Registered User Login
Ultrasound Image Runner

PRODUCTION OF SOUND WAVES


Note: Click any image to enlarge.

Categories of Sound:
  • INFRASOUND = < 20 Hz
  • AUDIBLE SOUND = 20 Hz to 20 kHz ; All other frequencies cannot be detected by the human ear
  • ULTRASOUND = > 20 kHz
  • DIAGNOSTIC ULTRASOUND = > 2MHz

Factors Affecting Propagation:
Stiffness and density properties of a material determine the speed of sound propagation in the material
  • Stiffness:  AKA bulk modulus; 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

  • Compressibility: The opposite of stiffness, Increasing compressibility causes a decrease in propagation speed
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 into 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 sound wave when pressure deforms them from voltage applied
  • Average propagation speed in a PE element is 4-6mm/ms
  • Ceramic Materials
       >        Lead zirconate titanate - #1 most common
       >        Barium titanate
       >        Lead metaniobate
       >        Lead titanate
  • Natural Elements
       >        Quartz - used in musculoskeletal and other high frequency superficial imaging probes
       >        Tourmaline
       >        Rochelle Salt

Piezoelectric Elements:
  • Piezoelectric characteristics are caused in certain ceramic materials when polarized at a temperature above its Curie point (avg. 360 degrees Fahrenheit)
  • If a transducer is reheated above the Curie point, depolarization may result and the element will lose the ability to produce sound waves
  • 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
Sound Waves:
  • 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, an area of high pressure and particle density on a pressure wave is referred to as a compression
  • Rarefaction: negative amplitude of wave, an area of low pressure and particle density on a pressure wave is referred to as a rarefaction
  • Cycle: one compression and one rarefaction

Acoustic Variables:
  • Four types of recordable changes occur with the mechanical interaction of the wave and the medium
  • Pressure, temperature, density and particle motion
  • Changes in these variables can lead to damage to the medium (bioeffects)
Types of Waves:
       EX: snake movement along the ground
       EX: Ocean wave traveling toward the beach, inchworm movement along the ground

Wave Parameters:

Amplitude:
  • Difference between the average value and the maximum value of the wave intensity (compression)
  • Difference between the average value and the minimum value of the wave intensity (rarefaction)
  • Height of the compression or depth of the rarefaction (+ or -)
  • Indicates relative intensity or strength of the wave
  • Peak to Peak Amplitude refers to the difference between the minimum and maximum value of the sound wave intensity
  • Decreases as it travels through tissue and attenuation occurs
  • Proportional to power/voltage; Power = Amplitude squared
  • If the amplitude decreases by 1/2 of the original value, the power decreases to 1/4 of its original value
  • If the amplitude decreases to 1/3 of the original value, the power decreases to 1/9 of its original value
  • Units: mm or cm
  • Operator adjustable with output power controls
  • Determined by the US system/probe
Power:
  • Rate energy transmitted into substance OR the rate work is performed
  • Units: W or mW
  • Operator adjustable by output power controls
  • Power = A2
  • Higher power settings are related to increased risk for the occurrence of bioeffects
  • Power diminishes as the wave travels through a medium
  • If power doubles, intensity doubles
  • If power triples, intensity triples
  • If amplitude doubles, power increases to 4X original value
  • If amplitude triples, power increases to 9X original value
Intensity:
  • The rate energy travels through a substance
  • Best indicator for the related risk of bioeffects
  • Power and area are inversely related in regards to intensity
  • Intensity = Power/Area
  • Power = (Amplitude)2 = Intensity of the beam
  • Units: mW/cm2
  • Diminishes as wave travels through media
  • Operator adjustable with output power and electronic focusing
  • The intensity at a more shallow focal zone will be higher than the intensity at a deeper focal zone on the same US beam
  • The wider the element, the greater the beam area
  • The larger the area of the beam, the lower the intensity of the beam when power is constant
  • If beam area doubles with no change in power, intensity is reduced to 50%
  • The smaller the area of the beam, the higher the intensity of the beam when power is constant
  • If the beam area is decreased by 50% with no change in power, intensity is doubled
Half Intensity Depth:
  • Defined as the depth at which the intensity of the beam is reduced to 1/2 the original intensity
  • Attenuation reduces beam intensity as it travels through tissue.
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
  • Only changes when the sound wave travels into a different media
  • Does NOT vary with frequency or wavelength of the sound waves
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 probe
  • Pulsed US frequency varies with the thickness of the element and the speed of sound in the element
  • Continuous US frequency is determined by electrical frequency applied to the element
  • Frequency remains constant at all depths as the sound wave travels through a medium
  • Frequency does NOT change if you change from pulsed to continuous transmission
  • Diagnostic US 2 - 15MHz
Period:
  • The time it takes for one cycle to occur
  • Period is the reciprocal of frequency
  • 1 / frequency = period
  • As frequency increases, period decreases
  • Units: sec, ms, µs
  • Diagnostic US 0.08 - 0.5 µs
  • NOT operator adjustable
  • Determined by the US system/probe
  • Lower frequency waves have longer periods and longer wavelengths
  • Remains constant as sound travels through medium
Wavelength:
  • The distance traveled by one cycle (one compression and one rarefaction)
  • Measured in mm
  • Average in US of soft tissue 0.1 - 0.8mm
  • Inversely related to frequency with sound propagation
  • The higher the frequency, the shorter the wavelength
  • Not operator adjustable
  • ONLY sound parameter determined by the US system/probe AND medium
  • Lower frequency waves have longer periods and longer wavelengths
  • Increases in length as the wave travels through tissue
  • Sound waves with shorter wavelengths are preferred to produce the best 2D image
Sound Beam Formation:

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 that 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
Huygen’s Principle:
  • Explains the hour-glass shape of the beam
  • Sound waves produced by ultrasound transducers originate as numerous points on the surface of a piezoelectric element
  • Each point serves as a source of small individual sound wavelets



Register Now! Click Here
Available products and price list on next page

Return to Top Reference List