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BASICS OF ULTRASOUND- Different modes of scanning | Wave Propagation, Wavelength | FOCUSING,HISTORY

BASICS OF ULTRASOUND
Ultrasound pulses of the type produced by the scanners described here are of a frequency form 2 to 10 MHz (1MHz is 1,000,000 cycles per second).

What is ultrasound

Ultrasound is the name given to high_frequency sound waves,over 20000 cycles per second (20 kHz).these waves, inaudible to humans,can be transmitted in beams and are use to sacan the tissue of the body.

  •  Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz". In air at atmospheric pressure, ultrasonic waves have wavelengths of 1.9 cm or less.
  • Ultrasound pulses of the type produced by the scanners described here are of a frequency form 2 to 10 MHz (1MHz is 1,000,000 cycles per second).The duration of the pulse is about 1 microsecond (a millionth of a second)and the pulses are repeated about 1thousand times alter the waves in different ways: the wave pass through the tissue at different speed (for ex,1540 meters per second through soft tissues).


Different modes of scanning
     The various modes show the returning echoes in different ways.

  1. A-mode
  2. B-mode
  3. Real-time
  4. M-mode   
Wave propagation
        Wave propagation describes the transmission and spread of ultrasound waves to different tissue. The differences in the way in which ultrasound interact with tissue influence the design of an ultrasound unit, affect the interpretation of the images and impose limitations on the usefulness of the method.

Ultrasound waves propagate as longitudinal waves in soft tissues. The molecules vibrate and deliver energy to each other so that ultrasound energy propagates through the body.

Wavelength   
        The wavelength of ultrasound is inversely proportional to its frequency. The higher the frequency, the wavelength. For eg; Ultrasound of 3MHz has a wavelength of 0.5mm in soft tissue, whereas ultrasound of 6MHz has a wavelength of 0.25mm. The shorter the wave length the better the resolution.

Focusing
       Ultrasound waves can be focused either by lenses and mirrors or electronically in composite transducers. In the same way that a thin beam of lite shows an object more clear than a widely scattered, unfocused beam, so with focused ultrasound: a narrow beam images a thin section of tissue and thus gives better details. 

History
          Acoustics, the science of sound, starts as far back as Pythagoras in the 6th century BC, who wrote on the mathematical properties of stringed instruments. Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound, not vision. Francis Galton in 1893 invented the Galton whistle, an adjustable whistle that produced ultrasound, which he used to measure the hearing range of humans and other animals, demonstrating that many animals could hear sounds above the hearing range of humans. The first technological application of ultrasound was an attempt to detect submarines by Paul Langevin in 1917. The piezoelectric effect, discovered by Jacques  and Pierre Curie in 1880, was useful in transducers to generate and detect ultrasonic waves in air and water.

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