1) Tissue Harmonic Imaging (THI) works by transmitting ultrasound at a fundamental frequency and receiving echoes at twice the fundamental frequency, known as the second harmonic. 2) As ultrasound propagates through tissue, a small amount of energy shifts from the fundamental frequency to the second harmonic frequency due to nonlinear behavior in tissue. 3) THI can improve image quality by reducing distortions that occur at the fundamental frequency, as the second harmonic signal distorts to a lesser extent.

1. US PHYSICS (7)
Dr. Kamal Sayed MSc US UAA
THI/mechanical index/acoustic parameters/

2. •
Tissue Harmonic Imaging (THI)
•
Is defined as Transmitting sound at a particular frequency
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(called the fundamental frequency), but creating an image
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From sound reflected at twice the fundamental frequency
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(called the harmonic or second harmonic).
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Fundametal frequency : The frequency of the transmitted
sound wave.
•
Harmonic frequency : Twice the transmitted freq. Also called
‘second harmonic.’

3. •
Example : A transducer transmits a sound pulse with a
•
a fundamental frequency of 2MHz.
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In the harmonic mode, an image created from 4 MHz sound
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reflections will be displayed.
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SEE SLIDE (4)
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As a sound wave travels in tissues, a miniscule amount of
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energy is converted from the fundamental frequency to
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the harmonic frequency due to non-linear behaviour.

5. •
When the fundamental image is suboptimal, the second
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harmonic may improve image quality.
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Harmonics work
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because the fundamental beam undergoes distortion
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(creating a bad image), while the harmonic signal distorts
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to a lesser extent.

6. •
TISSUE HARMONICS : The non-linear behavior of sound
propagating in the body
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causes energy to shift from the transmitted frequency to
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twice the transmitted frequency, the second harmonic.
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The further the sound wave travels, the more energy is
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transferred to the second harmonic.
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New frequencies, that
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were not originally present in the transmitted wave, are
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added as the wave propagates.

7. •
The non linear behaviour of sound propagating in the body
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Also causes more harmonics where the fundamental beam is
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strong. Few harmonics exist when the beam is weak -
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thus no harmonic side lobes.
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See slide (8)
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New frequencies magically "appear" in the sound beam after
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the beam is past the chest wall. The new frequency is
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twice that of the fundamental.

9. •
These new frequencies appear only where the beam is strong,
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in the main axis.
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Harmonics do not appear where the
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beam is weak, off axis and side-lobes. So, if we listen for
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the second harmonic only, the signal will arise only from
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the beam's main axis and will have substantially less
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distortion.
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Echoes most likely to produce artifacts are least likely
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to produce harmonics.
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Slide (10)

11. •
Pulse Inversion Harmonic Imaging
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a form of harmonic imaging where positive and
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Negative pulses are transmitted down each scan line.
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The negative pulse is the ‘inverse’ of the positive pulse.
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Harmonic images are created with this process.
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The major disadvantage of pulse inversion imaging is that the
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frame rate is half that of fundamental imaging .
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Thus pulsed inversion imaging degrades temporal
•
resolution, while improving spatial resolution (image
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detail accuracy) see slide (12) .
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13. •
CONTRAST HARMONICS
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Overall theory - send at a frequency, process reflections at
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twice the transmitted frequency. Transmitted sound
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strikes the bubble which behaves in a non-linear manner
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and creates harmonics.
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Mechanisms for harmonic production
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1. resonance - forced
oscillations
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2. bubble destruction

14. •
BUBBLE DISRUPTION Creates harmonics
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There is more bubble disruption with lower frequency &
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higher outputs (minimum pressure).
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Mechanical Index (MI) - directly proportional to the creation
of
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harmonics. The relationship between the shell and the
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internally trapped gas determines the contrast agent's
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stability and its longevity in the circulation.

15. •
MI = maximum rarefaction pressure / (frequency)^1/2
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Shell : Shells are designed to trap the gas within the bubble &
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prolong the bubble's persistence in the circulation.
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Gas bubbles without shells, (agitated saline), shrink and
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quickly vanish as the gas dissolves in blood.
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Shells are also designed to be flexible, so that they can
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change shape. More rigid shells tend to fracture.
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See slide (16)

17. •
The outgoing pulse must have little or no energy at the
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harmonic frequency
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Harmonic transducers have a narrower bandwidth
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The US system needs to pass the harmonic frequency but
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eliminate all other frequencies.

18. •
Acoustic parameters (variables)
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These are measureable quantities describing sound & include:
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period, frequency, amplitude,
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power, intensity, propagation speed, and wavelength.
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Period and Frequency
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Period (T ) is defined as the time it takes for one cycle to occur
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Since period is measured in time units, it is most often
described in microseconds
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(μs), or one millionth of a second
•

19. •
Frequency (f ) is defined
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as the number of cycles per second. Frequency is measured in
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hertz (Hz), kilohertz (kHz), or megahertz (MHz)
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Frequency and
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period are inversely related. Therefore, as frequency
increases, the period
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decreases, and as frequency decreases, the period increases
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Their relationship is also said to be reciprocal .
•
•

20. •
When two
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reciprocals are multiplied together, the product is 1.
Consequently, period multiplied by frequency equals 1.
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One cycle consists of one compression & one rarefaction.
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Propagation Speed
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Propagation speed (c) is defined as the speed at which a
sound wave travels
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through a medium .

21. •
All sound, regardless of its frequency, travels
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at the same speed through any particular medium.
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Therefore, a 20-Hz sound wave and a 20-MHz sound wave
travel at the same speed in a given medium.
•

22. •
Amplitude, Power, and Intensity
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Amplitude, power, and intensity all relate to the size or
strength of
the sound wave
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All three of these decrease as sound travels
•
through a medium.
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Amplitude (A) is defined as the maximum or minimum
deviation
•
of an acoustic variable from the average value of that
variable

23. •
For example, on a road trip, an average velocity may be 55
mph, but occasional increases of speed of up to 60 mph
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or decreases of speed down to 50 mph may occur.
•
•

24. •
In this situation, the amplitude would be 5 mph, because that
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is the maximum and minimum variation from the average
velocity. Note that
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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.

25. •
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
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So thin crystals vibrate at a higher high frequency & vice versa.
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Sound frequency is of 3 categories :
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1- ultrasound : frequency above 20 KHz
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2- infrasound : frequency beow 20 KHz
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3- audible sound is between 20 Hz & 20 KHz (20,000 Hz)
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Medical diagnostic US uses frquenices per mega hertz (MHz)
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26. •
The amplitude of these changes can be measured.
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When amplitude
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is discussed in ultrasound physics, it is commonly the
pressure amplitude
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that is being referenced.
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The units of amplitude are Pascals (Pa).

27. •
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Power (P) is defined as the rate at which work is performed or
energy is transmitted.
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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.

28. •
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).

29. •
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.

30. •
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 (31/32)

33. •
Medical diagnostic US is a pulse echo imaging technique used
depends on the amount of reflection
&
in sonar or sound
navigation (to send a pulse & receive echo)
But xrays depend on ionizing radiation to penetrate organs.
The piezoelectric crystal in the transducer converts the electrical
energy voltage into short pulses of mechanicl sound energy.
Then the back-reflected echos (mechanical sound energy) are
received also by the transducer & converted into electrical
energy once more.

34. High frequency probes (short wave length)are used for better
resolution & used to scan superficial structures
Low frequency probes have long wave lengthwith better
penetration to deeper structures but poorer resolution.
So best images are obtained using the highest frequency that
will penetrate the region of interest
Modern machines have new software called THI (tissue
harmonic imaging) which combines between good resolution
& better penetration thus improved image quality

35. •
Range of frequencies of medical diagnostic US used in
scanning different structures :
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1- general abdomen 2 – 5 MH
2- GB 3-5 MHz
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3- OBGYN up to 7.5 MHz (TVS)
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4- adult heart 2-3
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5- neck , breast , scrotum 5-15
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6- ophthalmology 7- 15
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7- endoluminal up to 30 MHz
•

36. •
Wavelength is the distance or space needed for one cycle to
occur.
•
In diagnostic ultrasound wavelength is measured in meters
(m) and millimeters (mm).
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It is important in image resolution.
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For given sound frequencies the higher the speed of sound
the longer the wavelength
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Wave length is directly proportional to speed of sound &
inversely proportional to frequency.

37. •
Wavelength is determined by 2 factors :
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1- transducer frequency
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2- speed of sound in a medium
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High freq has short wavelength.
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Low freq has long wavelength.
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Wavelength is an important factor affecting quality of
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US image : short wavelength means high frequency leading
to good resolution & poor penetration & vice versa
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Wavelength = speed of sound/frequency
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Slide (38)

38. (WAVELENGTH) Is the distance from the begining of a cycle to the end of that
cycle in millimeters (in mm)

39. •
Sound waves must have a medium to pass through.
•
The speed at which a sound wave travels through a medium is
called the propagation speed or velocity.
It is equal to the frequency times the wavelength.
In ultrasound it is measured in meters per second (m/s) or
millimeters per microsecond (mm/µ s)
•
In general, the propagation speed of sound through gases is
low, liquids higher and solids highest.
The average propagation speed for sound in body tissue is
1540 m/s, or 1.54 mm/µs.