Perfluorochemicals are used as microbubble filling gases because of their low solubility in blood and high vapor pressure. Various types of perfluorochemical gases like perfluorocarbon, perfluorobutane, perfluoropropane, and perfluorohexane are used to substitute the air in microbubbles to improve the stability and plasma longevity of the agents. Perfluorocarbons are liquids at room temperature but gas at body temperature. The large molecules of perfluorocarbons have slow diffusion and solubility which increase the enhancement time of the ultrasound contrast agent as compared to air.

See also Filling Gas, and PESDA.

Phospholipids are a major component of all biological membranes. When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other. This forms a membrane with hydrophilic heads on both sides. This membrane is partially permeable and very flexible.
Phospholipid containing microbubbles are in use as diagnostic ultrasound contrast agents. Phospholipids can be targeted to atheroma and other pathologic components to enhance atherosclerosis imaging. The majority of these echogenic liposomes range in diameter from 0.25 to 5.0 μm.

(PD) Power Doppler imaging (PDI) is a Doppler technique, sensitive to low blood flow, allowing a complete visualization of detailed vascular blood structure. This medical imaging method is useable for detecting microbubbles during myocardial contrast echocardiography.

See also Resistive Index.

Selective detection of the microbubble contrast medium can be enhanced by Doppler processing that removes signals with zero Doppler frequency shifts. This will remove tissue harmonics. By detecting overlong bursts of inverted pulses and using Doppler detection methods, very high sensitivity to microbubbles can be achieved. The bubbles can be detected at sufficiently low incident power levels to avoid destroying them. Pulse inversion Doppler has demonstrated the first real-time images of myocardial perfusion using perfluorocarbon gas agents.

See also Pulse Inversion Imaging, Myocardial Contrast Echocardiography, and Perfluorochemicals.

(PII) Pulse inversion imaging (also called phase inversion imaging) is a non-linear imaging method specifically made for enhanced detection of microbubble ultrasound contrast agents. In PII, two pulses are sent in rapid succession into the tissue; the second pulse is a mirror image of the first. The resulting echoes are added at reception. Linear scattering of the two pulses will give two echoes which are inverted copies of each other, and these echoes will therefore cancel out when added.
Linear scattering dominates in tissues. Echoes from linear scatterers such as tissue cancel, whereas those from gas microbubbles do not. Non-linear scattering of the two pulses will give two echoes which do not cancel out completely due to different bubble response to positive and negative pressures of equal magnitude. The harmonic components add, and the signal intensity difference between non-linear and linear scatterers is therefore increased. The resulting images show high sensitivity to bubbles at the resolution of a conventional image.
In harmonic imaging, the frequency range of the transmitted pulse and the received signal should not overlap, but this restriction is less in pulse inversion imaging since the transmit frequencies are not filtered out, but rather subtracted. Broader transmit and receive bandwidths are therefore allowed, giving shorter pulses and improved axial resolution, hence the alternative term wideband harmonic imaging. Many ultrasound machines offer some form of pulse inversion imaging.

See also Pulse Inversion Doppler, Narrow Bandwidth, Dead Zone, Ultrasound Phantom.