Perfusion imaging with pulse inversion Doppler and microbubble contrast agents: in vivo studies of the myocardium

Pulse Inversion Doppler (PID) is a recently developed, multi-pulse imaging technique for detecting microbubble contrast agents in tissue. By transmitting as few as 2 pulses per line of sight, then applying modified colour Doppler wall filters together with power or colour Doppler processing, PID can suppress linear tissue echoes while imaging nonlinear microbubble echoes in real-time, even when microbubble and tissue are moving at the same velocity. Filtering efficiency improves as more transmit pulses are used. By transmitting 4 or more high intensity pulses per line of sight and applying appropriate wall filters, both linear and nonlinear clutter can be suppressed while moving or disrupting microbubbles are imaged. The flexibility of pulse inversion Doppler allows imaging parameters to be optimized according to clinical application, transmit pulse intensity, and agent properties. Prototype PID imaging was implemented on a clinical scanner, and myocardial contrast imaging was performed in anaesthetized dogs with perfluorocarbon and air based agents. Fundamental and harmonic grey-scale, conventional Doppler and PID techniques were compared at different transmit intensities. At low transmit intensity, PID produced up to 15 dB more agent to tissue contrast than harmonic imaging, allowing realtime imaging of myocardial perfusion. At high transmit intensities, both PID and conventional Doppler provided strong myocardial enhancement. Effects were agent dependent. Doppler filter design and implementation are discussed.