Abstract
In photoacoustic imaging, the intensity of photoacoustic signal induced by optical absorption in biological tissue is proportional to light energy deposition, which is the product of the absorption coefficient and the local light fluence. Because tissue optical properties are highly dependent on the wavelength, the spectrum of the local light fluence at a target tissue beneath the sample surface is different than the spectrum of the incident light fluence. Therefore, quantifying the tissue optical absorption spectrum by using a photoacoustic technique is not feasible without the knowledge of the local light fluence. In this work, a highly accurate photoacoustic measurement of the subsurface tissue optical absorption spectrum has been achieved for the first time by introducing an extrinsic optical contrast agent with known optical properties. From the photoacoustic measurements with and without the contrast agent, a quantified measurement of the chromophore absorption spectrum can be realized in a strongly scattering medium. Experiments on micro-flow vessels containing fresh canine blood buried in phantoms and chicken breast tissues were carried out in a wavelength range from 680 nm to 950 nm. Spectroscopic photoacoustic measurements of both oxygenated and deoxygenated blood specimens presented an improved match with the references when employing this technique.
Highlights
Photoacoustic tomography (PAT), referred to as thermoacoustic or optoacoustic tomography, is an emerging biomedical imaging technique which has drawn considerable interest in the past decade [1,2,3,4,5,6,7,8,9]
We present a new technique that can directly measure the spectrum of local light fluence at a target tissue in a highly scattering medium by introducing an extrinsic optical contrast agent with known optical absorption spectrum
To validate the sensitivity and accuracy of our SPAT system in describing the optical absorption spectrum of a tissue, spectroscopic measurement of blood specimens was first conducted in a non-scattering medium
Summary
Photoacoustic tomography (PAT), referred to as thermoacoustic or optoacoustic tomography, is an emerging biomedical imaging technique which has drawn considerable interest in the past decade [1,2,3,4,5,6,7,8,9]. In PAT, a short-pulsed laser source is used to illuminate a biological sample. The laser-generated photoacoustic signals that are excited by thermoelastic expansion resulting from a transient temperature rise on the order of 10 mK can be measured by a wide-band ultrasonic transducer. The acquired signals can be used to rebuild the distribution of optical energy deposition within the sample. The amplitude and the time of arrival of the photoacoustic (PA) signal provide respectively the information about the intensity and the spatial distribution of tissue optical absorption [10,11]. PAT images have the advantages of the inherent high contrast of diffuse optical imaging and the high spatial resolution of ultrasonography
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