Ultrasound

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For other uses, see Ultrasound (disambiguation).

Ultrasound is sound with a frequency greater than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz).

Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications
A fetus in its mother's womb, viewed in a sonogram (brightness scan)
A fetus in its mother's womb, viewed in a sonogram (brightness scan)

Contents

[edit] Ability to hear ultrasound

Some animals such as dogs, dolphins, bats, and mice have an upper frequency limit that is greater than that of the human ear and thus can hear ultrasound. Some have said that children can hear some high-pitched sounds that older adults cannot hear, as in humans the upper limit pitch of hearing gets lower with age (a cell phone company has used this to create ring signals only able to be heard by younger humans[1]). This frequency limit is caused by the middle ear that acts as a low-pass filter. If ultrasound is fed directly into the skull bone and reaches the cochlea without passing through the middle ear, much higher frequencies (up to about 200 kHz) can be heard. This effect (sometimes called ultrasonic hearing) was first discovered by divers exposed to a high-frequency (ca. 50 kHz) sonar signal.

Sonogram of a foetus at 14 weeks (Profile)
Sonogram of a foetus at 14 weeks (Profile)

[edit] Diagnostic sonography

A fetus, aged 29 weeks, in a "3D ultrasound"
A fetus, aged 29 weeks, in a "3D ultrasound"
Main article: Medical Sonography

Medical sonography (ultrasonography) is a ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, their size, structure and any pathological lesions. They are also used to visualize a foetus during routine and emergency prenatal care. Ultrasound scans are performed by medical health care professionals called sonographers. Obstetric sonography is commonly used during pregnancy.

Ultrasound is generally dismissed as a "safe test" because it does not use ionizing radiation, which imposes hazards, such as cancer production and chromosome breakage. However, Ultrasound energy has two physiological effects: 1. Enhance inflammatory response, 2. Heats soft tissue. Ultrasound energy produces a mechanical pressure wave through soft tissue. This pressure wave causes microscopic bubbles in living tissues, and distortion of the cell membrane, influencing ion fluxes and intracellular activity. When ultrasound enters the body, it causes molecular friction and heats the tissues slightly. In some cases, it can also cause small pockets of gas in body fluids or tissues to expand and contract/collapse (cavitation). The long-term effects of tissue heating and cavitation are not known. Many national and international organizations regard ultrasound as an “unsafe test” not to be used in low risk pregnancies. However obstetric ultrasound can be used to identify many conditions that would be harmful to mother and baby. For this reason many healthcare professionals consider that the risk of leaving these conditions undiagnosed is much greater than the very small risk associated with undergoing the scan.

Ultrasound is used routinely in obstetric appointments during pregnancy, but the FDA discourages its use for non-medical purposes such as fetal keepsake videos and photos, even though it is the same technology used in hospitals.

Obstetric ultrasound is primarily used to:

  • Date the pregnancy.
  • Check the location of the placenta.
  • Check for the number of fetuses.
  • Check for physical abnormities.
  • Check the sex of the baby.
  • Check for fetal movement, breathing, and heartbeat.

Unfortunately, results are often wrong, producing a high number of false positives [Cochrane collaboration]. There are non-invasive methods to determine your pregnancy date, placenta location, number of fetuses, fetal movement, breathing, and heartbeat. A routine ultrasound exam only detects about half of all structural birth defects. Many women are told their baby will, or will not, have a birth defect when the reverse is actually true. Sex determination is only accurate after 13 weeks gestation [Ultrasound in Obstetrics and Gynecology 1999]. When balancing risk & reward; it is highly recommended to avoid the use of routine ultrasound for low risk pregnancies [ACOG].

A study on rodent fetus brains that are exposed to ultrasound showed signs of damage. Speculation on human fetuses can be in a range of no significant complications to variety of mental and brain disorder. The study shows that rodent brain cells failed to grow to their proper position and remained scattered in incorrect parts of the brain. The conditions of this experiment are different from typical fetal scanning because of the long dwell times. [National Institute of Neurological Disorders; Proceedings of the National Acadamy of Sciences]. Care should be taken to use low power settings and avoid pulsed wave scanning of the fetal brain unless specifically indicated in high risk pregnancies.

It should be noted that obstetrics is not the only used of ultrasound. Soft tissue imaging of many other parts of the body is conducted with ultrasound. Other scans routinely conducted are cardiac, renal and hepatic. There are also a few specialist centres undertaking dermal and ophthalmic ultrasound investigation. Figures released for period 2005-2006 by UK Government (Department of Health) show that non-obstetric ultrasound examinations contributed to more than 65% of the total number of ultrasound scans conducted

[edit] Biomedical ultrasound applications

Ultrasound also has therapeutic applications, which can be highly beneficial when used with dosage precautions[2]:

  • According to RadiologyInfo [1], Ultrasounds are useful in the detection of Pelvic abnormalities and can involve techniques known as abdominal (transabdominal) ultrasound, vaginal (transvaginal or endovaginal) ultrasound in women, and also rectal (transrectal) ultrasound in men.
  • Treating benign and malignant tumors and other disorders, via a process known as Focused Ultrasound Surgery (FUS) or HIFU, High Intensity Focused Ultrasound. These procedures generally use lower frequencies than medical diagnostic ultrasound (from 250 kHz to 2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by MRI, as in Magnetic Resonance guided Focused Ultrasound.
  • More powerful ultrasound sources may be used to clean teeth in dental hygiene or generate local heating in biological tissue, e.g. in occupational therapy, physical therapy and cancer treatment.
  • Extracorporeal shock wave lithotripsy uses a powerful focused ultrasound source to break up kidney stones[3].
  • Focused ultrasound sources may be used for cataract treatment by phacoemulsification.
  • Additional physiological effects of low-intensity ultrasound have recently been discovered, e.g. the ability to stimulate bone-growth and its potential to disrupt the blood-brain barrier for drug delivery.
  • Ultrasound is used in UAL (= ultrasound-assisted lipectomy), or liposuction.
  • Doppler ultrasound is being tested for use in aiding tissue plasminogen activator treatment in stroke sufferers. This procedure is called Ultrasound-Enhanced Systemic Thrombolysis.
  • Low intensity pulsed ultrasound is used for therapeutic tooth and bone regeneration.
  • Ultrasound can also be used for elastography. This can be useful in medical diagnoses, as elasticity can discern healthy from unhealthy tissue for specific organs/growths. In some cases unhealthy tissue may have a lower system Q, meaning that the system acts more like a large heavy spring as compared to higher values of system Q (healthy tissue) that respond to higher forcing frequencies. Ultrasonic elastography is different from conventional ultrasound, as a transceiver (pair) and a transmitter are used instead of only a transceiver. One transducer (a single element {or array of elements} acts as both the transmitter and receiver to image the region of interest over time. The extra transmitter is a very low frequency transmitter, and perturbs the system so the unhealthy tissue oscillates at a low frequency and the healthy tissue does not. The transceiver, which operates at a high frequency (typically MHz) then measures the displacement of the unhealthy tissue (oscillating at a much lower frequency). The movement of the slowly oscillating tissue is used to determine the elasticity of the material, which can then be used to distinguish healthy from unhealthy tissue.
  • Ultrasound has been shown to act synergistically with antibiotics in bacterial cell killing.[4]
  • Ultrasound has been postulated to allow thicker eukaryotic cell tissue cultures by promoting nutrient penetration.Scientific Article
  • Ultrasound in the low MHz range in the form of standing waves is an emerging tool for contactless separation, concentration and manipulation of microparticles and biological cells. The basis is the acoustic radiation force, a non-linear effect which causes particles to be attracted to either the nodes or anti-nodes of the standing wave depending on the acoustic contrast factor, which is a function of the sound velocities and densities of the particle and of the medium in which the particle is immersed.

[edit] Industrial ultrasound

Non-destructive testing of a swing shaft showing spline cracking
Non-destructive testing of a swing shaft showing spline cracking

Ultrasound is also used in Nondestructive testing to find flaws in materials. Frequencies of 2 to 10 MHz are common but for special purposes other frequencies are used. Inspection may be manual or automated and is an essential part of modern manufacturing processes. Most metals can be inspected as well as plastics and aerospace composites. Lower frequency ultrasound (50 kHz to 500 kHz) can also be used to inspect less dense materials such as wood, concrete and cement.

Ultrasound can also be used for heat transfer in liquids[5].

Researchers recently employed ultrasound in dry corn milling plant to enhance ethanol production [2].

[edit] Ultrasound flow meter

Main article: Ultrasound flow meter

[edit] Ultrasonic cleaning

Ultrasonic cleaners, sometimes mistakenly called supersonic cleaners, are used at frequencies from 20-40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments and industrial parts. An ultrasonic cleaner works mostly by energy released from the collapse of millions of microscopic cavitations near the dirty surface. The bubbles formed by cavitation collapse forming tiny jets directed at the surface. Home ultrasonic cleaners are available and cost about US $60 or more.

[edit] Ultrasound and animals

[edit] Bats

Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey.

[edit] Dogs

The dog whistle is used to call to a dog. It makes ultrasound at a frequency in the range of 16 kHz to 22 kHz that dogs can hear.

[edit] Dolphins and whales

It is well known that dolphins and some whales can hear ultrasound and have their own natural sonar system.

[edit] Fish

Several types of fish can detect ultrasound. Of the order Clupeiformes, members of the subfamily Alosinae (shad), have been shown to be able to detect sounds up to 180 kHz, while the other subfamilies (e.g. herrings) can hear only up to 4 kHz.[6]

[edit] Moths

There is evidence that ultrasound in the range emitted by bats causes flying moths to make evasive manoeuvres, because bats eat moths. Ultrasonic frequencies trigger a reflex action in the noctuid moth that cause it to drop a few inches in its flight to evade attack. [3]

[edit] Rodents/Insects

Ultrasound generator/speaker systems are sold with claims that they frighten away rodents and insects, but there is no scientific evidence that the devices work. Laboratory tests conducted by Kansas State University did show positive results for products from specific manufacturers. Controlled tests on some of the systems have shown that rodents quickly learn that the speakers are harmless. The positive results (Kansas State University) were limited to units which use constantly modulating frequencies. The frequency used however is often within the range that most children can hear, and can cause headaches.

[edit] Mosquitoes

There is a theory that ultrasound of certain frequencies, while not audible to humans, repel mosquitoes. There are computer programs available on the internet that claim to use this phenomenon for pest-control. There have been mixed reports about the effectiveness of this method towards mosquito-control.

[edit] Sonochemistry

Main article: Sonochemistry

Power ultrasound in the 20-100 kHz range is used in chemistry. The ultrasound does not interact directly with molecules to induce the chemical change, as its typical wavelength (in the millimeter range) is too long compared to the molecules. Instead:-

  • It causes cavitation which causes local extremes of temperature and pressure in the liquid where the reaction happens.
  • It breaks up solids and removes passivating layers of inert material to give a larger surface area for the reaction to occur over.

Both of these make the reaction faster.

[edit] Ultrasonic disintegration

Some sorts of ultrasound can disintegrate biological cells including bacteria. This has uses in biological science and in killing bacteria in sewage. High power ultrasound at frequency of around 20 kHz produces cavitation that facilitates particle disintegration. Dr. Samir Khanal of Iowa State University employed high power ultrasound to disintegrate corn slurry to enhance liquefaction and saccharification for higher ethanol yield in dry corn milling plants.

See examples:-

[edit] Ultrasound as a weapon

Main article: Sonic weaponry

[edit] Sonoluminescence

Main article: Sonoluminescence

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.

[edit] Modulated ultrasound carrying audio messages

Main article: Modulated ultrasound

[edit] Ultrasonic range finding

A common use of ultrasound is in range finding; this use is also called sonar. This works simiarly to radar: An ultrasonic pulse is generated in a particular direction. If there is an object in the path of this pulse, part or all of the pulse will be reflected back to the sender as an echo and can be detected. By measuring the difference in time between the pulse being transmitted and the echo being received, it is possible to determine how far away the object is.

Although range finding underwater is performed at both sub-audible and audible frequencies for great distances (1000 to 30000 meters), ultrasonic range finding is used when distances are shorter and the accuracy of the distance measurement is desired to be finer. Ultrasonic measurements may be limited from about a hundred to a thousand meters, but can be performed with one-centimeter to one-meter accuracy.

[edit] Other uses

Ultrasound when applied in specific configurations can produce exotic phenomena such as sonoluminescence. These phenomena are being investigated partly because of the possibility of bubble fusion (a nuclear fusion reaction hypothesized to occur during sonoluminescence).

[edit] Nonlinear Effects of Propagating Acoustic Wave

Because of their high amplitude to wavelength ratio, ultrasonic waves commonly display nonlinear propagation.

[edit] See also

[edit] References

  1. ^ "A Ring Tone Meant to Fall on Deaf Ears" (New York Times article)
  2. ^ Essentials of Medical Ultrasound: A Practical Introduction to the Principles, Techniques and Biomedical Applications, edited by M. H. Rapacholi, Humana Press 1982
  3. ^ Raymond Bonnett and Robert G Moore, Minimally Invasive Urological Surgery, Taylor and Francis, London, 2005
  4. ^ Citation list
  5. ^ Milton B. Larson, Study of the Effects of Ultrasonic Vibrations on Convective Heat Transfer in Liquids, (1960)
  6. ^ Mann DA, et al. (2001) Ultrasound detection by clupeiform fishes. JASA 109 (6), 3048-3054 | doi:10.1121/1.1368406

[edit] External links

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