Quantitative Doppler Methods in Cardiovascular Imaging

Abstract

Ultrasound imaging of blood flow in the heart and blood vessels has become an essential part of diagnosing diseases related to the circulatory system. By using different Doppler methods, the blood flow may be visualized or quantified. In this work we take advantage of the opportunities given by the introduction of parallel processing of ultrasound data to develop new quantitative Doppler methods.

Pulsed wave (PW) Doppler is a technique for measuring blood velocities, providing the full velocity spectrum in a specific region of interest. The maximum velocities may be found by delineation of the spectral envelope, and may be used to estimate the severity of stenoses or valve leakages. However, PW Doppler suffers from several challenges, which makes quantitative analysis problematic. To limit spectral broadening, we created a new method called 2-D tracking Doppler, which incorporates information from several parallel receive beams. Spectra with improved resolution and signal-to-noise ratio were produced for a large span of beam-to-flow angles. The new method was tested using in vitro and in vivo recordings. A signal model was derived and the expected Doppler power spectra were calculated, showing good agreement with experimental data.

Experiments were performed to investigate how the 2-D tracking Doppler method depends on the tracking angle. It was shown that the spectra have lowest bandwidth and maximum power when the tracking angle is equal to the beam-to-flow angle. This may facilitate new techniques for velocity calibration. It was shown that the velocity calibration errors may be lower for the 2-D tracking Doppler method than for a conventional PW Doppler approach, and especially for large beam-to-flow angles.

In heart disease, the quantification of valve regurgitation is a remaining challenge. In this thesis, we have investigated a new technique to estimate the size of regurgitant jets using spectral Doppler and parallel beamforming. A modality that uses high pulse repetition frequency 3-D Doppler was devised, to isolate the backscattered signal power from the vena contracta, that is the narrowest flow region of a regurgitant jet. A simulation study was performed to test and optimize the new method, suggesting a feasible setup for the transmit- and receive beams. Cross-sectional power Doppler images of simulated regurgitations of various sizes were generated, and the regurgitant volumes were accurately estimated. Since the velocity-time integral and the orifice area are extracted from a single recording, the proposed method may give more robust volume estimates than methods where the velocities and the area are measured from separate recordings.