A setup that allows for the co-registration of photoacoustic (PA) and speed-of-sound (SOS) section images is presented. By means of ultrasound (US) transmission imaging the distribution of the acoustic speed within an object can be obtained. Our method uses the PA effect for the generation of the traversing US waves. Short near-infrared (NIR) laser pulses emitted by a Nd:YAG laser system are used to illuminate external optical absorbing targets at various distances in front of the sample. At the same time the object under investigation is illuminated by a part of the same frequency-doubled laser pulse. A free laser beam, which is part of a Mach-Zehnder interferometer, is used for the detection of the US signals coming from and passing through the sample. Due to a cascaded arrangement of absorbing targets for laser ultrasound (LUS) generation a single laser pulse yields information for a projection of the SOS distribution. The resolution is determined by the number and width of LUS sources. Separation of the signals arriving at the integrating detector is possible because of their different times of flight. After collection of the data reconstruction of a two-dimensional SOS map is accomplished by applying an inverse Radon transform to the projections. For PA section imaging a cylindrical acoustic reflector behind the detector yields an acoustic focus in the observed slice. To the data gathered by detecting the reflected PA signals also the inverse Radon transform is applied to obtain a reconstructed image of the illuminated section. In this paper a detailed description of the setup is given and the results of experiments on two- and three-dimensional phantoms are presented.
Three-dimensional photoacoustic tomography with line sensors, which integrate the pressure along their length, has shown to produce accurate images of small animals. To reduce the scanning time and to enable in vivo applications, a detection array is built consisting of 64 piezoelectric line sensors which are arranged on a semi-cylinder. When measuring line integrated pressure signals around the imaging object, the three-dimensional photoacoustic imaging problem is reduced to a set of two-dimensional reconstructions and the measurement setup requires only a single axis of rotation. The shape and size of the array were adapted to the given problem of biomedical imaging and small animal imaging in particular. The length and width of individual line elements had to be chosen in order to take advantage of the favorable line integrating properties, maintaining the requested resolution of the image. For data acquisition the signals from the 64 elements are amplified and multiplexed into a 32 channel digitizer. Single projection images are recorded with two laser pulses within 0.2 seconds, as determined by the laser pulse repetition rate of 10 Hz. Phantom experiments are used for characterization of the line-array. Compared to previous implementations with a single line sensor scanning around an object, with the developed array the data acquisition time can be reduced from about one hour to about one minute.
Acoustic line detectors have been shown to be capable of providing accurate signals for three-dimensional photoacoustic tomography. Free and guided beam optical Mach-Zehnder interferometers (MZI) have been used as well as a waveguide Fabry-Perot interferometer (FPI). The ultimate sensitivity is expected from a FPI where the optical field in the resonator propagates in the acoustic coupling medium (water) surrounding the imaged object. Such a free-beam FPI is completely optically and acoustically transparent, while providing a higher sensitivity compared to the MZI due to the multiple beam interference. In this work the performance of a FPI for measurement of ultrasound waves is compared to a MZI. It is shown that an at least 4.5-fold higher signal to noise ratio is achieved compared to a MZI. The resolution of the FPI is simulated and measured, showing a constant diameter of the interferometer beam. Verification of the stability of the free beam FPI over longer time periods is demonstrated by acquiring a two-dimensional tomography image of a phantom. The sensitivity and stability of the setup makes it suitable for tomographic imaging.
A purely optical setup for simultaneous photoacoustic (PA) and laser-ultrasound (US) tomography is presented. It is shown that combined imaging can be achieved by using the same laser pulse for photoacoustic generation and for launching a broadband ultrasound pulse from an optically absorbing target. Detection of the laser-generated plane waves that have been scattered at the imaging object and of the photoacoustic signals emitted from the sample is done interferometrically. This way data for PA and US imaging are acquired within one single measurement. Distinction between the signals is possible due to their different times of flight. After data separation, image reconstruction is done using standard back-projection algorithms. The resolution of the setup was estimated and images of a zebra fish are shown, demonstrating the complementary information of the two imaging modalities.
We propose the further development of the optical detection setup towards photoacoustic (PA) and ultrasound (US) dual-modality
section imaging. Both imaging modalities use optical generation and detection of ultrasound waves. A onesided
chrome coated concave cylindrical optical lens is used as target to induce acoustic signals for US imaging and as
acoustic mirror that forms acoustic images. By probing the temporal evolution of the acoustic images with an optical
beam perpendicular to the acoustic axis and simultaneously rotating the object, data for reconstruction of PA and US
slice images are acquired. All acoustic signals are excited optically via the thermoelastic effect using laser pulses coming
from the same laser system.
A method is proposed that utilizes the advantages of optical ultrasound detection in two-dimensional photoacoustic section imaging, combining an optical interferometer with an acoustic mirror. The concave mirror has the shape of an elliptical cylinder and concentrates the acoustic wave generated around one focal line in the other one, where an optical beam probes the temporal evolution of acoustic pressure. This yields line projections of the acoustic sources at distances corresponding to the time of flight, which, after rotating the sample about an axis perpendicular to the optical detector, allows reconstruction of a section using the inverse Radon transform. A resolution of 120 µm within and 1.5 mm between the sections can be obtained with the setup. Compared to a bare optical probe beam, the signal-to-noise ratio (SNR) is seven times higher with the mirror. Furthermore, the imaging system is tested on a biological sample.
The method proposed in this work combines the advantages of optical detection (optical and acoustical transparency)
with 2D slice imaging, using an optical interferometer combined with an acoustic reflector. The concave reflector has the
shape of an elliptical cylinder and concentrates the acoustic wave generated around one focal line in the other one, where
an optical beam probes the temporal evolution of acoustic pressure. This yields line projections of the initial acoustic
pressure sources at distances corresponding to the time of flight. Image reconstruction from the signals recorded while
rotating the sample about an axis perpendicular to the optical detector requires only the application of the inverse Radon
transform. The resolution and sensitivity of the detection system were investigated in experiments on phantom samples.
Furthermore, the imaging system was tested on a real biological sample.
A piezoelectric detection system consisting of concentric rings is investigated for large depth of field photoacoustic
imaging. Compared to a single ring, the array with its dynamic focusing capability leads to a reduction of imaging
artifacts. Image resolution studies are performed in simulations and in experiments. Detector arrays with four and eight
rings were simulated to compare axial and lateral resolution. In simulation an improvement regarding the reduction of Xshaped
imaging artifacts for the eight ring detection system in comparison to a four ring detection is presented. To
compare the resolution axial and lateral profiles are shown and discussed. Furthermore signal processing methods are
demonstrated, such as coherence factor weighting, which improve resolution and further reduce artifacts. To demonstrate
the multiple ring detection system in experiment we used a 4 ring detection system and crossed horse hairs as phantom.
Different projection images and a 3D image of the phantom are presented.
A piezoelectric detector with cylindrical shape for photoacoustic section imaging is characterized. This detector is larger
than the imaging object in direction of the cylinder axis, giving rise to its integrating properties. Its focal volume has the
shape of a slice and the acquisition of signals for one section image requires rotation of an object about an axis
perpendicular to this slice. Image reconstruction from the signals requires the application of the inverse Radon transform.
It is shown that implementing the Abel transform is a suitable step in data processing, allowing speeding up the data
acquisition since the scanning angle can be reduced. The resolution of the detector was estimated in directions
perpendicular and parallel to the detection plane. An upper limit for the out of plane resolution is given and section
images of a zebra fish are shown.
Photoacoustic imaging is based on the excitation of ultrasound waves by irradiating objects with short laser pulses.
Absorbing laser energy causes thermal expansion, which leads to broadband ultrasonic waves, carrying information
about size, location and optical properties of the observed target. Images reveal purely optical contrast, yet the technique
is acoustic. Classical ultrasonic imaging generates images with purely acoustical contrast based on the impedance
differences of structures in observed samples. For developing a dual mode scanning acoustic microscope, which uses
simultaneously both contrast mechanism (acoustic pulse-echo and photoacoustic image contrast) ultrasonic pulses with a
large depth of field are advantageous. By illuminating special conically shaped transducers, so called axicons, with short
laser pulses, broadband ultrasonic pulses with a large depth of field at small lateral extension can be excited. These
special pulses, so called X-waves and their use in a microscope are investigated.
Photoacoustic imaging with a scanning, fixed focus receiver gives images with high resolution, without the need for
image reconstruction. For achieving high depth of field, a conically shaped piezoelectric ultrasound detector, the so
called axicon-detector, is investigated. It is characterized by a sustained line of focus with a length that depends only on
the geometry of the detector but not on the wavelength. Simulated and experimentally taken images of various objects
reveal X-shaped artifacts due to the conical surface of the detector. To improve the image quality a frequency domain
deconvolution can be applied, as the point spread function (PSF) of the detector is spatially invariant over the depth of
field. The reduction of the artifacts works well for simulated images but is not functional for experimental data yet.
Nevertheless, the detector gives images with precise shape and position of the investigated samples.
Currently two different types of integrating line sensors are used in photoacoustic tomography (PAT). Thin film
piezoelectric polymer sensors (PVDF) are characterized by compactness, easy handling and the possibility to
manufacture sensing areas with different shape. However, they are vulnerable to electrical disturbance and to scattered
light from the illuminated sample. Also optical sensors are used as integrating line sensors in combination with some
kind of interferometric setup. For example, one arm of a
Mach-Zehnder interferometer or the cavity of a Fabry-Perot
interferometer can be used as line detector. In both cases, the light wave either propagates freely in the liquid or is guided
in an optical fiber. Such sensors are quite immune against noise sources described above and suitable for high bandwidth
detection. One drawback is the limited mobility due to the complex arrangement of the setup.
This study is focused on the comparison of the different implementations of line detectors, mainly on directivity and
sensitivity. Shape and amplitude of signals generated by defined sources are compared among the various sensor types.
While the shape of the signals recorded with the optical free beam detector matches quite well to the simulation the
signals detected with the PVDF detector are affected by directivity effects. This causes a strong distortion of the signal
shape depending on the incident angle of the acoustic wave. How these effects influence the reconstructed projection
image is discussed.
Photoacoustic imaging with a scanning, fixed focus receiver gives images with high resolution, without the need for
reconstruction algorithms. However, the usually employed spherical ultrasound lenses have a limited focal depth that
decreases with increasing lateral resolution due to the inverse relation between numerical aperture and Rayleigh length.
In this study the use of an axicon detector is proposed, consisting of a conical surface onto which a piezoelectric polymer
film is attached. The detector is characterized in simulations and in experiments, demonstrating the expected high
resolution over an extended depth of focus. Simulated and experimental images reveal X-shaped artifacts that are due to
the conical detector surface. Since the point spread function (PSF) of the detector is spatially invariant over the depth of
field, a frequency domain deconvolution can be applied to the images. Although this clearly improves the image quality
in simulations, the reduction of artifacts was not so efficient in experiments. However, the detector is able to produce
images with accurate position and shape of objects. Moreover, the axicon transducer rejects signals from planar surfaces
(e.g. the skin surface) and favors signals from small, isolated sources.