In-vivo studies of new vector velocity and adaptive spectral estimators in medical ultrasound

Abstract

In this PhD project new ultrasound techniques for blood flow measurements have been investigated in-vivo. The focus has mainly been on vector velocity techniques and four different approaches have been examined: Transverse Oscillation, Synthetic Transmit Aperture, Directional Beamforming and Plane Wave Excitation. Furthermore two different adaptive spectral estimators have been investigated: Blood spectral Power Capon method (BPC) and Blood Amplitude and Phase Estimation method (BAPES). The novel techniques investigated in this thesis are developed to circumvent some of the main limitations in conventional Doppler ultrasound. That is angle dependency, reduced temporal resolution and low frame rate. Transverse Oscillation, Synthetic Transmit Aperture and Directional Beamforming can estimate the blood velocity angle independently. The three methods were validated in-vivo against magnetic resonance phase contrast angiography when measuring stroke volumes in simple vessel geometry on 11 volunteers. Using linear regression and Bland-Altman analyses good agreements were found, indicating that vector velocity methods can be used for quantitative blood flow measurements. Plane Wave Excitation can estimate blood velocities angle independently with a high frame rate. Complex vessel geometries in the cardiovascular system were explored in-vivo on four volunteers using the technique. Flow patterns previously visualized with magnetic resonance angiography and predicted by models of computational fluid dynamics, were shown for the first time with ultrasound. Additionally, new information on complex flow patterns in bifurcations and around venous valves was discovered. BPC and BAPES are adaptive spectral estimators which can produce spectrograms with a high temporal resolution. Spectrograms obtained in-vivo with the two techniques on ten volunteers were evaluated quantitatively and qualitatively and compared to the conventional spectral Doppler method. Descriptive statistics, kappa statistics and multiple t-tests were performed and it was shown that BAPES and BPC can produce useful spectrograms with a narrower observation window compared to the conventional spectral Doppler method. The thesis shows, that novel information can be obtained with vector velocity methods providing quantitative estimates of blood flow and insight in to the complexity of fluid dynamics. This could give the clinician a new tool in assessment and treatment of cardiovascular diseases. Also solutions to produce spectrograms with fewer emissions per estimate were given. This could potentially bring improvements to spectral blood estimation as an increase of the temporal resolution of the spectrogram or as an increase of the frame rate for the interleaved B-mode images.