High Frequency Ultrasound Elastography for Assessing the Mechanical Properties of Thin-Layer Tissues

Abstract

The mechanical properties of soft tissue are considered effective biomarkers for the diagnosis of various diseases Several ultrasound elastography techniques have been developed for the purpose of evaluating the tissue stiffness Among them acoustic-radiation-force (ARF)-based elasticity imaging including acoustic radiation force impulse (ARFI) imaging and shear wave elasticity imaging (SWEI) has been proposed and considered as a promising technique for clinical diagnosis Based on this technique mechanical properties of tissue can be quantified by measuring the localized tissue displacements or shear wave speed However characterizing the viscoelastic properties of thin-layer tissues (thickness at level of hundreds micrometer to few millimeter) has remained challenging for many years This is because most of these ARF-based elasticity imaging modalities were mainly developed to measure organ-level tissues (i e breast liver and kidney) and were unable to measure those thin-layer tissues which need either the high spatial resolution detection and the appropriate wave propagation model The thesis aims to accurately estimate the mechanical properties of thin-layer tissues by using ARF-based elasticity imaging technique To attain the goal a concept of dual-frequency ultrasound was proposed to excite ARF with lower ultrasound frequency and detect tissue dynamic response with higher ultrasound frequency which compensated the trade-off between the ultrasound frequency and the intensity of ARF The concept of dual-frequency ultrasound was realized by several newly-designed ultrasound transducers in the thesis To determine the intensity of ARFs required to induce sufficient displacements in vascular tissues including porcine aorta pulmonary artery coronary artery and different hematocrit of blood clots several intensities of ARF under different excitation parameter were generated using the high-resolution ARFI imaging system with a confocal dual-frequency (11 and 48 MHz) transducer and measured by a calibrated hydrophone Those acoustic intensities consistent with the regulations specified by the US Food and Drug Administration for intravascular ultrasound (IVUS) imaging applications were determined The stiffness distributions of arteries with blood clots and artificial arteriosclerosis were determined by the ARFI images as well To assess the mechanical properties of atherosclerosis an integration of IVUS and ARF-based elasticity imaging was proposed A dual-frequency IVUS transducer with 8 5- and 31-MHz elements was fabricated and used to simultaneously induce and monitor the shear wave propagation The integrated system was implemented using the rotating scan to achieve cross-section information of samples The phantom results demonstrated that the system can distinguish the regions with different stiffnesses through the wave-amplitude image and the wave-velocity image which was respectively reconstructed by measuring the peak displacement and the wave velocity of shear wave propagation Moreover stiffness distributions of the atherosclerotic aorta from the rabbit could be obtained from these elastographic images To accurately determine the viscoelasticity of thin-layer tissues Lamb wave model and our previously developed ARF-elasticity imaging system using a 4 5 MHz ring transducer and a 40 MHz needle transducer were integrated The phase velocity in the tissue was induced by the impulse method and harmonic method Based on the Lamb wave model the measured shear elasticities of thin-layer phantoms of different thicknesses were consistent with the results of the mechanical test and shear wave rheological model in bulk phantoms and the trend of measured shear viscosities was in good agreement with the results of the shear wave rheological model and measured attenuations By contrast the shear elasticity of thin-layer phantoms as estimated from the group velocity did not agree with the results of mechanical tests The shear elasticity and shear viscosity of porcine cornea and rabbit carotid artery are also reported by using both the impulse and the harmonic methods In the thesis a high-resolution ARF-based elasticity imaging was proposed and implemented using several dual-frequency ultrasound transducers With appropriate algorithms and system setups this imaging methodology can provide the stiffness distribution or the quantified mechanical properties of the thin-layer tissues including the cornea and several kinds of the artery All results indicate that the dual-frequency ultrasound is an efficient solution for ARF-based elasticity imaging to measure the mechanical properties of thin-layer tissues and demonstrates a promising future for improving diagnoses in multiple clinical applications

Date of Award	2018 Sept 21
Original language	English
Supervisor	Chih-Chung Huang (Supervisor)