Real-time clinical clutter reduction in combined epi-optoacoustic and ultrasound imaging

Flexible imaging of the human body, a requirement for broad clinical application, is obtained by direct integration of optoacoustic (OA) imaging with echo ultrasound (US) in a multimodal epi-illumination system. Up to date, successful deep epi-OA imaging is difficult to achieve owing to clutter. Clutter signals arise from optical absorption in the region of tissue irradiation and strongly reduce contrast and imaging depth. Recently, we developed a displacement-compensated averaging (DCA) technique for clutter reduction based on the clutter decorrelation that occurs when palpating the tissue. To gain first clinical experience on the practical value of DCA, we implemented this technique in a combined clinical OA and US imaging system. Our experience with freehand scanning of human volunteers reveals that real-time feedback on the clutter-reduction outcome is a key factor for achieving superior contrast and imaging dept

Full correction for spatially distributed speed-of-sound in echo ultrasound based on measuring aberration delays via transmit beam steering

Aberrations of the acoustic wave front, caused by spatial variations of the speed-of-sound, are a main limiting factor to the diagnostic power of medical ultrasound imaging. If not accounted for, aberrations result in low resolution and increased side lobe level, over all reducing contrast in deep tissue imaging. Various techniques have been proposed for quantifying aberrations by analysing the arrival time of coherent echoes from so-called guide stars or beacons. In situations where a guide star is missing, aperture-based techniques may give ambiguous results. Moreover, they are conceptually focused on aberrators that can be approximated as a phase screen in front of the probe. We propose a novel technique, where the effect of aberration is detected in the reconstructed image as opposed to the aperture data. The varying local echo phase when changing the transmit beam steering angle directly reflects the varying arrival time of the transmit wave front. This allows sensing the angle-dependent aberration delay in a spatially resolved way, and thus aberration correction for a spatially distributed volume aberrator. In phantoms containing a cylindrical aberrator, we achieved location-independent diffraction-limited resolution as well as accurate display of echo location based on reconstructing the speed-of-sound spatially resolved. First successful volunteer results confirm the clinical potential of the proposed technique

Monte Carlo modeling of polarized light propagation: Stokes vs Jones - Part II

In this second part of our comparative study inspecting the (dis)similarities between “Stokes” and “Jones,” we present simulation results yielded by two independent Monte Carlo programs: (i) one developed in Bern with the Jones formalism and (ii) the other implemented in Ulm with the Stokes notation. The simulated polarimetric experiments involve suspensions of polystyrene spheres with varying size. Reflection and refraction at the sample/air interfaces are also considered. Both programs yield identical results when propagating pure polarization states, yet, with unpolarized illumination, second order statistical differences appear, thereby highlighting the pre-averaged nature of the Stokes parameters. This study serves as a validation for both programs and clarifies the misleading belief according to which “Jones cannot treat depolarizing effects.

Spectral correction of OA signals based on multiple irradiation sensing: experimental validation

In this study we show that the spectral distortion of OA signals, caused by wavelength-dependent optical attenuation inside the bulk tissue, can be corrected based on OA imaging, when using multiple-irradiation sensing. The tissue is modeled as a strongly scattering background, in which a discrete number of blood vessels, characterized by a higher absorption than the background, are sparsely distributed. OA signals generated by these vessels, which serve as intrinsic “fluence detectors”, are recorded as a function of irradiation position. In order to account for realistic situations, we have developed a semi-empirical light diffusion model that is fitted to the recorded signals, so as to determine the background’s optical effective attenuation coefficient for arbitrarily shaped tissues. The experimental validation of this model was performed on tissue-mimicking phantoms. The results demonstrate a successful correction of the measured OA spectrum of the embedded vessel-like inclusions, in the presence of lateral geometrical boundaries and when vessel-like absorbing structures influence the light propagation

Multiple irradiation sensing of the optical effective attenuation coefficient for spectral correction in handheld OA imaging

Spectral optoacoustic (OA) imaging enables spatially-resolved measurement of blood oxygenation levels, based on the distinct optical absorption spectra of oxygenated and de-oxygenated blood. Wavelength-dependent optical attenuation in the bulk tissue, however, distorts the acquired OA spectrum and thus makes quantitative oxygenation measurements challenging. We demonstrate a correction for this spectral distortion without requiring a priori knowledge of the tissue optical properties, using the concept of multiple irradiation sensing: recording the OA signal amplitude of an absorbing structure (e.g. blood vessel), which serves as an intrinsic fluence detector, as function of irradiation position. This permits the reconstruction of the bulk effective optical attenuation coefficient μeff,λ. If performed at various irradiation wavelengths, a correction for the wavelength-dependent fluence attenuation is achieved, revealing accurate spectral information on the absorbing structures. Phantom studies were performed to show the potential of this technique for handheld clinical combined OA and ultrasound imaging

Study of clutter origin in in-vivo epi-optoacoustic imaging of human forearms

Epi-optoacoustic (OA) imaging offers flexible clinical diagnostics of the human body when the irradiation optic is attached to or directly integrated into the acoustic probe. Epi-OA images, however, encounter clutter that deteriorates contrast and significantly limits imaging depth. This study elaborates clutter origin in clinical epi-optoacoustic imaging using a linear array probe for scanning the human forearm. We demonstrate that the clutter strength strongly varies with the imaging location but stays stable over time, indicating that clutter is caused by anatomical structures. OA transients which are generated by strong optical absorbers located at the irradiation spot were identified to be the main source of clutter. These transients obscure deep in-plane OA signals when detected by the transducer either directly or after being acoustically scattered in the imaging plane. In addition, OA transients generated in the skin below the probe result in acoustic reverberations, which cause problems in image interpretation and limit imaging depth. Understanding clutter origin allows a better interpretation of clinical OA imaging, helps to design clutter compensation techniques and raises the prospect of contrast optimization via the design of the irradiation geometry