Pulse inversion Doppler: a new method for detecting nonlinear echoes from microbubble contrast agents

Abstract

A novel technique for the selective detection of ultrasound contrast agents, called pulse inversion Doppler, has been developed. In this technique, a conventional Doppler or color Doppler pulse sequence is modified by inverting every second transmit pulse. Either conventional or harmonic Doppler processing is then performed on the received echoes. In the resulting Doppler spectra, Doppler shifts from linear and nonlinear scattering are separated into two distinct regions that can be analyzed separately or combined to estimate the ratio of nonlinear to linear scattering from a region of tissue. The maximum Doppler shift that can be detected is 1/2 the normal Nyquist limit. This has the advantage over conventional harmonic Doppler that it can function over the entire bandwidth of the echo signal, thus achieving superior spatial resolution in the Doppler image. In vitro measurements comparing flowing agent and cellulose particles suggest that pulse inversion Doppler can provide 3 to 10 dB more agent to tissue contrast than harmonic imaging with similar pulses. Similar measurements suggest that broadband pulse inversion Doppler can provide up to 16 dB more contrast than broadband conventional Doppler. Nonlinear propagation effects limit the maximum contrast obtainable with both harmonic and pulse inversion Doppler techniques.

Cavitation Threshold of Microbubbles in Gel Tunnels by Focused Ultrasound

“…As the power is increased, bubbles scatter not only at the acoustic center frequency, but also at low multiples and simple fractions of the driving frequency. It is the detection of these non-linear components which is at the basis of contrast-specific modalities such as contrast harmonic (Burns et al 1996) and pulse inversion imaging and contrast Doppler (Hope Simpson et al 1999), which are used to suppress tissue signals and enhance blood echo, particularly at the level of the smaller blood vessels. As the pressure amplitude is further increased, contrast agent disruption takes place with the emission of a strong and brief nonlinear echo, which is relatively easy to detect and allows the evaluation of tissue perfusion because of the modification of the signal intensity of the image (Wei et al 1998).…”

Contrast-enhanced ultrasound for examining tumor biology

“…In other words, the signal from tissue is suppressed. Echoes reflected from microbubbles, however, will not cancel each other out, since they do not mirror each other [5,6] . Likewise, when signals from SAEs are recorded, no cancellation occurs, since the bubble is destroyed before the second pulse arrives.…”

Improved visualization of high-intensity focused ultrasound lesions

“…Generation of harmonics increases as pulse intensity increases, and while cumulative, are generated primarily in the region near the focus where pulse intensity is greatest. Tissue harmonic images are produced from the signal envelope after bandpass filtering or from the envelope of the additive signal of consecutive phase inverted pulses (Simpson et al 1999). …”