Passive element enriched photoacoustic computed tomography (PER PACT) for simultaneous imaging of acoustic propagation properties and light absorption\ud

We present a ‘hybrid’ imaging approach which can image both light absorption properties and acoustic transmission properties of an object in a two-dimensional slice using a computed tomography (CT) photoacoustic imager. The ultrasound transmission measurement method uses a strong optical absorber of small cross-section placed in the path of the light illuminating the sample. This absorber, which we call a passive element acts as a source of ultrasound. The interaction of ultrasound with the sample can be measured in transmission, using the same ultrasound detector used for photoacoustics. Such measurements are made at various angles around the sample in a CT approach. Images of the ultrasound propagation parameters, attenuation and speed of sound, can be reconstructed by inversion of a measurement model. We validate the method on specially designed phantoms and biological specimens. The obtained images are quantitative in terms of the shape, size, location, and acoustic properties of the examined heterogeneitie

Fast acousto-optic tissue sensing with tandem nanosecond pulses experiments and theory

Photoacoustics allows imaging with optical contrast deep in biological tissues. The signal strength depends on the fluence distribution which is unknown. To obtain more quantitative photoacoustic measurements the signal must be normalized with acousto-optic measurements. In Acousto-optics light is modulated with ultrasound and the amount of modulated light detected is a measure for the fluence at the ultrasound focus. In this thesis a novel acousto-optic method is explored that can use the same laser system as the photoacoustics and overcomes one of the largest obstacles to apply acousto-optics in living tissues. This is achieved by reducing the signal acquisition time orders of magnitude compared with existing acousto-optic methods. This is done by recording the interference patterns from a coherent but short tandem laser pulse that consist of two pulses only tens of nanoseconds apart. This method enables acoustoptic measurements in biological tissues and fluency corrected photo acoustics

Developing a stochastic model for acousto-optic tissue imaging

Direct optical measurements in scattering media offer poor resolution due to the high scattering. Ultrasound is scattered orders of magnitude less in tissue compared with light and therefore offers good resolution. Photoacoustics and acoustooptics are both relatively new hybrid techniques that enable measurements of optical properties in scattering media by combining ultrasound and light. Quantified measurements of the fluence and absorption coefficient however are desired and can not be performed by these separate techniques. A new approach to achieve this goal is to combine both hybrid techniques. By combining photoacoustic and acousto-optic measurements there is sufficient information to calculate the absorption coefficient and fluence at the ultrasound focus used for the acousto-optics. We require knowledge on the interaction of light and sound inside tissue, so the size of the so called tagging volume can be determined. This tagging volume is defined by the size and shape of the ultrasound focus used in the acousto-optic measurements. A stochastic model for acousto-optics is under development that used existing knowledge on the in the interaction between light and sound. By separating light transport and the interactions of light and sound and writing this interaction as a probability density function it is possible to find the effective geometrical properties of the tagging volume. At the moment multiple interaction mechanisms of sound and light are added to this model. In the future this model will be validated in phantoms and biological tissue