Editing Medical ultrasound (section)

=== Producing a sound wave ===
[[File:A modern medical ultrasound scanner.jpg|thumb|A modern medical ultrasound scanner]]
A sound wave is typically produced by a [[piezoelectric]] transducer encased in a plastic housing. Strong, short electrical pulses from the ultrasound machine drive the [[Ultrasonic sensor|transducer]] at the desired frequency. The [[frequencies]] can vary between 1 and 18 [[Megahertz|MHz]], though frequencies up to 50–100 megahertz have been used experimentally in a technique known as biomicroscopy in special regions, such as the anterior chamber of the eye.<ref>{{cite book |last1=Pavlin |first1=Charles |last2=Foster |first2=F. Stuart |year=1994 |title=Ultrasound Biomicroscopy of the Eye |publisher=Springer |isbn=978-0-387-94206-3}}</ref>

Older technology transducers focused their beam with physical lenses.{{citation needed|date=October 2021}} Contemporary technology transducers use [[digital antenna array]] techniques (piezoelectric elements in the transducer produce echoes at different times) to enable the ultrasound machine to change the direction and depth of focus. Near the transducer, the width of the ultrasound beam almost equals to the width of the transducer, after reaching a distance from the transducer (near zone length or [[Fresnel zone]]), the beam width narrows to half of the transducer width, and after that the width increases (far zone length or [[Fraunhofer diffraction|Fraunhofer's zone]]), where the lateral resolution decreases. Therefore, the wider the transducer width and the higher the frequency of ultrasound, the longer the Fresnel zone, and the lateral resolution can be maintained at a greater depth from the transducer.<ref>{{Cite journal |last1=Ng |first1=Alexander |last2=Swanevelder |first2=Justiaan |date=October 2011 |title=Resolution in ultrasound imaging |url=https://linkinghub.elsevier.com/retrieve/pii/S1743181617302068 |journal=Continuing Education in Anaesthesia, Critical Care & Pain |language=en |volume=11 |issue=5 |pages=186–192 |doi=10.1093/bjaceaccp/mkr030|doi-access=free }}</ref> Ultrasound waves travel in pulses. Therefore, a shorter pulse length requires higher bandwidth (greater number of frequencies) to constitute the ultrasound pulse.<ref name="Starkoff 2014">{{Cite journal |last=Starkoff |first=Brian |date=February 2014 |title=Ultrasound physical principles in today's technology |journal=Australasian Journal of Ultrasound in Medicine |language=en |volume=17 |issue=1 |pages=4–10 |doi=10.1002/j.2205-0140.2014.tb00086.x |pmc=5024924 |pmid=28191202}}</ref>

As stated, the sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner, in the [[beamforming]] or spatial filtering technique. This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth.

Materials on the face of the transducer enable the sound to be transmitted efficiently into the body (often a rubbery coating, a form of [[impedance matching]]).<ref name="auto">{{cite web |last1=Leskiw |first1=Chris |last2=Gates |first2=Ian |title=EP2542914A1 - System and method for using orthogonally-coded active source signals for reflected signal analysis |url=https://patents.google.com/patent/EP2542914A1/en |website=Google Patents |publisher=European Patent Office |access-date=6 March 2024}}</ref> In addition, a water-based gel is placed between the patient's skin and the probe to facilitate ultrasound transmission into the body. This is because air causes total reflection of ultrasound; impeding the transmission of ultrasound into the body.<ref>{{cite book |last1=Ostensen |first1=Harald |title=Basic physics of ultrasound imaging |date=2005 |publisher=Diagnostic Imaging and Laboratory Technology - World Health Organisation |location=Geneva |pages=25–26 |url=https://apps.who.int/iris/bitstream/handle/10665/43179/9241592990_eng.pdf |access-date=2 October 2021}}</ref>

The sound wave is partially reflected from the layers between different tissues or scattered from smaller structures. Specifically, sound is reflected anywhere where there are acoustic impedance changes in the body: e.g. [[blood cell]]s in [[blood plasma]], small structures in organs, etc. Some of the reflections return to the transducer.<ref name="auto"/>