Development of a noncontact ultrasound indentation system for measuring tissue material properties using water jet

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Development of a noncontact ultrasound indentation system for measuring tissue material properties using water jet


Author: Lu, Minhua
Title: Development of a noncontact ultrasound indentation system for measuring tissue material properties using water jet
Degree: Ph.D.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Ultrasonic imaging.
Ultrasonic waves -- Measurement.
Cartilage -- Mechanical properties.
Department: Dept. of Health Technology and Informatics
Pages: xxvii, 197 leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: To measure or image the mechanical properties of tissues has been attracting increasing research efforts during the recent decades. The purpose of this research was to develop a noncontact ultrasound indentation system using water jet compression. The key idea was to use a water jet as the indenter and simultaneously as the medium for ultrasound to propagate through so as to measure or map the mechanical properties of tissues. A high frequency focused ultrasound (20 MHz) transducer was used in the system for measuring tissue thickness and deformation at a high resolution. A pressure sensor installed within the water pipe could monitor the water pressure during the indentation process. The device was fixed on a high-resolution 3-D translating device, thus the system could easily conduct 1-D or 2-D scan across the tissue sample and obtain the modulus distribution of soft tissue. Tissue-mimicking phantoms were used for the validation of the ultrasound water jet indentation system. It was found that the stiffness ratio defined as the pressure applied on phantom surface to the local strain of the phantom correlated very well with the Young's modulus of the phantom measured using the uniaxial compression test. Therefore, the stiffness ratio was employed as an indicator of the tissue stiffness. The repeatability of the measurement was very good with Intra-class coefficient (ICC) > 0.99. The phantom study was also used to demonstrate the ability of the new system to map the distribution of the stiffness. Results showed that both the spatial resolution (0.4 mm) and the contrast resolution (6.7 kPa) were good enough for the modulus imaging of most soft tissues. The ultrasound water jet indentation system provided a noncontact way for the measurement of both mechanical and acoustical properties of soft tissues. It was used to assess the degeneration of articular cartilage in this study. It was found that, after the bovine patellar cartilages were treated by 0.25% trypsin solution for 4 hours, their stiffness ratios were significantly reduced by 44.3% +- 17.0%, and the peak-to-peak amplitude of ultrasound echoes reflected from the cartilage surface significantly decreased by 35.64% +- 19.51% in comparison with the intact samples. The system was also employed to monitor the healing of the bone-tendon junction tissues of rabbits after partial patellectomy. Low intensity pulsed ultrasound (LIPUS) and functional electrical stimulation (FES) were used to accelerate the healing of the bone-tendon junction structures. After studying the stiffness and Young's modulus of the remained patellar cartilage, the junction tissue and the tendon, it was found that the patellar cartilages and junctions of the postoperative samples were significantly softer than those of the control samples, and the junction tissues of the samples treated by LIPUS and FES were significantly stiffer than those of the postoperative samples without any treatment. With further improvement of the system and the development of an appropriate model to simulate the interaction between the water jet and soft tissues, the ultrasound water jet indentation system could be potentially used to assess the mechanical and acoustical properties of living tissues, tissues with small dimensions, bioengineered tissues, and other materials.

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