Contrast-enhanced Doppler Ultrasound Imaging using Plane Waves

There is a clear advantage in having an imaging method that would be capable of displaying perfusion and vascular imaging simultaneously. In ultrasound, Doppler can visualize veins and arteries, but cannot detect the capillary level perfusion. Conversely, microbubble contrast-enhanced ultrasound imaging enables low velocity perfusion to be detected and separated from the tissue echo, but has a limited ability to depict the vascular morphology within the perfused tissue. To date, it has been challenging to combine these two methods due to realistic imaging time constraints severely limiting the number of pulses available for Doppler estimates. However, recent breakthroughs in plane-wave synthetic ultrasound considerably increased the maximal imaging frame rate, enabling new imaging trade-off previously unavailable. In this thesis, we first implement pulse inversion Doppler using plane-wave ultrasound and demonstrate the ability of the technique in segmenting fast from slow blood flow, which has applications, for example, in the detection, diagnosis and assessment of anti-angiogenic treatment of focal lesions. As the ability to differentiate perfusion from the tissue signal is limited by the contrast-to-tissue ratio achieved by the contrast-enhanced pulsing sequence, independently of the Doppler ensemble length, we develop a new Doppler sequence based on amplitude modulation (AM) which improves bubble detection specificity compared to conventional Pulse Inversion Doppler. We also demonstrate that modifying the pressure of the acoustic pulse (e.g. AM) introduces a characteristic phase delay in the microbubble echo, with no equivalent in tissue. A novel microbubble imaging scheme taking advantage of this behaviour is shown to further improve the contrast-to-tissue ratio. Lastly, while it is known that long Doppler acquisitions enable accurate velocity estimation in blood (hematocytes), microbubbles are only quasi-stable during repeated ultrasound exposure. Short acoustic pulses are helpful in mitigating microbubble destruction, but concurrently introduce a classical error associated to uncertainty limit of waves. Existing contrast agents are also polydispersed in their physical properties, which adds another degree of statistical randomness in the received echoes. In this work, we assess how these factors affect our ability to measure the velocity of flowing microbubbles.

Department dc:contributor.department

Medical Biophysics

Year dc:date.issued

2016

Author dc:creator

• Tremblay-Darveau, Charles

Advisor dc:contributor.advisor

• Burns, Peter N

• Doppler
• Microbubble
• Perfusion imaging
• Plane-wave ultrasound
• Vascular morphology

Handle dc:identifier.uri

http://hdl.handle.net/1807/73183

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/73183