Ultrasound and Photoacoustic Techniques for Surgical Guidance Inside and Around the Spine

Technological advances in image-guidance have made a significant impact in surgical standards, allowing for safer and less invasive procedures. Ultrasound and photoacoustic imaging are promising options for surgical guidance given their real-time capabilities without the use of ionizing radiation. However, challenges to improve the feasibility of ultrasound- and photoacoustic-based surgical guidance persists in the presence of bone. In this thesis, we address four challenges surrounding the implementation of ultrasound- and photoacoustic-based surgical guidance in clinical scenarios inside and around the spine. First, we introduce a novel regularized implementation of short-lag spatial coherence (SLSC) beamforming, named locally-weighted short-lag spatial coherence (LW-SLSC). LW-SLSC improves the segmentation of bony structures in ultrasound images, thus reducing the hardware and software cost of registering pre and intra-operative volumes. Second, we describe a contour analysis framework to characterize and differentiate photoacoustic signals originating from cancellous and cortical bone, which is critical for a safety navigation of surgical tools through small bony cavities such as the pedicle. This analysis is also useful for localizing tool tips within the pedicle. Third, we developed a GPU approach to SLSC beamforming to improve the signal-to-noise ratio of photoacoustic targets using low laser energies, thus improving the performance of robotic visual servoing of tooltips and enabling miniaturization of laser systems in the operating room. Finally, we developed a novel acoustic-based atlas method to identify photoacoustic contrast agents and discriminate them from tissue using only two laser wavelengths. This approach significantly reduces acquisition times in comparison to conventional spectral unmixing techniques. These four contributions are beneficial for the transition of a combined ultrasound and photoacoustic-based image-guidance system towards more challenging scenarios of surgical navigation. Focusing on bone structures inside and surrounding the spine, the newly combined systems and techniques demonstrated herein feature robust, accurate, and real-time capabilities to register to preoperative images, localize surgical tool tips, and characterize biomarkers. These contributions strengthen the range of possibilities for spinous and transthoracic ultrasound and photoacoustic navigation, broaden the scope of this field, and shorten the road to clinical implementation in the operating room

CMUT array design and fabrication for high frequency ultrasound imaging

High frequency ultrasound imaging is utilized in a broad range of applications from intravascular imaging to small animal imaging for preclinical studies. Capacitive micromachined ultrasonic transducers (CMUTs) possess multiple preferable characteristics for high frequency imaging systems, such as simpler fabrication methods, simpler integration to electronics, and greater variety of array geometries. Adequate performance and optimization of CMUT based systems require a comprehensive analysis of multiple design parameters. This research utilizes a nonlinear lumped model, capable of simulating the pressure output, electrical input-output, and echo response to a planar reflector of CMUT arrays with arbitrary membrane shape and array geometry, to determine the performance limitations of high frequency CMUT arrays and the effect of different design parameters on its performance. Receiver performance is analyzed through parameters extracted from simulations, namely, thermal mechanical current noise, plane wave pressure sensitivity, and pressure noise spectrum. Transmitter performance is analyzed through pressure output simulation, and the overall performance is analyzed through the simulated pulse-echo response from a perfect planar reflector and the thermal mechanical current noise limited SNR. It is observed that the frequency response is dominated by two vibroacoustic limiting mechanisms: Bragg’s scattering, determined by array lateral dimensions, and crosstalk actuated fundamental and antisymmetric array modes, determined by individual membrane dynamics. Based on the limiting mechanism frequencies, a simplified design methodology is developed and used to design two CMUT array sets covering a broad frequency range of 1-80MHz. These CMUT arrays are fabricated and their limiting mechanisms are experimentally verified through pressure and admittance measurement and simulation comparison. CMUT arrays for guidewire IVUS application are implemented and successfully interfaced with ASICs to demonstrate imaging at 40MHz. Considering that CMUT array performance is also susceptible to the electrical termination conditions, the simulation model is utilized to investigate the effect of different impedance matching scenarios. Receiver performance of the integrated CMUT array and termination circuitry is analyzed through the system’s SNR and acoustic reflectivity.Ph.D

Integrated Electronics for Wireless Imaging Microsystems with CMUT Arrays

Integration of transducer arrays with interface electronics in the form of single-chip CMUT-on-CMOS has emerged into the field of medical ultrasound imaging and is transforming this field. It has already been used in several commercial products such as handheld full-body imagers and it is being implemented by commercial and academic groups for Intravascular Ultrasound and Intracardiac Echocardiography. However, large attenuation of ultrasonic waves transmitted through the skull has prevented ultrasound imaging of the brain. This research is a prime step toward implantable wireless microsystems that use ultrasound to image the brain by bypassing the skull. These microsystems offer autonomous scanning (beam steering and focusing) of the brain and transferring data out of the brain for further processing and image reconstruction. The objective of the presented research is to develop building blocks of an integrated electronics architecture for CMUT based wireless ultrasound imaging systems while providing a fundamental study on interfacing CMUT arrays with their associated integrated electronics in terms of electrical power transfer and acoustic reflection which would potentially lead to more efficient and high-performance systems. A fully wireless architecture for ultrasound imaging is demonstrated for the first time. An on-chip programmable transmit (TX) beamformer enables phased array focusing and steering of ultrasound waves in the transmit mode while its on-chip bandpass noise shaping digitizer followed by an ultra-wideband (UWB) uplink transmitter minimizes the effect of path loss on the transmitted image data out of the brain. A single-chip application-specific integrated circuit (ASIC) is de- signed to realize the wireless architecture and interface with array elements, each of which includes a transceiver (TRX) front-end with a high-voltage (HV) pulser, a high-voltage T/R switch, and a low-noise amplifier (LNA). Novel design techniques are implemented in the system to enhance the performance of its building blocks. Apart from imaging capability, the implantable wireless microsystems can include a pressure sensing readout to measure intracranial pressure. To do so, a power-efficient readout for pressure sensing is presented. It uses pseudo-pseudo differential readout topology to cut down the static power consumption of the sensor for further power savings in wireless microsystems. In addition, the effect of matching and electrical termination on CMUT array elements is explored leading to new interface structures to improve bandwidth and sensitivity of CMUT arrays in different operation regions. Comprehensive analysis, modeling, and simulation methodologies are presented for further investigation.Ph.D