SPI 1 TEST 2

What is the criteria for refraction?

• Oblique incidence/angles

• Different propagation speeds between the medium & oblique incidence

What is refraction?

Transmission with a bend

What is the criteria for reflection?

• Different acoustic impedances between the medium & tissues

• Sound beam must hit boundary at 90 degrees

What’s are other words for 90 degrees?

Orthogonal

Perpendicular

When would you have a wider angle of refraction?

When going from a low to high propagation speed

When would you have a smaller angle of refraction?

When going from a high to low propagation speed

What is the primary cause for attenuation?

Absorption (heating)

What are other causes of attenuation?

Reflection

Refraction

Interference

If frequency increases, what happens to attenuation?

Increases

If you increase the frequency, what happens to the absorption?

Increases

If you increase the intensity, what happens to the absorption?

Increases

What tissue has the highest absorption rate?

Bone

What medium has the lowest attenuation rate?

Water

What occurs with Positive dB?

Increase in gain/power/intensity, stronger echoes

What occurs with negative dB?

Decrease in gain/power/intensity, attenuation, weak echoes

What does a 3 dB increase mean?

2x

What does a 6 dB increase mean?

4x

What does a 9 dB increase mean?

8x

What does -3 dB mean?

1/2

What does -9 dB mean?

1/8

What does -10 dB mean?

1/10

What law is the REFRACTION equation applied to?

Snell’s Law

What law is the REFLECTION equation applied to?

Luez’s law

What reflection is applied to Luez’s Law?

Specular reflection

Equation for Luez’s Law (law of reflection)

Angle of incidence = angle of reflection

What is Snell’s Law?

Ratio of propagation speeds for two media = ratio of Sines of two angles (angle of incidence & angle of transmission)

What are the two angles used in Snell’s Law?

Angle of Incidence & Angle of Transmission

Types of reflection in ultrasound

Specular reflection

Diffuse reflection (scattering)

Rayleigh scattering (back scattering)

What does reflection look like sonographically?

Mirror like reflection

Large and smooth

What does scattering look like sonographically?

Rough, irregular surface

Think: tofu texture

What does scattering help depict?

parenchyma of an organ

Is diffuse reflection angle dependent?

No, it bounces in many different directions

What’s another word for scattering?

Non specular reflection

What is Rayleigh scattering proportional to?

Frequency to the fourth power

(any increase to the transducer frequency, multiply it by the 4th power to get Rayleigh scattering)

If the transducer is 2 MHz frequency, what is the Rayleigh scattering?

16

2 to the 4th power = 16

If the transducer is 4 MHz frequency, what is the Rayleigh scattering?

256x

4 to the 4th power = 256

What is Rayleigh scattering useful for?

Detecting blood flow in Doppler

Unit for acoustic impedance

MRayls or Rayls

Equation for impedance

Density x Propagation Speed = Impedance

( p x c = z)

What does no impedance look like sonographically?

No reflections, Anechoic

What does large impedance look like sonographically?

Strong reflection, hyperechoic

What does small impedance look like sonographically?

Weak reflections, hypoechoic

What is the result of small impedance mismatches?

Inability to visualize structure

What is considered an oblique angle?

Anything that isn’t 90 degrees

What’s a refractive artifact that occurs at the critical angle?

Edge shadowing

When does total internal reflection occur?

When angle of incidence is greater than critical angle

What’s another term for the critical angle?

Grazing Incidence

What does attenuation mean?

Loss of power as it travels through medium

What is attenuation coefficient?

Amount of attenuation per cm per MHz frequency

(AKA the total loss as you travel deeper)

Unit for attenuation coefficient

dB/cm

What is always negative dB?

Attenuation is always negative dB (represents loss)

What’s the “speed limit” in ultrasound?

< 77,000 cm/s

What does the speed limit take into account?

Imaging depth

Number of scan lines & focal zones

Frame rate

What’s temporal resolution?

Ability to separate events and locations in time

What is color display line?

Multiple acoustic lines

(AKA group of pulses travel together in a packet to generate one color display line)

What are other names for color display line?

Packet or ensemble

What are Scanned Modalities?

Acquire information over a plane

What is significantly reduced during scanned modalities?

Frame rate (temporal resolution)

What is Non-scanned Modalities?

Acquire information only along a line

What is the risk of non-scanned modalities?

Increased risk of thermal bio effects (because it scans the same line over & over)

Examples of scanned modalities

2D imaging

3D imaging

Color flow imaging

Examples of non-scanned modalities

CW Doppler

PW Doppler

Spectral tissue Doppler

M mode

A mode

What is frame rate?

Number of frames per seconds

(AKA how many images taken per second)

Unit for frame rate

Hz

What’s the typical range for frame rates?

30 to 60 FPS

(AKA 30 to 60 Hz)

What’s another term for frame rate?

Frame frequency

What is frame time?

Time it takes to create a single frame/scan

Unit for frame time

Seconds

How long does it take to TRAVEL 1 cmd?

6.5 microseconds

How long does it take to IMAGE 1 cmd?

13 microseconds

What is pulse?

Number of cycles that travel together

Is there any impedance mismatch in angle of reflection?

Yes, impedance is the primary reason for reflection

What affects pulse duration?

The source (transducer frequency)

How are ultrasound images created?

Multiple acoustic lines create one image

What is bandwidth?

Range of frequencies in a pulse

If a transducers frequency is 2-5 MHz, what’s the bandwidth?

3

What’s the unit for duty factor?

No unit (unitless)