Experimental verification of tomographic measurement methods in ultrasound

Abstract: This thesis describes two different measurement methods applied to ultrasound. The first method, Ultrasound doppler vector tomography, uses ultrasound to measure blood flow and the second method, Light diffraction tomography, measures the pressure field in an ultrasound beam with laser light. Both methods depend on tomography algorithms for reconstruction of the measured region. A prime motive for Ultrasound doppler vector tomography was to develop an automatic ultrasound based diagnostic scanning system for breast cancer screening. The special tomography algorithm used for detection of directional blood flow is described as well as the developed experimental system. Flow phantoms were measured and the results were compared with simulations, which show good agreement. The ongoing development of the method concerns improved tomography algorithms, better tissue-mimicking flow phantoms and ways to increase the spatial resolution. The main part of the thesis is about the development and use of Light diffraction tomography, which can be used for characterization of ultrasound fields. To our knowledge, only three laboratories worldwide have published results with this method and the subjects of the current work are to define, describe and expand the limits. Some of the most important advantages with light diffraction tomography are the non-perturbing character and the possibility to do absolute pressure measurements with high spatial resolution in both water and air. These advantages were exploited to measure 10 MHz transducers in water, close to the surfaces with high spatial resolution and to measure ultrasound in air in the frequency range 40 kHz – 2 MHz. The results show that under certain circumstances light diffraction tomography can be used for absolute measurements with an uncertainty on the order of 10% in water and 13% in air. Complex ultrasound nearfields were characterized and peaks separated 160 µm were resolved. The applicability on airborne ultrasound was investigated showing unique results regarding absolute measurements, spatial resolution, sensitivity, phase measurements and frequency response. The thesis also includes the Department of Electrical Measurements’ experience in the EC project “An assessment of medical ultrasonic field measurement methods”, the purpose of which was to evaluate a specific ultrasound measurement standard.