Showing papers by "F. Levent Degertekin published in 2021"

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

Abstract: 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.

Analysis of Negative Capacitance-Based Broadband Impedance Matching for CMUTs

Abstract: Tight integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with integrated circuits can make active impedance matching feasible for practical imaging devices. In this article, negative capacitance-based impedance matching for CMUTs is investigated. Simple equivalent circuit model-based calculations show the potential of negative capacitance matching for improving the bandwidth along with electrical power transfer and acoustic reflectivity, but the model has limitations especially for acoustic reflectivity evaluation. For more realistic results, an experimentally validated CMUT array model is applied to a small 1-D CMUT array operating in the 5–15 MHz range. The results highlight the difference between electrical power transfer and acoustic reflectivity as well as the tradeoffs in signal-to-noise ratio (SNR). According to the results, ideal negative capacitance termination matched to the CMUT capacitance provides the broadest bandwidth and highest SNR if acoustic or electrical reflections are of no concern. On the other hand, negative capacitance and resistance matching to minimize acoustic reflectivity provides both lower reflection and closer to ideal SNR as compared with electrical power matching. It is observed that acoustic matching also reduces acoustic crosstalk and improves array uniformity. While several challenges for integrated circuit implementation are present, negative capacitance-based impedance matching can be a viable broadband active impedance matching method for CMUTs operating in conventional and collapsed mode as well as other ultrasound transducers with mainly capacitive impedance.

Multiple electrically tunable parametric resonances in a capacitively coupled electromechanical resonator for broadband energy harvesting

Abstract: Parametric excitation (PE) has widely been employed as a method of mechanical pre-amplification in nonlinear vibration energy harvesting systems. However, despite their advantages, most current PE systems are limited to degenerate parametric operation within a narrow frequency band around the primary instability tongue. In this paper, we simulate and experimentally demonstrate a parametrically driven capacitive electromechanical resonator having multiple electrical degrees of freedom. Multiple modes allow for several frequency bands in which the electrical resonator is driven into nondegenerate (combination) parametric resonance (PR) in addition to degenerate resonance, thereby enabling operation over a broader range of frequencies while maintaining the same mechanical footprint. These frequency bands and PR thresholds are tunable by simply changing the electrical circuit parameters and PR can be achieved in the presence of high mechanical damping making the method more adaptable than purely mechanical approaches. Experimental results are extended by simulations indicating that proper selection of operating parameters can enable the merging of instability tongues to produce a broadband region of PR for elastic wave energy harvesting thereby obtaining superior performance when compared to an equivalent single degree of freedom PE energy harvester.

Optimization of High Frequency CMUT Array Geometry for Guidewire IVUS

Abstract: High frequency ultrasound arrays on small scale probes can find many applications where high resolution is desired with limited penetration. CMUTs provide many advantages for such systems including electronics integration for miniaturization and fabrication flexibility for different array geometries. CMUT models have been proposed to model and optimize overall frequency response, however more detailed analysis and optimization of the imaging performance considering overall array geometry are required for practical applications. In this study, we describe a detailed design model and PSF based layout optimization of a 40-MHz 1-D CMUT array that can be used on a guidewire for IVUS imaging. For the design, a lateral array size of 300 µm, suitable for 0.014″ IVUS guidewires, is considered. Dynamic analysis of CMUT membranes and bandwidth optimization for required design specifications are performed using a nonlinear lumped large signal model. Two different membrane configurations are considered having $15\ \mu \mathrm{m}$ and $17.5\ \mu \mathrm{m}$ circular membranes. PSF simulations are performed to optimize array element elevation length along the depth of field. Simulated artery phantom is designed and the performance of the array configurations is verified with imaging simulations.

Dual mode CMUT Array Operation for Skull Imaging and Passive Acoustic Monitoring in Transcranial Ultrasound

Abstract: Transcranial focused ultrasound (tFUS) therapy shows great potential in several applications, including blood brain barrier (BBB) opening. In these applications two important aspects are the registration of the skull to CT images for aberration correction and monitoring the microbubble (MB) activity using methods like passive acoustic mapping (PAM). Currently the image registration is performed using MRI as the tFUS ultrasound arrays with piezoelectric transducers are not optimized as wideband transducers for higher resolution imaging rather they are tuned to MB excitation and detection frequencies. The potential of capacitive micromachined ultrasonic transducer (CMUT) as wideband and low noise receiver for tFUS has been already realized. However, CMUT has some unique characteristics such as control of operation mode which in turn can change its frequency in a very broad range, from MB excitation, to detection and skull imaging for image registration. In this work, we demonstrate the feasibility of multi-mode operation of CMUT operation for tFUS, where the same commercially available CMUT is used for both MB activity monitoring for PAM as well as skull imaging. Furthermore, experiments show that the same array can potentially be used for both MB activity excitation/detection in addition to skull imaging. This is achieved by using the same CMUT in conventional and collapsed mode effectively utilizing a frequency range of 0.5 MHz to 5 MHz.

Acousto-optic Modulator Based Magnetic Field Sensor Using π-Phase Shifted Fiber Bragg Grating

Abstract: A fiber optic magnetic field sensor utilizing an FBG based acousto-optic modulator is introduced. Sensitivity of 1.84mV/nT with minimum detectable magnetic field strength of 6.1pT/√Hz and dynamic range of 117dB/√Hz are demonstrated at 23MHz.