For clinical optoacoustic imaging, linear probes are preferably used because they allow versatile imaging of the human body with real-time display and free-hand probe guidance. The two-dimensional (2-D) optoacoustic image obtained with this type of probe is generally interpreted as a 2-D cross-section of the tissue just as is common in echo ultrasound. We demonstrate in three-dimensional simulations, phantom experiments, and in vivo mouse experiments that for vascular imaging this interpretation is often inaccurate. The cylindrical blood vessels emit anisotropic acoustic transients, which can be sensitively detected only if the direction of acoustic radiation coincides with the probe aperture. Our results reveal for this reason that the signal amplitude of different blood vessels may differ even if the vessels have the same diameter and initial pressure distribution but different orientation relative to the imaging plane. This has important implications for the image interpretation, for the probe guidance technique, and especially in cases when a quantitative reconstruction of the optical tissue properties is required.
Photoacoustic imaging, based on ultrasound detected after laser irradiation, is an extension to diagnostic ultrasound for imaging the vasculature, blood oxygenation and the uptake of optical contrast media with promise for cancer diagnosis. For versatile scanning, the irradiation optics is preferably combined with the acoustic probe in an epi-style arrangement avoiding acoustically dense tissue in the acoustic propagation path from tissue irradiation to acoustic detection. Unfortunately epiphotoacoustic imaging suffers from strong clutter, arising from optical absorption in tissue outside the image plane, and from acoustic backscattering. This limits the imaging depth for useful photoacoustic image contrast to typically less than one centimeter. Deformation-compensated averaging (DCA), which takes advantage of clutter decorrelation induced by palpating the tissue with the imaging probe, has previously been proposed for clutter reduction. We demonstrate for the first time that DCA results in reduced clutter in real-time freehand clinical epiphotoacoustic imaging. For this purpose, combined photoacoustic and pulse-echo imaging at 10-Hz frame rate was implemented on a commercial scanner, allowing for ultrasound-based motion tracking inherently coregistered with photoacoustic frames. Results from the forearm and the neck confirm that contrast is improved and imaging depth increased by DCA.
Clinical photoacoustic (PA) imaging relies on illuminating objects at depth. To do this, it is important to optimise the illumination geometry with respect to the sensitivity pattern of the acoustic receiver, taking optical scattering into account. The three-dimensional point spread function (3D PSF) measured at various depths as a function of the optimisation variables, is being explored to determine its usefulness for this purpose. The 3D PSF of a reflection mode photoacoustic scanner was measured by acquiring a series of PA images of the tip of a 0.25mm radius graphite rod placed at a depth of 2 cm, by translating the photoacoustic linear array transducer and illumination optics in the elevational direction. This was done for a series of angles and separations of the fibre optic illuminators, for a background medium of 1% intralipid, which simulates, to first order, the optical scattering that would be experienced in tissue. The background noise was found to be influenced by the illumination geometry, and may have been associated with PA clutter generated by absorption in the background medium. The angle of illumination and distance separating fibre optic illuminators were found to be weakly optimum at around 76 degrees and 15.5mm respectively, where the PSF amplitude passed through a weak maximum. As expected, the shape of the 3D PSF was found to be independent of illumination geometry. However, the combination of using the tip of a graphite rod as a point object, and plotting the 3D PSF as a means of locating the peak signal, appears to be a successful method of studying the effect of illumination variables on signal strength. Ultimately when complete, this optimisation should enable the clarity images at the depth of interest to be maximised.
To assess the malignancy and progression of a tumour, parameters such as the size and number density of the
microvessels are expected to be important. The optical absorption due to the blood that fills the microvessels can be
visualised by optoacoustic imaging (OA). We have previously reported that increasing the inhomogeneity of absorption
within a large absorbing volume produces evidence of reduced acoustic coherence which results in improved contrast
and boundary detectability. Here we propose to take advantage of the expectation that the detailed nature of the
inhomogeneity should influence the frequency spectrum of the OA signal. The overall aim of this work is to determine
whether an analysis of the frequency spectrum of the emitted optoacoustic signal can be used to determine the scale of
this absorption inhomogeneity, in particular parameters such as the characteristic size and separation of the absorbers
(microvessels). In the preliminary study reported here, various gelatine-intralipid phantoms containing cylindrical wallless
tubes filled with an ink solution were measured in water with a linear array ultrasound detector, using pulsedillumination
that had been adjusted for an optimal distribution of light fluence with depth. Simulations of the
experiments were also conducted, using a time domain acoustic propagation method. The results confirm that
optoacoustic signals bear information on the sizes and distribution of the absorbers in their frequency spectra. It is shown
that a simple way to determine the diameter of a single cylindrical absorber is to estimate the quefrency of the peak in the
cepstrum of the measured signal. Further work is proposed to extend this to the statistical estimation of mean diameter
and mean separation for an ensemble of similar absorbers and to absorbers with a diameter that is smaller than the axial
resolution of the acoustic receiver.
Optoacoustic (OA) imaging allows optical absorption contrast to be visualised using thermoelastically generated
ultrasound. To date, optoacoustic theory has been applied to homogeneously absorbing tissue models that may describe,
for example, large vessels filled with blood, where the whole target will act as a coherent source of sound. Here we
describe a new model in which the optical absorbers are distributed inhomogeneously, as appropriate to describe
microvasculature, or perhaps the distribution of molecularly targeted OA contrast agents inside a tumour. The degree of
coherence over the resulting distributed acoustic source is influenced by parameters that describe the scale of the
inhomogeneity, such as the sizes of the absorbers and the distances between them. To investigate the influence of these
parameters on OA image appearance, phantoms with homogeneously and imhomogeneously absorbing regions were
built and imaged. Simulations of the same situation were conducted using a time domain acoustic propagation method.
Both simulations and experiments showed that introducing inhomogeneity of absorption produces more complete images
of macroscopic targets than are obtained with a homogeneous absorption. Image improvement and target detectability
were found to reach a maximum at an intermediate value of the length-scale of the inhomogeneity that was similar to the
axial resolution of the acoustic receiver employed. As the scale of inhomogeneity became finer than this the target's
detectability and appearance began to revert to that for homogeneous absorption. Further understanding of this topic is
believed to be important for optimising the design of clinical optoacoustic imaging systems.