Ultrasound physics involves the generation and propagation of sound waves. Sound is a mechanical wave that requires a medium and causes the vibration of molecules. Key acoustic variables that describe sound waves include pressure, density, and distance, which change as waves cause molecular compression and rarefaction. Essential parameters used to characterize ultrasound waves are period, frequency, amplitude, wavelength, and propagation speed. Understanding ultrasound physics is necessary for proper use of diagnostic ultrasound.

1. ULTRASOUND PHYSICS (1)
Dr. Kamal Sayed MSc US UAA
ok
sound/waves/MD us/wave parameters/speed/
frequency/period/power/intensity/amplitude/
FR/resolution/imaging depth/real time/ ok

2. What is the principle of ultrasound?
An electric current passes through a cable to the transducer and is applied to the crystals, causing
them to deform and vibrate. This vibration produces the ultrasound beam. The frequency of
the ultrasound waves produced is predetermined by the crystals in the transducer.
•
Physics is the branch of science concerned with the nature and properties
of matter and energy.
•
The subject matter of physics includes mechanics, heat, light and other
radiation, sound, electricity, magnetism, and the structure of atoms.
•
Physics is the natural science that studies matter, its motion and behavior
through space and time, and the related entities of energy and force
.
•
Physics is one of the most fundamental scientific disciplines, and its main
goal is to understand how the universe behaves. Wikipedia
•
Physics is a science that deals with matter and energy and their
interactions. a : the physical processes and phenomena of a particular
system. b : the physical properties and composition of something.
•

3. •
the father of physics is Galileo Galilei
•
The 7 branches of physics
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Mechanics. Motion and its causes; interactions between objects.
•
Thermodynamics. Heat and temperature.
•
Vibrations and Waves Phenomena. Specific types of repetitive
motions- springs, pendulums, sound.
•
Optics. Light (including mirrors), lenses, colors.
•
Electromagnetism. ...
•
Relativity. ...
•
Quantum Mechanics.

4. •
To be expert in US yu must be expert in US physics
•
To define medical diagnostic US yu must know what is sound ,
wave
•
‫ماهوالصوت‬ ‫؟‬ ‫الموجة‬ ‫ماهي‬ ‫؟؟‬ ‫صوتية‬ ‫موجات‬
‫موجة‬ ‫هي‬
wave
‫صوت‬ ‫وفيها‬
sound
Any motion which repeats in cycles is called a wave
it starts from zero point up to 360 degrees (this is called one cycle)
& when repeated it is called a wave
Therefore : a wave is a disturbance or variation that transferes energy
progressively from point to point in a medium
A medium is a substance or region through which a wave is
transmitted

5. •
These are mechanical waves
•
A wave is produced due to a disturbance from a source– eg.
(stone dropped into a stagnant water leads to waves
propagating from zero point to the end).
•
The stone caused a pressure on water surfae which carried
the energy from point zero to the far point.
•
This process results in :
•
1-wave propagation direction
•
2- particle motion
•
3- source of the wave

6. •
Therefore energy is transferred over distance BUT matter is NOT
because water particles move in a vibration motion (all media are
composed from many small molecules bound to each other with
intermolecular elastic forces/
•
‫صالبة‬ ‫اكثر‬ ‫الوسط‬ ‫كان‬ ‫كلما‬ ‫شديدة‬ ‫الجزيئات‬ ‫ربط‬ ‫قوة‬ ‫كانت‬ ‫كلما‬
more stiff
•
& when pressure is applied one molecule presses the next & so on
In vibration motion transferring the energy & NOT the water
molecules thus producing a wave).
•
This wave can be recorded in a graph shape thereby we can
measure the various wave variables (wave length , period, etc
•
A wave can be described by certain variables (parameters) or
•
‫متغيرات‬
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That change over time & space. These variables help to understand
how waves behave under various conditions

7. •
Wave variables are :
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Amplitude , frequency , & period
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1) Amplitude : is the degree of loudness, it
•
Is the sterngth or wave height which increases with magnitude of
pressure
•
2) frequency : (repetition of cycles) is the number of cycles per
second.
‫الواحدة‬ ‫الثانية‬ ‫في‬ ‫الكاملة‬ ‫الدورات‬ ‫وعدد‬ ‫التردد‬ ‫يعنى‬
•
3) period : the time it takes for a sound wave to complete one cycle
•
Period is the reciprocal of frequency : period = 1/frequency
•
In meters per second or mm/ms or ---
•
At 5 MHz the period is 0.2 ms/ the higher the frequency the
shorter the period
•
•

8. •
Ultrasonic sound is a cyclic sound pressure with a high
frequency than the upper limit of human hearing equal to
20KHz
•
Seven parameters that describe sound waves:
•
Period /frequency / amplitude /power / speed / intensity /
wavelength
•
Acoustic variables identify sound waves
•
Acoustic parameters describe the particular features of
•
sound waves.

9. The 3 main acoustic variables
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Pressure = Force per unit area or the Pascals (Pa) or pounds
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concentration of force per square inch (lb/in2)
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Density = Mass per unit volume Kilograms per centimeter
•
cubed (kg/cm3)
•
Distance = How far apart objects are Feet, inches,
centimeters,
•
or miles

10. •
Period
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Definition The time required to complete a single cycle.
•
Period can also be described as the time from the start of a
•
cycle to the start of the next cycle.
•
Example The period of the moon circling the earth is 28 days.
•
The period of class in high school may be 50 minutes.
•
Units : microsec, seconds, hours—all units of time
•
Typical Values 0.1 to 0.5microsec
•
0.0000001 to 0.0000005sec =
•
1 x 10-7 to 5 x 10-7sec =
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Determined By Sound source
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Cannot be Changed by Sonographer. Slide (11)

11. In Microseconds (μs), one millonth of a second
Period The time it takes for one cycle to occur

12. •
Frequency
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The number of certain events that occur in a particular time
•
duration.
•
In diagnostic ultrasound, the frequency of a wave is described
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as the number of cycles of an acoustic variable that occur
•
in one second
•
Units of frequency : Hz/sec
•
Frequency is Determined By Sound source
•
Cannot be Changed by Sonographer. Slide (13)
•

13. The number of cycles per second in Hz,KHz,MHz

14. •
The relationship between frequency and period.
•
period = 1/frequency
•
(T = 1/F)
•
increasing the frequency will reduce the period & vice versa
•
Formula for frequency.
•
frequency = propagation speed/wavelength = F=
C/Lambda
•
Propagation speed = elasticity (stiffness) / density
•
Propagation speed = The speed by which sound wave travels
through media in Meters per second (m/s) or millimeteres per
microsecond (mm/microsec)

15. •
TWO types of waves :
There are
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1- mechanical waves : eg sound , ocean & seisemic waves
•
These waves are longitudina & cannot propagate in vaccum. longitudinal
waves carrying energy & need a physical medium to be transferred
through it.
•
molecules are displaced horizontally parallel to the wave direction
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So : sound is a Mechanical Longitudinal Pressure wave transmitted in a /
Medium (medium is a must) sound is able to converge & diverge
* only certain sound frequencies are audible to human ear.
slide (16)
‫البشرية‬ ‫لالذن‬ ‫المسموع‬ ‫التردد‬ ‫يعني‬
‫ويساوي‬
20
‫الي‬
20،000
HERTZ
Typically The audible range for human ear hearing is :
20 & 20000 HERTZ
•

16. (LONGITUDINAL WAVE ) If a gust of wind blows over a field of tall grass , the stalks
bend back and forth in the same direction the wind is blowing

17. •
2- electromagnetic waves : eg X ray , gamma rays radio & light
•
Electromagnetic wave is a transverse wave & can propagate in
vaccum .
•
They are transverse waves.
•
Molecules move up & down ie perpendicular to the
wave
direction (90 degrees to the wave direction)
this represents electromagnetic waves like light, X-rays,
gamma rays.
•
slide (18/19)

18. (TRANSVERSE WAVE) spectators in a stadium dot the wave by moving arms up & down
at Right angles 90 degrees ie perpendicular to the horizontal direction

19. Transverse Wave Particles move in a direction perpendicular (at right
angles) to the direction of the wave (upper image)
Longitudinal Wave Particles move in the same direction as the wave (
lower image)
:

20. •
The intensity (strength)of sound we hear depends on the 1ST
pressure applied (source of sound)
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A very strong Earthquake in tusanami south east acia in 2010
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deep in the center of the ocean caused strong vibrations in
water molecules lead to very high waves which covered up to
the 3RD floor of the towers.

21. •
On earth air is a good medium for propagation of Sound.
Sound propagates in air with a speed different than in other
media depending on the properties of each medium.
•
Frequency is the number of cycles per second & measured in
units called Hertz (1 Hz = 1 cycle per second)
•
10 to power 3 = 1000 KHz
•
10 to power 6 = 1000000 = MHz (one million)
•
10 to power 9 = billion (miliard) = GHz =
•
= one thousand million=1000,000,000
•
10 t0 the power 12 = Tera = THz
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A 3 MHz probe gives 3 million cycles per second

22. •
Velocity of Sound in Various Materials
Material # Velocity (m/s)
----------------
_______
air 331
fat 1450
water (50°C) 1540
human soft tissue 1540
brain 1541
liver 1549
kidney 1561
blood 1570
muscle 1585
lens of eye 1620
skull-bone 4080
brass 4490
aluminum 6400

23. •
Air molecules are far apart with week intermolecular elastic
bonds & so air medium is compressible & not stiff
•
The molecules move in vibration motion & when pressure is
applied it becomes compressed & pushed together so that
due to temporary increase in density & the area is called area
of compression
•
When pressure is released that area of the wave becomes
more dense & called area of rarefaction due to temporary
change in density

24. •
After wave moves through the medium the medium particles
return to their resting position
•
Sound is a mechanical wave moves fast & generally doesnot
produce visible changes in the medium & is difficult to
visualise.
•
Sound wave is propagated by the same concept as the
previous example of the stone thrown into water :
•
Sound is a mechanical wave causes medium molecules to
vibrate & sound energy is transferred from point to point.
•
Sound needs a medium to be transferred & cannot be
transferred through vacuum
•

25. •
Anything related to sound is described as acoustic
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Acoustic pressure is the local pressure deviation from the
ambient (normal) pressure caused by the sound wave or to
quantify the pressure of the wave .
•
Maximum pressure elevation is called the amplitude, & the
greater the pressure the highest is the amplitude &
magnitude of the wave.
•
Amplitude is measured in units of pascal.

26. •
Sound
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Is A type of wave that carries energy from place to place.
•
• Created by the vibration of a moving object.
•
• Sound waves are comprised of compressions (increases in
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pressure or density ) and rarefactions (decreases in
•
pressure or density ).
•
• Sound cannot travel through a vacuum—sound must
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travel through a medium.
•
• Sound is a mechanical, longitudinal wave.
•
• Sound travels in a straight line.
•
Sound is a form of radiant energy.
•
Sound is NOT ionizing radiation.

27. •
What Is Sound?
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Sound is a form of energy. It is a pressure wave, created by a
mechanical action,
•
and is therefore called a mechanical wave. Sound is produced
when a
•
vibrating source causes the molecules of a medium to move
back and forth.
•
This backward and forward movement of the molecules
creates waves of
•
sound energy that travel, or propagate, through the medium.
A medium is

28. •
A medium is any form of matter: solid, liquid, or gas.
•
Sound requires a medium in which
•
to propagate; therefore, it cannot travel in a vacuum.
•
When sound energy propagates through a medium, it does so in
longitudinal
•
waves, meaning that the molecules of the medium vibrate back and
•
forth in the same direction that the wave is traveling . In summary,
•
sound is a mechanical, longitudinal wave. Longitudinal waves should
•
not be confused with transverse waves where molecules in a medium
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vibrate at 90 to the direction of the traveling wave

29. •
Acoustic variables are changes that occur within a medium as
a result of sound travelling through that medium
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The three primary acoustic variables are :
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pressure, density, and distance .
•
As stated in the previous
•
section, when sound energy propagates through a medium, it
causes the molecules to move back & forth.
•
Each back and forth movement completes
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one wave or one cycle of movement.

30. •
Each cycle consists of two parts:
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a compression, where the molecules are pushed closer
together, and a rarefaction,
•
where they are spread wider apart.
•
The molecules, as
•
they are squeezed together and separated, cause changes in
the pressure within the medium.

31. •
Similarly, molecules undergoing compression and
•
rarefaction show variations in density.
•
Density is defined as mass per unit
•
volume.
•
This movement of molecules, or particle motion, is due to
propagating
•
sound energy.
•
Distance is defined as how far apart objects are, and it
•
is the measurement of particle motion. Distance may also
be referred to as
•
vibration or displacement

32. •
Acoustic parameters (variables)
•
These are measureable quantities describing sound & include:
•
period, frequency, amplitude,
•
power, intensity, propagation speed, and wavelength.
•
Period and Frequency
•
Period (T ) is defined as the time it takes for
ohttps://www.slideshare.net/KamalEldirawine cycle to occur
•
Since period is measured in time units, it is most often described in
microseconds
•
(μs), or one millionth of a second
•
Frequency (f ) is defined
•
as the number of cycles per second . Frequency is measured in
•
hertz (Hz), kilohertz (kHz), or megahertz (MHz)
•

33. •
Frequency and
•
period are inversely related. Therefore, as frequency
increases, the period
•
decreases, and as frequency decreases, the period increases
•
Their relationship is also said to be reciprocal .
•
When two
•
reciprocals are multiplied together, the product is 1.
Consequently, period
•
multiplied by frequency equals 1.

34. •
One cycle consists of one compression & one rarefaction.
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Propagation Speed
•
Propagation speed (c) is defined as the speed at which a
sound wave travels
•
through a medium . All sound, regardless of its frequency,
travels
•
at the same speed through any particular medium. Therefore,
a 20-Hz sound
•
wave and a 20-MHz sound wave travel at the same speed in a
given medium

35. •
Propagation speeds tend to be the fastest in solids, such as
•
bone, and slowest in gases or gas-containing structures, such as the
lungs
•
In the body, sound travels at slightly different speeds through
•
the various organs and tissues. The units for propagation speed are
meters
•
per second (m/s) or millimeters per microsecond (mm/μs).
•
The average speed of sound in all soft tissue is considered to be
1540 m/s or 1.54 mm/μs.
•
This number was derived by averaging all of the actual propagation
speeds
•
of the tissues in the body.

36. •
Amplitude, Power, and Intensity
•
Amplitude, power, and intensity all relate to the size or strength of
the sound wave
•
All three of these decrease as sound travels
•
through a medium.
•
Amplitude (A) is defined as the maximum or minimum deviation
•
of an acoustic variable from the average value of that variable
•
For example, on a road trip, an average velocity may be 55 mph,
but occasional
•
increases of speed of up to 60 mph or decreases of speed down to
50
•
mph may occur.

37. •
In this situation, the amplitude would be 5 mph, because that
•
is the maximum and minimum variation from the average
velocity. Note that
•
the amplitude is not the difference between the maximum
and the minimum extremes.
•
As sound propagates through a medium, the acoustic
variables
•
(distance, density, and pressure) will vary, and therefore, they
may increase
•
or decrease.

38. •
To measure amplitude (the maximum variation of an acoustic
variable)is from baseline to peak.
•
With respect to sound resonance frequency of an ultrasound
transducer is determined by the peizoelectric crystal thickness
•
So thin crystals vibrate at a higher high frequency & vice versa.
•
Sound frequency is of 3 categories :
•
1- ultrasound : frequency above 20 KHz
•
2- infrasound : frequency beow 20 KHz
•
3- audible sound is between 20 Hz & 20 KHz (20,000 Hz)
•
Medical diagnostic US uses frquenices per mega hertz (MHz)
•

39. •
The amplitude of these changes can be measured.
•
When amplitude
•
is discussed in ultrasound physics, it is commonly the
pressure amplitude
•
that is being referenced.
•
The units of amplitude are Pascals (Pa).

40. •
•
Power (P) is defined as the rate at which work is performed or
energy is transmitted.
•
As a sound wave travels through the body, it loses some of its
energy.
•
Therefore, power decreases as the sound wave moves
through the body.
The power of a sound wave is typically described in units of
watts (W) or milliwatts (mW).
Power is proportional to the amplitude squared .
Therefore, if the amplitude doubles, the power quadruples.

41. •
The intensity (I ) of a sound wave is defined as
•
Intensity is the rate at which energy passes through unit area.
•
Average intensity of a sound beam is the total power in the beam divided
by the cross-sectional area of the beam. Power is the rate at which
energy is transferred.
•
the power of the wave divided
•
by the area (a) over which it is spread, or the energy per unit area.
•
Intensity is proportional both to power (I P) and to amplitude
•
squared (I A2). Intensity is measured in units of watts per centimeter
•
squared (W/cm2) or milliwatts per centimeter squared (mW/cm2).
Intensities
•
typically range from 0.01 to 100 mW/cm2 for diagnostic ultrasound.
Intensity
•
is discussed in more detail later in this chapter (See section “More about
•
Intensity”).

42. •
Amplitude.
This schematic illustrates how sound can be depicted as a sine
wave whose peaks
and troughs correspond to areas of compression and
rarefaction, respectively.
•
As sound energy propagates through tissue, the wave has a
fixed wavelength that is determined by the frequency and
amplitude that is a measure of the magnitude of pressure
changes.
•
slide (43/44)