Ultrasound
:''For other meanings of "ultrasound" or "ultrasonic", see
ultrasound (disambiguation).
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A baby in its mother's womb, viewed in a sonogram (brightness scan) |
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A baby, aged 29 weeks, in a "3D ultrasound" |
Ultrasound is
sound with a
frequency greater than the upper limit of human hearing, this limit being approximately 20
kilohertz (20,000 hertz).
Some animals, such as
dogs,
dolphins,
bats, and
mice have an upper limit that is greater than that of the human
ear and thus can hear ultrasound. 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). 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.
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Sonogram of a fetus at 14 weeks (Profile) |
Medical sonography (
ultrasonography) is a useful 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 fetus during pregnancy. Ultrasound scans are performed by medical health care professionals called
sonographers.
Obstetric sonography is commonly used during
pregnancy.
*A new 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.
Ultrasound also has therapeutic applications:-
*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.
*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 system Qs (healthy tissue) that respond to higher forcing frequencies. Ultrasonic elastography is different from conventional ultrasound, as both a transceiver (pair) and transmitter are used instead of one 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 latter is a very low frequency transmitter. The latter acts to perturb the system such that the unhealthy tissue oscillates at a low frequency (single transmitter) while the healthy tissue does not resonate at such low frequencies. The transceiver, which operates at a high frequency (typically MHz) them 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 differentiate between healthy and unhealthy tissue.
Ultrasound is also used in industry 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.
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 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.
Rodents
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; controlled tests have shown that rodents quickly learn that the speakers are harmless. However, the pitch used is well within the range that most children can hear, and can cause
headaches.
Insects
There is evidence that ultrasound in the range emitted by
bats causes flying
moths to make evasion maneuvers, because bats eat moths.
Dogs
The
dog whistle is used to call to a
dog. It makes ultrasound at a
frequency that dogs can hear.
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.
Some sorts of ultrasound can disintegrate
biological cells including
bacteria. This has uses in
biological science and in killing bacteria in
sewage. See for example:-
*http://www.sanyogallenkamp.com/proddetail.asp?prodid=236
*http://www.iwaponline.com/wst/04209/wst042090073.htm
When an acoustic wave propagates through a material, it acts as a force that creates localized pressure. When a material is under pressure (as compared to some lower equilibrium pressure), the speed of sound increases because the molecules transmitting the energy are closer together. As a result, the wave travels faster during the high pressure phase than for the lower pressure section of oscillation. Consider a sinusoidal wave with a high peak. As a result of this nonlinear effect, the peaks of the wave travel faster than the dips (near zero). When the peaks travel faster, the shape of the wave changes, as the higher amplitude sections shift farther forward than the lower pressure part of the wave and the signal approaches more of a square wave than a sinusoidal one. Fourier analysis will show that this single-frequency wave will be changed into one that has much more than a single impulse in the frequency domain! This implies a non-linear system, as a linear one cannot output frequencies that were not a part of the input signal.
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).
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.
Bats use a variety of ultrasonic ranging (
echolocation) techniques to detect their prey.
*
Infrasound (sound at extremely low frequencies)
*
Light*
Physics*
Radiology Web Site Directory*
Ultrasound Job Outlooks*
Foundation for Focused Ultrasound Research*
Radiology Resources for Students and Professionals*
Medical Engineer - Clinical Ultrasound for Blood Flow*
Ultrasound Equipment from Siemens Medical*
3D Ultrasound Pictures*
21 week scan (Uses
Java)
*
New Yahoo email group about ultrasound*
Human and veterinary ultrasound systems*
Ultrasound Can Affect Fetal Brain Development
