UltraSound

• Ultrasound is

  • Longitudinal
  • Frequency greater then 20kHz
• • An ultrasound transducer is used to emit pulses of ultrasound into the body
  • Pulses are reflected by boundaries between different tissues
  • Transducer detects reflected pulse then reflected intensity and delay time can be used to give information about type of tissue and depth.
  • A computer takes this data to produce an image
• A-Scan

  • Ultrasound pulses in one direction
  • Used to find depth or distance by producing a trace eg thickness of skin or muscle
• B-Scan

  • Ultrasound pulses in many directions with multiple detectors
  • Produces a 2d or 3d image eg pre-natal scanning
• Ultrasound transducer uses the piezoelectric effect to emit and detect ultrasound
• Piezoelectric effect - When a pd is applied to a material it causes compression or extension
• Emission of Ultrasound

  • Piezoelectric crystal is connected to alternating pd
  • Causing crystal to vibrate
  • Emitting ultrasound
• Detection of Ultrasound - Piezoelectric can happen in reverse - compession or extension causes a pd to be induced

  • Ultrasound is absorbed by crystal
  • Causing cystal to vibrate
  • Induces an alternating pd
  • This is send to a computer to produce a trace (A) or image (B)
• The depth of a tissue can be determined using s=vt

  • s - distance from emitter to tissue to detector (depth is s/2)
  • v = speed of sound in the tissue
  • t = time for the ultrasound pulse to reach the tissue and be reflected back
• Acoustic inpedance is a property of a material defined as the density of the material multiplied by the speed of sound through it
• The greater the differance in acoustic impedances of two materials the greater the intensity of the reflected ultrasound at the boundary between the materials
• The fraction of ultrasound reflected is the differance between the acoustic impedances squared divided by the sum of the acoustic impedances squared
• Transmitted intensity is 1-reflected intensity
• Impedance matching - if there is any air between the transducer and skin most of the ultrasound will be reflected and little will be transmitted into the body
• An imedance matched gel is applied to the skin

  • The acoustic impedance of the gel and skin are similar
  • So very little ultrasound is reflected by the skin at the boundary
  • So most is transmitted into the body
• The resolution of a scan (smallest distinguishable feature) is equal to the wavelength of the ultrasound. Shorter wavelengths give finer detail but only last shorter depths
• Dopler effect - A change in frequency/wavelength due to relative motion of source and observer
• Measuring the flow of blood in arteries using ultrasound

  • Transducer placed at angle to artery and emits pulse
  • Pulse is reflected by moving blood
  • Transducer detects reflected pulse which has changed frequency and wavelength
  • The change in frequency is related to speed of blood