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.

(THI) Tissue harmonic imaging (also called native harmonic imaging) is a signal processing technique which addresses ultrasound limitations like penetration and resolution. Tissue harmonic imaging reduces noise and clutter by improving signal to noise ratio and resolution. The signal penetration in soft tissue increases as the transmit frequency is decreased, by simultaneous decreased image resolution. As an ultrasound wave propagates through the target media a change occurs in the shape and frequency of the transmitted signal. The change is due to the normal resistance of tissue to propagate sound energy. This resistance and the resulting signal change is called a harmonic oscillation.
For harmonic imaging the input frequency doubles the output frequency, for example a transmit frequency of 3.0 MHz. which would provide maximum penetration will return a harmonic frequency of 6.0 MHz. The returning higher frequency signal has to only travel one direction to the probe. The advantages of high frequency imaging and the one-way travel effect are decreased reverberation, beam aberration, and side lobes, as well as increased resolution and cystic clearing.

(MI) The mechanical index is an estimate of the maximum amplitude of the pressure pulse in tissue. It is an indicator of the likelihood of mechanical bioeffects (streaming and cavitation). The mechanical index of the ultrasound beam is the amount of negative acoustic pressure within a ultrasonic field and is used to modulate the output signature of US contrast agents and to incite different microbubble responses.
The mechanical index is defined as the peak rarefactional pressure (negative pressure) divided by the square root of the ultrasound frequency.
The FDA ultrasound regulations allow a mechanical index of up to 1.9 to be used for all applications except ophthalmic (maximum 0.23). The used range varies from 0.05 to 1.9.
At low acoustic power, the acoustic response is considered as linear. At a low MI (less than 0.2), the microbubbles undergo oscillation with compression and rarefaction that are equal in amplitude and no special contrast enhanced signal is created. Microbubbles act as strong scattering objects due to the difference in impedance between air and liquid, and the acoustic response is optimized at the resonant frequency of a microbubble.
At higher acoustic power (MI between 0.2-0.5), nonlinear oscillation occurs preferentially with the bubbles undergoing rarefaction that is greater than compression. Ultrasound waves are created at harmonics of the delivered frequency. The harmonic response frequencies are different from that of the incident wave (fundamental frequency) with subharmonics (half of the fundamental frequency), harmonics (including the second harmonic response at twice the fundamental frequency), and ultra-harmonics obtained at 1.5 or 2.5 times the fundamental frequency. These contrast enhanced ultrasound signals are microbubble-specific.
At high acoustic power (MI greater than 0.5), microbubble destruction begins with emission of high intensity transient signals very rich in nonlinear components. Intermittent imaging becomes needed to allow the capillaries to be refilled with fresh microbubbles. Microbubble destruction occurs to some degree at all mechanical indices. A mechanical index from 0.8 to 1.9 creates high microbubble destruction. The output signal is unique to the contrast agent.

According to Shannon's sampling theorem, the sampling frequency should be twice the frequency being sampled. The nyquist frequency is the maximum frequency that can be sampled without aliasing. In ultrasound imaging, it is defined as half of the pulse repetition frequency.
NF = PRF/2 (nyquist frequency = pulse repetition frequency/2)
This is the so-called Nyquist limit. If the velocity of flow exceeds the Nyquist limit, the direction and velocity are inaccurately displayed and appear to change direction. Color flow Doppler capitalizes on this effect. This allows detecting flow disturbances from laminar to turbulent flow.

See also Aliasing Artifact, Repetition Rate, and Sampling Rate.

Harmonic imaging relies on detection of harmonics of the transmitted frequency produced by bubble oscillation. This method is widely available on ultrasound scanners and uses the same array transducers as conventional imaging. A major limitation of the use of ultrasound contrast agents is the problem that signals from the microbubbles are mixed with those from tissue. Echoes from solid tissue and red blood cells are suppressed by harmonic imaging.
In harmonic mode, the system transmits at one frequency, but is tuned to receive echoes preferentially at double that frequency, and the second harmonic echoes from the place of the bubble. Typically, the transmit frequency lies between 1.5 and 3 MHz and the receive frequency is selected by means of a bandpass filter whose center frequency lies between 3 and 6 MHz.
Color Doppler and real-time harmonic spectral Doppler modes have also been implemented and show a level of tissue motion suppression not available in conventional modes.

See also Harmonic B-Mode Imaging, and Harmonic Power Doppler.