In the reconstruction process of photo acoustic experiments, it was observed that adding a passive element to the
experimental setup, improves the quality of the reconstruction of the object. This contribution analyzes this effect
in some detail. We consider a cylindrical configuration. We start from an artificial and theoretically constructed
optical absorption distribution that radiates sound waves when interrogated by the optical pulse. We analyze in
the experimental setup the addition of the passive element to this example. The reported investigation is a part
of a larger study on the existence, uniqueness and stability of photo acoustic inverse source reconstructions.
We present a 'hybrid' imaging system, which can image both optical absorption properties and acoustic
transmission properties of an object in a two-dimensional slice using a computed tomography photoacoustic
imager. The ultrasound transmission measurement method uses a strong absorber of light which is placed in the
path of the light illuminating the sample. This acts as a source of ultrasound, whose interaction with the sample
can be measured at the far-end of the sample using the same ultrasound detector used for photoacoustics. Such
measurements are made at various angles around the sample in a computerized tomography approach. The
ultrasound transients at the multi-element detector at all projections are analyzed for both times-of-arrival and
amplitude. Using a fan-beam projection reconstruction algorithm we obtain hybrid images of optical absorption,
speed-of-sound and acoustic attenuation. We validate the method on an appropriate phantom.
Photoacoustic imaging is a relatively new medical imaging modality. In principle it can be used to image the
optical absorption distribution of an object by measurements of optically induced acoustic signals. Recently
we have developed a modified photoacoustic measurement system which can be used to simultaneously image
the ultrasound propagation parameters as well. By proper placement of a passive element we obtain isolated
measurements of the object's ultrasound propagation parameters, independent of the optical absorption inside
the object. This passive element acts as a photoacoustic source and measurements are obtained by allowing the
generated ultrasound signal to propagate through the object. Images of the ultrasound propagation parameters,
being the attenuation and speed of sound, can then be reconstructed by inversion of a measurement model.
This measurement model relates the projections non-linearly to the unknown images, due to ray refraction
effects. After estimating the speed of sound and attenuation distribution, the optical absorption distribution
is reconstructed. In this reconstruction problem we take into account the previously estimated speed of sound
distribution. So far, the reconstruction algorithms have been tested using computer simulations. The method
has been compared with existing algorithms and good results have been obtained.
Photoacoustic imaging is an upcoming medical imaging modality with the potential of imaging both optical and
acoustic properties of objects. We present a measurement system and outline reconstruction methods to image
both speed of sound and acoustic attenuation distributions of an object using only pulsed light excitation. These
acoustic properties can be used in a subsequent step to improve the image quality of the optical absorption
distribution. A passive element, which is a high absorbing material with a small cross-section such as a carbon
fiber, is introduced between the light beam and the object. This passive element acts as a photoacoustic source
and measurements are obtained by allowing the generated acoustic signal to propagate through the object. From
these measurements we can extract measures of line integrals over the acoustic property distribution for both
the speed of sound and the acoustic attenuation. Reconstruction of the acoustic property distributions then
comes down to the inversion of a linear system relating the obtained projection measurements to the acoustic
property distributions. We show the results of applying our approach on phantom objects. Satisfactory results
are obtained for both the reconstruction of speed of sound and the acoustic attenuation.
Photoacoustics is a hybrid imaging technique that combines the contrast available to optical imaging with
the resolution that is possessed by ultrasound imaging. The technique is based on generating ultrasound from
absorbing structures in tissue using pulsed light. In photoacoustic (PA) computerized tomography (CT) imaging,
reconstruction of the optical absorption in a subject, is performed for example by filtered backprojection. The
backprojection is performed along circular paths in image space instead of along straight lines as in X-ray CT
imaging. To achieve this, the speed-of-sound through the subject is usually assumed constant. An unsuitable
speed-of-sound can degrade resolution and contrast. We discuss here a method of actually measuring the speed-of-
sound distribution using ultrasound transmission through the subject under photoacoustic investigation. This
is achieved in a simple approach that does not require any additional ultrasound transmitter. The method uses
a passive element (carbon fiber) that is placed in the imager in the path of the illumination which generates
ultrasound by the photoacoustic effect and behaves as an ultrasound source. Measuring the time-of-flight of this
ultrasound transient by the same detector used for conventional photoacoustics, allows a speed-of-sound image
to be reconstructed. This concept is validated on phantoms.
The effects of an inappropriately chosen speed-of-sound in photoacoustic imaging reconstructions are to cause
blurring of images and impairment of contrast. Here we outline a new methodology to measure the speed-of-sound
in a photoacoustic imager with little or no additional cost and without the need to perform extra measurements.
The method uses a strong absorber of light which is placed in the path of the light illuminating the sample.
This acts as a source of ultrasound whose interaction with the sample can be measured at the far-end of the
sample using the same ultrasound detector used for photoacoustics. This yields time-of-arrival measurements of
the ultrasound transient at the multi-element detector. Such measurements are made at various angles around
the sample in a computerized tomography approach. Reconstruction of the time-of-arrival or speed-of-sound
tomogram of the object can be made by implementing a fan-beam projection reconstruction algorithm. We
present the concept and validate the method on a speed-of-sound phantom.