PRODUCTION OF SOUND WAVES

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:

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 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
Amplitude decreases as it travels through tissue and attenuation occurs
Proportional to power/voltage
Power = Amplitude squared
P = A²
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
As beam power increases, intensity increases
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:

Speed of the wave as it travels through a medium
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:

General definition - number of vibrations per second of an energy waveform
Sound frequency - The number of cycles per second in a soundwave
Each cycle consists of a compression and a rarefaction
Frequency of sound is measured in Hertz
1 Hertz = 1 cycle per second
1kHz = 1000 cycles per second
1MHz = 1,000,000 cycles per second
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
The choice of frequency for an exam is dependent on the penetration and resolution needed for an optimal exam

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
The US system determines the frequency of the wave and the propagation speed varies with the medium in which the wave travels
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

Associated with the acoustic speckle artifact and reduced signal to noise ratio

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

The questions below are reading comprehension questions intended to help evaluate your retention of the material presented on the page. To access our registry exam practice questions or the CME post test, please refer to your login instructions for information on accessing the dedicated testing site (if your subscription included exam access).

Correct answer is "Stiffness and density".

Correct answer is "Stiffness".

Correct answer is "Sound waves carry energy from one point to another".

Correct answer is "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 ".

Correct answer is "The PW transducer has 1 PE element, while the CW transducer has 2 PE elements".

Correct answer is "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. ".

Correct answer is "Amplitude".

Correct answer is "Attenuation".

Correct answer is "Quadrupled".

Correct answer is "Intensity = Power/Area, mW/cm2".

Correct answer is "Increases as wave travels through media".

Correct answer is "1540 m/s".

Correct answer is "thickness of the element and the speed of sound in the element, electrical frequency applied to the element".

Correct answer is "All the above".

Correct answer is "The time it takes for one cycle to occur".

Correct answer is "Shorter wavelengths are preferred to produce the best 2D image".

Correct answer is "Speckle".