H. Lamela
Optoelectronic and laser technology group (GOTL), Carlos III de Madrid University, 28911 Leganes, Madrid, Spain

Abstract:

Optoacoustic tomography [1] is a promising non-invasive non-ionizing imaging technique to visualize biological soft tissues. It combines the advantages of optical absorption contrast and optical spectroscopy with the spatial resolution of ultrasound imaging techniques. The optical absorption of the laser pulse by the different elements, which the tissue is composed of, produces heating and due to the thermoelastic effect broadband ultrasonic pulses (several megahertz) are generated. These ultrasonic pulses propagate to the surface of the tissue where they are detected by ultrasonic transducers. The detection technology traditionally used in conventional ultrasonic imaging is based on piezoelectric transducers, which are highly sensitive but have narrow bandwidth due to their resonant nature. However, the optoacoustic generated signals require broadband detectors to image the different sizes of absorption regions inside the body. The detectors based on thin piezoelectric polymer film, like PVDF, are wideband however the signal to noise ratio increases and the sensitivity decrease as their size is reduced. This is a problem at high ultrasonic frequencies when the detector size must be shorter than half of the wavelength or when miniaturizing for integration in an array of sensors. Another drawback to piezoelectric sensors, related to their electrical nature, is that they are not immune to electromagnetic interference.

The optical detection of ultrasound has been studied as an alternative to piezoelectric technology. We can distinguish two kinds of ultrasound optical sensors, ones that monitories pressure induced displacements of a membrane or resonant optical cavity; others work based on a pressure induced index refraction variation in or around the sensor material. In the first group the following are included: etalons [2, 3], fiber Bragg gratings [4], and dielectric multilayer interference filters [5]. Intrinsic fiber optic interferometric sensors [6] are in the second group. All these optical sensors, contrary to piezoelectric transducers, are not affected by external electromagnetic disturbances or other artifacts like electrical noise and thermal signals produced by the direct laser pulse illumination. In particular, the fabrication of intrinsic fiber optic interferometric sensors is straightforward and involves the use of low cost materials. Moreover the sensitivity of these sensors can be improved by appropriate folding or coiling of the fiber increasing the surface area in which the acoustic field interacts with the optical fiber [7]. Besides this, intrinsic fiber optic Mach-Zehnder and Fabry-Perot sensors have been proposed as integrating line detectors for optoacoustic imaging tomography [8]. High resolution optoacoustic 3- dimensional images have been obtained rotating the object and scanning in one direction with a free laser beam as line detector interrogated by a Mach-Zender interferometer [9].

In this work we present for the first time results on the detection of optoacoustic signals using fiber optic interferometric sensors which are designed for the frequency range from 100 KHz to 5 MHz. Moreover the sensitivity can be higher using polymer optical fiber instead of silica optical fiber sensors [10] reaching deeper and fainter optoacoustic signals. In this paper, we propose a novel fiber optic interferometric ultrasonic imaging system with a finite aperture to reconstruct optoacoustic images for breast cancer detection. This new system has been compared with images obtained using a PVDF phantom probe imaging system LOIS (Laser Optoacoustic Imaging System, Fairway Medical Technologies, Houston, TX, USA)) [11]. (See Fig. 1)
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References:

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[11]      S. A. Ermilov, T. HKhamapirad, A. Conjusteau, M. H. Leonard, R. Lacewell, K. Mehta, T. Miller, and A. Oraevsky, "Laser optoacoustic imaging system for detection of breast cancer", Journal of Biomedical Optics, vol. 14, pp. 024007-024001-024014, 2009.