Tuning Viscoelasticity with Minor Changes in Speed of Sound in an Ultrasound Phantom Material

The acoustic properties of ultrasound phantom materials have always been important, but with new applications interrogating tissue mechanical properties, viscoelasticity has also become an interesting feature to consider. Along with Young's modulus, the viscous component of tissue is affected by certain diseases and can therefore be used as a biomarker. Furthermore, viscoelasticity varies between tissue types and individuals, and therefore it would be useful with a phantom material that reflects this physiological range. Here we describe a gel for ultrasound imaging with a range of mechanical properties given by mixing different ratios of two oil-based gels, clear ballistic and styrene-ethylene/butylene-styrene (SEBS). The gels were mixed in five different proportions, ranging from 0-100% of either gel. For each of the gel compositions, we measured time of flight to determine speed of sound, narrowband ultrasound transmission for attenuation, stress-relaxation for viscoelasticity, mass and volume. Analysis of the stress-relaxation data using the generalized Maxwell model suggests that the material can be described by five parameters, E0, E1, E2, η1 and η2, and that each of these parameters decreases as more SEBS is incorporated into the mixed material. Instantaneous Young's modulus (the sum of E0, E1 and E2 in our model) ranges between 49 and 117 kPa for the different ratios, similar to values reported for cancerous tissue. Despite the large span of obtainable mechanical properties, speed of sound is relatively constant regardless of composition, with mean value estimates (± 95 % CI) between 1438 ± 9 and 1455 ± 3 m/s for pure and mixed gels. This was attributed to a variation in density and Poisson's ratio, following from the relation linking them to speed of sound and elasticity. Furthermore, both speed of sound and attenuation were within a suitable range for ultrasound phantoms. Combining this ballistic gel with SEBS copolymer in oil allows for control of mechanical properties, both elastic and viscous as evaluated by the material model. Furthermore, it does so without compromising ease of use, longevity and safety of the pre-made gel

Towards real-time magnetomotive ultrasound imaging

Magnetomotive ultrasound (MMUS) imaging indirectly enables visualization of magnetic nanoparticles (MNPs) with ultrasound. An external time varying magnetic field displaces MNPs and thus their closest surrounding, the induced displacement is tracked in the US data and color-coded on B-mode images. However, images are currently processed offline, which is time consuming and precludes clinical use of MMUS. In this work, the previously proposed MMUS algorithm (DOI: TUFFC.2013.2591) is automated and implemented online on the ULA-OP scanner

Towards real-time magnetomotive ultrasound imaging

Enabling detection of nanoparticles with ultrasound can open new application avenues for the ultrasound technique. Magnetomotive ultrasound (MMUS) is a technique under development which indirectly visualizes magnetic nanoparticles. In MMUS, an external time-varying magnetic field acts to displace the nanoparticles, and thus their closest surrounding. This induced displacement is subsequently detected and the nanoparticle location may then be revealed. The MMUS technique has shown to be promising in both phantom and animal studies but limited efforts have been made on optimizing the technique for clinical applications in the sense of providing real-time bedside imaging. In this work, the previously proposed MMUS algorithm is automated and implemented online on the ULA-OP scanner. To evaluate the online implementation, a phantom made of styrene-ethylene/butylene-styrene and mineral oil with a 2 % magnetic ferrite particle inclusion was used. MMUS displacement was calculated in the entire image area, 192×230 pixels, and in a sub-region of 130×90 pixels, covering the inclusion. It was found that the automated online implementation computes one full MMUS image in 2.8 seconds and the sub-region in 1.17 seconds, which should be compared to 1-2 minutes in post processing mode. An immediate on-screen change in the magnetomotive displacement could be observed as the applied magnetic field was altered

Mitigating skin tone bias in linear array in vivo photoacoustic imaging with short-lag spatial coherence beamforming

Photoacoustic (PA) imaging has the potential to deliver non-invasive diagnostic information. However, skin tone differences bias PA target visualization, as the elevated optical absorption of melanated skin decreases optical fluence within the imaging plane and increases the presence of acoustic clutter. This paper demonstrates that short-lag spatial coherence (SLSC) beamforming mitigates this bias. PA data from the forearm of 18 volunteers were acquired with 750-, 810-, and 870-nm wavelengths. Skin tones ranging from light to dark were objectively quantified using the individual typology angle (ITA°). The signal-to-noise ratio (SNR) of the radial artery (RA) and surrounding clutter were measured. Clutter was minimal (e.g., −16 dB relative to the RA) with lighter skin tones and increased to −8 dB with darker tones, which compromised RA visualization in conventional PA images. SLSC beamforming achieved a median SNR improvement of 3.8 dB, resulting in better RA visualization for all skin tones