Definition of Side lobes and the principle behind its production during ultrasound imaging. Side lobes artifact and its result on image. Explanation of harmonic imaging, its production and the techniques use to eliminate fundamental frequency to produce optimal harmonic images.

2. Outline
 Side Lobes
 What is Side lobe?
 How are side lobes produce?
 Side lobes artifact
 Harmonic Imaging Techniques
 Bandwidth Filtering
 Side by Side Cancellation
 Pulse Coded Harmonic

3. Side Lobes
Multiple beams of low-amplitude US energy that project radially
from the main beam axis
Side lobe beams are low-intensity beams that surround the
central beam
Weak, off axis beam

4. Side Lobes
Production of Side Lobe
1. Electrical Energy Piezoelectric Element Sound Waves
2. Transducer produce a “main lobe” of sound wave
3. Additionally, secondary lobes or “side lobes” are also
produced. (Radial contraction and expansion of piezoelectric
crystal)
4. Less intense, multiple sound beams which occur outside of
the main beam.

6. Side Lobes Artifacts
Strong reflectors present in the path of these low-energy, off-axis
beams may create echoes detectable by the transducer.
These echoes will be displayed as having originated from within the
main beam.
Artifact appear as a hyperechoic object within an anechoic or
hypoechoic structure e.g. urinary bladder and gallbladder

8. Side Lobe Artifact
Side lobe artifacts occur where side lobes reflect
sound from strong reflector that is outside of the
central beam, and where the echoes are displayed as
if they originated from within the central beam.

9. Side lobes artifact seen in the mucinous
ovarian tumor
Side lobe artifact in a cyst, leading to
possible misdiagnosis of septation cyst

11. Harmonic Imaging
The creation of an image from sound reflections at twice the
frequency of the transmitted sound.
The transmitted frequency is called the fundamental frequency.
The frequency of sound created by the transducer and transmitted
into the body.
As sound wave travels a small amount of energy is converted from
fundamental frequency to harmonic frequency.

12. Harmonic Imaging
Tissue nonlinearity distorts the fundamental sinewave pattern during
propagation through tissue,
Reason being the speed of propagation is slightly greater in the
compressed regions of the tissue than in the expanded regions.
Over time, this difference in propagation speed will distort even an
ideal sine wave into a sawtooth wave producing harmonic frequency
Resulting waveform distortion depends on the emitted pulse amplitude
and distance traveled by the fundamental Wave.

13. Harmonic Imaging
The fundamental and the second harmonic frequencies are
received together in the time domain as a combined distorted
wave.
High-quality harmonic imaging depends on complete elimination
of the echoes at the fundamental frequency

14. Harmonic Imaging
Various techniques are used to remove the fundamental
wave in harmonic imaging, including;
Bandwidth receive filtering
Pulse inversion
Side-by-side phase cancellation
Pulse-coded harmonics
Power Modulation Harmonics

15. Bandwidth Filtering
Technique to produce a narrow bandwidth to filter out the
spectrum of frequencies that arise from the fundamental beam
Higher-frequency harmonic echoes are used to generate the image.
In this technique, noise diminishes and image is enhanced.
However, narrowing the received bandwidth reduces axial
resolution.

16. Bandwidth Filtering

17. Pulse Inversion
Pulse inversion is a technique in which two pulses with a 180° phase
difference 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,
Harmonic waves at even frequency multiples of the fundamental frequency
are retained.
Axial resolution is not degraded and tissue contrast is better preserved.

18. Side-by-Side Cancellation
Similar to pulse inversion
This method sends both signals together at the same time
with opposite phases.
Then adds the echoes from the two opposite polarity
pulses to cancel the fundamental echoes, leaving only the
harmonic information.

19. Side-by-Side Cancellation
Fundamental Echo Harmonic Echo

20. Pulse Coded Harmonic
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 by a special
decoder that is part of the equipment
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

21. Fundamental and Pulse subtraction upper abdomen images

22. Advantages of Harmonic Imaging
 Harmonic get a “free ride” through the superficial tissues,
thereby remaining distortion free with less noise
The use of Harmonics increases the SNR
 Beams that are most likely to create harmonics are least likely to
create artifacts e.g. Side lobes artifacts
Harmonic improve poor quality images

23. References
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harmonic imaging in ultrasound scannersonography: evaluation of image
quality compared with conventional sonography. AJR Am J Roentgenol
1998;171:1203-6.
Anvari A, Forsberg F, Samir E. A. A primer on the physical principles of tissue
harmonic imaging. RadioGraphics. Nov-Dec 2015; 35:1955–1964.
DOI:10.1148/rg.2015140338.
Hann LE, Bach AM, Cramer LD, Siegel D, Yoo HH, Garcia R. Hepatic
sonography: comparison of tissue harmonic and standard sonography
techniques. AJR Am J Roentgenol 1999;173:201-6.