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Ultrasound - How it works

• Ultrasound uses high frequency sound waves (1 to 5 megahertz - MHz) and their echoes.
• The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines.

• The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).
• Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.
• The reflected waves are picked up by the probe and relayed to the machine.
• The distance from the probe to the tissue or organ (boundaries) is calculated by the machine, using the speed of sound in tissue (1540 m/s) and the time of each echo's return (usually millionths of a second).
• The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image like the one shown below:

Thanks to Sheila and Andy Ball for image

How the sound waves are produced and detected

• In the probe, there are one or more quartz crystals called piezoelectric crystals.
• When an electric current is applied to these crystals, they change shape rapidly.
• The rapid shape changes, or vibrations, of the crystals produce sound waves that travel outward.
• Conversely, when sound or pressure waves hit the crystals, they emit electrical currents.

Doppler Ultrasound

• When the object reflecting the ultrasound waves is moving, it changes the frequency of the echoes, creating a higher frequency if it is moving toward the probe and a lower frequency if it is moving away from the probe.
• How much the frequency is changed depends upon how fast the object is moving.
• Doppler ultrasound measures the change in frequency of the echoes to calculate how fast an object is moving. Doppler ultrasound has been used mostly to measure the rate of blood flow through the heart and major arteries

Major Uses of Ultrasound

• Obstetrics and Gynecology
  - measuring the size of the fetus to determine the due date
  - determining the position of the fetus to see if it is in the normal - head down position or breech
  - checking the position of the placenta to see if it is improperly developing over the opening to the uterus (cervix)
  - seeing the number of fetuses in the uterus
  - checking the sex of the baby (if the genital area can be clearly seen)
  - checking the fetus's growth rate by making many measurements over time
  - detecting ectopic pregnancy, the life-threatening situation in which the baby is implanted in the mother's Fallopian tubes instead of in the uterus
  - determining whether there is an appropriate amount of amniotic fluid cushioning the baby
  - monitoring the baby during specialized procedures e.g. during amniocentesis (sampling of the amniotic fluid with a needle for genetic testing)
  - seeing tumours of the ovary and breasts
• Cardiology
  - seeing the inside of the heart to identify abnormal structures or functions
  - measuring blood flow through the heart and major blood vessels
• Urology
  - measuring blood flow through the kidney
  - seeing kidney stones
  - detecting prostate cancer early
  
  In addition to these areas, there is a growing use for ultrasound as a rapid imaging tool for diagnosis in emergency rooms

Click here for images