Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" (audible) sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults.

The production of ultrasound is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium, a property also used by animals such as bats for hunting. The most well known application of ultrasound is its use in sonography to produce pictures of fetuses in the human womb. There are a vast number of other applications as well.

Medical sonography (ultrasonography) is an ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, to capture their size, structure and any pathological lesions with real time tomographic images. Ultrasound has been used by radiologists and sonographers to image the human body for at least 50 years and has become one of the most widely used diagnostic tools in modern medicine. The technology is relatively inexpensive and portable, especially when compared with other techniques, such as magnetic resonance imaging (MRI) and computed tomography (CT). Ultrasound is also used to visualize fetuses during routine and emergency prenatal care. Such diagnostic applications used during pregnancy are referred to as obstetric sonography.

As currently applied in the medical field, properly performed ultrasound poses no known risks to the patient.

Sonography is generally described as a "safe test" because it does not use mutagenic ionizing radiation, which can pose hazards such as chromosome breakage and cancer development. However, ultrasonic energy has two potential physiological effects: it enhances inflammatory response; and it can heat soft tissue. Ultrasound energy produces a mechanical pressure wave through soft tissue. This pressure wave may cause 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. This effect is typically very minor as normal tissue perfusion dissipates most of the heat, but with high intensity, it can also cause small pockets of gas in body fluids or tissues to expand and contract/collapse in a phenomenon called cavitation; however this is not known to occur at diagnostic power levels used by modern diagnostic ultrasound units.

Obstetric ultrasound is primarily used to:

Date the pregnancy (gestational age)

Confirm fetal viability

Determine location of fetus, intrauterine vs ectopic

Check the location of the placenta in relation to the cervix

Check for the number of fetuses (multiple pregnancy)

Check for major physical abnormalities.

Assess fetal growth (for evidence of intrauterine growth restriction (IUGR))

Check for fetal movement and heartbeat.

Determine the sex of the baby

Unfortunately, results are occasionally wrong, producing a false positive

False detection may result in patients being warned of birth defects when no such defect exists. Sex determination is only accurate after 12 weeks gestation.

It should be noted that obstetrics is not the only use of ultrasound. Soft tissue imaging of many other parts of the body is conducted with ultrasound. Other scans routinely conducted are cardiac, renal, liver and gallbladder (hepatic). Other common applications include musculo-skeletal imaging of muscles, ligaments and tendons, ophthalmic ultrasound (eye) scans and superficial structures such as testicle, thyroid, salivary glands and lymph nodes. Because of the real time nature of ultrasound, it is often used to guide interventional procedures such as fine needle aspiration FNA or biopsy of masses for cytology or histology testing in the breast, thyroid, liver, kidney, lymph nodes, muscles and joints.

Ultrasound scanners have different Doppler-techniques to visualize arteries and veins. The most common is colour doppler or power doppler, but also other techniques like b-flow are used to show bloodflow in an organ. By using pulsed wave doppler or continuous wave doppler bloodflow velocities can be calculated.
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Hello
Thanks for nice query.     
Ultrasound uses ultra short sound waves at very high frequency this passes at different speed through different organs or density .& reflected back to produce some kind of images which are studied.
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Hello.
Thanks for writing to us.
An ultrasound uses high frequency sound waves to detect the echogenecity of various internal organs based on which the results of the scan are interpreted. Different tissues absorb these sound waves upto different extent and the reflected waves are interpreted by the tranducer to produce an image.
I hope this information has been both informative and helpful for you.
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Dr. Rakhi Tayal
drtayalrakhi@gmail.com