Attenuation Imaging with Pulse-Echo Ultrasound based on an Acoustic Reflector

Ultrasound attenuation is caused by absorption and scattering in tissue and is thus a function of tissue composition, hence its imaging offers great potential for screening and differential diagnosis. In this paper we propose a novel method that allows to reconstruct spatial attenuation distribution in tissue based on computed tomography, using reflections from a passive acoustic reflector. This requires a standard ultrasound transducer operating in pulse-echo mode, thus it can be implemented on conventional ultrasound systems with minor modifications. We use calibration with water measurements in order to normalize measurements for quantitative imaging of attenuation. In contrast to earlier techniques, we herein show that attenuation reconstructions are possible without any geometric prior on the inclusion location or shape. We present a quantitative evaluation of reconstructions based on simulations, gelatin phantoms, and ex-vivo bovine skeletal muscle tissue, achieving contrast-to-noise ratio of up to 2.3 for an inclusion in ex-vivo tissue.Comment: Accepted at MICCAI 2019 (International Conference on Medical Image Computing and Computer Assisted Intervention

Frequency-Dependent Attenuation Reconstruction with an Acoustic Reflector

Attenuation of ultrasound waves varies with tissue composition, hence its estimation offers great potential for tissue characterization and diagnosis and staging of pathology. We recently proposed a method that allows to spatially reconstruct the distribution of the overall ultrasound attenuation in tissue based on computed tomography, using reflections from a passive acoustic reflector. This requires a standard ultrasound transducer operating in pulse-echo mode and a calibration protocol using water measurements, thus it can be implemented on conventional ultrasound systems with minor adaptations. Herein, we extend this method by additionally estimating and imaging the frequency-dependent nature of local ultrasound attenuation for the first time. Spatial distributions of attenuation coefficient and exponent are reconstructed, enabling an elaborate and expressive tissue-specific characterization. With simulations, we demonstrate that our proposed method yields a low reconstruction error of 0.04dB/cm at 1MHz for attenuation coefficient and 0.08 for the frequency exponent. With tissue-mimicking phantoms and ex-vivo bovine muscle samples, a high reconstruction contrast as well as reproducibility are demonstrated. Attenuation exponents of a gelatin-cellulose mixture and an ex-vivo bovine muscle sample were found to be, respectively, 1.4 and 0.5 on average, from images of their heterogeneous compositions. Such frequency-dependent parametrization could enable novel imaging and diagnostic techniques, as well as help attenuation compensation other ultrasound-based imaging techniques

Improving elevation resolution in phased-array inspections for NDT

The Phased Array Ultrasonic Technique (PAUT) offers great advantages over the conventional ultrasound technique (UT), particularly because of beam focusing, beam steering and electronic scanning capabilities. However, the 2D images obtained have usually low resolution in the direction perpendicular to the array elements, which limits the inspection quality of large components by mechanical scanning. This paper describes a novel approach to improve image quality in these situations, by combining three ultrasonic techniques: Phased Array with dynamic depth focusing in reception, Synthetic Aperture Focusing Technique (SAFT) and Phase Coherence Imaging (PCI). To be applied with conventional NDT arrays (1D and non-focused in elevation) a special mask to produce a wide beam in the movement direction was designed and analysed by simulation and experimentally. Then, the imaging algorithm is presented and validated by the inspection of test samples. The obtained images quality is comparable to that obtained with an equivalent matrix array, but using conventional NDT arrays and equipments, and implemented in real time.Fil: Brizuela, Jose David. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Camacho, J.. Consejo Superior de Investigaciones Científicas; EspañaFil: Cosarinsky, Guillermo Gerardo. Comisión Nacional de Energía Atómica; ArgentinaFil: Iriarte, Juan Manuel. Comisión Nacional de Energía Atómica; ArgentinaFil: Cruza, Jorge F.. Consejo Superior de Investigaciones Científicas; Españ

Ultrasonido cuantitativo y los efectos de la no linealidad acústica

Descargue el texto completo en el repositorio institucional de la University of Illinois: https://hdl.handle.net/2142/115056El desarrollo de nuevas técnicas de imagen utilizando señales de ultrasonido que tienen diferentes fuentes de contraste de imagen es médicamente significativo. Por ejemplo, aquellas basadas en el mapeo de parámetros de ultrasonido cuantitativo (QUS). Se sabe además que diferentes tejidos tienen diferentes coeficientes de no linealidad (B/A), que pueden distorsionar en menor o mayor grado a los estimados de QUS. En la primera parte de la tesis, se determinó las condiciones que pueden disminuir la distorsión no lineal de los estimados de QUS provocada por diferenies niveles de presión acústica y se exploró un método de calibración in situ que minimizaría tal distorsión no lineal. En la segunda parte del trabajo, se presentaron dos métodos para calcular el B/A basados en observaciones relacionadas con estimados de QUS. El primer método fue un enfoque heurístico basado en la estimación del exceso del coeficiente de atenuación. El segundo método para la estimación del B/A se realizó en el dominio del tiempo utilizando el principio de conservación de la energía y un cálculo implícito de la señal del segundo armónico. Usando este último método se logró construir imágenes paramétricas del promedio acumulado de B/A en medios heterogéneos.Diagnostic ultrasound is the most heavily utilized imaging modality in medicine worldwide second only to digital X-ray. Conventional B-mode imaging relies on small impedance differences (often <5%) between tissues to provide image contrast. This perceived contrast is further reduced because B-mode images are replete with speckle. The development of novel imaging techniques using ultrasound signals that have different sources of image contrast and may not be affected by speckle is medically significant; for example those based on mapping quantitative ultrasound (QUS) parameters. However, different types of tissues or tissues under different disease states are known to have different coefficients of nonlinearity (B/A). For example, fatty tissues have a B/A of 11, liver has a B/A of 6.6, and water has a B/A of 5. At low pressures it can be assumed that QUS parameters are not changed by acoustic nonlinear distortion, whereas at higher pressures the nonlinear distortion transfers energy from the fundamental frequency of the ultrasound wave into higher harmonics. These distortions can affect the bias and variance of spectralbased QUS estimates, such as the backscatter coefficient and attenuation coefficient. In the first part of this dissertation, we aim to determine conditions that can decrease acoustic nonlinear effects. We explored an in situ calibration approach which would minimize nonlinear distortion on QUS estimates. In the second part of this dissertation we present two methods to calculate the B/A based on observations related to QUS estimation. The first method was an heuristic approach based on estimating the excess attenuation coefficient and required two well-characterized reference phantoms. The second method for B/A estimation needed only one reference phantom and was performed mainly in the time-domain using the conservation of energy principle. Using the later method we can construct parametric images to map the cumulative average B/A versus depth.Perú. Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica (Concytec). Beca de doctorado en el extranjero 2016-2 CIENCIACTIV

Investigation of ultrasonic properties of MAGIC gels for pulse-echo gel dosimetry

This thesis describes investigations into the design and evaluation of novel ultrasonic methods for 3-dimensional ionising radiation dose verification. Pulse-echo ultrasound methods were investigated for the measurement and analysis of complex radiation therapy dose delivery.The physical properties of MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel dosimeters have been characterized. The variations of speed of sound, ultrasonic attenuation coefficient and density of MAGIC gel with radiation dose and temperature have been quantified. This extends work that has previously been reported for the properties of this gel to the effect of measurement temperature on the results. The facilities to perform these measurements were specified, constructed and evaluated as part of the project.The measurement of radiation dose using ultrasound back scatter from an interface between the polymer gel dosimeter and an inert reflector is demonstrated. To enable the measurement of radiation dose using pulse-echo ultrasound methods a novel inert material has been specified, manufactured and characterised. This material is matched to the acoustic impedance of MAGIC gel to produce the most dose-sensitive reflections.The reflections from the interface between the inert reflector and dose-dependent MAGIC gel have been analysed using both a single element transducer and a commercial ultrasound scanner. Both measurement systems demonstrate the same dose and temperature dependence of the ultrasonic reflection. A methodology has been developed to relate pixel values from the ultrasound scanner to the amplitude of the reflected ultrasound signal. A phantom consisting of an array of threads formed from the inert backscattering material has been designed and constructed and a method of extracting pixel data from images of the array acquired using a commercial ultrasound scanner has been developed, so that multiple imaging positions could be used to perform a 3-dimensional assessment of radiation dose distributions.It has been demonstrated that a pulse-echo technique using a commercial ultrasound scanner shows promise for radiation gel dosimetry. Further investigation and alternative polymer gel and inert reflector combinations may improve these techniques

New methods for deep tissue imaging

Microscopes play vital role biological science and medicine. For single photon microscopies, the scattering of photons makes regions of interest located a few hundred microns beneath the surface inaccessible. Multi-photon microscopes are widely used for minimally invasive in vivo brain imaging due to their increased imaging depth. However, multi-photon microscopes are hampered by limited dynamic range, preventing weak sample features from being detected in the presence of strong features, or preventing the capture of unpredictable bursts in sample strength. In the first part of the thesis, I present a solution to vastly improve the dynamic range of a multi-photon microscope while limiting potential photodamage. Benefits are shown in both structural and in-vivo functional mouse brain imaging applications. In the second section of the thesis work, I explore a completely different approach towards deep tissue imaging by changing the type of radiation from light to ultrasound. Inspired by an optical phase contrast technique invented in the lab, I developed an unprecedented ultrasound imaging system that can visualize the ultrasound phase contrast in the sample. The ultrasound phase contrast technique is able to visualize local sound speed variations instead of local reflectivity. Compared with existing sound speed tomography systems, our technique eliminates the cumbersome sound speed reconstruction process. The research work in this section contains three parts. In the first part, we designed a low-cost single element scanning system as proof of concept. In the second part, we implemented the ultrasound phase contrast imaging system on a commercial linear phased transducer array and an imaging apparatus designed for samples with finite thickness. In the third part, we studied the feasibility of ultrasound phase contrast imaging in arbitrarily thick tissue. We presented a complete workflow of theoretical study, simulation, prototyping and experimental testing for all three parts.2020-02-28T00:00:00

Broadband acoustic measurement of an agar-based tissue mimicking material - a longitudinal study

Commercially available ultrasound quality assurance test phantoms rely upon the long-term acoustic stability of tissue-mimicking-materials (TMMs). The measurement of the acoustic properties can be technically challenging and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique is proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range of 4.5 – 50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and the attenuation were very similar and within the IEC recommended value. However, the results obtained using the proposed modified technique demonstrated greater stability over the 1-year period when compared with the results acquired using the standard technique