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Title: Design and fabrication of micro-scale high frequency ultrasonic diagnostic devices for in-vivo pathology
Author: Ssekitoleko, Robert Tamale
ISNI:       0000 0004 5362 7210
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2014
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Transducer arrays operating above 15 MHz enable real time high resolution imaging of tissue, capable of resolving features below 200μm. Clinical applications such as oncology and gastroenterology could significantly benefit from the improved resolution for high frequency ultrasound (HFUS) characterization of tissues. However, this is presently challenging due to the limited penetration depth of HFUS and limited access. Since the device dimensions scale with imaging wavelength, it becomes feasible to integrate HFUS arrays with interventional tools such as biopsy needles. Although there are many design and fabrication challenges associated with incorporating transducers with interventional tools such as biopsy needles, it creates opportunities for timely and accurate characterisation of tissue, leading to in-vivo pathology. This study reports progress in the development of fabrication processes for miniature linear arrays suitable for integration with biopsy needles. While patterning high frequency transducer arrays based on piezocomposites has been shown to be feasible, there remain many challenges to miniaturize the interconnect and cabling of an ultrasound probe suitable for in vivo pathology. Novel packaging techniques for integrating an ultrasound array into a needle were developed. Wafer scale fabrication was adopted to reduce the overall cost of fabrication. Microfabrication and precision micromachining processes were developed to overcome the technical challenges in fabricating miniature arrays operating up to 25 MHz. Array elements are defined by precision dicing and the necessary external flex circuit cabling was fed through the needle. A flexible printed circuit is connected to back surface electrodes using low-temperature bonding methods. A flex circuit connected to the 1-3 piezocomposite was patterned with 60 μm pitch to define array elements suitable for a 25 MHz linear array. The polyimide flexible printed circuit, with fine pitch traces, was twisted into a helical structure so that it can fit within the core of the biopsy needle and permit large numbers of elements and electrode traces. The spiral-helical flexi-circuit design was developed as a way to fit multiple conductive tracks into a needle. The definition of fine-pitch conductive tracks on polyimide polymer was achieved using dry-film photoresist and the application of a megasonic transducer to provide agitation and small bubbles for copper etching. Investigation and evaluation of low temperature bonding methods was undertaken. This overcomes the problem of using high temperature methods on the temperature sensitive single crystal materials. Bonding techniques such as ultrasonic bonding and magnetically aligned anisotropic UV curable epoxy were investigated. A Resolution integral was applied to simulated beam plots as a way of evaluating transducers at a design stage. This considers the ultrasound beams and a measure of the beam at -6 dB is taken as the lateral resolution. This is measured over the depth of field. A transducer with a higher resolution integral would have a narrow beam over a long distance The process was validated with a single element transducers made from fine-scale single crystal composites involving PMN-PT and Manganese doped Lead Indium Niobate-Lead Magnesium NiobateLead Titanate (Mn-PIN-PMN-PT). These were fabricated using the conventional dice and fill method, and incorporated into needles and tested. These composites had pitches as small as 50 μm with kerf of 18 μm. Images were generated using these transducers. Arrays operating at 5 MHz and 15 MHz were fabricated. The fabrication process development and testing demonstrated the feasibility of a linear array integrated into a biopsy needle. The extension of the fabrication processes to higher frequency arrays.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available