Post Quiz 1 Content (Units 5 + 6)

What are the differences between these two sine waves?

-amplitude and therefore intensity?

-frequency therefore resolution and penetration are different too

-wavelength

-period

What’s the similarities between these two sine waves? How are they different?

-amplitude is the same

-frequency is the same

-period is the same
-they are out of phase with each other. If they are added together the net result is zero

The ultrasound beam is created by _______ each emitted from their own source.

-many waves

The point sources of ultrasound waves are called _____    _______.

-Huygens’ sources

The _____ in a probe is the Hugens’ Source.

-crystals

Each wave emitted from Huygens’ sources are referred to as ______      ______.

-Huygen’s wavelets

The divergence of the wavelets depends on the _____ of the source.

-size

What was the analogy for wavelets divergence?

-throwing a small pebble into a body of water. Small ripples will travel quickly

-throwing a large log into a body of water. This will make a big splash but the waves won’t travel as quickly as the ripples do

The spreading of the beam as it travels from the source is referred to as ______.

-divergence

The ______ the source of sound the less divergence there is.

-larger

-think of pebble vs log analogy

What occurs after a beam with planar wave fronts passes through a small aperture (opening)?

-diffraction

Are divergence and diffraction the same?

-no

Are divergence and diffraction the same?

-no

How would you define Huygen’s Principle?

-each point source creates its own sound wave/beam

-multiple wavelets from many sources form a new wave-front

-little waves will interfere and create a new wave front

How is Hugens’ Principle related to the design of probes?

-probes have an array of crystals mounted on the transducer face

-each crystal emits its own wavelets, and they all add together to create a larger beam (wavefront)

What does interference refer to?

-when 2 or more waves interact. Adds the motion of one wave to the other

What are the 2 types of interference?

1)     Constructive

2)     Destructive

What kind of beams are the best for getting through the body tissues so we can differentiate structures the best?

-short skinny beams (Skinny legends)

When two waves are _____, this results in constructive interference.

-in phase

During constructive interference the contributions of each wave result in an ______ in amplitude and intensity.

-increase

During destructive interference, the contributions of each wave results in a _______ in amplitude and intensity.

-decrease

When two waves are ______, this results in destructive interference.

-out of phase

What is the result of two out of phase waves with the same amplitudes?

-cancel out so net result is zero

What is the result of two waves that are neither in or out of phase? This is how ______ signals are detected in ultrasound.

-beat frequency

-Doppler

What is responsible for the natural focusing of the beam?

-interference

These types of waves occur when two waves propagate in the same medium but in opposite directions. What happens when they are added together?

-standing waves

-certain points the amplitude is zero, at other points the amplitude is doubled

The weakening of sound as it travels is referred to _______.

-attenuation

How can attenuation affect our image?

1) limits our imaging depth

2) Must be compensated for (TGCs)

3) Can be useful in diagnosis

The ____ and _____ are relative units of measurement expressing the loudness (intensity level) of sound waves.

-bel (B)

-decibel (dB)

1 Bel is = to how many dB?

-10 dB

Hearing threshold is = to how many dB?

-0

A whisper is = to how many dB?

-30

Normal conversation is = to how many dB?

-60

A lawn mower is = to how many dB?

-90

A concert is = to how many dB?

-115

Pain is = to how many dB?

-130

The ____ uses dB to express the number of shades of gray displayed on the monitor.

-dynamic range

Another way to describe dynamic range is the?

-amount of contrast

-the higher the dynamic range the more black and white the image will be

What 2 parameters use decibels to express the amount of amplification required to optimize the returning echoes?

-TGC and gain

What is the forumla to calculate Bels?

Bel = log(new_i/original_i)

Since 1 Bel = 10 dB how can we rewrite the Bel formula?

dB = 10log (new_i/original_i)

What else can the Bel/decibel formula be used to describe?

-power and voltage

What is the formula to calculate power by using dB?

-dB = 10log (new_P/original_P)

What is the formula to calculate voltage by using dB?

- dB = 20log (new_V/original_V)

What do we need to remember to change when using the power/voltage dB equation?

-it is 10log for power and 20log for voltage

A __ dB drop is = to 1/2 the original intensity.

-3dB

A ___ dB drop is = to 0.1 of the original intensity.

-10dB

If the power is at 100% (100W), what is the dB = to?

-0dB

If the power is at 50W, what is the dB = to? Keep in mind 100W is = to 0dB.

-        Negative 3dB

The power output of an amplifier is 100mW while the input power is 0.1 mW, what is the amplifier gain in decibels?

dB = 10 log(New_P/Original_P)

dB = 10log(100mW/0.1mW)

dB = 10log(1000)

dB = 10(3)

dB = 30

An electric attenuator has a power output of 0.01mW with an input power of 100mW. What is the attenuation in decibels?

dB = 10log(New_P/Original_P)

dB = 10log (0.01mW/100mW)
dB = 10log (0.0001)

dB = 10(-4)

dB = -40

The amount of attenuation that occurs with each one-centimeter travels is known as the _______.

-attenuation coefficient

In soft tissue there is _____ of attenuation for every 1 cm travelled per 1MHz. Therefore, in soft tissue the attenuation coefficient is = to _______.

-0.5dB

-1/2frequency

What is the formula to calculate the total attenuation?

- Tatt = Att. Coef. X Path length (cm)

What is the formula for total attenuation in soft tissue?

- Tatt = 1/2f x path length (cm)

Calculate the decibel loss back at the probe for a 2.5 MHz ultrasound beam imaging to a depth of 6cm in soft tissue.

TAtt = ½ f x path length (cm)

Tatt = ½ (2.5MHz) x 12 cm

Tatt = 1.25MHz x 12 cm

Tatt = 15 dB

Now plug -15dB into dB = 10log(I_new/I_OG)

-15 dB = 10 log(I_new/100mW/cm²)

-1.5 dB = log (I_new/100)

0.032 = I_new/100

3.2 mW/cm² = I_new

Calculate the intensity back at the probe for a 3 MHz ultrasound beam imaging to a depth of 5 cm in soft tissue and an original intensity of 50 mW/cm².

Tatt =½ f x path length (cm)

Tatt = ½ (3 MHz) x 10 cm

Tatt = 1.5MHz x 10 cm

Tatt = 15 dB

Now plug -15dB into dB = 10log(I_new/I_OG)

-15 dB = 10 log(I_new/50mW/cm²)

-1.5 dB = log (I_new/50)

0.032 = I_new/50

1.6 mW/cm² = I_new

The distance  sound must travel in a material to reduce the intensity to half its original value is known as the ___________.

-half-value layer

The half-value layer is = to how many dB?

-3dB

What can vary attenuation?

-nature of the tissue (dead or alive)
-frequency

-depth

What are 5 reasons we have attenuation? hint: WARRS

1)absorption

2)reflection

3)refraction

4)scatter

5)wave-front divergence

The conversion of sound energy to heat and is the dominant factor in attenuation accounting for approximately 80% is known as _______.

-absorption

What are 3 factors that effect absorption?

1)viscosity

2)relaxation time on molecules

3)frequency

The ease in which molecules can slide past one another is _______.

-viscosity

Increased viscosity provides greater ______, more _____ and therefore increased _______.

-resistance

friction

-attenuation

_____ is how sound energy is converted to heat.

-friction

When a mechanical force is applied to a molecule it will vibrate. The time it takes to come to rest is the called the _______________.

-relaxation time

What happens if molecules can’t come to rest before the next compression phase? What does this cause?

-more energy us required to reverse its direction.

-heat

Since relaxation time is relatively constant in soft tissues, what is more likely to influence the amount of absorption?

-frequency

If frequency is increased, then there is ____ time available for molecules to recover during the relaxation phase. What is the result?

-less

-more absorption

What are the 2 types of reflection?

1)specular

2)non-specular

When a sound beam hits a large, smooth surface this is referred to as ________.

-specular reflection

What is a good example of a specular reflector?

-the diaphragm

The intensity of the reflected sound depends on 2 things:

1)angle of incidence

2)acoustic impedance (stopping) of two media

The angle of incidence will equal the ________.

-angle of reflection

When would a reflection potentially not return to the probe?

-when reflections of sound have non-perpendicular incidence

What helps improve reflection?

-perpendicular incidence

How fast sound moves through a medium will depend on the medium’s ____ and ____.

-density

-stiffness

Acoustic impedance is also known as _______.

-characteristic impedance

Characteristic Impedance describes the relationship between _______ and the speed of particle vibrations in a sound wave (Speed propagation).

-acoustic pressure

What is the formula for impedance?

impedance (Z) = density (p) x velocity (c)

What are the units of impedance?

-Rayls (Z)

Z _______ with increasing density/velocity.

-increases

The larger the Z value difference at the interface of 2 media the _____ the reflection.

-bigger

The amount of sound that reflects at an interface is called the ________.

-intensity reflection coefficient (IRC)

How can we calculate the IRC?

IRC = reflected intensity (I_r)/Incident intensity (I_i)

Or

IRC = (Z₂-Z₁/Z₂+Z₁)²

If we know the amount of sound that is reflected then we can calculate the amount ______. This is known as the _____________.

-transmitted

-intensity transmission coefficient (ITC)

What’s the formula for ITC?

ITC = 1 – IRC

The more sound reflected, the _____ transmitted.

-less

What happens if the impedance is equal across the interface?

-no reflection will occur

Calculate the amount of sound reflected (IRC) at a bone/soft tissue interface. Bone Z = 7.8 x 10 ^6 Rayls, ST Z= 1.63 x 10^6 Rayls.

IRC = (Z₂-Z₁/Z₂+Z₁)² x 100

IRC = (7.8 x 10 ^6 - 1.63 x 10^6 / 7.8 x 10 ^6 + 1.63 x 10^6)² x 100

IRC = (6.17/9.43)² x 100

IRC = 0.42 x 100

IRC = 43% reflected

IF IRC is 43%, calculate the percent of sound transmitted (ITC).

ITC = 1-IRC
ITC = 1 – 0.43

ITC = 0.57 or 57%

Determine the amount of reflected sound (IRC) at an air/ST interface. Air Z = 0.0004 x 10 ^6 Rayls, ST Z= 1.63 x 10^6 Rayls.

IRC = (Z₂-Z₁/Z₂+Z₁)² x 100

IRC = (0.0004 x 10 ^6 - 1.63 x 10^6 / 0.0004 x 10 ^6 + 1.63 x 10^6)² x 100

IRC = (-1.6296/1.6304)² x 100

IRC = 0.999x 100

IRC = 99.9% reflected

Why does reflection occur?

-a difference of Z values across an interface

True/False, it is possible for 2 tissues to have different sound velocities but have the same Z value?

-true, this is why we need gel when we look at the reflection from air

What’s the difference between reflection and refraction?

-reflection looked at the sound returning to the probe from an interface

-refraction deals with the sound that is transmitted across the interface

Sound obeys what law?

-Snell’s Law of optics

What is Snell's Law?

Sin θ_i/ Sin θ_t = v_i/v_t

What happens if the velocities across the interface are equal?

- then no refraction will occur (if V1 = V2)

What happens if the velocity of the first medium is greater than the second?

- sound will bend towards the normal (fast to slow, to the normal I go)

- think of car analogy, it turns towards the slower tire