The Doppler interrogation frequency is the frequency of the transmitted acoustic energy relative to the measured Doppler frequency shift. This frequency is most but not necessarily, the nominal transducer frequency.

See also Doppler Effect and Doppler Ultrasound.

Doppler Shift is the change in the perceived frequency relative to the transmitted frequency. The Doppler shift is dependent on the insonating frequency, the velocity of moving blood, and the angle between the sound beam and direction of moving blood.
Doppler equation:
Doppler shift frequency: fD = fr - f0 = 2f0v/c
Where fD is the Doppler shift frequency = the difference between transmitted and received frequencies.
Ultrasound system use the following equation:
Doppler shift frequency with incident angle: fD = 2f0v/c cosØ
Where f is the transmitted frequency, v is the blood velocity, c is the speed of sound in tissue, cosØ is the Cosine of the blood flow to beam angle.
The Doppler angle (theta) is the angle of incidence of the beam upon the object. If the beam is parallel to the flowing blood, the angle theta is zero, and the determination of flow is most accurate. If the angle of incidence is greater, the results are less reliable. Doppler shift results with an angle greater than 20° should not be used for the calculation.

See also Doppler Interrogation Frequency, Zero Crossing Detector, Doppler Effect, Doppler Ultrasound and Motion Discrimination Detector.

(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
The effect of different frequencies on tissue penetration:
The higher the frequency the less the penetration, the lower the frequency the greater the penetration. As frequency increases, resolution improves but the imaging depth or penetration decreases. The lower the axial resolution, the more detail can be seen.
Usual frequencies for pediatric ultrasound: 5.0mHz to 7.5mHz and 10mHz.
Usual frequencies for adult ultrasound: 2.0mHz to 3.0mHz.

See also Doppler Interrogation Frequency, Multi-frequency Probe, and Huygens Principle.