transducer. Mechanical resonance of the piezoelectric transducer is matched to the operating frequencies of commonly used MRI systems to increase the sensitivity of the sensor. Sensitivity of the sensor is measured as 1.27 mV/V/m, with a minimum detectable electric field of 4.4 mV/m/√/Hz. Directivity of the sensor is measured with a 18 dB orthogonal component rejection. The dynamic range of the sensor is calculated as 117 dB/Hz, which allows the measurement of electric fields up to 3.2 kV/m. In MRI studies, the AO sensor was able detect local hot spots around a reference implant accurately with high signal-to-noise ratio. AO sensor exhibited similar or better performance when compared with commercially available MRI compatible electric field sensors. Furthermore, the small size of the sensor with the flexible fiber optic link could allow in situ measurements of electric fields during critical interventional procedures such as pacemaker lead or deep brain stimulator placement as an MRI dosimeter during diagnostic scans.

Application of Low Temperature Processed 0-3 Composite Piezoelectric Thick Films in Flexible, Non-planar, High Frequency Ultrasonic Devices.

Low-temperature, flexible, 0-3 composite piezoelectric materials can decrease the size, cost, and complexity of high-frequency acoustic devices on temperature sensitive substrates such as those in catheter based ultrasonic devices and acoustooptic sensors. In this paper, the application of low-temperature 0-3 connected composite thick films in flexible, non-planar, high frequency ultrasonic devices is reported. A flexible high-frequency ultrasound transducer and an acousto-optic radio-frequency (RF) field sensor are demonstrated utilizing PZT-based composite thick films. Flexible composite films have been fabricated with thicknesses between 20-100µm utilizing screen-printing, stencil-printing, and dip-coating techniques. Composite films' piezoelectric d33 coefficient is measured, with results between 35-43 pC/N. Ultrasonic transducers utilizing these films demonstrate broadband acoustic response. A composite transducer is fabricated on flexible polyimide and wrapped around a 3mm catheter. Pulse-echo experiments demonstrate viability of these films as both as an actuator and a sensor in flexible devices. The composite material is further dip-coated onto an optical fiber Bragg grating to form a flexible acousto-optic RF field sensor. The sensor demonstrates RF field sensing in the 20-130 MHz range. The results from these experiments indicate significant potential for future flexible, high frequency ultrasonic devices utilizing low temperature 0-3 composite piezoelectric materials on temperature sensitive substrates.

Real-time device tracking under MRI using an acousto-optic active marker.

PURPOSE: This work aims to demonstrate the use of an "active" acousto-optic marker with enhanced visibility and reduced radiofrequency (RF) -induced heating for interventional MRI. METHODS: The acousto-optic marker was fabricated using bulk piezoelectric crystal and π-phase shifted fiber Bragg grating (FBGs) and coupled to a distal receiver coil on an 8F catheter. The received MR signal is transmitted over an optical fiber to mitigate RF-induced heating. A photodetector converts the optical signal into electrical signal, which is used as the input signal to the MRI receiver plug. Acousto-optic markers were characterized in phantom studies. RF-induced heating risk was evaluated according to ASTM 2182 standard. In vivo real-time tracking capability was tested in an animal model under a 0.55T scanner. RESULTS: Signal-to-noise ratio (SNR) levels suitable for real-time tracking were obtained by using high sensitivity FBG and piezoelectric transducer with resonance matched to Larmor frequency. Single and multiple marker coils integrated to 8F catheters were readout for position and orientation tracking by a single acousto-optic sensor. RF-induced heating was significantly reduced compared to a coax cable connected reference marker. Real-time distal tip tracking of an active device was demonstrated in an animal model with a standard real-time cardiac MR sequence. CONCLUSION: Acousto-optic markers provide sufficient SNR with a simple structure for real-time device tracking. RF-induced heating is significantly reduced compared to conventional active markers. Also, multiple RF receiver coils connected on an acousto-optic modulator can be used on a single catheter for determining catheter orientation and shape.

Micro-mirror-array based off-axis flat lens for near-eye displays.

We developed an off-axis diffractive lens using a micro-mirror array on a flat substrate. MMA creates an on-axis converging beam from a 45 degrees off-axis diverging illumination beam and functions similar to a large and bulky elliptical mirror. The array consists of individual micro-mirrors with normal directions that vary across the component. The size, normal direction and the center height of each micro-mirror are optimized to achieve a phase matching condition so that the smallest focal spot size is achieved at the design wavelength. Design can also be optimized for full color applications using a synthetic design wavelength. A sample MMA of size 3 mm by 5 mm is fabricated using grayscale lithography. The designed MMA is used to illuminate a computer-generated hologram in a near-eye display system. Experimental results verify the premises of the designed component.