Broadband All-Optical Plane-Wave Ultrasound Imaging System Based on a Fabry–Perot Scanner

Abstract

A broadband all-optical plane-wave ultrasound imaging system for high-resolution 3D imaging of biological tissues is presented. The system is based on the planar Fabry-Perot (FP) scanner for ultrasound detection and the photoacoustic generation of ultrasound in a Carbon-Nanotube-Polydimethylsiloxane (CNT-PDMS) composite film. The FP sensor head was coated with the CNT-PDMS film to act as an ultrasound transmitting layer for pulse-echo imaging. Exciting the CNT-PDMS coating with nanosecond laser pulses generated monopolar plane-wave ultrasound pulses with MPa-range peak pressures, and a-6dB bandwidth of 22 MHz, that were transmitted into the target. The resulting scattered acoustic field was detected across a 15 mm × 15 mm scan area with a step size of 100 and an optically defined element size of 64. The-3dB bandwidth of the sensor was 30 MHz. A 3D image of the scatterer distribution was then recovered using a k-space reconstruction algorithm. To obtain a measure of spatial resolution, the instrument line-spread function (LSF) was measured as a function of position. At the centre of the scan area the depth dependent lateral LSF ranged from 46 to 65 for depths between 1 and 12 mm. The vertical LSF was independent of position and measured to be 44 over the entire field of view. To demonstrate the ability of the system to provide high-resolution 3D images, phantoms with well-defined scattering structures of arbitrary geometry were imaged. To demonstrate its suitability for imaging biological tissues, phantoms with similar impedance mismatches, sound speed and scattering properties to those present in tissue, and ex-vivo tissue samples were imaged. Compared to conventional piezoelectric based ultrasound scanners this approach offers the potential for improved image quality and higher resolution for superficial tissue imaging. Since the FP scanner is capable of high-resolution 3D photoacoustic imaging of in-vivo biological tissues, the system could ultimately be developed into an instrument for dual-mode all-optical ultrasound and photoacoustic imaging.

A Review of Transparent Sensors for Photoacoustic Imaging Applications

“…The noncontact and nondestructive characteristics of most types of optical US detection technology give it broad application prospects in nondestructive testing and evaluation, and it can perform real-time detection of an object under test in an environment of radiation, high temperature, and high pressure [8,11]. At the current stage, common types of optically transparent US sensors are FP interferometer sensors, MRRs, and grating-based sensors [22][23][24][25][39][40][41][42][43][44]. Optical detection methods have the following advantages over conventional US detection methods:…”

“…With the good directionality of the laser, FP sensors can achieve long-distance measurement [41,45,46]; • Most FP sensors and MRR sensors are noncontact and so can avoid the error and pollution caused by the contact between the couplant and the workpiece during traditional US detection, therefore realizing truly nondestructive testing [37,41,42,47];…”

“…In recent years, various kinds of materials can be used as the mirrors in the FP cavity, such as silica, silver, and graphene. The sensitivity of the sensor can be improved using a smaller thickness and larger diameter of diaphragm [40][41][42]45,46,[48][49][50]. In some studies, the introduction of MEMS technology can produce diaphragms with a smaller thickness and a smaller diameter to achieve a higher detection frequency.…”

“…Pham et al fabricated a FP sensor, which consists of two highly reflecting dichroic mirrors separated by a 20 µm thick spacer made of Parylene C. Most of the range of frequency attenuation in the transmission process in the carbon nanotube polydimethylsiloxane (CNT-PDMS) film is relatively weak; for example, a 100 µm film attenuates frequencies up to 22 MHz by less than 1.7 dB [39]. The FP sensor also uses the CNT-PDMS composite film as the US transmitting layer for pulse-echo imaging [41]. The −6 dB bandwidth of the FP sensor is 22 MHz, which is a much wider bandwidth than for conventional piezoelectricbased US detectors.…”

“…In addition, the sensor has higher resolution for tissue imaging and better image quality than conventional piezoelectric-based US scanners. It can achieve high resolution in tissue up to depth of 10 mm in vivo PAI [41]. Zhang et al fabricated a FP sensor, which is composed of a 38 µm thick Parylene C film, two dichroic mirrors, and a polymethylmethacrylate (PMMA) backing stub.…”