ultrasounds

• wave 


  a periodic/cyclic disturbance that travels through a medium
• transverse waves


  displacement is perpendicular to the direction of wave motion
• longitudinal waves


  displacement is in the same plane as wave velocity
• wavelength 


  distance in metres between consecutive max or mins
• wave velocity 


  speed at which the wave travels through the medium
• amplitude


  distance between 0 and the max or min
• frequency 


  number of complete wave cycles that pass a fixed point
• period 


  time required to complete one cycle
• sound waves are 


  longitudinal waves
• infra-sound 


  frequency <20Hz
• sonic frequencies

  20Hz - 20000Hz
• ultrasonic frequency 


  frequency >20000Hz
• example of transverse waves

  water and light waves, Mexican waves
• example of longitudinal waves

  sound, ultrasound, train carriages
• short wavelength 


  high frequency
• high frequency

  means good resolving power but high absorption which means poor penetration
• best compromise for frequency

  between 1 and 10 MHz
• resolution 


  the ability to distinguish between two points in space
• spatial resolution 


  the ability of an ultrasound system to distinguish between two points at a particular depth in tissue
• spatial resolution divided into three components

  lateral resolution (X), axial resolution (Y) and elevation resolution (Z)
• lateral resolution 


  the ability to distinguish between two objects perpendicular to the path of the ultrasound beam
• axial resolution 


  the ability to distinguish between two objects parallel to the path of the ultrasound beam
• elevation resolution 


  the ability to distinguish between two objects perpendicular to the scan plane, depends on depth
• piezo-electric effect


  the ability of certain materials to generate an electric charge in response to applied mechanical stress
• ultrasound transmitters generate their high frequency vibrations using


  the piezo-electric effect
• ultrasound receivers also detect ultrasound echoes using


  the piezo-electric effect
• when applied with a voltage

  the piezoelectric element vibrates to generate a soundwave
• when applied with vibration (an ultrasonic wave)

  the piezoelectric element generates a voltage
• things that happen to an ultrasound pulse that meets a boundary between two media

  transmission, reflection, absorption
• acoustic impedance 


  a measure of how hard it is for the ultrasound wave to travel through the medium
• amount of reflection and transmission that takes place depends on

  the acoustic impendance of the two media
• if there's a big difference in acoustic impedance then most of the energy will be reflected
• if there's no difference in acoustic impedance then all the energy will be transmitted