Alfonso Rodriguez-Molares

Plane-Wave Imaging Challenge in Medical Ultrasound

Plane-Wave imaging enables very high frame rates, up to several thousand frames per second. Unfortunately the lack of transmit focusing leads to reduced image quality, both in terms of resolution and contrast. Recently, numerous beamforming techniques have been proposed to compensate for this effect, but comparing the different methods is difficult due to the lack of appropriate tools. PICMUS, the Plane-Wave Imaging Challenge in Medical Ultrasound aims to provide these tools. This paper describes the PICMUS challenge, its motivation, implementation, and metrics.

Quantification of the ultrasound induced sedimentation of Microcystis aeruginosa

It has been known for more than 40 years that vacuolate organisms can be induced to sediment with ultrasound. However, robust indicators are still needed to compare the efficacy of different treatments. A repeatable index is proposed that makes it possible to quantify the ultrasonic induced sedimentation. The procedure is used to monitor the long term sedimentation of Microcystis aeruginosa after sonication. Results reveal that the sedimentation process continues after gas vesicles have fully recovered, although at a slower rate.

The angular apodization in coherent plane-wave compounding [Correspondence]

This article describes the relation between apodization in conventional focused imaging and apodization in coherent plane-wave compounding (CPWC). We pose the hypothesis that equivalent transmit beams can be produced with both methods if the transmit apodization is adequately transformed. We derive a relation between apodization in CPWC and in synthetic transmit aperture imaging (STAI), which we argue to be equivalent to conventional optimal multifocus imaging. We find that under certain conditions, the transformation of the apodization becomes trivial and the same window used in STAI can be applied for CPWC but extended to the whole angle sequence. We test the hypothesis with in silico data and find that the transformed apodization accurately mimics the objective transmit apodization, with differences in the lateral resolution between 3% and 6%.

The dynamic range of adaptive beamformers

All adaptive beamformers aim at increased resolution and better contrast. However, the same effect can be produced by dynamic range stretching. Adaptive beamformers are often tested on wire targets or uniform cysts, which do not allow to study dynamic range alterations. This work shows that dynamic range stretching may account for some of the benefits of some adaptive beamformers in the literature. We propose a phantom allowing us to do a dynamic range calibration. We demonstrate this calibration for the Mallart-Fink Coherence Factor (CF), the Camacho-Fritsch Phase Coherence Factor (PCF) and CaponsMinimum Variance (MV) beamformer.We show that the increased resolution and contrast reported for the CF and PCF are to a great extent due to the alterations of the dynamic range.

The dark region artifact in adaptive ultrasound beamforming

An undesired effect, the dark region artifact (DRA), has been under-communicated in our research community. The DRA appear next to acoustically strong targets for some of the many adaptive beamformers introduced in the literature. This study investigates the DRA for a collection of adaptive beamformers and shows that this effect originates because some of the methods fail to estimate which signals arise in the mainlobe and which originates from sidelobes. The DRA results in darker regions in the ultrasound images, indicating the wrong acoustical amplitude. Therefore, the measured contrast can falsely appear higher for adaptive beamformers affected by the DRA.

Reconstruction of specular reflectors by iterative image source localization

A method is presented to reconstruct the geometry of specular reflectors with an ultrasonic array based on the image source principle. The ultrasonic beam is focused at a point in space emulating a point source within the medium. The transmitted wave interacts with the specular reflector and propagates back to the array as if it were generated by an image source. The reflected wave is analyzed with a sound source localization algorithm to estimate the image source location, and the reflector geometry is extracted using the mirror equation for spherical reflectors. The method is validated experimentally and its accuracy is studied. Under ideal conditions the method provides an accurate reconstruction of the position, orientation, and radius of curvature of specular reflectors, with errors Δr < 0.2 mm, Δα < 3°, and ΔR/R0 < 0.2, respectively. The method performs very well in the presence of high levels of thermal and speckle noise, with no degradation of the reconstruction as long as SNR(th) > -3 dB (signal-to-thermal-noise ratio) and SNR(sp) > 7 dB (signal-to-speckle-noise ratio). An iterative scheme based on the proposed method is presented to reconstruct the geometry of arbitrary reflectors by subdividing the reflector boundary into smaller segments. The iterative scheme is demonstrated both numerically and experimentally.

The ultrasound toolbox

In the last decade the number of beamforming methods has exploded. Many innovative ideas have been proposed, but we lack the tools to compare the different techniques efficiently. The PICMUS challenge (IUS 2016, Tours) was a pioneering step that made clear that two things are required to establish a fair comparison: a common data format, and a body of methods to process that data.

Needle detection by image source localization

In conventional B-mode images, needles are poorly visualized due to the directivity of specular reflectors. A correct localization of needles is important for a number of ultrasound-guided clinical procedures, such as biopsies, regional anesthesia, and fetal blood transfusion.

Adaptive Color Gain for Vena Contracta Quantification in Valvular Regurgitation

Severe valvular regurgitation can lead to pulmonary hypertension, atrial fibrillation and heart failure. Vena contracta width is used to estimate the severity of the regurgitation. Parameters affecting visualization of color Doppler have a significant impact on the measurement. We propose a data-driven method for automated adjustment of color gain based on the peak power of the color Doppler signal in the vicinity of the vena contracta. A linear regression model trained on the peak power was used to predict the orifice diameter. According to our study, the color gain should be set to about 6 dB above where color Doppler data completely disappears from the image. Based on our method, orifices with reference diameters of 4, 6.5 and 8.5 mm were estimated with relative diameter errors within 18%, 12% and 14%, respectively.

In-vivo 3D cardiac vector flow imaging — A comparison between ultrasound and phase-contrast MRI

Phase-Contrast MRI (PC-MRI) is capable of 4D flow imaging and considered a gold standard for blood velocity measurements, but is currently cumbersome and time consuming. Ultrasound is bedside applicable, but is typically limited to a 1D velocity measurement. In this work we measure 3D blood flow velocity fields using high frame rate 3D ultrasound blood speckle tracking (BST) without contrast and compare to PC-MRI in a healthy volunteer.