Wood, Benjamin

The Doppler twinkling artifact (TA) plays a crucial role in detecting some biopsy markers including those made of polymethyl methacrylate (PMMA), facilitating precise lymph node localization in diagnostic and therapeutic procedures. This study presents comprehensive signal processing analysis methods to quantify and characterize the TA phenomenon using raw, per-channel radiofrequency (RF) data acquired from PMMA markers scanned with an L7-4 linear array transducer. We evaluated signals at different stages along the image reconstruction pipeline including (1) beamforming, (2) beamformed RF signal conversion into in-phase/quadrature (IQ) signals, and (3) Doppler velocity calculations and image reconstruction. We analyzed changes of pulses within an ensemble and between ensembles (i.e., frames). We also evaluated the impact of wall filters with various cutoff frequencies on color and power Doppler image reconstruction. Comparative analysis between PMMA markers and a non-twinkling control sample demonstrated distinct TA signatures or lack thereof in color and power Doppler images. The IQ data from PMMA marker locations exhibiting TA signatures showed substantially higher variability compared to non-twinkling samples, with standard deviations approximately 20 times higher between ensembles and 5 times higher within ensembles. This quantitative analysis framework provides valuable insight into the TA signal, potentially leading to more reliable and accurate diagnostic interpretations.

The color Doppler twinkling artifact has been attributed to existing microbubbles or cavitation has been reported for objects with rough surfaces like kidney stones, sandpaper, and polymethyl methacrylate (PMMA) bone cement. Passive cavitation detection (PCD) is a method for determining the presence of bubble oscillation from subharmonic, ultraharmonic, and inharmonic components in the Fourier domain. We evaluated multiple PMMA samples with varying twinkling amounts as well as a non-twinkling metal rod. Ultrasound scans were conducted using a Verasonics L11-4v probe on a V1 system from 3–9 MHz Doppler transmit frequency. A Sonic Concepts Y-107 PCD probe connected to a JSR DPR300 pulser/receiver collected data during Doppler ultrasound transmission. The Fourier spectra showed increased magnitude trends in the midband frequency range between the fundamental, f, and second harmonic, 2f, on the twinkling samples compared to non-twinkling metal rod. The midband increase was highest at frequencies that showed the most twinkling. This work indicates that the presence of color Doppler twinkling signal on PMMA gives rise to additional peaks in the Fourier domain, particularly in the midband range. Additionally, as there were no distinct subharmonic peaks, the increased midband magnitudes could be due to cavitation on the rough surfaces of the PMMA.

The color Doppler twinkling artifact has been attributed to existing microbubbles or cavitation occurring on objects like kidney stones, some breast biopsy markers, and sandpaper. The grooves of helical constructs that twinkle may provide sufficient locations for bubble retention and/or cavitation. We developed six half-cylinders that replicate the geometry of twinkling helical constructs with a micro 3D-printing process to explore how their characteristics relate to twinkling. Four copies of each design including a control were created. The cylinders had pitch (groove-to-groove distance) of 87.5–343 μm and amplitude (groove depth) of 41.5–209 μm. The cylinders were submerged in degassed water and optically imaged before and after ultrasound to visualize bubbles on the cylinders. The cylinders remained submerged while scanning with color Doppler at frequencies from 3.1 to 6.3 MHz with a General Electric Logiq E9 scanner and 9L linear array transducer. Two designs that showed twinkling characteristics were further examined. The presence or absence of bubbles on these designs was confirmed with microscopy, and the resulting twinkling behavior was investigated. This work shows strong evidence that both existing visible bubbles and either cavitation or ultrasound wave interactions with patterned or rough surfaces are significant factors in producing the twinkling signature.

Characterization of mechanical properties of tissue has become an important area of study over the last two decades. Shear wave elastography (SWE) is being used in many clinical applications to characterize, and multiple mechanical properties are being reported. To more comprehensively characterize soft tissues, we have focused on frequency-domain methods for evaluating viscoelasticity. We measure the shear wave velocity and attenuation and the variation with frequency. We will present results with the Kelvin-Voigt fractional derivative model and general power laws to characterize data from SWE experiments in tissue-mimicking phantoms and exvivo and in vivo tissue. Tissue mimicking phantoms from CIRS, inc., that have varying amounts of viscoelasticity will be investigated. Measurements from ex vivo and in vivo liver and kidney will also be used for this study. We will examine the range of the power-law exponents associated with these samples and the different models to provide understanding of the range of these parameters and how well tissue mimicking phantoms realize the parameters measured in soft tissues.

In recent years, work has been done to understand the mechanisms of Doppler ultrasound twinkling artifacts (TAs) and why they appear over kidney stones. In this work, twinkling artifacts were evaluated as a possible method of locating and characterizing kidney stones. Doppler ultrasound scanning was used to evaluate 47 stones of different types and sizes. An isolated stone study was used to understand the behavior of the TAs while varying transmit voltages. An ex vivo kidney study was conducted to determine if TAs were localized to the stones. An ex vivo randomized stone placement study was used to evaluate the robustness of detecting and locating stones. The TAs were also shown to be isolated to the stone when placed in an ex vivo kidney. The randomized stone placement study showed that this method could find all 47 used stones with only two false positives. A few limitations to this method were found with issues accurately sizing stones as well as difficulties in specificity for characterizing the stones. Further work will be done on these limitations by improving the Doppler acquisition and processing code as well as evaluating the use of TAs in human studies.

Local Phase Velocity Imaging (LPVI) is a novel method of shear wave elastography (SWE) analysis, which provides phase velocity images at multiple frequencies. The primary components of this method include using two acoustic radiation force (ARF) pushes near the edges of the field-of-view. Fourier analysis of the spatiotemporal data is conducted to obtain maps of the phase velocity at multiple frequencies. A robustness study was conducted to evaluate the effectiveness of LPVI in elastic and viscoelastic phantoms with different mechanical properties, ARF focal depths and F-numbers, and shear wave frequencies using a linear array transducer. The results showed effective phase velocity reconstructions for most scanning parameters in the phantoms evaluated. We evaluated mean and standard deviations of phase velocities in the different phantoms for regions-of-interest defined in the images. Issues were seen with shear wave signal being distorted from the incident push when using low F-numbers as well as issues with attenuating shear waves with higher F-numbers and deeper focal depths. Future directions of this work will be to evaluate the robustness of LPVI in inclusion phantoms as well as testing on a curved array transducer.