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.

(PRF) The pulse repetition frequency is the number of pulses per second. The usual range for echocardiographs is between 200 and 5000 pulses per second. The PRF varies with the type of mode in operation.

See also Q-value.

Pulse volume recording is a method in which air-filled cuffs are placed segmentally on a limb and a difference in the limb volume associated with arterial pulsation is translated into a waveform. Evaluation of waveform change in this plethysmographic technique indicates the probability of the presence of an obstruction.

See also Plethysmography.

Pulsed ultrasounds are cycles of ultrasound separated in time with gaps of no signal. Pulsed sound waves are generated by short, strong pulses of sound from a phased array of piezoelectric crystals. The transducer, though emitting ultrasound in rapid pulses, acts as a receiver most of the time. In sonography, pulsed ultrasound is used to perform diagnostic or therapeutic procedures.

See also Pulse Average Intensity, Release Burst Imaging.