What is of Intensities Has the Same Value for Continuous Wave Us

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:

• 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
• 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

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Source: https://ultrasoundregistryreview.com/physicsTrial.html

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