Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound energy while the other half is continuously receiving the reflected signals. If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Pulsed wave techniques allow the accurate measurement of blood flow at a specific area in the heart and the detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

See also Doppler Velocity Signal and Doppler Effect.

The earliest introduction of vascular ultrasound contrast agents (USCA) was by Gramiak and Shah in 1968, when they injected agitated saline into the ascending aorta and cardiac chambers during echocardiographic to opacify the left heart chamber. Strong echoes were produced within the heart, due to the acoustic mismatch between free air microbubbles in the saline and the surrounding blood.

Huygens principle states that an expanding sphere of waves behaves as if each point on the wave front were a new source of radiation of the same frequency and phase. The principle explains how a flat ultrasound transducer can transmit a narrow ultrasound beam, which in the near field is confined to the dimensions of the transducer surface.
Spherical wavelets are emitted from numerous point sources on the transducer surface. They interfere to form a narrow, slightly converging beam of ultrasound in the near field. The wavefronts in the beam are nearly parallel. A precondition for this interference is that the transducer surface is much larger than the ultrasound wavelength.

See also Interference Artifact.

Ultrasound elastography is a specialized imaging technique that provides information about tissue elasticity or stiffness. It is used to assess the mechanical properties of tissues, helping to differentiate between normal and abnormal tissue conditions.
The basic principle behind ultrasound elastography involves the application of mechanical stress to the tissue and measuring its resulting deformation. This is typically achieved by using either external compression or shear waves generated by the ultrasound transducer.
There are two main types of ultrasound elastography:
Both strain elastography and shear wave elastography offer valuable insights into tissue characteristics and can assist in the diagnosis and characterization of various conditions. In clinical practice, ultrasound elastography is particularly useful for evaluating liver fibrosis, breast lesions, thyroid nodules, prostate abnormalities, and musculoskeletal conditions. By providing additional information about tissue stiffness, ultrasound elastography enhances the diagnostic capabilities of traditional ultrasound imaging. It allows for non-invasive assessment, improves the accuracy of tissue characterization, and aids in treatment planning and monitoring of various medical conditions.
See also Ultrasound Accessories and Supplies, Sonographer and Ultrasound Technology.

Christian Johann Doppler first described the effect of motion of sound sources and the frequency change of the sound to the observer.
Doppler ultrasound uses this effect to detect and measure blood flow, and the major reflector is the red blood cell. Doppler ultrasound depends on the fact that if the reflecting surface is moving in relation to the transducer (blood flowing in a vessel) the frequency of the received ultrasound wave will be different from that of the transmitted wave. If blood cells are moving towards the transducer, they increase the frequency of the returning signal. As cells move away from the transducer, the frequency of the returning signal decreases.

See also Quadrature Detection and Doppler Techniques.