Thin polymer etalons are demonstrated as high-frequency ultrasound sensors for three-dimensional (3-D) high-resolution photoacoustic imaging. The etalon, a Fabry-Perot optical resonator, consists of a thin polymer slab sandwiched between two gold layers. It is probed with a scanning continuous-wave (CW) laser for ultrasound array detection. Detection bandwidth of a 20-µm-diam array element exceeds 50 MHz, and the ultrasound sensitivity is comparable to polyvinylidene fluoride (PVDF) equivalents of similar size. In a typical photoacoustic imaging setup, a pulsed laser beam illuminates the imaging target, where optical energy is absorbed and acoustic waves are generated through the thermoelastic effect. An ultrasound detection array is formed by scanning the probing laser beam on the etalon surface in either a 1-D or a 2-D configuration, which produces 2-D or 3-D images, respectively. Axial and lateral resolutions have been demonstrated to be better than 20 µm. Detailed characterizations of the optical and acoustical properties of the etalon, as well as photoacoustic imaging results, suggest that thin polymer etalon arrays can be used as ultrasound detectors for 3-D high-resolution photoacoustic imaging applications.
Here we present an ultrasound detection system with an optical end capable of parallel probe. An erbium-doped fiber
amplifier, driven by a tunable laser, outputs light at 27 dBm. A lens collimates the light to probe a 6-μm thick SU-8
etalon and controls the parallel detection area (total array size). A two-lens system guides the reflected light into a
photodetector and controls the active area (array element size) on the etalon surface. A translation stage carries the
photodetector to detect signals from different array elements. The output of the photodetector is recorded using an
oscilloscope. The system's noise equivalent pressure was estimated to be 6.5 kPa over 10~50 MHz using a calibrated
piezoelectric transducer when the -3 dB parallel detection area was 1.8 mm in diameter. The detection bandwidth was
estimate to exceed 70 MHz using a focused 50 MHz piezoelectric transducer. Using a single probe wavelength, a 1D
array with 41 elements and a 1.06 mm aperture length was formed to image a 49 μm black bead photoacoustically. The
final image shows an object size of about 95 μm in diameter. According to the results, realizing high-frequency 2D
optoacoustic arrays using an etalon is possible.
We present a fiber-based optical detection system for high-frequency 3D ultrasound and photoacoustic imaging.
Optically probing the surface of a thin polymer Fabry-Perot etalon defines the acoustic array geometry and element size.
We have previously demonstrated wide bandwidth signal detection (>40 MHz) and element size on the order of 15 &mgr;m.
By integrating an etalon into a photoacoustic imaging system, high-resolution 3D images were obtained. However, the
previous system is limited for clinical applications because the etalon is rigidly attached to a free-space optical scanning
system. To move etalon detector technology toward a practical clinical device, we designed a remote-probe system based
on a fiber bundle. A fiber bundle, composed of 160,000 individual light guides of 8-&mgr;m diameter, delivers the optical
probe to the etalon. Light coupled into a single guide creates an active element on the etalon surface. We successfully
measured the ultrasound signals from 10 MHz and 50 MHz ultrasound transducers using a laser tunable around 1550 nm.
With further progress on reducing the size of the etalon, it will be possible to build a practical device for in vivo high-frequency
3D ultrasound and photoacoustic imaging, especially for intravascular and endoscopic applications.
Recent advances in fabrication techniques have accelerated development of optical generation and detection of
ultrasound, a promising technology to construct high-frequency arrays for high resolution ultrasound imaging. A
two-dimensional (2-D) gold nanostructure has been fabricated to optically generate high frequency ultrasound. The
structure consists of 2-D arrangements of gold nanoparticles, sandwiched between a transparent substrate and a 4.5 &mgr;m
thick PDMS layer. A pulsed laser beam is focused onto the optically absorbing gold nanostructure, and consequently, a
localized volume is heated, and thermal expansion launches an acoustic wave into the overlying layer. The high optical
extinction ratio of the gold nanostructure provides a convenient method to construct an integrated transmit/receive
optoacoustic array. A thin polymer Fabry-Perot etalon is used for optoacoustic detection. The etalon is an active optical
resonator, where the relatively low elasticity of the polymer and the high quality factor of the resonator combine to
provide high ultrasound sensitivity. An integrated device combining the gold nanostructure and the etalon has been
fabricated. Preliminary results demonstrate its promise as an all-optical ultrasound transducer.
Multi-dimensional, high frequency ultrasound arrays are extremely difficult to fabricate from conventional piezoelectrics. For over a decade, our lab has explored optical detection as an alternate technology for high frequency applications. We have developed several different types of acoustically coupled optical resonators to provide the sensitivity and bandwidth required for biomedical imaging. Waveguide and fiber lasers, thin Fabry-Perot etalons constructed from polymers, and thin microring resonators imprinted into polymers have all been used as ultrasound transducer arrays. Their performance rivals the theoretical conversion efficiency of piezoelectric devices but with bandwidths approaching 100 MHz, array element dimensions approaching 10 um, and no electrical interconnects. In this paper we present results on several resonant optical ultrasound transducer (ROUT) arrays, emphasizing their potential use in photoacoustic imaging. These results strongly suggest that a high resolution photoacoustic microscope can be constructed using a ROUT in a footprint appropriate for endoscopic and minimally invasive applications.
An optoacoustic detector denotes the detection of acoustic signals by optical devices. Recent advances in fabrication techniques and the availability of high power tunable laser sources have greatly accelerated the development of efficient optoacoustic detectors. The unique advantages of optoacoustic technology are of special interest in applications that require high resolution imaging. For these applications optoacoustic technology enables high frequency transducer arrays with element size on the order of 10 μm. Laser generated ultrasound (photoacoustic effect) has been studied since the early observations of A.G. Bell (1880) of audible sound generated by light absorption . Modern studies have demonstrated the use of the photoacoustic effect to form a versatile imaging modality for medical and biological applications. A short laser pulse illuminates a tissue creating rapid thermal expansion and acoustic emission. Detection of the resulting acoustic field by an array enables the imaging of the tissue optical absorption using ultrasonic imaging methods. We present an integrated imaging system that employs photoacoustic sound generation and 2D optoacoustic reception. The optoacoustic receiver consists of a thin polymer Fabry-Perot etalon. The etalon is an optical resonator of a high quality factor (Q = 750). The relatively low elasticity modulus of the polymer and the high Q-factor of the resonator combine to yield high ultrasound sensitivity. The etalon thickness (10 μm) was optimized for wide bandwidth (typically above 50 MHz). An optical scanning and focusing system is used to create a large aperture and high density 2D ultrasonic receiver array. High resolution 3D images of phantom targets and biological tissue samples were obtained.