Study of Various Detection Mechanisms for Photoacoustic Imaging

Abstract

Photoacoustic imaging (PAI) is a developing imaging technique that has been researched for several clinical applications including oncology, neurology, dermatology and ophthalmology. PAI combines the benefits of pure optical and acoustic imaging to attain optical absorption contrast images. There are three different modalities in photoacoustic imaging which are categorized according to the type of image reconstruction and the focus. Photoacoustic computed tomography (PACT) uses reconstruction-based image formation while photoacoustic microscopy (PAM) uses focused-based image formation. Photoacoustic microscopy can be further divided into optical-focused imaging, optical-resolution photoacoustic microscopy (OR-PAM) and acoustic-focused imaging, acoustic resolution photoacoustic microscopy (AR-PAM). The two essential components in the photoacoustic imaging system are the excitation laser, which is typically implemented as a pulsed laser, and the detector. Various detection mechanisms have been investigated for photoacoustic imaging, ranging from contact mode physical sensors to non-contact forms of detection. In this research project, photoacoustic imaging with a piezoelectric transducer and a novel non-contact detection mechanism, photoacoustic remote sensing (PARS) were studied for optical-resolution photoacoustic microscopy, OR-PAM applications. Conventional photoacoustic imaging uses a piezoelectric transducer to pick up the pressure induced by the photoacoustic effect. PARS system, on the other hand, captures the changes in the elasto-optical refractive index modulation caused by the photoacoustic initial pressure. The photoacoustic signals can be detected in two modes, transmission and reflection modes. When the excitation laser hits the sample, the photoacoustic signals are generated in different directions. In transmission mode, photoacoustic signals are detected below the sample while reflection mode is the sensing of photoacoustic signals that have bounced off the sample. A 2.25 MHz piezoelectric transducer was used in transmission mode to image carbon fibers network. For the PARS system, the piezoelectric transducer was replaced with a 637 nm continuous-wave laser. The continuous-wave detection laser was co-focused and co-scanned with an excitation laser to image the same carbon fiber networks in reflection mode.

The second objective of this research project focuses on the preliminary investigation of a micro-electro-mechanical system device, capacitive micromachined ultrasound transducers (CMUTs) for PAI applications. In comparison with conventional piezoelectric transducer, CMUTs generally have a larger bandwidth which will in turn attribute to a better axial resolution for applications such as PAM imaging. Furthermore, unlike piezoelectric transducer, CMUTs have a similar acoustic impedance as tissue. Therefore, there is no need for an additional matching layer. A 3.4 MHz CMUT fabricated with nitride-to-oxide wafer bonding technology was analyzed and characterized for PAI in this research project.