Physics Test Chapter 5&6

Intensity

• The strength of a sound beam, or concentration of energy in a particular area (cross section) of the sound beam

  • mW/cm²

• The sound beam will have greater or less intensity at different locations and times

Why does Intensity Matter in Ultrasound?

• understanding power and intensity is crucial for creating clear images of the inside of the body

• Too much intensity = harmful —- Too little —- not a clear picture

• Ultrasound machines: control the power and intensity of the sound waves to ensure safety and effectiveness

• Sonographers can control the output power on the machine, so they need to understand how these changes effect the intensity and bioeffects

• evaluate and understand exposure to sound energy (BIOEFFECTS)

Intensity Equation

Intensity = power/beam area

• W/cm² or mW/cm²

Varying Intensities

• Intensity is not uniform across space

  • at different depths of imaging, there will be differing intensities

• Intensity is not uniform in time

  • depends on send and receive times

Spatial

space/distance, intensity at a specific place which changes depending on location

Peak

max value

Average

middle value

Temporal

time (both transmit and receive)

Pulsed

only transmit time (avg intensity over the transmit time or “on” time)

Spatial Peak Intensity (I_sp)

beams location of maximum intensity

• center of the beam

Spatial Average Intensity (I_sa)

average intensity across entire area

• *HINT* this will always be less than spatial peak intensity

Temporal Peak Intensity (I_tp)

intensity at exact instant of max intensity

I_max (I_m)

• average intensity over the most intense half cycle

• also a way of measuring peak intensity

Pulse Average Intensity (I_PA)

• average intensity during the transmit time (pulse duration)

Temporal Average Intensity (I_ta)

• average intensity over the entire transmit and receive times (PRP)

Highest to Lowest Values

I_tp —→ I_max —→ I_pa —→ I_ta

I_sptp (spatial peak temporal peak)

maximum intensity based on location and time (highest of all intensities)

I_sppa (spatial peak pulse average)

location of maximum intensity averaged over transmit time

I_spta (spatial peak temporal average)

maximum intensity based on location and averaged over ALL time (both transmit and receiving)

I_sata (spatial average temporal average)

average of entire cross sectional area of the sound beam and average over all time (this has the lowest of all intensities)

_____ is often referenced with bioeffects (how ultrasound effects the tissue through which it travels

SPTA

Spatial Average Pulse Average (SAPA)

• average intensity across the beam (SA)

• averaged during pulse duration (PA)

Spatial Average Temporal Peak (SATP)

• average intensity across the beam (SA)

• Peak intensity at point in time (TP)

Intensities are reported in all different ways in relation to ____ and ____

time and space

Intensity is the main _____ when talking about ______ in tissue

parameter , bioeffects

_____ is the most commonly referred to and the most relevant in regards to _____

SPTA , bioeffects

_____ is ALWAYS greater than ____

peak , average

• SPTP (greatest value)

• SATA (lowest value)

10 Commandments of Intensity According to Edelman

1. Intensities reported in various ways with respect to time and space

2. intensities important when studying bioeffects (SPTA most relevant)

3. Units for ALL intensities w/cm²

4. SPTP has highest value - peaks are higher than averages

5. The beam uniformity coefficient (SP/SA factor) describes the spread of beam in space - it is unitless with a value greater than 1

6. Duty Factor describes relationship of beam intensity with time - unitless between 0 and 1

7. Continuous Wave - beam is always on so pulse average and temporal average are the same (SPTA=SPPA and SATA=SAPA)

8. When pulsed and continuous have the same SPTP intensities, the continuous wave will have the higher SPTA intensity (because continuous wave is always on)

9. Temporal Considerations: Temporal peak intensity is max intensity in time

• I_m intensity is averages over the most intense half-cycle

• Pulse average intensity (I_pa) is averaged only during the pulse duration (on time)

• Temporal average intensity (I_ta) is averaged during PRP (both on and off time

• 10. Spatial Considerations: Spatial peak intensity (I_sp) is the maximum in space

• spatial average intensity (I_sa) is averaged over the cross-sectional area of the beam

What Principle does DMS rely on?

as an ultrasound wave moves through human anatomy, small echoes return from the boundaries between different mediums

The system _______ THEN it ________

keeps track of all the parameters and variables of the transmit wave

measures all the variables and parameters of the receive wave

Behavior of Sound

• sound waves propagate (travel) through the body and ATTENUATE

• these waves move from one medium to another and can change directions

• As sound hits different tissue types and reflect back to the transducer, the signal is then strengthened or amplified to be processed and displayed clearly

• Reflections are created at boundaries between two media (tissue types)

• These reflections create images

Attenuation and Amplification

• The Decrease in the bigness of a sound as it travels

  • bigness being amplitude, power, intensity

  • wave loses energy as it travels deeper

• The further sound travels, the more attenuation occurs

• Not ALL sound attenuates - some is reflected back to the transducer

• The reflected signals are weak and must be amplified

• The amplified signals are processed to become an image

• will result in negative number value because the wave is a WEAKENING as it travels

• attenuation and speed = NOT RELATED

Decibel Notation

way to express changes in sound intensity or power

• describes how much sound is attenuated or how much the system is amplifying the sound wave

• dB make it easier to express these changes

• uses a logarithmic scale

log10(100) =

2

Decibels

• measure the strength of the sound beam created by transducers

• logarithmic

• decibel notation measures relative change and requires two intensities

  • reference or starting intensity

  • actual intensity at time of measurement

Decibel Equation

dB = 10 * Log (actual intensity/starting intensity)

Decibel Typical Values

• When signal strength doubles = 3 dB

• When signal strength is cut in half = -3 dB

• When signal strength increases 10 times = 10 dB

• When signal strength reduced one tenth = -10 dB

Describe changes in signal strength

Decibels Synonyms

“change in intensity” or “change in signal strength”

Decibel Shortcuts (memorize)

• + 3dB increase by factor of 2

• • -3dB decrease TO ½

• + 6 dB increase by a factor of 4

• -6dB decrease TO ¼

• + 9dB increase by a factor of 8

• -9dB decrease TO 1/8

• + 10 dB increase by a factor of 10

• -10dB decrease TO 1/10

• +20dB increase by a factor of 100

• -20dB decrease TO 1/100

• +30 dB increase by a factor of 1000

• -30dB decrease TO 1/1000

What Determines Attenuation?

1. Distance traveled/path length

• The farther the sound travels, the greater the attenuation, and the

weaker the beam

• Distance and attenuation are DIRECTLY related

2. The frequency of sound

• Attenuation in soft tissue depends on the wave’s frequency

• Frequency and attenuation are DIRECTLY related

Attenuation Relationship

MORE distance = MORE attenuation

GREATER frequency = GREATER attenuation

• less penetration

What Contributes to Attenuation?

1. reflection

2. scattering

3. absorption

Reflection

• occurs when sound strikes a boundary between two mediums, a portion of the wave may be reflected back toward the sound source

• beam weakens as energy is reflected back to the source

• basis for ultrasound imaging

• reflection in soft tissue depends on the tissue interface that it strikes

two types of reflection

1. specular reflection

2. diffuse reflection (also called backscatter)

Specular Reflection

• When sound hits a smooth boundary, it reflects in only one direction,

in an organized manner

• angle of incidence = angle of reflection

• Reflection back to the transducer depends on the angle of incidence

Why do Specular Reflectors cause Strong echos that are easily detected by the sound source?

• Because if the beam is perpendicular, the echo comes straight back to the source

• If the beam is not perpendicular, the reflection will bounce off at an angle, not toward the source

Diffuse Reflection

• reflection that results from sound hitting an irregular surface (backscatter)

• Advantage —> allows for reflections from suboptimal angle

• Disadvantage —> backscatter produces lower strength signals than specular reflectors

• echoes from non 90 degree angles get back to the source even though not all reflected sound will return to the source

Scattering

• tissue interface is small (same or less than the wavelength of the incident sound beam)

• random redirection of sound in many directions

• air in lung tissue scatters sound

• Higher Frequency = more scatter

Rayleigh Scattering

• structure is MUCH smaller than the beams wavelength

• redirects sound equally in all directions

• Rayleigh Scattering ∝ frequency^4

• ex: blood cells

Absorption

• primary cause of attenuation

• ultrasound energy is converted to another form of energy (usually heat)

• doesn’t give diagnostic info

• HIGH frequency absorption = POOR PENETRATION

  • heat loss is greatest in bone

Absorption Relationship

INCREASE absorption = INCREASE attenuation

• MORE absorption = LESS imaging DEPTH (less penetration)

• slightly increasing frequency = great increase in absorption

Air and Bone Absorption and Attenuation

HIGH

Attenuation in lung

HIGH due to scattering and absorption

Attenuation in body fluid and water

LOW

total attenuation depends on

• distance traveled

• frequency

• the media through which the sound is traveling

Attenuation Coefficient

A simple way of reporting the attenuation of sound

• # of decibels of attenuation that occurs when sound travels 1 cm

• UNITS: dB/cm

• ADVANTAGES: the value is CONSTANT regardless of how far the sound wave travels

Attenuation Coefficient Equation

Total Attenuation = attenuation coefficient (dB/cm) x distance (cm)

Attenuation Coefficient in Soft Tissue

• ½ the frequency in MHz (0.5 dB/cm per MHz)

Attenuation in Media Chart

Half Value Layer Thickness

• simplifies attenuation

• distance sound travels in a soft tissue that to reduce the sound to half its original strength

  • OR

• distance required for -3dB attenuation

EQUATION: HVL (cm)= 3dB/atten coeff.

Half Value Layer Thickness Units

of length and distance (cm)

Half Value Layer Thickness Typical Values

.25 to 1cm

Half Value Layer Thickness Synonyms

• penetration depth

• depth of penetration

• half boundary layer

• 3dB rule

  • HVL (cm) = 3dB/atten coeff.

Half Value Layer Factors

1. tissue

2. frequency

Half Value Layer (tissue factor)

GREATER attenuation in tissue = LESS DISTANCE the sound needs to travel to be decreased by ½

Half Value Layer (frequency factor)

HIGHER frequency = SMALLER HVL

• higher frequency means shorter distance before the intensity is bc the higher the frequency the greater the attenuation-happens

  faster)

Reflection

redirection of sound to the transducer to create an image

Transmission

portion of the wave that continues on into the tissue

What determines how much sound will reflect and how much sound

with transmit?

Impedance

Impedance

• the acoustic resistance to sound traveling in a medium

• reflections depend on impedance between two mediums

  • greater difference = greater reflection

• 2 medium with equal impedance = NO REFLECTION

Impedance Equation

Impedance = density of the medium (kg/m³) x speed of sound in that medium (m/s)

Impedance units

rayls (z)

Impedance determined by…

medium

Impedance values

1,250,000 - 1,750,000 rayls

Impedance is AKA

characteristic impedance

Angle of Incidence

• angle at which the wave strikes the boundary or interface

  • determines behavior of pulse

Normal Incidence

• sound beam strikes boundary at exactly 90 degrees

• synonyms:

  • perpendicular

  • orthogonal

  • right angle

  • 90 degrees

Oblique Incidence

• sound beam strikes boundary at an angle other than 90 degrees

• synonyms:

  • not at right angles

  • non-perpendicular

Incident Intensity

the sound wave’s intensity the instant before it strikes a boundary

Reflected Intensity

the intensity of the sound beam that returns back to where it came from after it strikes the boundary

Transmitted Intensity

The intensity of the sound beam that keeps going and continues forward in the SAME direction after striking the boundary

Incident Intensity Equation

Incident Intensity = reflected intensity + transmitted intensity

IT (GREEN) = RI (RED) + TI(PURPLE)

Intensity Reflection Coefficient (IRC)

percentage of intensity that bounces back when a sound beam strikes the boundary between two media

• in two soft tissues = 1% or less

• MORE reflection = GREATER %

Intensity Transmission coefficient (ITC)

• The percentage of intensity that passes in the forward direction (keeps going) when a beam strikes a boundary

• in two soft tissues: 99% or more

Relationship Between IRC and ITC

• the intensity of those two must equal the intensity of the original sound source

• 100% = IRC (%) + ITC (%)

Reflection with identical impedances

None

Reflection with slightly different impedances

Small reflection

Reflection with very different impedances

LARGE reflection

Two Principles of Reflection with Oblique Incidence

1. Conservation of energy (all energy must be accounted for)

2. Reflection angle = incidence angle (think of looking in a mirror)

Refraction

a change in direction of the wave propagation when traveling from one medium to another (transmission with bending of the beam)

2 Conditions of Refraction

1. Oblique Incidence (CANNOT BE NORMAL)

2. Different propagation speeds of the two media

Refraction is more in _____ due to ____

bone/soft tissue , because of the significant difference in media

Snell’s Law

• quantifies the physics of refraction

• every angle has an associated sine

Speed 1 = Speed 2

No refraction

transmission angle = incidence angle

Speed 1 < Speed 2

transmission angle greater than incident angle

Speed 1 > Speed 2

transmission angle is less than incident angle