Phase in ultrasound describes where the sound wave is in its cycle of amplitude change. Different waves oscillate at different frequencies, so time is often not a suitable measure of phase.
The phase shift is a difference in the phase or the temporal offset of the peaks of a waveform along one scan line.

See also Coherence, and Histogram.

A piezoelectric crystal changes the physical dimensions when subjected to an electric field. When deformed by external pressure, an electric field is created across the crystal. Piezoelectric ceramic and crystals are used in ultrasound transducers to transmit and receive ultrasound waves.
The piezoelectric crystal in ultrasound transducers has electrodes attached to its front and back for the application and detection of electrical charges. The crystal consists of numerous dipoles, and in the normal state, the individual dipoles have an oblique orientation with no net surface charge.
In ultrasound physics, an electric field applied across the crystal will realign the dipoles and results in compression or expansion of the crystal, depending on the direction of the electric field. For the transmission of a short ultrasound pulse, a voltage spike of very short duration is applied, causing the crystal to initially contract and then vibrate for a short time with its resonant frequency.

See also Composite Array, Transducer Pulse Control, and Temporal Peak Intensity.

Piezo means pressure, so piezoelectric means that pressure is generated when electrical energy is applied to a quartz crystal. When electrical energy is applied to the face of the crystal, the shape of the crystal changes as a function of the polarity of the applied electrical energy. As the crystal expands and contracts it produces compressions and rarefactions, and creates sound waves. When this material is struck by sound waves it creates electrical currents.
Thus, a piezoelectric crystal can produce a pulse of mechanical energy (pressure pulse) by electrically exciting the crystal (transmitter), and they can produce a pulse of electrical energy by mechanically exciting the crystal (receiver). This ultrasound physics principle is called the piezoelectric effect (pressure electricity), which was discovered by Pierre and Jacques Curie in 1880, and is used to generate ultrasound waves. Instead of quartz crystals, piezoelectric ceramics such as barium titanate or lead zirconate titanate are also used, which are crystalline materials with similar piezoelectric properties.

See also Temporal Peak Intensity.

Sound and ultrasound waves consist of a mechanical disturbance of a medium such as air. The disturbance passes through the medium at a fixed speed causing vibration. The rate at which the particles vibrate is the frequency, measured in cycles per second or Hertz (Hz).
The pressure of sound is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2*10^-5 Pascal (20 mP). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.
Sound becomes inaudible to the human ear above about 20 kHz and is then known as ultrasound. Diagnostic imaging uses much higher frequencies, in the order of MHz.
See also Spatial Peak Intensity.

Sound frequencies:

Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.