Acoustical Physics Exam 2

Spatial Peak Intensity

I▿SP; beams intensity at the location where it is maximum

Spatial Average Intensity

I▿SA; average intensity across the beams entire cross-sectional area

Temporal Peak Intensity

I▿TP; measuring intensity at the time of its maximal value

I▿m or I▿max

average intensity during the most intense half cycle

Pulse Average Intensity

I▿PA; average intensity during pulse duration

Temporal Average Intensity

I▿TA; average intensity during the entire pulse repetition period (on and off time)

Combination: Isptp

measured where intensity is maximum and the exact time of most powerful

Combination: Isppa

measured where intensity is maximum and averaged over the transit time (pulse duration)

Combination: Ispta

measured at maximum intensity and averaged over all time (transmit and receive)

Combination: Isata

measure over the entire cross section area and average over all time

Measuring intensity: SPTP

spatial peak, temporal peak; most intense

Measuring intensity: SATP

spatial average, temporal peak

Measuring intensity: SPTA

spatial peak, temporal average

Measuring intensity: SATA

spatial average, temporal average; least intense

Measuring intensity: SPPA

spatial peak, pulse average

Measuring intensity: SAPA

spatial average, pulse average

Commandments #1

respect for time and space

Commandments #2

different units if intensity are important for bioeffects

Commandments #3

units for intensity are w/cm³

Commandments #4

SPTP- most intense

SATA- least intense

Commandments #6

duty factor describes the relationship of beam intensity with time

Commandments #7

For continuous wave ultrasound (CW) pulse average and temporal average are the same, therefore; SPTA=SPPA & SATA=SAPA

Commandments #8

- When Pulsed wave (PW) and continuous wave (CW) have the same SPTP intensity, the CW has the higher SPTA intensity

- When PW and CW have the same SATP intensity, the CW has the higher SATA intensity

Commandments #9

Temporal considerations-

I▿TP, I▿m, I▿PA, I▿TA

Commandments #10

Spatial considerations-

I▿SP, I▿SA

Decibels (dB)

common unit for measuring strength of sound beams, electrical signals, and brightness

Positive decibels

reports signals that are increasing

Negative decibels

reports signals that are decreasing

Attenuation

decrease in intensity, power, and amplitude as sound travels

Attenuation is _____related to Distance

Attenuation is directly related to distance

Attenuation is _____related to Frequency

Attenuation is directly related to frequency

Attenuation is determined by two factors, what are they

Path length and frequency of sound

More attenuation equals

longer distances and higher frequencies

Less attenuation equals

shorter distances and lower frequencies

Attenuation has 3 contributions, what are they

Reflection, Scattering, and Absorption

Reflection

weakens the portion of the soundwave that continues in a forward direction

Specular Reflection

boundary is smooth, reflected in only one direction and organized

Diffused Reflection

irregular surface and radiates in more than one direction

Rayleigh Scattering

occurs when the structures dimensions are much smaller that the beams wavelength. This redirects the soundwave equally in all direction

Absorption

when ultrasonic sound gets converted into another energy form

Absorption is _____ related to frequency

Absorption is Directly related to frequency

Attenuation coefficient

the number of decibels of attenuation that occurs when sound travels 1cm

Half value layer thickness

the distance that sounds travels in a tissue that reduces the intensity of sound to ½ of its original value

Half value layer thickness depends on

depends on the medium and the frequency of sound

Transmission is critical to the ability of what

critical to the ultrasounds ability to image a structure located deep in the body

Impedance

acoustic resistance to sound traveling in a medium

any angle that isn’t 90 degrees is considered

oblique

Incidence

angle the wave strikes the boundary determines the behavior of the pulse

Normal incidence strikes the boundary at;

exactly 90 degrees

Oblique incidence

the incidence sound beam strikes the boundary at any angle other than 90 degrees

Refraction

change in direction of wave propagation when traveling from one medium to another

Refraction only occurs if two conditions are satisfied, what are they

oblique incidence and different propagation speeds

Snell’s Law

Conditions that change the angle- Trans angle = incident angle

the beam travels straight

Conditions that change the angle- trans angle > incident angle

speed of medium 2 is greater than medium 1

Conditions that change the angle- trans angle < incident angle

speed of medium 2 is less than medium 1

Go-return time (time of flight)

elapsed time from pulse creation to pulse reception

PRP is _____ related to depth

PRP is Directly related to depth

PRF is _____ related to depth

PRF is Inversely related to depth

13 microsecond rule

for every 13ɥs of go-return time, the object is 1cm deep in the body

The total distance is __x the reflector depth

2x the reflector depth; it has to travels there and back

Transducers

any device that converts one form of energy into another

Reverse Piezoelectric effect

PZT materials change shape when voltage is applied to them

Components of a transducer: Case

metal or plastic; protects the patient and inside the probe

Components of a transducer: Electrical shield

thin, metallic, and inside the case; helps prevent electrical noise

Components of a transducer: Acoustic Insulator

thin, cork or rubber; prevents vibration in the case from inducing an electrical voltage

Components of a transducer: PZT (active element)

the crystals; ½ s wavelength thick

Components of a transducer: Wire

electrical connection between the PZT and the ultrasound system

Components of a transducer: Matching layer

Infront of PZT, ¼ wavelength thick; increases efficiency of sound energy transfer between active elements and the body; protects the active element

Components of a transducer: Backing Material

reduces “ringing”; prevents from excessive shaking

Consequences of backing material

decreased sensitivity

wide bandwidth

Low quality factor

Curie Temp / Curie Point

temperature at which PZT is polarized

Continuous wave (CW)

constant electrical signal excites or drives the active element

Pulsed Wave (PW)

short duration spike that travels though the wire and strikes the PZT

2 characteristics of PZT that determines the frequency of sound

Speed and Thickness

Speed is ____ related to frequency

speed is directly related to frequency

Thickness is ____ related to frequency

thickness is inversely related to frequency