Physics Exam 2

Is the intensity of a sound beam uniform with pulsed ultrasound?

NO, the intensity varies depending on the measurement point within the beam.

What is the Isp (spatial peak intensity)?

The beam’s intensity at the location where it is maximum

What is the Isa (spatial average intensity)?

The beam’s average intensity across the entire cross-sectional area

True or False- spatial peak intensity is always higher than the spatial average intensity

TRUE

What is the Itp (temporal peak intensity)?

The intensity of the beam at the instant in time of its maximal value

What is the Imax?

The AVERAGE intensity during the MOST intense half-cycle

What is the Ipa (pulse average intensity)?

The average intensity during the transmit time

What is the Ita (temporal average intensity)?

The average intensity during the talking AND listening time (PRP)

Rank the 4 temporal intensities from largest to smallest.

Itp, Imax, Ipa, Ita

What are the units for all intensities?

Watts/cm²

Which intensity is the most important regarding thermal bio-effects?

SPTA (spatial peak, temporal average)

Which intensity has the highest value?

SPTP (spatial peak, temporal peak)

Which intensity has the lowest value?

SATA (spatial average, temporal average)

Rank the following intensities from lowest to highest: SPTA, SPPA, SAPA, SATA

SATA, SAPA, SPTA, SPPA

True or False- the pulse average and temporal average intensities are the same for continuous wave ultrasound (SPTA = SPPA, SATA = SAPA)

TRUE, since the beam is always on for CW ultrasound

Decibel notation is ___.

logarithmic, a relative measurement, a comparison, a ratio of the final and initial intensities

decibels

a standard measurement tool used to report CHANGES in the strength of the beam; compares two values but does not give an exact value

positive decibels

report signals that are getting BIGGER

negative decibels

report signals that are getting SMALLER

what happens to a wave’s intensity when there is a +3 dB relative change?

doubles

what happens to a wave’s intensity when there is a +10 dB relative change?

increases ten-fold (ten times larger)

what happens to a wave’s intensity when there is a -3 dB relative change?

halved; reduced to ½ its original value

what happens to a wave’s intensity when there is a -10 dB relative change?

reduced to 1/10 its original value

what happens to a wave’s intensity when there is a +6 dB relative change?

4 times bigger

what happens to a wave’s intensity when there is a -6 dB relative change?

reduced by ¼ its original value

the acoustic power indicates a level of -6 dB, the ultrasound system is pulsing at __% off its original power.

25%

attenuation

the decrease in intensity, power, and amplitude as sound propagates

what are the 2 factors that determine attenuation?

path length (distance) and frequency

what is the relationship between distance and attenuation?

DIRECT; the farther sound travels, the greater the attenuation

what is the relationship between frequency and attenuation?

DIRECT; the higher the frequency, the higher the attenuation (hence lower penetration)

what are the 3 processes that contribute to attenuation?

reflection, scattering, absorption

reflection

as a sound strikes a boundary, a portion of the wave’s energy may be reflected back to the sound source

what are the 2 types of reflection?

specular and diffuse

specular reflection

when the boundary is SMOOTH, the sound is reflected in only ONE direction in an organized manner. Think of a mirror!

do specular reflections at an angle return to the transducer?

NO

specular reflections are seen well when the sound strikes the reflector at __ degrees

90

what else can cause specular reflections to occur?

when the pulse’s wavelength is much smaller than the irregularities in the boundary

what is the greatest of all attenuators?

air

what is the least of all attenuators?

water

why is bone a big attenuator?

Due to absorption and reflection

why are the lungs a big attenuator?

due to scattering

diffuse reflection (backscatter)

when a wave reflects off an IRREGULAR surface, it radiates in MORE THAN ONE direction

what is the advantage of diffuse reflections?

interfaces at suboptimal angles to the sound beam can still produce reflections that will return to the transducer

what is the disadvantage of diffuse reflections?

backscattered signals have a lower strength than specular reflections

scattering

if the boundary between 2 media have irregularities with a size SIMILAR to or SMALLER than the pulse’s wavelength, then the wave may be CHAOTICALLY redirected in ALL directions

how is scattering related to frequency?

DIRECTLY related; higher frequency sound beams scatter much more than lower frequency sound beams (hence higher attenuation)

speckle

grainy appearance from scattering

rayleigh scattering

occurs when a sound wave strikes a reflector that is MUCH SMALLER than the pulse’s wavelength, the sound energy is diverted in ALL directions but stays ORGANIZED

what causes rayleigh scattering in clinical ultrasound?

RBCs

how is rayleigh scattering related to frequency?

DIRECTLY related; rayleigh scattering is related to frequency raised to the 4th power; when frequency doubles, rayleigh scattering is 16 times larger (2×2×2×2)

absorption

occurs when ultrasonic energy is converted into another energy form like heat

how is absorption related to frequency?

DIRECTLY related

attenuation coefficient

the number of decibels of attenuation that occurs when sound travels one centimeter. THE ONLY COEFFICIENT/FACTOR WITH UNITS! (dB/cm)

how is the attenuation coefficient related to the frequency?

DIRECTLY related

does the attenuation coefficient change when path length changes?

NO

what is the equation for total attenuation?

total attenuation (dB) = atten. coefficient (dB/cm) x path length/distance (cm)

if the path length is 5 cm and the attenuation coefficient is 3 dB/cm. what is the total attenuation?

15 dB

A 7 MHz sound beam travels through two media. It attenuates 4 dB in medium A and 6 dB in medium B. What is the total attenuation that the sound beam undergoes as it travels through both media?

4 dB + 6 dB = 10 dB

what is the attenuation coefficient in SOFT TISSUE?

half of the frequency (MHz); atten. coefficient = frequency (MHz) / 2

A sonographer is scanning an abdominal/cardiac exam and is using a 4 MHz frequency transducer. What would be the attenuation coefficient?

4 MHz / 2 = 2 dB/cm

In soft tissue, a 4 MHz ultrasonic wave starts with an intensity of 8 mW/cm². What is the total attenuation, after traveling a depth of 10 cm?

atten. coefficient = 4 MHz / 2 = 2 dB/cm

total attenuation = atten. coefficient x path length = 2 dB/cm × 10 cm = -20 dB (negative b/c energy is lost with attenuation)

Rank the following mediums from lowest to highest attenuation:

water, blood, soft tissue, muscle, bone and lung, air

half-value layer thickness

the distance that sound travels in a tissue that reduces the intensity of sound to ½ its original value AKA the depth of tissue that results in 3 dB of attenuation to the intensity

what causes a thin half value?

high frequency sound, media with high attenuation rate

what causes a thick half value?

low frequency sound, media with low attenuation rate

impedence

the acoustic resistance to sound traveling in a medium. Calculated by multiplying the density of a medium by the speed at which sound travels in the medium

impedence is associated with __.

only the medium

oblique angles

angles with a measure other than 90 degrees. Acute and obtuse angles are oblique angles

normal incidence

the incident sound beam strikes the boundary at EXACTLY 90 degrees. AKA perpendicular

oblique incidence

the incident sound beam strikes the boundary at an angle OTHER THAN 90 degrees

incident intensity

the sound wave’s intensity immediately BEFORE it strikes a boundary

reflected intensity

the portion of the incident intensity that, AFTER striking a boundary, changes direction and RETURNS BACK from where it came from

transmitted intensity

the portion of the incident intensity, that AFTER striking a boundary, CONTINUES FORWARD in the same direction it was traveling

what is the formula for incident intensity?

incident (starting) intensity = reflected intensity + transmitted intensity

intensity reflection coefficient (IRC)

the PERCENTAGE of the intensity that BOUNCES BACK when a sound beam strikes the boundary between 2 media

intensity transmission coefficient (ITC)

the PERCENTAGE of intensity that PASSES THROUGH when the beam strikes the boundary between 2 media

what must adding the IRC and ITC equal?

100% = IRC + ITC

At the boundary between two media, the principle of “___” applies

conservation of energy; this is why IRC + ITC = 100%

in soft tissue, what percentage of the incident ultrasound energy is reflected at a soft tissue boundary?

1% or less

what percentage of the incident ultrasound energy is reflected at a air-tissue boundary?

99%

what percentage of the incident ultrasound energy is reflected at a bone-tissue boundary?

50%

reflection with normal incidence only occurs when ___.

the two media at the boundary have DIFFERENT acoustic impedances

will reflection occur with normal incidence if the two media have identical impedences?

NO

a ___ reflection will occur with normal incidence if the impedances are slightly different

small

a ___ reflection will occur with normal incidence if the impedances are substantially different

large

with normal incidence, __ of the sound is TRANSMITTED when the two media have the SAME impedance

ALL

does reflection and transmission occur with OBLIQUE incidence?

I DON’T KNOW! Reflection and transmission MAY or MAY NOT occur with oblique incidence

if reflection and transmission DOES occur with oblique incidence, the reflection angle will ___ the incident angle

EQUAL

refraction

a CHANGE in direction of a wave propagating from one medium to another; refraction is transmission with a bend!!!

what are the 2 conditions that are needed for refraction to occur?

oblique incidence and different propagation speeds of the 2 media

the physics of refraction are described by ___.

Snell’s law: transmission angle / incident angle = propagation speed 2 / propagation speed 1

if propagation speed of medium 2 is LESS than propagation speed of medium 1, then the transmission angle is ___ than the incident angle

LESS

if propagation speed of medium 2 is GREATER than propagation speed of medium 1, then the transmission angle is ___ than the incident angle

GREATER

will refraction occur when medium 1 and 2 have the SAME propagation speeds?

NO