DEVELOPMENT OF ALL-OPTICAL QUANTITATIVE ULTRASOUND IMAGING SYSTEM

Abstract

Ultrasound (US) is a well-established deep-tissue imaging modality in biomedicine. It distinguishes different tissue types based on their echogenicity, but this approach provides limited diagnostic sensitivity and accuracy. The majority of the US transducers nowadays rely on lead zirconate titanate (PZT) ceramic elements to transmit and receive ultrasound. Unfortunately, significant limitations arise from these transducers due to their frequency characteristics and complex fabrication process. A recently introduced technique, Quantitative Ultrasound (QUS) Measurement, shows a great promise to improve US-based tissue diagnosis, but it requires a transducer with a large spectrum bandwidth, which is a feature not available in PZT transducers. Recent research has shown that optical methods for ultrasound generation and detection may overcome these limitations. The significant advantages provided by the optical approaches are the tunable center frequency, large spectrum bandwidth, high sensitivity, and miniaturization friendly. More importantly, they are exceptionally suitable for quantitative measurements. The primary objective of this Ph.D. research is to develop an all-optical ultrasound transducer (AOUT) for quantitative ultrasound measurements and imaging and, eventually, for tissue characterization. In order to accomplish this goal, an optical ultrasound transmitter (OUT) and a sensitive optical refractometry ultrasound detector (RUD) were developed. For ultrasound generation, an OUT was designed and built based on the photoacoustic (PA) effect. The OUT developed consists of two-layer Candle soot-PDMS coated on a glass substrate and can generate a maximum pressure (Pmax) of 0.16 MPa, a center frequency (f0) of 35 MHz, and a spectrum bandwidth (fBW) of over 38 MHz. For ultrasound detection, a RUD was designed and built based on the probe beam deflection technique (PBD). The RUD developed in this study has a sensitivity of 55 Pa and an axial resolution of 0.27 mm. By combining OUT and RUD, an AOUT was designed and fabricated. The imaging and the quantitative measurement capabilities of the AOUT were validated using three ultrasound phantoms. The first phantom contains 106-125 µm diameter soda-lime microspheres, the second phantom contains 70-90 µm diameter microspheres, and the last phantom contains 38-45 µm diameter microspheres. The reconstructed images show the profiles of the phantoms with reasonable accuracy, and the frequency spectra of the backscattering signals acquired from these phantoms were distinctively different, which confirms the feasibility of using AOUT to perform quantitative ultrasound measurements.