ULTRASOUND PHYSICS CH 3 DESCRIBING SOUND WAVES

Pulses of acoustic energy

Ultrasound uses what to create images?

Pulse

A collection of cycles that travel together.

Although there are multiple cycles that make up the pulse, the pulse moves as one.

Must have a beginning and end or sound is a continuous wave, which cannot produce an image.

2 components of pulsed ultrasound

On time or transit time

Off time or receive time

Seven parameters

Describe sound waves

Period

Time it takes a wave to complete a single cycle.

The time from the start of one cycle to the start of the next.

Units of period

Time, seconds, milliseconds, hours , days

Values of period

Microseconds or millionths of a second

Period is determined by what?

The sound source and cannot be changed, only by changing transducer

Frequency

Number of cycles in a second. Number of times an event occurs in a space of time

Units of frequency

Hertz

Ultrasound uses frequency ranges of 2 million - 15 million hertz.

Frequency is determined by what

Sound source and cannot be changed unless change transducer

Infrasound

Below 20 hertz

Below the range of human hearing (snakes and whales)

Ultrasound

Above 20 KHz

Above the range of humans (bats, dog, deer)

Audible

20 HZ - 20 KHz

Frequency affects what

Penetration and image quality

Reciprocals

The relationship between frequency and period.

They're inversely related

Typical period in ultrasound

.06- .5 microseconds

Amplitude, power, and intensity

Called the Bigness Parameters that describe the magnitude or size of a sound wave

Amplitude

Difference between the maximum value and the average of an acoustic variable. Difference between average and minimum value.

Units of amplitude

Pressure (pascals), Density (g/cm^3), particle motion (cm, inches- any distance), and decibels (dB)

Decibels

Most commonly used for amplitude in sonography when we speak of the amplitude increasing or decreasing as it propagates through the body.

Pressure amplitude

Ranges from 1 million to 3 million pascals

Amplitude is determined by what?

The sound source initially but is also determined by the medium as it becomes weaker as it travels through the body.

Can be controlled by sonographer

Peak to peak amplitude

Is the difference between maximum and minimum values of an acoustic variable.

2 times the value of amplitude

Power

Rate at which work is performed.

Units of power

Watts

Values of power

0.004-0.9 watts (4-90 Milliwatts)

Power is determined by what?

Initially the sound source, power decreases as sound propagates through the body. Sonographer can adjust by the power knob.

Power and amplitude

Proportional to each other, describe the size of the wave

If you double the power, what happens to your amplitude

Increases by 4 times

Intensity

Concentration of energy in a sound beam

Formula for intensity

Divide the beams power by the beams cross sectional area

Intensity = watts/ cm^2

Changes in intensity depend on

Characteristics of medium and shape of the beam. Sonographer can alter the intensity

Intensity is proportional to?

amplitude squared

And power

When power and intensity double, amplitude is?

1/4

Wavelength

Distance it takes to complete a single cycle.

Measured in distance

Unit for wavelength

Any unit of distance

Ultrasound uses wavelengths of what in soft tissue?

0.1- 0.8 mm

Wavelength is determined by what

The sound source and the medium.

Sonographer cannot adjust the wavelength, its built onto the transducer

Lower the frequency the _____ the wavelength?

longer

Higher the frequency the ____ wavelength?

Shorter

Shorter the wavelength ____ the period?

Shorter

Propagation speed

Distance that sound travels through a medium in one second.

AKA velocity or speed

Units of propagation speed

Meters per second, mm per micro second, or any distance divided by time

Propagation speed is determined by

The medium through which it travels and cannot be changed

If the medium is the same, the ____ is the same?

The speed of sound is the same

Lung

500 m/s

Fat

1450 m/s

Soft tissue

1540 m/s

Liver

1560 m/s

Blood

1560 m/s

Muscle

1600 m/s

Tendon

1700 m/s

Bone

3500 m/s

Air

330 m/s

Water

1480 m/s

Metals

2,000 — 7,000 m/s

Travels fastest in?

Solids

Travels slower in ?

Liquids

Travels slowest in?

Gases

Characteristics of the medium affect the what?

Propagation speed

Bulk modulus

Stiffness

Elasticity and compressibility

Opposite if stiffness

Density relates to?

Weight of the medium

Stiffer the medium?

Faster the speed

Denser the medium?

Slower the speed

Although density and stiffness both affect propagation speed, which has the bigger effect?

Stiffness

Formula for frequency

Frequency (MHz) x Period (sec) =1

Formula for period

Period (sec) = 1/ frequency (Hz)

Formula for frequency

Frequency = 1/time period

Power is proportional to

amplitude squared

Intensity is proportional to

Amplitude squared

Power and intensity are

proportional

Formula for intensity

Intensity ( watts/ cm^2) = power (watts) / beam area

Formula for wavelength

Wavelength (mm) = 1.54 mm per micro second/ frequency (MHz).

Formula for propagation speed

Propagation speed (m/s) = frequency (Hz) x wavelength (meters, mm)