Describe the main method ultrasound images are created and how the depth and brightness/echogencity of an object is determined.

Pulse-echo mode is the main method ultrasound images are created:

• Piezoelectric effect = Burst of RF signal / voltage applied to the transmitter’s ceramic crystals – causes crystals to expand or contract depending on the polarity of the voltage. The crystal then resonates, converting the electricity to an ultrasound beam (spatial pulse length = number of pulses x wavelength of pulses). The frequency of the ultrasound waves is predetermined by the crystals and crystal thickness in the transducer
• Ultrasound waves are produced in pulses because the same crystals are used to generate and receive sound waves (can’t do both at the same time).
  • transducers generally emit ultrasound only 1% of the time, the rest of the time is spent receiving the returning echoes
• In the time between pulses, the ultrasound beam enters the patient and is reflected back to the transducer (echoes) which cause the crystals to deform again and produce an electrical signal that is converted to an image on the monitor
  • The time it takes for the echo to return (sound propagation time) is used to calculate the DEPTH of the object (longer time for echo means deeper object as the ultrasound travels a longer distance) (velocity m/sec) = frequency (cycles/sec) x wavelength (m)
  • The strength of the echo highlights the BRIGHTNESS/ECHOGENICITY of the object (stronger pulses = brighter = higher echogenicity = higher intensity of reflection)
• The ultrasound machine repeats this for the adjacent transducer elements and so on until all elements are done and hence the whole image is formed.

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