Wave Motion and Sound

Electromagnetic Waves

Can travel in a vacuum. Produced by the vibration of charged particles. Do NOT require a medium to transport their energy. Transverse waves Ex: Light waves

Acoustic Variables

Changes due to a mechanical interaction of the sound wave through a medium. Describe the rhythmic changes in time of the sound wave. Pressure Density Distance (particle motion) Temperature

Continuous

Continuously sending- only transmitting

Decrease Frequency

Increase Penetration, Decrease Image Quality (resolution) LOWER FREQUENCY - BETTER PENETRATION

As Frequency Increases, What happens to Attenuation???

Increases

How are Frequency and Wavelength Related

Inversely

Reflected Beam

Is the part of the sound beam coming (echoing) back towards the transducer with its reflected intensity after striking the interface between 2 different media.

Constructive Interference

Occurs when both waves intersect "in-phase." Crests match and troughs match. The wave's amplitudes reinforce each other, building a wave of even greater amplitude. This results in an increase in amplitude. It will also increase the intensity of the sound beam.

Power (P)

Power is proportional to the wave's amplitude squared P proportional A2 if you double (2) the amplitude, the power increases by a factor of 4 If you triple (3) the amplitude, the power increases by a factor of 9. If you quadruple (4) the amplitude, the power increases by a factor of 16 .

Propagation speed is determined by

Propagation speed is determined by the density and stiffness of the medium

Rarefaction

Rarefaction is a point in a medium through which the wave has decreased density and pressure. Molecules are stretched apart

Sound Waves

Requires a medium to travel: solid, liquid or gas They are longitudinal waves. Mechanical waves because: Cannot travel through a vacuum. Molecules vibrate back and forth from a fixed position. Travel through a medium in which the molecules are alternately compressed and rarefied. They travel in a straight line.

Reverberation

Reverberation is the mixing of reflected sounds with its original sound adding to the volume of the tone. Multiple reflections of sound - sounds after the original sound is produced - sound seems to be prolonged- Excessive reverberation can be a distraction, (use carpert or absorbing material)

Stiffness

Rigidity of an object;"hardness"; the ability of a material to maintain its shape even when pressure is applied to it Stiffness is the opposite of flexibility or pliability. Materials with greater flexibility or pliability have lower speeds. Stiffness is directly related to velocity. speed is lowest in gases (air, lungs) higher in liquids (water, blood, tissue, fat) highest in solids (metal, bone)

The disbursion of ultrasound in every which way while it is attenuated, absorbed through material.

Scattering

Refraction

Similar to scatter- wave hits uneven surfaces - the bending of the wave as it travels from one medium to another- arced type encounters - refracts ultrasound wave into different directions so it can not retrun to the probe. Bending of transmitted sound beam due to the wave hitting a different boundary of different densities. Changes the direction the wave travels. Only occurs if beam is NOT at a normal incidence (perpendicular to boundary)—therefore the beam strikes the interface at oblique incidence. Velocities of each media are different.

Pressure

Sound is a traveling pressure variation that transfers energy (not particles) through a medium (matter). P = F/A As the amount of force in a sound wave increases, the pressure and the sound gets louder. As the amount of area covered decreases (compression area), the pressure increases and the sound gets louder.

wave equation

Speed = Wavelength Frequency s = λ*f or v = λ*f

What is constant through any given medium?

Speed of Sound (c)

Rigidity of an object

Stiffness

Crest

The crest is the point that exhibits the maximum amount of positive (or upward) displacement from the rest position

Propagation Speed (c)

The speed at which sound moves through a given medium. (aka: sound's speed, velocity of sound, acoustic velocity) Units: m/s or mm/μsec All sound, regardless of frequency, travels at the same velocity (or speed) through the same medium. If frequency changes, then wavelength changes. For a wave, the speed is the distance traveled (wavelength) by a given point on the wave in a given period of time (period). S = λ ÷ T

Period (T)

The time it takes to complete one cycle. The time it takes something to happen; it is a time quantity. Period is the seconds/cycle. Period Frequency = 1 (inverse related) Frequency is the reciprocal of the time between crests or the period of the vibration. T = 1/F or F = 1/T If period increases, then frequency decreases. Use "opposite" units: Mega - Micro Ex: 4 MHz = ¼ μsec Units of period: seconds (s), msec (milliseconds), μsec (microseconds) Determined by sound source (transducer) and not affected by the medium in which it travels through.

Longitudinal Wave

a wave in which particles of the medium move in a direction parallel to the direction that the wave moves. Longitudinal waves are always characterized by particle motion being parallel to wave motion The wavelength of a longitudinal wave can be determined by measuring the distance between any two corresponding points on adjacent waves.

Angel of Reflection

angel of reflection is always related to the angel of incidence occurs from a surface that is large and smooth relative to the wavelength

Scatter

as the wave hits uneven surfaces the wave is scattered in differnet directions decreasing wave power. not going to return to ultrasound probe

Interface

border between two structures- bone, tissue

Wave Equation- Diagnostic Ultrasound

c = λ*f c = 1.54 mm/μsec or 1540 m/s (MEMORIZE) c is the average value describing the speed of sound in soft tissue no matter the transducer.

Scatter is directly related to

frequency

The speed of an object refers to

how fast an object is moving and is expressed as the distance traveled per time of travel. Speed = Distance / Time Wavelength Speed = Wavelength / Period S = λ ÷ T

Amplitude (A)

the peak pressure, strength, or loudnessis the strength of a sound wave. It is the maximum displacement of the particles of the wave the (Returning sound waves with increased amplitude appears brighter than surrounding tissue)

Piezoelectric Effect

responsible for transmission of the ultrasound wave Ultrasound transducers have a piezoelectric material in them. This allows electrical energy (or pulses) to be converted to mechanical vibrations (sound waves resonating at certain frequencies) and then convert the mechanical vibrations back into electrical energy. The most notable piezoelectric material is quartz.

Sound Wave

same velocity/speed in tissue (how fast depends on Stiffness, Density)

Pulsed sound waves

send and received waves that reflect off body structures provide an anatomical image using the piezoelectric effect.

The greater the ____ the faster the sound

stiffness bone muscle liver fat lung

Loudness Parameters

Amplitude Power Intensity Pressure

Speed of sound in human tissue

1540m/s or 1.54m/s

Numeric Values of dB

2 4 8 10 100 (positive)

Audible sound: (What humans hear!)

20 Hz − 20,000 Hz

Infrasound: (Subsonic):

<20 Hz You can feel these vibrations more than you can hear them.

Ultrasound: (Supersonic):

>20,000 Hz (or >20 kHz)

Waves

A disturbance (oscillation or vibration) that travels through a medium, transporting energy from one location (its source) to another location without transporting matter. Waves are considered mechanical.

Medium

A substance or material that carries the wave.

Factors of Attenuation

Absorbtion, Refraction, Scatter, Reflection

Diffuse Reflection

Back) Scattering

Decibels

Based on the logarithmic scale

Ultrasound is measured in

Decibels always negative

Increase Frequency

Decrease Penetration, Increase Image Quality (resolution) HIGHER FREQUENCY - BETTER RESOLUTION

The greater the _____ the slower ther sound

Density

Doppler Shift Equation

Difference in the frequency that the machine puts out to the frequency of the echo that comes back. Two factors that affect shift Frequency from machine and velocity of blood moving Ex: The transducer is emitting a frequency of 5 MHz and the returning frequency is 5.003 MHz. What is the Doppler shift? A: + 3,000 Hz (or 3 kHz). Meaning the red blood cells are moving towards (positive) the transducer.

How are Period and Wavelength Related

Directly (Period and wavelength measure the exact same area on a wave but in different units of measurement (time vs. length).

How are Power and Intensity Related

Directly related.

Transverse Wave

Equilibrium: The position of the object at rest if there is no disturbance moving through it. Once a disturbance is introduced, the particles of the object begin to vibrate up and down or oscillate

Diffraction

Happens when a wave hits a sharp obstacle, corner, or gap. Diffraction is the greatest when the gap is about the same size as the wavelength of the wave. The waves bend around the object or spread out when they pass through the gap. The wave changes shape.

Compression

Higher the compressibility/speed of sounds decreases. Compression is a point in a medium through which a wave has increased density and pressure. Molecules are squeezed together

Inverse Square Law

If you double (2) the distance, the intensity decreases by a factor of 1/4.

Intensity (I)

Measure of the energy the sound wave is carrying. Intensity = power I = P area A Units: watts/meter squared (W/m2) Determined by the source (ultrasound system) and is important when discussing bioeffects. Can be changed by the operator using the power control. This will change the amplitude of the wave. Intensity is proportional to the wave's amplitude squared.

Destructive Interference

Occurs when both waves intersect "out-of-phase." The peak of one wave lines up with the trough of the other, (the maximum & the minimum amplitudes of both waves occur at different times). This results in a decrease in amplitude or a canceling out of the wave altogether contributes to ultrasound attenuation. occurs when two or more waves combine to produce a wave with a smaller displacement

Rayleigh Scattering

Occurs when dimensions of an object are much smaller than the wavelength of the beam Most common form: Red Blood Cells! Redirects sound beam equally in all directions Increases dramatically with increase in frequency Rayleigh Scattering Frequency4 More scatter because more reflections - not traveling as deep so hitting more surfaces faster

Amplitude (A)

The length of one complete wave cycle. A wave has a repeating pattern. UNITS: mm or meters Determined by both the source AND the medium.

Fundamental Frequency

The longest wave possible in a standing wave with one antinode and ½ wavelength and has the lowest frequency possible called the fundamental frequency. The FF determines the pitch of the basic musical note and is called the first harmonic.

Frequency ()

The number of cycles that occur in a specific duration of time. It is a rate! Frequency refers to how often something happens; Frequency is the cycles/second. A complete cycle is one crest and one trough. In ultrasound, a wave's frequency is the number of cycles that occur in one second. (4 cycles per second = 4Hz) Units: cycles per second = Hertz (Hz); KiloHertz (kHz); MegaHertz (MHz) Clinical Imaging frequencies: 1.0 - 15.0 MHz Frequency = Velocity Wavelength f = v / λ or v = λf The lower the frequency the longer the wavelength. The deeper into the body it will go (penetration), but lose resolution. Determined by the Sound Source

Reflection

The porperty of sound to bounce back from the surface of which it falls - power decrease as some waves power is reflected, how image is produced, When a wave front strikes a boundary perpendicular to the wave front, the wave may be absorbed, transmitted or reflected (back to the sound source) or a mixture of some or all of these 3 depending on the boundary medium. Hard surfaces reflect more Soft surfaces absorb more Part of the beam is still transmitting through the tissue and is weakened at each interface.

Wavelength ()

The wavelength of a transverse wave is measured as the distance from crest to crest or from trough to trough. OR the distance from a point on a wave to the corresponding point on the next cycle of the wave.

Loudness Parameters

These parameters illustrate size and strength of a wave: Amplitude Power Intensity Pressure

Back) Scattering

Wave reflected back at many different directions. This type of reflection occurs from surfaces that are rough with respect to the wavelength. AKA Diffuse Reflection! Advantage: reflections at suboptimal angles will still be received by the transducer so data is not lost. Disadvantage: backscatter signals are at a lower strength than specular reflections.

Density

When density goes up, sped of sound goes down- Comparing mass per unit volume: Density = mass/volume Formula: p= m/V Units: g/cm3 or g/mL or kg/m3 or kg/L is associated with the pressure variation. As sound travels through a medium, its pressure increases and decreases causing variations in density.

Incident Beam

created by transducer- sound is sent into the body original intensity of the sound beam before it strikes an interface Full power, once it hits a structure, fat, liuquid, a reflection comes back to the transducer. Can loose strength if hits soft tissue becasue of absorbtion. can come back if hits strong surface becuase it can not be absorbed.

Attenuation

decrease in intensity & amplitude of a sound wave as it travels through a medium and is absorbed. It is the weakening or reduction of the wave due to the interaction with the medium. The body absorbs the ultrasound energy making the waves disappear. These waves don't "return" to the transducer. The more body tissues the wave has to cross the more the attenuation the wave suffers. ATTUNATION measured in decibels (-dB: the intensity is weakening & therefore decreasing. The negative sign is assumed when referring to attenuation).

Frequency & Speed

distinctly different quantities

Doppler Effect

is produced by a moving source of waves: There is an apparent upward shift in frequency when the sound wave source approaches you and There is an apparent downward shift in frequency when the sound wave source moves away from you. Note: The effect does not result because of an actual change in frequency of the source. Occurs for all waves including electromagnetic waves. Red blood cells are the moving source If the red blood cell is moving towards us, the transducer will think there is an increase in frequency or positive Doppler shift.

Transmitted Beam

is the part of the beam that continues on traveling in the same direction after striking a boundary between 2 media

Absorption

power decreases as wave power is absorbed by body tissues Absorption reduces the amount of energy there is to reflect a sound wave echo. This reduces penetration and signal strength. The energy being absorbed causes tissue heating, a potential risk of thermal tissue damage, or thermal bioeffects because of the increase in intensity.

What is an echo?

reflected sound arriving at the listener after the direct sound. can be distinguished from the original sound Single reflection of sound waves

Standing Waves

reflected waves that interfere with incoming waves of the same frequency. Ex: a stringed instrument has strings fixed at both ends. String is plucked and waves of different frequencies travel on the string. Some fade and some resonate setting up patterns of waves. Occur at the resonant (or natural) frequency.

Acoustic Impedance (Z)

the resistance of a medium to bringing its particles into motion-(vibrate and ossilate) acoustic resistance to sound traveling in a medium The amount of reflection depends on how mismatch in propagation speed the materials are on both sides of the boundary. This mismatch of the materials is called the acoustic impedance of the two mediums. Determines what % of the incident wave is transmitted and what % is reflected at an acoustic interface. When there is a large difference between the acoustic impedances of the 2 interfacing media, there is a large reflection and a small transmission. When no acoustic impedance mismatch occurs at an interface, there is 0% reflection and 100% transmission occurring.If there is no acoustic impedance mismatch there will be no reflection or only 100% transmission. property of the medium- of many properties that affect how sound waves travel through it.

Troughs

trough is the point on the medium which exhibits the maximum amount of negative (or downward) displacement from the rest position

Interference

two waves overlap at the same location and at the same time. Thus, they combine. The resulting overlap is the formation of a single, new wave that is the sum of the two original waves. Interference of two waves can result in an increase or decrease in amplitudes

Dispersion

waves with different wavelengths creating different propagation speeds cause the waves to spread out.

Resonance

when the frequency of an external force matches the natural frequency of an object. Energy is transferred very efficiently A phenomenon that occurs when two objects naturally vibrate at the same frequency