Ultrasound Physics: Key Concepts and Relationships

What is a sound wave?

A mechanical wave that requires a medium to travel.

Can sound travel through a vacuum?

No. Sound requires a medium.

What does a direct relationship mean?

As one variable increases, the other also increases.

What does an inverse relationship mean?

As one variable increases, the other decreases.

What is a reciprocal?

The value obtained by flipping a fraction; for example, the reciprocal of f is 1/f.

What is the unit for period?

Microseconds (µs).

Can the sonographer adjust period directly?

No. Period is source-determined.

What is frequency?

The number of cycles occurring per second.

What is the unit for frequency?

Hertz (Hz), commonly megahertz (MHz) in diagnostic ultrasound.

What is the typical diagnostic ultrasound frequency range?

Approximately 2-15 MHz.

What is ultrasound?

Sound with a frequency greater than 20 kHz.

What is infrasound?

Sound with a frequency below 20 Hz.

What is the human hearing range?

Approximately 20 Hz to 20 kHz.

What is the relationship between frequency and period?

They are inversely related and reciprocal: period = 1/frequency.

If frequency increases, what happens to period?

Period decreases.

What are the three bigness parameters?

Amplitude, power, and intensity.

What is amplitude?

The magnitude or strength of a sound wave.

What is peak-to-peak amplitude?

The total distance from the positive peak to the negative peak; it is 2 times the amplitude.

What is power?

The rate of energy transfer.

What is the unit for power?

Watts (W).

How is power related to amplitude?

Power is proportional to amplitude squared.

What is intensity?

The concentration of energy in a sound beam.

What is the unit for intensity?

Watts per square centimeter (W/cm²).

What is the intensity equation?

Intensity = Power / Area (I = P/A).

How are amplitude, power, and intensity related?

They are directly related; increasing amplitude increases power and intensity.

If amplitude doubles, what happens to intensity?

Intensity increases by 4 times.

If power doubles, what happens to intensity when area is unchanged?

Intensity doubles.

What is wavelength?

The distance of one complete cycle.

What are common units for wavelength?

Millimeters (mm) or centimeters (cm).

What determines wavelength?

The source frequency and the medium.

Can the sonographer directly adjust wavelength?

No.

What is the relationship between frequency and wavelength?

They are inversely related when propagation speed is constant.

What is the wavelength of 1 MHz sound in soft tissue?

1.54 mm.

What is the wavelength formula in soft tissue?

Wavelength (mm) = 1.54 / frequency (MHz).

Why is a shorter wavelength useful?

It is associated with better detail/resolution.

What is propagation speed?

The rate at which sound travels through a medium.

What determines propagation speed?

The characteristics of the medium, especially stiffness and density.

What is the average propagation speed in soft tissue?

1540 m/s, or 1.54 mm/µs.

How does sound speed compare among solids, liquids, and gases?

Sound travels fastest in solids, slower in liquids, and slowest in gases.

How does stiffness affect propagation speed?

Greater stiffness increases propagation speed.

How does density affect propagation speed when stiffness is equal?

Greater density decreases propagation speed.

What is the speed equation?

Speed = frequency × wavelength (c = fλ).

What is attenuation?

The weakening of a sound beam as it travels through tissue.

What are the three processes that contribute to attenuation?

Reflection, scattering, and absorption.

How does path length affect attenuation?

Attenuation increases as path length increases.

How does frequency affect attenuation?

Attenuation increases as frequency increases.

What does dB represent in ultrasound?

A logarithmic way of expressing relative changes in intensity or amplitude.

What does a +3 dB change mean for intensity?

Intensity doubles.

What does a +10 dB change mean for intensity?

Intensity increases by 10 times.

What does a -3 dB change mean for intensity?

Intensity is reduced to one-half.

What does a -10 dB change mean for intensity?

Intensity is reduced to one-tenth.

What is specular reflection?

Reflection from a smooth boundary in an organized direction.

What are examples of specular reflectors?

Liver capsule, diaphragm, vessel walls, and fascia.

What is a limitation of specular reflection?

If the beam is not directed back toward the transducer, the echo may not return and the structure may be poorly visualized.

What is diffuse reflection?

Reflection from an irregular surface that sends sound in multiple directions.

What is an advantage of diffuse reflection?

Some of the sound can return to the transducer even when the interface is not perfectly perpendicular.

What is a disadvantage of diffuse reflection?

The returning echoes are weaker than strong specular reflections.

What is scattering?

Random redirection of sound in many directions by small structures or interfaces.

When does scattering occur?

When the reflecting interface is approximately equal to or smaller than the wavelength.

How does frequency affect scattering?

Higher frequency generally produces more scattering.

What is Rayleigh scattering?

Scattering from structures much smaller than the wavelength, such as red blood cells.

How is Rayleigh scattering related to frequency?

It is proportional to frequency to the fourth power (f⁴).

If frequency doubles, how does Rayleigh scattering change?

It increases by 16 times.

What is absorption?

Conversion of sound energy into other forms of energy, especially heat.

How does absorption relate to frequency?

Absorption increases as frequency increases.

What is the attenuation coefficient?

A value describing how much attenuation occurs per unit distance in a medium.

What are the units of the attenuation coefficient?

dB/cm.

What attenuation coefficient relationship is given in the lecture?

Attenuation coefficient = frequency / 2 (dB/cm).

What is the half-value layer?

The depth of tissue that produces 3 dB of attenuation.

What factors affect the half-value layer?

The medium and the frequency.

What is acoustic impedance?

The resistance a medium presents to sound propagation.

What is the acoustic impedance equation?

Z = density × propagation speed (Z = ρc).

What are the units of acoustic impedance?

Rayls; clinical values are commonly expressed in Mrayls.

What happens when two media have the same acoustic impedance?

At normal incidence, no reflection occurs at the boundary.

What happens when the acoustic impedance difference between two media is large?

A larger percentage of the incident sound is reflected.

What is normal incidence?

The sound beam strikes the boundary at 90° to the interface.

What is oblique incidence?

The sound beam strikes the boundary at an angle other than 90°.

What is incident intensity?

The intensity of sound arriving at a boundary.

What is reflected intensity?

The intensity of sound that returns from the boundary.

What is transmitted intensity?

The intensity of sound that continues into the second medium.

What is the relationship among incident, reflected, and transmitted intensity?

Incident intensity = reflected intensity + transmitted intensity.

What does IRC stand for?

Intensity Reflection Coefficient.

What does ITC stand for?

Intensity Transmission Coefficient.

What happens at normal incidence when acoustic impedance is identical on both sides?

There is no reflection.

What is refraction?

A change in the direction of a sound beam as it enters a different medium.

What two conditions are required for refraction?

Oblique incidence and different propagation speeds in the two media.

What is the law of reflection?

The angle of reflection equals the angle of incidence.

What is time-of-flight?

The time required for a pulse to travel to a reflector and return to the transducer.

Why is time-of-flight important?

The ultrasound system uses it to determine reflector depth.

What is the 13 µs rule?

A 13 µs round-trip travel time corresponds to approximately 1 cm of depth in soft tissue.

Why is the total travel distance twice the displayed depth?

The pulse travels to the reflector and then returns to the transducer.

What is PRP?

Pulse repetition period: the time from the start of one pulse to the start of the next pulse.

How does PRP change with imaging depth?

PRP increases as imaging depth increases.

How does PRF change with imaging depth?

PRF decreases as imaging depth increases.

What imaging mode displays echoes as brightness?

B-mode, or brightness mode.

What does compression/dynamic range control?

The range of echo amplitudes that are displayed as different shades of gray.

What happens when compression is increased?

More gray shades are displayed and image contrast decreases.

What happens when compression is decreased?

Fewer gray shades are displayed and image contrast increases.

What does overall gain do?

Brightens or darkens echoes throughout the entire image.

What does TGC do?

Adjusts amplification at specific depths to compensate for attenuation.