Biomedical Photoacoustic Imaging Research Articles

Fabry Perot polymer film sensors for ultrasound field characterization and biomedical photoacoustic imaging

A contact ultrasound sensing technique based upon the detection of acoustically induced changes in the optical thickness of a Fabry Perot polymer film sensing interferometer has been developed for ultrasound measurement and imaging applications in medical and industrial fields. The technique provides a broadband (>50 MHz) response and excellent detection sensitivities (<1 kPa), comparable to those of piezoelectric PVDF transducers. A key feature is that the sensing geometry is defined by the area of the polymer sensing film that is optically addressed. As a consequence, very small element sizes (in principle down to the optical diffraction limit of a few μm) can be obtained without compromising detection sensitivity—a useful advantage over piezoelectric transducers. Additionally, by spatially sampling over a relatively large aperture, a high-density ultrasound receive array can readily be configured. These attributes lend the technique to a variety of applications such as mapping the output of broadband ultrasound sources, transmission ultrasound imaging, and biomedical photoacoustic imaging. The latter is particularly promising, offering the prospect of high-resolution imaging of optically absorbing soft tissue structures such as blood vessels for the study of tumors and other tissue abnormalities characterized by changes in the structure and oxygen status of the microvasculature.

Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging

A 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 x 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-microm-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-microm Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing.

Development of a laser photothermoacoustic frequency-swept system for subsurface imaging: Theory and experiment

In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging.

Two-dimensional photoacoustic imaging by use of Fourier-transform image reconstruction and a detector with an anisotropic response.

Theoretical and experimental aspects of two-dimensional (2D) biomedical photoacoustic imaging have been investigated. A 2D Fourier-transform-based reconstruction algorithm that is significantly faster and produces fewer artifacts than simple radial backprojection methods is described. The image-reconstruction time for a 208 x 482 pixel image is approximately 1 s. For the practical implementation of 2D photoacoustic imaging, a rectangular detector geometry was used to obtain an anisotropic detection sensitivity in order to reject out-of-plane signals, thereby permitting a tomographic image slice to be reconstructed. This approach was investigated by the numerical modeling of the broadband directional response of a rectangular detector and imaging of various spatially calibrated absorbing targets immersed in a turbid phantom. The experimental setup was based on a Q-switched Nd:YAG excitation laser source and a mechanically line-scanned Fabry-Perot polymer-film ultrasound sensor. For a 800 microm x 200 microm rectangular detector, the reconstructed image slice thickness was 0.8 mm up to a vertical distance of z = 3.5 mm from the detector, increasing thereafter to 2 mm at z = 10 mm. Horizontal and vertical spatial resolutions within the reconstructed slice were approximately 200 and 60 microm, respectively.