Pulse Inversion Doppler for Blood Flow Detection in the Macro- and Micro-vasculature with Ultrasound Contrast Agents

Visualization of macro-vessels is difficult using existing contrast agent imaging methods without blooming or other artifacts. Early use of standard color flow equipment led to saturation and blooming, obscuring small vasculature. More recently, low Mechanical Index (MI<0.15) harmonic techniques have been devised to visualize nearly stationary microbubbles in the microcirculation in real time (frame rates>8Hz). These methods provide a fleeting view of the larger feeding vessels at the beginning of a bolus before the microcirculation fills obscuring their view (∼5-10 secs). Moderate power techniques (MI∼0.2-0.3) are used to visualize the macrovasculature by destroying the bubbles in the microcirculation, but then perfusion is lost. We present a method using a low-MI pulse-inversion acquisition with ultrasound contrast agents, which enables the simultaneous visualization of blood flow in both the macroand micro-vasculature. A combination of RF and Doppler filters enables the separation of the fundamental and harmonic flow components from tissue and stationary microbubbles. These flow signals are combined with conventional pulse inversion processing to make an image displaying both perfusion and flow. Using Definity at an MI of 0.07, this technique was compared against color flow without contrast for the detection of macrovascular blood flow in a human liver. Scanning at low MI reduced clutter in the fundamental and harmonic signals compared to high MI color flow by 25 dB and 45 dB respectively. Adding the contrast agent, even at very low MI, increased flow signals relative to color flow for the two components up to 15 dB and 4 dB, respectively. This gives overall improvement of flow to clutter ratios of 40 dB and 49 dB for the fundamental and harmonic components respectively, yielding improved flow sensitivity and penetration.