Echography (also called sonography) allows visualizing deep structures of the body by recording the reflections (echo) of ultrasound waves directed into the tissues. A medical diagnostic sonogram (echogram), as in echocardiography and echoencephalography, utilizes a frequency range of 1 to 10 MHz.

Ultrasound enhancement is the increase in reflection amplitude from objects that lie behind a weakly attenuating structure. Contrast agent particles enhance the reflectivity of blood.
The enhancement artifact may occur in structures below a cyst.

Harmonic B-mode imaging takes advantage of the non-linear oscillation of microbubbles. During harmonic imaging, the sound signal is transmitted at a frequency of around 1.5 to 2.0 MHz and received at twice this frequency. The microbubbles also reflect waves with wavelengths different from the transmitted one, the detectors can be set to receive only the latter ones and create only images of the contrast agent.
Using bandpass filters the transmitted frequency is separated from the received signal to get improved visualization of vessels containing ultrasound contrast agents (USCAs). The signal to noise ratio during the presence of microbubbles in tissue is four- to fivefold higher at the harmonic compared with the basic frequency.
Using harmonic B-mode imaging, harmonic frequencies produced by the ultrasound propagation through tissue have to be taken into account. The tissue reflection produces only a small amount harmonic energy compared to USCAs, but has to be removed by background subtraction for quantitative evaluation of myocardial perfusion.

See also Non-linear Propagation.

A point scatterer is a reflector with a diameter much smaller than the ultrasound wavelength. The reflection from blood is a typical example of point scattering. Red blood cells are with 7μm versus 0.44 mm wavelength at 3.5 MHz, smaller than any US wavelength. The individual cells are not only the point scatterers, ultrasound is scattered whenever there is a change in acoustic impedance, and in blood such changes are caused by variable cell concentration. These local fluctuations in cell concentration have a spatial extent that is also much smaller than the ultrasound wavelength, and they therefore act as point scatterers.
A point scatterer gives rise to spherical wavelets spreading out in all directions with the scatterer itself at the center of the sphere. The spherical wavelets from one single point scatterer are much too weak to be detected by the transducer, but constructive interference between numerous wavelets will produce backscattering of higher amplitude echoes with parallel wavefronts, also in the direction of the ultrasound transducer.

See also Rayleigh Scattering.

In the field of medical ultrasound imaging, the term 'probe' specifically refers to the ultrasound transducer and represent the handheld device that emits and receives ultrasound waves during an examination.
The probe encompasses various components such as the elements, backing material, electrodes, matching layer, and protective face that are responsible for both emitting and receiving the sound waves. Aperture, known also as the footprint, is the part of the probe that is in contact with the body. When the emitted sound waves encounter body tissues, they generate reflections that are received by the probe, which then generates a corresponding signal. In most cases, the probe emits ultrasound waves for only about 10% of the time and receives them for the remaining 90%.
Probes are available in different shapes and sizes to accommodate various scanning situations. The footprint is linked to the arrangement of the piezoelectric crystals and comes in different shapes and sizes e.g. linear array transducer//convex transducer. The transducer plays a huge role in image quality and is one of the most expensive parts of the ultrasound machine. Mechanical probes steer the ultrasound beam driven by a motor and are capable of producing high-quality images, but they are prone to wear and tear. Mechanical probes have been mostly replaced by electronic multi-element transducers, but mechanical 3D probes still remain for abdominal and Ob-Gyn applications.
In summary, the terms 'ultrasound transducer,' 'probe,' and 'scanhead' are often used interchangeably to refer to the same component of the ultrasound machine. Probes consist of multiple components and are available in different shapes and sizes depending on the sonographer's needs.

See also Handheld Ultrasound, Ultrasound System Performance, Omnidirectional, Probe Cleaning, and Multi-frequency Probe,