Tools for improving the diagnosis of atherosclerotic plaque using ultrasound

Søren Kragh Jespersen

    Research output: Book/ReportPh.D. thesis

    Abstract

    This Ph.D. project was carried out as an industrial research Ph.D. project at B-K Medical and the Department of Information Technology, Technical University of Denmark (DTU), and has focused on medical diagnostic ultrasound investigation of atherosclerotic disease in the carotid arteries. Two major topics have been investigated: an ultrasound pulse-echo simulation tool and a new compound imaging technique for improving visualization of atherosclerotic disease.A tool for simulation of the received electrical signal in a pulse-echo ultrasound system, due to a reflector surface of arbitrary geometry, has been developed. The method is denoted the Diffraction Response Interpolation Method (DRIM) and is based on the pulse-echo diffraction impulse response method. The DRIM is a computationally efficient tool for calculating the integral of the spatially varying pulse-echo diffraction impulse response over the reflector surface, offering improvements in computation time on the order of 30 to 400 times over a simple summation approach (depending on reflector geometry and orientation). As the DRIM is based on linearity, effects such as second-order diffraction, shadowing, transmission medium inhomogeneities etc. can not directly be incorporated in the modeling. Results obtained using the DRIM have been compared to results from other simulation techniques wherever possible and agreement was found to be high. Further, DRIM results have been compared to experimental results obtained using cylindrical reflectors and the correspondence was found to be high for reflectors near the acoustical axis of the transducer and moderate for reflectors far off-axis.An ultrasound imaging modality, Multi-Angle Compound Imaging (MACI), has been developed for imaging of vascular structures and investigated theoretically and experimentally. The MACI method operates by recording information about a given tissue region using a number of beam angles (typically 3 to 11) and combining the information into a single compound image (so-called spatial compounding). During the project a flexible experimental multi-channel ultrasound system, named ``XTRA'', has been designed, implemented, tested, and subsequently used for \emph{in vitro} investigation of the MACI technique. The MACI method has been investigated on various tissue mimicking phantoms, on porcine tissue samples and on human carotid plaque specimens. Generally, the results show that compared to conventional B-mode imaging MACI features reduced image speckle and better definition of tissue interfaces. The point spread function (PSF) for MACI was investigated when scanning through water and through an inhomogeneous tissue mimicking medium and compared to the PSF for conventional imaging. An increase of 30% in the axial width and 10% in the lateral width of the PSF was found, thus, the point resolution capability is only reduced marginally. Visualization of tissue interfaces was investigated using rubber tube phantoms, porcine aorta, and human plaque specimens. The MACI images show improved definition of the interfaces in the cases where one or more of the beams had near-normal incidence on the interface, i.e. an improved visualization over an angular range of interface orientations roughly corresponding to the range of beam angles used. The speckle statistics and the speckle reduction have been investigated experimentally and compared to existing theory, and high agreement was found. A phantom containing simulated low-contrast lesion regions was scanned and the detectability of the lesion regions analyzed subjectively and quantitatively. The quantitative analysis showed that seven out of eight regions were detectable when using MACI, compared to only three out of eight when using conventional B-mode imaging. Finally, two human carotid plaque specimens were scanned in 3D (by mechanical movement of the transducer in one direction). The MACI images were subjectively found to give a more consistent definition of the remaining lumen in the arteries and a less noisy depiction of the reflectivity inside the plaque deposits and the vessel wall. In conclusion, Multi-Angle Compound Imaging appears to be a promising imaging modality for improving the diagnosis of atherosclerotic disease in the carotid arteries.
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherTechnical University of Denmark
    Number of pages174
    Publication statusPublished - 1997

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