Parameters used to completely
characterize a sound wave
Describing Sound Waves
Chapter 3
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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
transducer combinations
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Source and Medium
• The source of a sound wave is the ultrasound
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.
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 also have a reciprocal
relationship
• When the two parameters are multiplied
together the result is 1
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
Period and frequency are inversely related to
each other.
• As frequency increases the period will
decrease
• As frequency decreases the period will
increase.
The frequency of this wave is 8Hz and the
period is 1/8 of a second
If a wave takes 1/5 of a second to complete one cycle
the period is 1/5 second and the frequency is 5 per
1/ x 5 = 1
second.
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Amplitude
Parameters that describe the magnitude of a
sound wave are:
• amplitude
• power
• 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)
• intensity
Peak to peak amplitude
Amplitude continued
• 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
• Determined by the source and not the medium
• not adjustable with some ultrasound systems and
transducer combinations
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Power continued
• 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
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 continued
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
Power continued
• When power increase amplitude increases
and when power decreases amplitude
decreases.
• Power is proportional to amplitude2
– 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 continued
• 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 continued
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
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Wavelength
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 continued
• 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) =
Wavelength continued
• 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
1.54 mm/us
frequency (MHz)
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
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Propagation speed continued
• 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 continued
• 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 continued
Tissue Type
Lung
Fat
Soft Tissue (average)
Liver
Blood
Muscle
Tendon
Bone
Speed (m/s)
500
1,450
1,540
1,560
1,560
1,600
1,700
3,500
Characteristics of a medium
• Stiffness is the ability of an object to resist
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
Propagation speed continued
Material
Speed (m/s
Air
330
Water
1,480
Metals
2,000 to 7,000
Characteristics of a medium continued
• 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
• As density & speed will '
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