Editing Medical ultrasound (section)

== Sound in the body ==
[[File:A medical ultrasound linear array probe, scan head, transducer.jpg|thumb|Curvilinear array transducer]]
Ultrasonography ([[sonography]]) uses a probe containing multiple acoustic [[transducer]]s to send pulses of sound into a material. Whenever a sound wave encounters a material with a different density (acoustical impedance), some of the sound wave is scattered but part is reflected back to the probe and is detected as an echo. The time it takes for the [[Echo (phenomenon)|echo]] to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between acoustic impedances, the larger the echo is. If the pulse hits gases or solids, the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to progress further.{{citation needed|date=July 2024}}

The frequencies used for medical imaging are generally in the range of 1 to 18&nbsp;MHz Higher frequencies have a correspondingly smaller wavelength, and can be used to make more detailed sonograms. However, the attenuation of the sound wave is increased at higher frequencies, so penetration of deeper tissues necessitates a lower frequency (3–5&nbsp;MHz).

Penetrating deep into the body with sonography is difficult. Some acoustic energy is lost each time an echo is formed, but most of it (approximately <math>\textstyle 0.5 \frac{\mbox{dB}}{\mbox{cm depth}\cdot\mbox{MHz}}</math>) is lost from acoustic absorption. (See [[Acoustic attenuation]] for further details on modeling of acoustic attenuation and absorption.)

The speed of sound varies as it travels through different materials, and is dependent on the [[Acoustic impedance|acoustical impedance]] of the material. However, the sonographic instrument assumes that the acoustic velocity is constant at 1540&nbsp;m/s. An effect of this assumption is that in a real body with non-uniform tissues, the beam becomes somewhat de-focused and image resolution is reduced.

To generate a [[2D computer graphics|2-D]] image, the ultrasonic beam is swept. A transducer may be swept mechanically by rotating or swinging or a 1-D [[phased array]] transducer may be used to sweep the beam electronically. The received data is processed and used to construct the image. The image is then a 2-D representation of the slice into the body.

[[3D computer graphics|3-D]] images can be generated by acquiring a series of adjacent 2-D images. Commonly a specialized probe that mechanically scans a conventional 2-D image transducer is used. However, since the mechanical scanning is slow, it is difficult to make 3D images of moving tissues. Recently, 2-D phased array transducers that can sweep the beam in 3-D have been developed. These can image faster and can even be used to make live 3-D images of a beating heart.

[[Doppler effect|Doppler]] ultrasonography is used to study blood flow and muscle motion. The different detected speeds are represented in color for ease of interpretation, for example leaky heart valves: the leak shows up as a flash of unique color. Colors may alternatively be used to represent the amplitudes of the received echoes.