Describing Sound Waves. Period. Frequency. Parameters used to completely characterize a sound wave. Chapter 3. Period Frequency Amplitude Power

Describing Sound Waves

Chapter 3

Parameters used to completely

characterize a sound wave

• Period • Frequency • Amplitude • Power • Intensity • Speed • Wave Length

Period

• Defined as the time it take one wave vibrate a single cycle. The start of one cycle to the start of the next cycle

• Measured in units of time. ex. Milliseconds, seconds.

• Determined by the source only and not the medium

• It is not adjustable by the sonographer

Frequency

As pertains to the ultrasound wave

• Defined as the number of complete cycles in one second.

• The unit of measure is the hertz. ex 1 cycle in 1 second = 1 hertz

• Typical clinical values range from 2 to 15MHz • Determined by the source and not the medium • not adjustable with some ultrasound systems and

Source and Medium

system and transducer combination.

• The sonographer can adjust some of the system parameters while others are fixed.

• The material or tissue through which the sound travels is refereed to as the medium.

Where did the term ultrasound

come from

• The human ear can only hear sound when it

is within an audible range. 20 Hz to 20 kHz

• sound that has a frequency below 20 Hz is

referred to as infrasound

• Sound that is greater than 20 kHz is referred

to as ultrasound

Frequency is important to the

very essence of ultrasound

imaging. It has pronounced

effects on both image quality and

depth of penetration

Period and frequency are inversely related to

each other.

• As frequency increases the period will

decrease

• As frequency decreases the period will

increase.

• Period and frequency also have a reciprocal

relationship

• When the two parameters are multiplied

together the result is 1

If a wave takes 1_/

5of a second to complete one cycle the period is 1_/

5second and the frequency is 5 per second. 1_/

5x 5 = 1

The frequency of this wave is 8Hz and the

period is 1/8 of a second

Parameters that describe the magnitude of a

sound wave are:

• amplitude

• power

• intensity

Amplitude

• Defined as the of a given wave either above the baseline or below the baseline

• The unit of measure is variable and includes

– pressure - pascals – density -g/cm3

– particle motion - cm, inches - any distance

• Amplitude can also be expressed in decibels (db)

Peak to peak amplitude

Amplitude

• Typically in clinical imaging pressure amplitude ranges from 1 million pascals (1MPa) to 3 million pascals (MPa).

• Determined only by the source

– the amplitude however does decrease as it propagates through the body and is also affected by the density of the medium as well • Adjustable with a control on ultrasound systems

allowing the sonographer to alter the initial amplitude

Power

• Is the rate of energy transfer, or the rate at which work is done

• The unit of measure is watts

• Typical clinical imaging values range from 0.004 to 0.090 watts

– also expressed as 4 to 90 milliwatts

Power

• Determined by the source and not the medium – However power does decrease as it propagates through the body

and is also affected by the density of the medium as well • It is adjustable with a control on the ultrasound

systems allowing the sonographer to alter the initial power of the wave

Power

• When power increase amplitude increases

and when power decreases amplitude

decreases.

• Power is proportional to amplitude

– as you increase the amplitude of a wave by a factor of 4 you have increased the power by 16. 4x4=16

– as you decrease the amplitude of a wave to ½ of the original value you have changed to power to ¼ its original value, ½ x ½ = ¼

Intensity

• Defined as the concentration of energy in a sound beam.

– Calculated by dividing the beams power by the beams cross-sectional area

• The units of intensity are watts/square centimeter (W/cm2₎

– watts from the power and cm2_{from the beam area}

– intensity (W/cm2) = power (w) area (cm2₎

• It depends on the power of the beam and over how big an areas it is spread out on

Intensity

• Typical clinical intensity values range from 0.01 to 300 W/cm2

• Determined by the source initially but its characteristics changes as it propagates through the medium and if one adjusts the beam shape. • It is adjustable with ultrasound systems allowing

the sonographer to either increase or decrease the initial intensity wave.

Intensity

Relationship between intensity and power

• Intensity is proportional to power

• if a the power of a wave doubles the

intensity also doubles

• if the power of a wave is cut in half so too

the intensity is cut in half

Intensity

Relationship between intensity and amplitude

• Intensity is proportional to the amplitude of

the wave squared.

• if the wave’s intensity is doubled the

amplitude is increased four times

Wavelength

• The distance or length of one complete cycle • Units of measurements include but not limited to

mm, meters

• In diagnostic imaging the wavelength in soft tissue range form 0.15 to 0.8 mm

• Is determined by both the source and medium • The sonographer cannot adjust the wavelength of

a particular transducer. (even a multihertz transducer has fixed wavelengths)

Wavelength

Wavelength

• Wavelength is inversely related to frequency • In soft tissue sound with a frequency of 1

MHz has a wavelength of 1.54mm • This can be converted into a formula to

calculate the wavelength of may MHz transducer

– wavelength (mm) = 1.54 mm/us

frequency (MHz)

Wavelength

• Wavelength plays an intricate role in role in ultrasound imaging

• The higher the frequency the shorter the wavelength and the higher the image quality • The lower the frequency the longer the

wavelength and the lower the image quality

Wave length of 3 different frequencies

Propagation speed

• Defined as the distance a sound wave

travels in 1 second through a medium

• Speed is measured in meters per second,

mm/μs, or distance divided by time

• Depending on the type of tissue the sound is

traveling through the typical speed ranges

from 500 m/s to 4000m/s

Propagation speed

• Speed is affected only by the medium

through which it travels

• Irrespective of the frequency all sound

travels at the same speed through the same

medium

• Speed changes only when it travels form

one medium to another medium of different

density

Propagation speed

• The average speed of sound in soft tissue is

1,540 m/s or 1.54 mm/μs or 1/54 km/s

• Sound travels fastest in solids slower in

liquids and the slowest is gasses

Speed(m/s) = frequency (Hz) x wavelength (m)

Propagation speed

3,500 Bone 1,700 Tendon 1,600 Muscle 1,560 Blood 1,560 Liver 1,540 Soft Tissue (average)

1,450 Fat 500 Lung Speed (m/s) Tissue Type

Propagation speed

2,000 to 7,000 Metals 1,480 Water 330 Air Speed (m/s Material

Characteristics of a medium

compression

– ex. a golf ball is stiff and Jell-O is not

• Stiffness and speed are directly related • If you & stiffness you & speed

• Elasticity and compressibility are terms used to describe a type of stiffness

– The skin is elastic – The appendix is compressible

Characteristics of a medium

• Density is the weight of a material

• When equal volumes of material are

compared the dense material will weight the

most

– Ex bone is more dense than muscle • Density and speed are inversely related