Ultrasound Physics Flashcards

Flashcards based on Ultrasound Physics lecture notes.

What is the role of impedance (Z) in ultrasound physics?

Determines the amount of incident beam reflected back and transmitted to the next medium.

What is the formula for calculating impedance (Z)?

z = ρv (density of the medium multiplied by the sound propagation speed).

What are the units of impedance?

Rayls (Z)

What is the typical range of impedance values in soft tissue?

Between 1,250,000 and 1,750,000 rayls (1.25 - 1.75 Mrayls).

What is an interface in ultrasound imaging?

A boundary where two tissues come together.

What interactions occur at tissue boundaries in ultrasound?

Absorption, reflection, scattering, diffraction, divergence, and refraction.

Which interaction may increase or decrease intensity of the beam?

Interference

What is perpendicular incidence?

The sound beam strikes the boundary between two media at 90 degrees.

What happens to reflected and transmitted sound during perpendicular incidence?

Reflected sound bounces back to the first medium, transmitted sound does not change direction.

On what factors do the intensities of reflected and transmitted sound depend during perpendicular incidence?

Incident intensity at the boundary and the impedances of the media involved.

What is the relationship between the angle of incidence and the angle of reflection with specular reflectors?

Angle of incidence equals the angle of reflection.

What is the primary factor determining the amount of reflected echoes and transmitted echoes?

The difference in acoustic impedance of the two media.

What happens if the impedance is exactly the same for both media?

All the incident sound is transmitted with no reflection.

What happens when media have the same impedances?

Energy transfer is most effective.

What are other terms for normal incidence?

Perpendicular, orthogonal, right angle.

Why, for perpendicular incidence, does the amount of reflection depend on the surface and not the thickness of the media?

The surface.

What matters for echoes that are reflected at the bounday?

The difference what really matters; not the value of 'Z'.

What is incident intensity?

The sound's intensity immediately before it hits the interface.

What is reflected intensity?

The intensity of the reflected echo.

What is transmitted intensity?

The intensity of the beam that continues forward in the same direction.

How much of the incident beam is reflected in clinical imaging?

Less than 1% of the incident beam.

What is the Intensity Reflection Coefficient (IRC)?

The fraction of the incident intensity that is reflected.

What is the Intensity Transmission Coefficient (ITC)?

The fraction of the incident intensity that is transmitted.

How much of the incident beam is transmitted in clinical imaging?

Most of the incident beam.

What should you remember regarding intensity reflection and transmission coefficients?

These are simply reflection questions, whatever remains after transmission, must be reflected!

If Intensity Reflection Coefficient increases, what happens to Intensity Transmission Coefficient?

Intensity Transmission Coefficient decreases.

What happens if the impedances are equal in both media and the incident intensity is perpendicular?

There is no reflected echo.

What is the relationship between incident and transmitted intensity when there is no reflected echo?

Transmitted Intensity would be same as the Incident Intensity.

What do we know about Transmission and reflection with oblique incidence?

Transmission and reflection may or may not occur with oblique incidence, but there are no "simple" rules.

What should you remember regarding reflection with oblique incidence?

With oblique incidence, we are uncertain as to whether reflection will occur. Simply say "I don't know!"

What is the relationship between incident, transmitted and reflected intensities?

Incident Intensity = Transmitted + Reflected.

What is the relationship between incident and reflection angle?

Incident Angle = Reflection Angle.

How common is Oblique Incidence in ultrasound?

Is very common in ultrasound.

What is the relationship between the incident angle and the reflected angle in oblique incidence?

The Incident Angle is equal to the Reflected Angle.

Describe the relationship between reflection and impedance for oblique incidence.

With Oblique Incidence, reflection can occur even if the impedances are the same for both media; and, the opposite is true too. Reflection may not happen even when the impedances are different.

What two things remain constant in oblique incidence?

If reflection occurs, the reflected angle will be equal to the incident angle, and the reflected and transmitted energy should equal 1 or 100%.

What are characteristics of a specular reflection?

Smooth boundary, one direction, organized.

What are characteristics of a diffuse reflection?

Irregular boundary, more than one direction, backscatter.

What is refraction?

The change of direction of the traveling sound wave as it goes from one medium to another.

When does the change of direction of sound happen relating to refraction?

When there is different speed of propagation of sound in the two media.

What happens regarding refraction if the velocities of the two media are the same?

No refraction occurs even though the impedances may be different.

What happens regarding refraction at perpendicular (normal) incidence?

Refraction will not occur.

What does Snell's Law calculate?

Calculates the refracted angle in relation to the angle of incidence and to the speeds of sound in the two media.

Of what use is refraction for imaging ultrasound?

Internal focusing using curved crystal.

What is critical angle reflection/refraction?

When a sound beam strikes a smooth acoustical interface at angles other than perpendicular, more of the energy is reflected than transmitted.

What two conditions should be present for critical angle reflection/refraction?

The velocity in medium1 being slower than the velocity in medium2; and, The angle of incidence beyond the so called critical angle.

If two elements are present, what happens to the beam?

The beam travels along the interface and no energy is transmitted.

What is 'time-of-flight'?

The time needed for a pulse to travel from the transducer to the reflector and back to the transducer.

What is the range equation formula for calculating distance to boundary?

Distance to boundary (mm) = (go-return time (μs) x speed (mm/μs)) / 2

What is the simplified range equation formula in soft tissue?

Time (us) x 0.77 mm/μs

What is the purpose of the Range Equation?

Determine the distance to a reflector or boundary where the echo was produced.

What is the range equation formula?

d = vt

Why is there a /2 factor in the equation?

The system needs only the distance.

What is the pulse round trip time?

The pulse round trip travel time is 13μs for each centimeter of distance from source to reflector

When the depth of view is superficial, how long is PRP?

PRP is SHORT.

When the depth is deep, how long is PRP?

PRP is LONG.

What is the PRP formula?

Imaging depth(cm)x13μs/cm

What is Pulse Repetition Period (PRP)?

The time from beginning of one pulse to the beginning of the next one.

What is Pulse Repetition Frequency (PRF)?

The number of pulses per second.

When the depth of view is superficial, how is PRF?

PRF is HIGH.

When depth is deep, how is PRF?

PRF is LOW.

What is the PRF formula?

77,000 cm/s / imaging depth (cm)

What is the defintion of resolution?

The ability to image accurately (accuracy, not merely quality)

What is axial resolution?

The ability to distinguish two structures that are close to each other front to back, parallel to, or along the beam's main axis.

What synonyms are there for axial resolution?

Longitudinal or axial

What units are used for axial resolution?

mm, cm all units of distance

Can a sonographer change axial resolution???

A new transducer is needed to change axial resolution.

What are typical values for Axial resolution?

0.05-0.5mm

What does "short pulse" mean?

Short pulse

What does Axial Resolution refer to?

The minimum reflector separation along the direction of the scan line to produce separate echoes.

What does Axial Resolution allow?

allows to differentiate between two reflectors close together along the scan line (parallel to the sound beam). It is measure in millimeters.

How can the axial resolution we need to be improved?

Shorten the spatial pulse length (increasing the frequency).

What is the Spatial Pulse Length equation?

Wavelength x # cycles in pulse

How can wavelength be reduced to improve spl?

Increasing frequency.

How is number of cycles reduced in the pulses?

Damping.

How can Axial Resolution can be modified?

Increasing the frequency.

Increasing the frequency results leads to what regarding penetration?

Shorter penetration.

What is the equation for Axial Resolution?

Half the spatial pulse length.

What is the time-of-flight (T)?

The time needed for a pulse to travel from transducer to reflector and back.

What is the average speed of ultrasound in soft tissue?

1.54 mm/μs.

State formula of distance.

Distance = Speed x Time

What does 'd' represents

The depth of the reflector.

In the basic form, the equations assume what?

Sound is not traveling at an oblique incidence

Why would the system be inaccurate, if not?

Because, to display an image, the ultrasound system computes the reflector depth assuming the sound travels directly to the reflector and back the the transducer.

How does refraction affect image accuracy?

It changes the direction of sound travel.

The system is calibrated to display returning sound (the echo) at correct locations using 1.54 mm/us. This uses what assumption?

Assumed speed of sound in soft tissue.

If not so, what is the consequences of not having a uniformed sound speed?

Anatomy may be displayed at the incorrect depth when sound does not propagate at 1.54 mm/us.

If a reflector is 2 cm deep, the pulse-round trip travel time is how many microseconds?

26 microseconds