Physical Principles of Tissue Harmonic Imaging and Harmonics production

1. TISSUE HARMONIC IMAGING

2. TISSUE HARMONIC IMAGING
• Harmonic imaging is a (image optimization) technique in ultrasound that
provides images of better quality as compared with conventional
ultrasound technique.

4. RESOLUTION

5. Spatial resolution
Contrast resolution
Temporal resolution
Resolution in space
Resolution of grey
shades
Resolution in time
RESOLUTION

6. SPATIAL RESOLUTION
Axial resolution Lateral resolution

7. Ability of the imaging system to differentiate the echoes returning from different parts
of the body and display there amplitude in different shades of gray
CONTRAST RESOLUTION

8. TEMPORAL RESOLUTION
Represents the ability of the ultrasound system to distinguish
between instantaneous events of rapidly moving structure.
consecutive unique images
Continuous moving
image on real time
image monitor

9. SPATIAL RESOLUTION
TEMPORAL
RESOLUTION
CONTRAST
RESOLUTION
RESOLUTION
1. Frequency
2. Focus
Zone/Number
3. Compound
imaging
4. THI
5. Line Density
6. Edge enhancement
1. Gain
2. Dynamic Range
3. Grey Map
4. Colour B mode
imaging
5. Speckle reduction
1. Depth
2. Sector width

10. Increased
frequency
Increased - -
Increased focal
point
Increased - Decreased
Compound
Imaging
Increased Increased Decreased
Tissue Harmonic
Imaging
Increased Increased Decreased
Gain - Increased -
Dynamic Range - Increased -
Grey Scale - Increased -
Colour B Imaging - Increased -
Increasing Depth Decreased - Decreased
Increasing Sector
Width
Decreased - Decreased
Increasing Line
Density
Decreased - Decreased
Spatial resolution Contrast resolution Temporal resolution

11. PHYSICS
HARMONICS (A new grayscale imaging technique)
• Harmonic imaging exploits non-linear propagation of ultrasound through the body tissues.
• The high pressure portion of the wave travels faster than the low pressure portion resulting in
distortion of the shape of the wave.
• This change in waveform leads to the generation of harmonics (multiples of the fundamental or
transmitted frequency) from a tissue.
• Transmitting a band of frequencies centered at 2MHz will result in production of harmonic
frequency bands at 4 MHz, 6MHz . This doubled frequency sound is called the second harmonic
• At present, the 2nd harmonic is being used to produce the image because the subsequent
harmonics are of decreasing amplitude and insufficient to generate a proper image.
• These harmonic waves that are generated within the tissue, increase with depth to a point of
maximum intensity and then decrease with further depth due to attenuation. Hence there is an
optimum depth below the surface at which maximum intensity is achieved.

15. Tissue harmonic images are obtained by collecting harmonic signals that
are tissue generated and filtering out the fundamental echo signals that
are transducer generated resulting in crisper images.

16. TECHNICAL CONCEPTS AND TERMINOLOGY
• Fundamental frequency is the original frequency of the acoustic beam emitted from the
transducer. Harmonic wave generation is an acoustic phenomenon. Harmonic waves are integer
multiples of the fundamental frequeancy.
• The second harmonic (twice the fundamental frequency) is currently used for tissue harmonic
imaging (THI). With THI, the fundamental frequency is eliminated with image processing
techniques. THI advantages include improved signal-to-noise ratio and artifact reduction.

17. Linear (left) and nonlinear (right) US wave
propagation.
The transmitted pulse consists of a range of frequencies
centered around fc.
In a linear medium, the echo pulse frequency is the
same as the fundamental frequency but has lower
energy,
whereas the nonlinear medium results in harmonic
waves of higher frequency and lower energy in addition
to the fundamental frequency.
fc = fundamental frequency, 2fC = example of a
harmonic frequency.

18. HARMONIC FREQUENCIES
• Harmonic frequencies are integer multiples of the fundamental frequency (ie, if the fundamental
frequency is f, the harmonics have frequencies of 2f, 3f, and so on).
• The amplitudes of the harmonic waves are almost always lower than those of the fundamental
frequency waves.
• Subharmonic frequencies are integer fractions of the fundamental frequency (eg, f/2, f/3, and so
on).
• Subharmonic imaging is generally suitable for deep imaging because there is less attenuation of
lower-frequency subharmonic signals.
• Frequency multiples of the subharmonic frequency component (eg, 3f/2, 5f/2, 7f/2, and so on) are
called ultraharmonic frequencies.
• US ultrasound wave travels trough more tissues, more harmonics are generated
• The production of harmonics is proportional to the square of the fundamental intensity

20. Diagram shows the harmonic energy profile.
Higher-amplitude US pulses produce more harmonic waves.
Therefore, harmonic waves are predominantly created in the
central, most intense portion of the beam.
which causes a narrower imaging plane and reduced artifacts
because of side lobes and grating lobes.
Note the increase in harmonic wave energy with increasing
depth.
At the skin, tissue harmonics are virtually zero; their intensity
increases with depth to the point where tissue attenuation
predominates and harmonic amplitudes decrease.

21. FUNDAMENTAL WAVE– ELIMINATION
TECHNIQUES
Bandwidth Receive Filtering
Pulse Inversion
Side-by-Side Phase Cancellation
Pulse-coded Harmonics.

22. • Bandwidth Receive Filtering.—
Bandwidth receive filtering is a signal processing technique in which lower frequencies that are more likely to
have emerged from the fundamental beam are filtered out, and higher-frequency harmonic echoes are used to
generate the image.
In this technique, noise diminishes and enhancement is improved. However, narrowing the received bandwidth
reduces axial resolution because axial resolution can be estimated as the speed of sound in the tissue divided by
two times the bandwidth.
Selection of an appropriate cutoff frequency is a compromise between harmonic frequency signal loss and
contamination by the fundamental frequency signal. To overcome this limitation, pulse-inversion methods were
developed.

25. • Pulse Inversion.—
Pulse inversion is a technique in which two pulses with a 180° phase difference (ie, opposite phase) are emitted
sequentially into the tissue along the same line.
The summation of these received pulses results in fundamental echoes canceled out with odd harmonic
frequency components, and the retention of harmonic waves at even frequency multiples of the fundamental
frequency, with a doubling of the amplitude of even frequency multiple harmonic waves.
This technique is also termed a phase cancellation or a temporal cancellation technique. The principal advantage
of this technique is that axial resolution is not degraded and tissue contrast is better preserved.
pulse-inversion harmonic imaging is highly dependent on a fixed tissue frame, and tissue motion can markedly
degrade the US image. In addition, some reduction in frame rate will occur.

29. • Side-by-Side Phase Cancellation.—
Side-by-side phase cancellation is similar to pulse inversion, but two pulses with opposite phase are
transmitted along adjacent lines of sight. These adjacent lines are then added to cancel fundamental echoes
and odd harmonics.
This technique is a spatial cancellation technique. As with pulse inversion, side-by-side cancellation
preserves harmonic frequency bandwidth.

30. Pulse-coded Harmonics.—
• The pulse-encoding technique transmits relatively complex pulse sequences into the body with a
unique and recognizable code imprinted on each pulse.
• The unique code is then recognized in the echoes . Because the fundamental echoes have a
specific code, they can be identified and canceled.
• The remaining harmonic echo is then processed to form the image.

31. CLINICAL APPLICATIONS
THI improves image quality and conspicuity, and has been shown to be
useful in multiple clinical scenarios, including
(1) Obesity
(2) Hollow structures (e.g., Cysts, gallbladder, urinary bladder) ( figure 4-
1 ), and
(3) The deep-seated major vessels (inferior vena cava [IVC] and
abdominal aorta) ( figure 4-2 ).

32. A, Image without harmonic imaging: the fundus and neck areas (arrows) of the
gallbladder (GB) and intraportal venous area (PV, arrow) appear echogenic and
cloudy.
B, With harmonic technique, the figure shows a clear GB and portal venous
structure. In addition, the tiny calcification on the anterior wall of the
GB (arrowhead) is well shown on the harmonic image in B but invisible in the blurred
image in A.

33. Figure 4-2Comparison of image conspicuity without/with harmonic
imaging in the sagittal plane of the left liver and the long axis of the
inferior vena cava (IVC). The arrows point to the intra-IVC area, which is
obviously cloudy and blurring in the nonharmonic image (A) compared
with the harmonic image (B).

34. ADVANTAGES OF THI
• Improved contrast resolution
• Beneficial effects on artifacts
• Reduced noise in the near field
• Improved imaging of deeper tissue
• Improved lateral resolution and reduced section thickness

35. DISADVANTAGES OF THI
• Axial resolution is decreased by THI because of the narrowed bandwidth.
• Fundamental frequency imaging may be clinically more efficacious than harmonic imaging in other
situations, such as diffuse fatty liver, because of compromise of axial resolution from filtration-related
bandwidth reduction, and higher attenuation of the higher frequency harmonic component.

36. THI SAFETY
Both the thermal and mechanical indexes remain the same for THI as for conventional B-mode US, and
therefore THI is considered safe for routine clinical use.

37. Ms. Mamta Panda
mpanda483@gmail.com