Note: Click any image to enlarge.
Interference:
  • Occurs when two waves with different frequencies are produced at the same time and combine to form a new wave
  • Destructive:  Original waves are waves are out of phase; Resultant amplitude is smaller than ONE of the individual waves,
  • Constructive: the individual waves become tangent to each other and have the same phase relationship; Resultant amplitude is larger than BOTH of the individual waves
Types of Waves:
  • Longitudinal – particles move parallel to the direction of the wave movement
       EX: snake movement along the ground
  • Transverse –  particles move perpendicular to the direction of the wave movement
       EX: Ocean wave travelling toward the beach



Propagation Speed:
  • Determined by medium
  • 1540 m/s standard speed in soft tissue (c)
  • Fastest to Slowest
       >        Solids
       >        Liquids
       >        Gases
  • Range in human body: 500m/s – 4000m/s
Intensity:
  • The rate energy travels through a substance
  • Intensity = Power/Area
  • The larger the area evaluated, the lower the intensity of the beam
  • Power = (Amplitude)2 = Intensity of the beam
  • Units: mW/cm2
  • Operator adjustable


Frequency:
  • Number of vibrations per second of an energy waveform
  • Frequency of sound is measured in Hertz or cycles per second
  • Each cycle consists of a compression and a rarefaction
  • Units: Hz, kHz, MHz
  • Not operator adjustable
  • Determined by the US system/probe



Period:
  • The time it takes for one cycle to occur
  • As frequency increases, period decreases
  • Units: sec, ms, µs
  • Not operator adjustable
  • Determined by the US system/probe



Wavelength:
  • The distance traveled by one cycle (one compression and one rarefaction
  • Measured in mm
  • The higher the frequency, the shorter the wavelength
  • Not operator adjustable
  • Determined by the US system/probe



PRODUCTION OF SOUND WAVES


Categories of Sound:
  • INFRASOUND = < 20 Hz
  • AUDIBLE SOUND = 20 Hz to 20 kHz
  • ULTRASOUND = > 20 kHz
  • DIAGNOSTIC ULTRASOUND = > 1MHz

Factors Affecting Propagation:
Stiffness and density properties of a material determine the speed of sound propagation in the material
  • Stiffness:  There is a significant increase in sound velocity with small increases in stiffness of the material

  • Density:  An increase in density results in a slight decrease in the sound velocity
Sound Propagation:
  • Mechanical wave requires a medium to travel, cannot travel in a vacuum
  • Sound waves move through matter by causing molecules to vibrate successively along their path
  • Sound waves carry energy from one point to another, no matter or particles are carried along the waveform

Piezoelectric Effect:
  • The conversion of electrical energy into mechanical energy – transmission of the sound beam
  • The conversion of mechanical energy in electrical energy – receiving the reflected beam information
  • Electricity is applied to the piezoelectric material which vibrates (expands and contracts) to produce mechanical sound or pressure waves
  • Returning sound waves cause mechanical vibrations (acoustic pressure) of the piezoelectric material that are converted into the electrical signal for the display
PE Materials:
  • Certain types of materials produce a voltage when pressure deforms them
  • Ceramic Materials
       >        Lead zirconate titanate - #1
       >        Barium titanate
       >        Lead metaniobate
       >        Lead titanate
  • Natural Elements
       >        Quartz
       >        Tourmaline
       >        Rochelle Salt

Piezoelectric Elements:
  • Piezoelectric characteristics are caused in certain ceramic materials when polarized at a temperature above its Curie point
  • If a transducer is sterilized by heating it above the Curie point, depolarization may result.
  • Continuous Wave production requires two piezoelectric elements, one to transmit and one to receive
  • Pulsed Wave production uses one piezoelectric element and alternates using it to transmit and receive sound waves
Huygen’s Principle:
  • Sound waves produced by ultrasound transducers originate as numerous points on the surface of a piezoelectric element.  Each point serves as source of small individual sound wavelets.


Power:
  • Rate energy transmitted into substance OR the rate work is performed
  • Units: W or mW
  • Operator adjustable
  • P = A2


Amplitude:
  • Height of the compression or depth of the rarefaction (+ or -)
  • Indicates relative intensity or strength of the wave
  • Decreases as it travels through tissue and attenuation occurs
  • Directly proportional to power
  • Units: mm or cm
  • Operator adjustable
  • Determined by the US system/probe



Sound Waves:
  • Acoustic variables: pressure, temperature, density and particle motion
  • Waves are formed by variations in the acoustic variables
  • Longitudinal: particle motion parallel to wave motion
  • Mechanical: requires a medium to travel
  • Cannot travel in vacuum
  • Carry energy not matter
  • Travel in straight lines
  • Compression: positive amplitude of wave
  • Rarefaction: negative amplitude of wave
  • Cylce: one compression and one rarefaction
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Reference List
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Compressibility: The opposite of stiffness, Increasing compressibility causes a decrease in propagation speed
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