Physics Chapter 3 Exam

Why are waves important in ultrasound?

DMS is based on how waves interact with various structures in the body, changing the parameters of the sound wave can lead to changes in the quality og the images

List the 7 acoustic parameters

frequency, period, wavelength, amplitude, propogation speed, intensity, power

Period

Time required to complete a single cycle or time froom start of one cycle to start of next

List the Units, values, determination and adjustibility of period

units: seconds, milliseconds, hours, days

values: 0.06-0.5 microseconds

Determined by: sound source only

Not adjustable

period

Frequency

number of cycles per second

List the Units, values, determination and adjustibility of frequency

units: Hz

values: 2 MHz - 15 MHz

determined by: sound source only

not adjustable

increased frquency =

less penetration through the body

Frequency of a sound wave refers to ___

how many compressions or rarefactions occur per time (usually per second)

Higher frequency =

more compressions per second

lower frequency =

less compressions per second

What is the frequency of this wave ?

8 compresions / 4 seconds = 2 Hz

Higher frequencies produce

better images of higher quality (opposite for lower)

How are frequency and period related ?

inversely related (Hertz per second is the inverse of time)

if one remains constant the other does not change

period x frequency = 1

As frequency increases, what happens to period ?

decreases

As period decreases, what happens to frequency ?

increases

Period =

1/frequency

Frequency =

1/period

Find the period of a 2MHz wave

2 : reciprocal of 2 = ½ (or 0.5)
M: reciprocal of Mega = micro (remember pairs)

Hz: reciprocal of Hz = seconds

Period = 0.5 micro seconds

Find the period of a 20kHz frequency wave

20: reciprocal of 20 = 1/20 (0.05)

k: reciprocal of k = milli

Hz: reciprocal of Hz = seconds

Period = 0.05 milli seconds

Find the frequency of a wave with a period of .5 microseconds

.5: reciprocal = 1/.5 (2)

micro: reciprocal = mega

second: reciprocal of second = Hz

Frequency: 2 MHz

What are the 3 “bigness” parameters ?

amplitude, power, intensity

describe the beams strength

how are amplitude, power, and intensity related to each other ?

directly related

Amplitude

measured from the baseline to max value (or minimum of value)

List the Units, values, determination and adjustibility of amplitude

units: any acoustic variable

values: 1 MPa - 3 MPa

determined by: sound source

is adjustable

Amplitude is determined by ___

the sound source

higher voltage = high amplitude

Amplitude decreases as

the sound propogates through the body (attenuation)

How is amplitude adjusted ?

output power: if you increase the amplitude of an ultrasound echo, the image will get brighter

amplitude

peak to peak amplitude

What are the 3 ways to calculate amplitude ?

1) Max - Mean

2) Mean - Min

3) (Max - min) / 2

Power

rate that work is performed, or rate of energy transfer

List the Units, values, determination and adjustibility of power

units: Watts, mW

values: 0.004 - 0.90 W, 4-90 MW

determined by: sound source inititally

is adjustable

How are amplitude and power related ?

power = amplitude²

power controls the amplitude of a wave

If amplitude is increased by a factor of 3, how much has power increased by

9 (3² = 9)

What is the power value if the sonographer cuts the amplitude in half ?

(1/2)²= 1/4

Higher power =

stronger ultrasound beam = better penetrations (but at the increased risk of bio effects)

Intensity

measure of the concentration of energy (power) in a second

beams power / beams cross-sectional area = intensity

List the Units, values, determination and adjustibility of intensity

units: watts/cm²

values: 0.02 - 300 W/Cm²

determind by: sound source

is adjustable

high power over a small area =

great intensity BUT increases the risk of bioeffects

What is the relationship between instensity and amplitude

intensity is proportional to amplitude²

increase intensity by

decreasing the area (narrowing sound beam) and focusing it on a certain point

Wavelength

distance or length of one complete cycle, measure from one point of the wave to the next idntical point (lambda)

List the Units, values, determination and adjustibility of wavelength

units: meters, mm, units of length

values: 0.1 - 0.8 mm

determined by : both the source and the medium

not adjustable

How are wavelength and fequency related

inversely related

What is the rule of finding the wavelength from frequency ?

wavelength = 1.54/frequency

Shorter wavelengths produce

better image quality

wavelength

Wavelength equation

wavelength = propogation speed (c) / frequency (f)

Propogation speed

distance a sound wave travels through a medium in 1 second

answers how long it will take to get from one location to another

List the Units, values, determination and adjustibility of propogation speed

units: meters/second, mm/mms or any distance divided by time

values: 500 m/s - 4000 m/s

determined by: medium only

not adjustable

what is the average speed of ultrasound in soft tissue?

1.54 mm/microsecond

1540 m/sec

Distance =

rate (speed) x time (machine knows how long the echo took to return)

how does stiffness and density affect speed

sound travels fastest in solids (tissue)

medium speed in liquids (water)

slowest speed in gases (air)

Examples of the medium having an affect on speed

very stiff + not dense = fast speed (bone)

not stiff + very dense = slow speed (air)

two formulas of speed

propogation speed (c) = stiffness/density

propogration speed = frequency x wavelength

speed of sound in different biological media

x

Stiffness

ability of an object to resist compressions

inelasticity

directly related to speeed

Bulk modulus

describes the change in materials volume under external stress

same as stiffness

Bulk modulus equation

change in pressure divided by the fractional change in volume

____ materials = larger bulk modulus

stiffer

_______ materials = smaler bulk modulus

more elastic materials

Is propogation velocity impacted more by stiffness or density ?

stiffness (stiffness plays a greater role)

equation 2 for Bulk Modulus

c = square root B(bulk modulus)/p (density)

Bulk Modulus table

x

Density equation

mass/volume

larger mass within the same volume _____

increases density

Density and speed are _____ related

inversely

Two objects of similar volume can have

very different densities

Rule of thumb #1

bone is not compressible and has tight moelcular bonds so speed is fastest (bone is stiff but not dense)

Rule of thumb #2

speed of sound is slowest in gas or air (gas is dense and not stiff)

Identical sound waves travel at

unequal speeds through different media

Important graph #1

x

important graph #2

x