Optoacoustic (OA) imaging in combination with diagnostic pulse-echo ultrasound is most flexibly implemented with irradiation optics and acoustic probe integrated in epi-style in a combined probe. Unfortunately, clinical epi-OA imaging depth is typically limited to one centimetre owing to clutter signals that originates from the site of tissue irradiation. In past years we have developed displacement-compensated averaging (DCA) for clutter reduction, based on the clutter decorrelation that occurs when palpating the tissue using the ultrasound probe. This method has now been implemented on a research ultrasound system for real time scanning with freehand guidance of the linear probe. Volunteer results confirm that clutter is significant in clinical OA imaging, and that DCA significantly improves image contrast as compared to conventional averaging. Clutter reduction is therefore a basic requirement for a successful combination of OA imaging with pulse-echo ultrasound.
Sound speed as a diagnostic marker for various diseases of human tissue has been of interest for a while. Up to now, mostly transmission ultrasound computed tomography (UCT) was able to detect spatially resolved sound speed, and its promise as a diagnostic tool has been demonstrated. However, UCT is limited to acoustically transparent samples such as the breast. We present a novel technique where spatially resolved detection of sound speed can be achieved using conventional pulse-echo equipment in reflection mode. For this purpose, pulse-echo images are acquired under various transmit beam directions and a two-dimensional map of the sound speed is reconstructed from the changing phase of local echoes using a direct reconstruction method. Phantom results demonstrate that a high spatial resolution (1 mm) and contrast (0.5 % of average sound speed) can be achieved suitable for diagnostic purposes. In comparison to previous reflection-mode based methods, CUTE works also in a situation with only diffuse echoes, and its direct reconstruction algorithm enables real-time application. This makes it suitable as an addition to conventional clinical ultrasound where it has the potential to benefit diagnosis in a multimodal approach. In addition, knowledge of the spatial distribution of sound speed allows full aberration correction and thus improved spatial resolution and contrast of conventional B-mode ultrasound.
In a multi-modal combination of optoacoustic (OA) and pulse-echo ultrasound (US) imaging, epi-mode irradiation with the irradiation optics integrated with the acoustic probe has the advantage of flexible clinical application on any part of the body that is already accessible to US. In epi-mode strong clutter limits the OA imaging depth to often around one centimetre. We investigated clutter in automated scanning of volunteer forearms using a real-time combined OA and US system. The results agree well with our theory that clutter arises from strong optical absorption at the location of tissue illumination. As a consequence, we show that an intermediate separation distance between imaging plane and irradiation region leads to superior OA image contrast compared to an irradiation close to the imaging plane.
Optoacoustic (OA) imaging will experience broadest clinical application if implemented in epi-style with the irradiation optics and the acoustic probe integrated in a single probe. This will allow most flexible imaging of the human body in a combined system together with echo ultrasound (US). In such a multimodal combination, the OA signal could provide functional information within the anatomical context shown in the US image, similar to what is already done with colour flow imaging. Up to date, successful deep epi-OA imaging was difficult to achieve, owing to clutter and acoustic aberrations. Clutter signals arise from strong optical absorption in the region of tissue irradiation and strongly reduce contrast and imaging depth. Acoustic aberrations are caused by the inhomogeneous speed of sound and degrade the spatial resolution of deep tissue structures, further reducing contrast and thus imaging depth. In past years we have developed displacement-compensated averaging (DCA) for clutter reduction based on the clutter decorrelation that occurs when palpating the tissue using the ultrasound probe. We have now implemented real-time DCA on a research ultrasound system to evaluate its clutter reduction performance in freehand scanning of human volunteers. Our results confirm that DCA significantly improves image contrast and imaging depth, making clutter reduction a basic requirement for a clinically successful combination of epi-OA and US imaging. In addition we propose a novel technique which allows automatic full aberration correction of OA images, based on measuring the effect of aberration spatially resolved using echo US. Phantom results demonstrate that this technique allows spatially invariant diffraction-limited resolution in presence of a strong aberrator.
KEYWORDS: Blood vessels, Transducers, Acoustics, Tissues, In vivo imaging, Tissue optics, Signal detection, Imaging systems, Absorption, 3D image processing
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.
For real-time optoacoustic imaging of the human body, a linear array transducer and reflection mode optical
irradiation is usually preferred. Such a setup, however, results in significant image background, which prevents
imaging structures at the ultimate depth limited only by the signal noise level. Therefore we previously
proposed a method for image background reduction, based on displacement-compensated averaging (DCA) of
image series obtained when the tissue sample under investigation is gradually deformed. Optoacoustic signals
and background signals are differently affected by the deformation and can thus be distinguished. The proposed
method has now been applied to imaging artificial tumors embedded inside breast phantoms. Optoacoustic
images are acquired alternately with pulse-echo images using a combined optoacoustic/ echo-ultrasound device.
Tissue deformation is accessed via speckle tracking in pulse echo images, and optoacoustic images are
compensated for the local tissue displacement. In that way optoacoustic sources are highly correlated between
subsequent images, while background is decorrelated and can therefore be reduced by averaging. We show that
breast image contrast is strongly improved and detectability of embedded tumors significantly increased, using
the DCA method.
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