Power modulation is a non-linear method, based on a multi-pulse technique where the acoustic amplitude (and hence power) of the transmitted pulses is changed. Full and half amplitudes pulses are used to induce changes in the response of the contrast agent. The received echoes from the emitted half amplitude pulse are adjusted with the full amplitude pulse and this pairs of pulses are subtracted. Power modulation is used to separate contrast agent echoes at low mechanical index, allowing real-time perfusion imaging.
Power modulation can be used with a low frequency wide band transducer to increase the depth and transmit the sound beam homogenous allowing ultraharmonic imaging.

The ultrasonic power is the total amount of sound energy emitted by the transducer per unit of time.

See also Ultrasonic Heating.

(CPA) Color power angio is a Doppler measurement, which employs the Doppler effect to assess whether blood is moving towards or away from the probe. By calculating the frequency shift of a particular sample volume, its speed and direction can be determined and visualized. This is particularly useful in cardiovascular studies and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension. The CPA information is displayed as a color image superimposed on the 2D gray scale image.

See also Color Power Doppler.

(CPD) CPD is a type of color Doppler to visualize the presence of detectable blood flow. The flow information is based on the amplitude or strength of echoes received from moving cells and not on frequency shifts. Power Doppler is very sensitive to flowing blood but does not provide velocity or directional information.
CPD is less angle dependent than traditional color Doppler, but more sensitive to motion artifacts. Color power angio (CPA) provides better sensitivity to slow flow states.
The color maps for CPD are represented by a single continuous color (colour, Brit.). Because CPD does not provide directional information, no aliasing artifact occurs.

See also Directional Color Power Doppler.

(HPD) Harmonic power Doppler is currently one of the most sensitive techniques for detecting ultrasound contrast agents. HPD works by transmitting multiple pulses toward the object to be imaged and detecting the pulse-to-pulse changes in the received echo signals.
Second harmonic bandbass filtering is applied to the received signals to exploit the non-linear behavior of scattering from bubbles (clutter). Harmonic power Doppler operates best at high output levels because of increased contrast destruction, and pulse amplitudes close to the maximum allowed are used much of the time.
With a high mechanical index, non-linear propagation of the sound will cause significant harmonic components from tissue, and the contrast agent to tissue ratio will decrease.
Also called Harmonic Power Angio. See also Multiple Frame Trigger.