• A type of imaging test to examine the internal organs using very high frequency sound waves.
  • Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes. The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines.: sound navigation and ranging


 Sound is a mechanical form of energy

Sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.


20Hz – 20000Hz human hearing sound

Lower 20Hz- infrasound

Higher 20000Hz – ultrasound


DIAGNOSTIC ULTRASOUND – Diagnostic ultrasound uses 1-20MHz frequency.

Wavelength is the distance between successive wave crest.

AMPLITUDE – distance between peak and midpoint of the wave or highest point of the wave

Period is the time taken for one complete cycle and is equal to 1/f. – 1 PERIOD

Frequency is the number of occurrences of a repeating event per unit of time. – DAY

ECHO – The reflection from tissue is called echo, which form the image.

Velocity is inversely proportional to compressibility and it depend on temperature of the medium. Inversely Proportional : when one value decreases at the same rate that the other increases.

  • Velocity higher in bone and lower in gas
  • Frequency do not change, but change of velocity may occur due to change of wavelength.

SPECKLE – Interference of scattered echoes from different site is called speckle.

In abdomen imaging, strong echoes arise from gas bubbles. Kidney, pancreas, spleen, and liver constitutes complex tissues containing scattering sites, gives rise to speckled texture. Bladder, blood vessels and cysts (fluids) have no internal structure, hence no echoes and appear as black.

Higher the frequency, greater the attenuation

A velocity of 1540m/sec is used as an average for body tissues.

Higher frequencies are more attenuated than low frequencies, resulting in increased SPL. This will degrade image resolution.


1794 – lazzaro spallanzani discovered non-audible sound. Echolocation in bat

1877 – pierre curie discovered piezoelectric effect

1917 – langevin produced ultrasound device using piezoelectric.

1942 – karl dussik investigates ultrasound transmission of the brain.

1958 – Dr. ian Donald obs/gyn.

1980 – real time ultrasound possible

1986 – first 3d image of foetus.

1990 – 3D and 4D ultrasound emerge.


  • The ultrasound machine transmits high-frequency sound pulses into your body using a probe.
  • 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 machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1,540 m/s) and the time of the each echo’s return (usually on the order of millionths of a second).
  • The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.

PRINCIPLE OF ULTRASOUND/ physics of ultrasound in short

  • USE high frequency sound waves and their echoes.
  • transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.
  • Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.