An electronically steered, wearable transcranial doppler ultrasound system

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 137-144).This thesis details the design of a transcranial Doppler (TCD) ultrasound system to measure cerebral blood flow velocity (CBFV) at the middle cerebral artery (MCA). TCD sonography has been clinically indicated in a variety of neurovascular diagnostic applications. Acceptance of conventional TCD methods, however, has been primarily impeded by several constraints, including restrictive system form factors, measurement reliability concerns, and the need for a highly-skilled operator. The goal of this work is to reduce the effects of such limitations through the development of a highly-compact, wearable TCD ultrasound system for autonomous CBFV measurement. A first-generation, eight channel printed circuit board prototype system has been designed, fabricated, and experimentally tested. Characterization of the prototype system using a Doppler flow phantom resulted in a normalized root-mean-square error of < 3.5% over the range of expected in vivo MCA flow velocities. Extension of the initial prototype to higher channel count systems and the development of phased array beamformation and algorithmic vessel location are also examined in this work. The emergence of simple, robust, and non-invasive neurovascular diagnostic methods presents an enormous opportunity for the advancement of neurovascular monitoring, particularly in applications where - due to restrictions in current diagnostic modalities - standard monitoring procedures have not yet been established.by Sabino Joseph Pietrangelo.S.M

All-optical ultrasound transducers for high resolution imaging

University of Minnesota Ph.D. dissertation. December 2014. Major: Biomedical Engineering. Advisor: Shai Ashkenazi. 1 computer file (PDF); xi, 109 pages.High frequency ultrasound (HFUS) has increasingly been used within the past few decades to provide high resolution (< 200 µm) imaging in medical applications such as endoluminal imaging, intravascular imaging, ophthalmology, and dermatology. The optical detection and generation of HFUS using thin films offers numerous advantages over traditional piezoelectric technology. Circumvention of an electronic interface with the device head is one of the most significant given the RF noise, crosstalk, and reduced capacitance that encumbers small-scale electronic transducers. Thin film Fabry-Perot interferometers - also known as etalons - are well suited for HFUS receivers on account of their high sensitivity, wide bandwidth, and ease of fabrication. In addition, thin films can be used to generate HFUS when irradiated with optical pulses - a method referred to as Thermoelastic Ultrasound Generation (TUG). By integrating a polyimide (PI) film for TUG into an etalon receiver, we have created for the first time an all-optical ultrasound transducer that is both thermally stable and capable of forming fully sampled 2-D imaging arrays of arbitrary configuration. Here we report (1) the design and fabrication of PI-etalon transducers; (2) an evaluation of their optical and acoustic performance parameters; (3) the ability to conduct high-resolution imaging with synthetic 2-D arrays of PI-etalon elements; and (4) work towards a fiber optic PI-etalon for in vivo use. Successful development of a fiber optic imager would provide a unique field-of-view thereby exposing an abundance of prospects for minimally-invasive analysis, diagnosis, and treatment of disease

Beamforming for 3D Transesophageal Echocardiography

In this thesis, we study beamforming techniques that offer opportunities for 3D transesophageal echocardiography imaging, especially to achieve higher frame rates. In 3D TEE with a matrix transducer, two main challenges are to connect a large number of elements to a standard ultrasound system and to achieve a high volume rate (>200 Hz). We develop a prototype miniaturized matrix transducer for pediatric patients with micro-beamforming to reduce the channel count. Initially, we propose two dual stage beamforming techniques for 1D arrays to produce high-quality images with reduced channel count: one using fixed focused receive and another with a simple summation in receive (no delays). Because of their inapplicability to the prototype transducer, we propose multiline 3D ultrasound beamforming schemes that utilize the micro-beamforming capabilities. The proposed beamforming schemes use an angle-weighted combination of the neighboring overlapping sub-volumes to suppress the crossover artifacts that are typical for parallel beamforming and produce high-quality images at a high volume rate (~300 Hz). A similar beamforming scheme adapted for a newly designed prototype matrix adult TEE probe is used for in vivo 3D imaging of the heart of a healthy adult pig to produce good quality 3D images at a high frame rate. The proposed 3D beamforming scheme can easily be adapted for matrix probes with micro-beamforming capabilities to produce good quality volume images at a high volume rate, even for a very different layout of the transmit and receive arrays

Towards Non-invasive Ultrasonic Characterization of Carotid Atherosclerotic Plaque

Vulnerable atherosclerotic plaques are thought to be prone to rupture due to various compositional and morphological factors. One key characteristic is thought to be the presence of a soft, lipid rich core in the plaque. Acoustic radiation force impulse (ARFI) and thermal strain imaging (TSI) are non-invasive ultrasound-based imaging modalities. ARFI imaging measures the tissue response to an ultrasonically generated mechanical perturbation. In TSI, the tissue temperature is increased and image contrast is a result of the temperature and composition dependence of the speed of sound. Initial efforts to develop a TSI system utilized two separate ultrasound transducers for heating and imaging. We developed signal processing to improve estimates of thermal strain obtained from this system and showed that TSI could be used to detect lipids in ex vivo human arterial tissue samples. However, the translational obstacles encountered by this system outweighed the potential imaging utility. In order to address these challenges, we developed temporally interleaved multi-foci beamforming which could be implemented on a standard imaging array to generate a broad, homogeneous ultrasound beam for either ARFI pushing or TSI heating. We showed that this beamforming approach could enable simultaneous acquisition of ARFI and thermal strain data while substantially improving the frame rate for ARFI imaging. In order to better understand the factors that affect signal quality in TSI and ARFI imaging, we conducted separate phantom studies for each imaging modality. We showed that with a temperature rise <1oC, TSI could differentiate between phantoms with different lipid percentages. Additionally, we showed that pulse inversion harmonic imaging could be used to improve TSI signal quality in the presence of clutter. Finally, we showed that high frame rate ARFI imaging was able to achieve a 45-fold improvement in frame rate at the cost of increased estimation bias and jitter, and decreased image contrast. These studies indicate that multi-foci beamforming can be used to enable simultaneous TSI and ARFI imaging on current clinical systems. This imaging sequences developed in this dissertation facilitate non-invasive assessment of both the composition and mechanical properties of tissue which might be especially useful for characterization of vulnerable plaques